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Near-Term Interstellar Probes: Some Gentle Suggestions

When Greg Matloff’s “Solar Sail Starships: Clipper Ships of the Galaxy” appeared in JBIS in 1981, the science fictional treatments of interstellar sails I had been reading suddenly took on scientific plausibility. Later, I would read Robert Forward’s work, and realize that an interstellar community was growing in space agencies, universities and the pages of journals. Since those days, Matloff’s contributions to the field have kept coming at a prodigious rate, with valuable papers and books exploring not only how we might reach the stars but what we can do in our own Solar System to ensure a bright future for humanity. In today’s essay, Greg looks at interstellar propulsion candidates and ponders the context provided by Breakthrough Starshot, which envisions small sailcraft moving at 20 percent of the speed of light, bound for Proxima Centauri. What can we learn from the effort, and what alternatives should we consider as we ponder the conundrum of interstellar propulsion?

by Dr. Greg Matloff

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Marc Millis, Paul Gilster and their associates of the Tau Zero Foundation are to be congratulated on the recent award of a $500,000 NASA grant to investigate the prospects for a near-term interstellar probe. As one of the co-authors of The Starlight Handbook, the author of Deep-Space Probes and many interstellar related papers, a former NASA consultant in this field and an Advisor to Project Starshot, I would like to offer some gentle and very personal suggestions about how to best spend this money. Since it is unlikely that I can attend this year’s Tennessee Valley Interstellar Workshop, I have elected to submit these concepts to Centauri Dreams.

Motivation

The basic reason for an early interstellar endeavor is knowledge acquisition. Data acquired by a star-probe en route to its destination includes in situ measurements of the interstellar medium including ions, neutral atoms, dust grains and cosmic rays. Of particular interest to designers of eventual human-carrying star arks is measurements of the directionality of high-Z cosmic rays. If these originate from discrete sources in and beyond our galaxy rather than being omni-directional, the problem of shielding a space ark will be more readily solved.

Another possible function of such a probe is extra-galactic astrometry. If the probe carries a telescope, the very-long baseline observations possible when pairing with solar-system instruments during interstellar cruise should yield valuable data regarding distances and kinematics of extra-galactic objects.

During the interstellar transfer after the probe’s distance from the Sun exceeds 550 AU, the Sun’s Gravitational Focus can be applied to obtain greatly amplified images of astrophysical objects occulted by the Sun. Trajectory deviations farther along the probe’s interstellar track might indicate the presence of elusive dark matter.

Upon arrival in the destination planetary system, investigation of planets within the target star’s habitable zone will be the highest priority. Does life evolve on any water-rich world within the liquid-water temperature range, if that world has an atmosphere? Or are special conditions such as a massive satellite a requisite?

If living planets are commonplace, do technology and civilization naturally evolve? Because we have received no unambiguous signals from hypothetical advanced extraterrestrial civilizations and intelligent ETs are apparently rare or non-existent in our solar system, our early interstellar robots should be configured to investigate the “Eerie Silence” (as Paul Davies has dubbed it) and Fermi’s Paradox (“where is everybody?”). Do advanced ETs perhaps evolve in a non-technological direction, or do they generally self-destruct? Or do they generally elect to remain radio silent and not engage in interstellar exploration and colonization?

Destination

I will next consider the probable destination for a probe that we might conceivably launch in the 2050-2100 time frame. Our early probes should almost certainly be directed towards the nearest stars—the Proxima/Alpha Centauri triple star system.

This system, which is estimated to be about 6 billion years old, consists of two central Sun-like stars (Alpha A and Alpha B) and a red dwarf companion (Proxima). Alpha A and B orbit their common center of mass in an elliptical orbit with a period of about 80 years. At their closest (periapsis), Alpha A and Alpha B are separated by about 9 Astronomical Units. At their farthest (apoapsis), their separation is in excess of 30 AU. Each of the central Centauri suns could have planets orbiting within their habitable zones. Alpha A/B Centauri is about 4.27 light years from the Sun.

Proxima Centauri is a bit closer at 4.24 light years from the Sun. It is quite possible (but not definite) that this star is gravitationally bound to the Alpha A/B even though its current separation from Alpha A/B is about 15,000 Astronomical Units.. During the summer of 2016, the discovery of a planet with a probable mass 30% greater than Earth orbiting Proxima Centauri within that star’s habitable zone was announced. A less-than-poetic designation for this planet is Proxima b Centauri.

Although several research teams are investigating the possibility of habitable worlds attending Alpha A or Alpha B Centauri, the discovery of Proxima b was totally unexpected. Since the nearest star to the Sun has a probable planet orbiting within its habitable zone, it is reasonable to conclude that such worlds are very common in our galaxy.

Achievable Interstellar Transit Duration

Our early extrasolar probes— Pioneer 10/11, Voyager 1/2, and New Horizons— don’t really count as starships. Yes, they have left or will eventually leave our solar system and move freely through the Milky Way galaxy. But their propulsion systems—chemical rockets combined with giant-planet gravity assists are not effective enough for true star voyaging. Even the fastest of these would require about 70,000 years to reach Proxima/Alpha Centauri if it happened to be pointing in the right direction (which it isn’t).

A human colony ship, often called an interstellar ark or world ship, could probably be designed using near-term technology such that it could survive a millennial journey to our nearest stellar neighbor. But such a long travel time for a robotic probe would be difficult to sell to the scientific community since most research participants would prefer to see some results within their lifetimes.

So the Breakthrough Initiatives project Breakthrough Starshot pushes technology to its limits on numerous fronts in order to design a starcraft capable of traversing the enormous distance between the Sun and Proxima/Alpha Centauri in about 20 years.

Everything about Starshot is enormously challenging. A hyperthin sail with dimensions up to a few meters on a side must be generated. It must have near perfect reflectivity, high emissivity, low areal mass thickness and very high melting point. This is necessary for it to survive a several minute exposure to a 50-100 GW laser beam without melting. By the way, it must also have enormous tensile strength in order to support the nano-payload during the acceleration process. The sail must also be configured to maintain stability within the beam.

The laser array would likely be mounted atop a Southern Hemisphere mountain, in order to point at Alpha Centauri. Adaptive optics must be used not only to compensate for the effects of Earth’s atmosphere but to insure that the beam completely fills the sail during the acceleration process at distances measured in millions of kilometers. Also, since a single continuous wave 50-100 GW laser is somewhat beyond current capabilities, thousands of smaller lasers must be synced together to produce the beam.

Assuming that the sail survives the acceleration process, it must possess ample on-board intelligence to perform several tasks independent of Mission Control. First, it should reorient itself to travel edge-on rather than broadside through interstellar space. This is necessary to reduce the effects of dust grain impacts. Although interstellar dust is rare in our galactic vicinity, even a single grain moving at 0.2c (60,000 kilometers per second) relative to the sail has an enormous wallop.

But we’re not done yet. Approaching Proxima/Alpha Centauri, the sail must reorient itself once again to allow its instrument suite to survey the environment of the destination stars and to send the results towards Earth. A very tall order indeed for a ~gram-massed nano payload.

None of the above challenges present physical impossibilities. The question is whether they can all be achieved in a single nano-spacecraft within the next few decades.

So any NASA-funded interstellar initiative intended for possible implementation within the next few decades should not attempt to duplicate the goals of Project Starshot. Rather than a 20-year travel duration, a 100-year flight time might be more realizable in the near term. Mission planners need to realize that even this is quite a challenge. A 100-200 year travel duration might be a reasonable goal.

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Image: Artist’s concept of the Breakthrough Starshot sail under beamed acceleration. Credit: Breakthrough Initiatives.

Proposed Propulsion Systems

Many propulsion systems have been proposed to enable interstellar exploration and colonization. Only a few have any hope of being feasible in the near term. Before we get to the near-term possibilities, it might be nice to review some of the more exotic suggestions.

Space Warps, Wormholes, and Hyperdrives

It would indeed be lovely if one of these devices emerged from the realms of science fiction and Hollywood special effects into the real world. Then we could wander the star lanes with the same dispatch that we book a flight to Europe.

Unfortunately, all of these short-cuts through space-time require either enormous amounts of energy, exotic forms of matter or new physics. It seems wise to continue research in these possibilities. There is no telling when or if a breakthrough might occur. But it would be unwise to hold our collective breaths.

Thrust Machines

In the 1960’s, we were treated to the famous Dean Drive. Now engineers in several international locations are testing the Shawyer EM Drive. These and similar devices apparently violate one of the basic laws of classical mechanics: Conservation of Linear Momentum. Although excess unidirectional thrust seems to be generated by the EM Drive, Marc Millis has described in this blog numerous possible causes for this effect that do not violate this law.

Before any proposed thrust machine can be seriously considered for application to interplanetary or interstellar propulsion, it must demonstrate excess thrust in outer space conditions. Two venues for preliminary in-space tests are stratospheric balloons and sub-orbital rockets. If these succeed, a follow-on demonstration would be a dedicated cubesat containing the device deployed in Low Earth Orbit.

The Matter/Antimatter Rocket

This physically possible interstellar propulsion system utilizes total conversion of matter to energy in the reaction between matter and antimatter. Sadly, we are a very long way from the capability of creating the necessary mass of antimatter in a reasonable time frame. If we applied humanity’s best antimatter factory (the Large Hadron Collider) to the the task of full-time antimatter production, we might have a gram of the stuff after 100 million years.

Another problem is storing the antimatter. Charged sub-atomic particles can be stored in Penning Traps for periods of weeks. These devices use crossed electric and magnetic fields to contain the particles. If applied in space travel, how would the trap’s fields compensate for variable spacecraft acceleration? Also, might stray cosmic rays heat and divert the anti-ions so that they explosively interact with the walls of the containment vessel?

Perhaps it’s a good thing that application matter-antimatter technology does not seem a near-term possibility. Our security would be jeopardized enormously (and probably terminally) if terrorists could smuggle city-killing weapons in thimble-sized containers.

Ramjets and EM Sails

By far the most elegant of physically possible interstellar spacecraft is Robert Bussard’s fusion ramjet. This craft utilizes an electromagnetic (EM) scoop to collect interstellar hydrogen over a large area and redirect the plasma to a proton-proton fusion reactor. Energized fusion products (helium nuclei) are exhausted out the rear of the craft. An ideal ramjet, accelerating at 1g could reach near-optic velocities in about a year Earth time. Because of relativistic effects, the craft could cross the galaxy within the crew’s lifetime, according to on-board clocks.

Sadly, there are a few problems with the proton-fusing ramjet. First and most significant is the difficulty of igniting the proton-proton thermonuclear reaction. This reaction, which powers main sequence stars such as our Sun, is many orders of magnitude more difficult to ignite than the fusion reactions we currently experiment with. One way around this is to consider lower performance ramjet alternatives such as the ram-augmented interstellar rocket (RAIR) that carries on-board fusion fuel and uses scooped protons as additional reaction mass.

But even that approach is limited by the limitations of EM scoops that have been suggested to date. Most (including those considered by this author) function better as proton reflectors or drag sails—very good for interstellar deceleration but not too effective for achieving high velocities. The one exception to this is Brice Cassenti’s toroidal scoop, suggested in the late 1990’s. But because this scoop utilizes an array of superconducting wires projected in front of the spacecraft, only accelerations of the order 0.01 g are possible.

In the near future, the best we can likely hope for to apply ramjet technology is in-space experiments using electric and magnetic sails to reflect the solar wind. This might encourage the perfection of both an interplanetary propulsion option requiring no on-board fuel and experimental tests of an approach to interstellar deceleration.

Beamed Propulsion

It is unclear whether Project Starshot’s imaginative enterprise will be successful. Even if a beam projector is located on a high mountain, it is not known how rapidly it can be adjusted to compensate for atmospheric turbulence. Another unknown is whether the beam-steering mechanism will be efficient enough to keep the beam output directed at Alpha/Proxima Centauri for several minutes. Finally, much analysis is required to insure that the beam is centered on the sail and fills the sail during the acceleration process.

Any funded consideration of interstellar probes would be wise, however, to investigate terrestrial and in-space experiments to demonstrate the utility of beamed propulsion. These could be far less ambitious and expensive than the Project Starshot concepts.

For example, imagine two cubesats launched simultaneously into Low Earth Orbit. One contains a wafer sail. Its neighbor deploys a very low-power laser or maser projector. The beam is focused on the unfurled sail. It should be possible to monitor both sail acceleration and stability in the beam.

Another possibility is to repeat an experiment originally planned for the failed Planetary Society Cosmos-1 Earth-orbiting solar-photon sail. After the sail is unfurled, a microwave beam from a terrestrial radio telescope could be focused on the sail. If sail stability and acceleration can be demonstrated, this will advance the possibility of Earth-escape by low-orbit photon sails as well as furthering the cause of interstellar travel.

Theoretical researchers might also expand the concept of particle-beam propulsion. Because electrically charged sub-atomic particles carry significantly more linear momentum than photons, it would be interesting to develop an understanding of particle-beam collimation over interplanetary and interstellar distances.

But there is a geopolitical obstacle to the construction of a ~gigawatt laser-, maser-, or particle-beam projector in space. Such a device could be applied to accelerating a starship or diverting an Earth-threatening asteroid; it could also be construed as a weapon.

If such an enormous beam projector could be constructed in space and could maintain its aim for decades, a hybrid interstellar propulsion system might ultimately become feasible. This is the laser ramjet. In such a vehicle, interstellar ions collected by a Cassenti EM scoop could be accelerated by energy beamed from the solar system.

Fission-Electric Propulsion

Nuclear fission has been an available energy source for more than 70 years. The solar-electric rocket (or ion drive) has been used successfully on several interplanetary probes. One reasonable approach to interstellar travel is to remove the solar panels and connect the ion drive’s thruster to a nuclear-fission reactor. In such a device, the reactor energy output would ionize propellant atoms (or molecules) and accelerate the resulting ions out the rear of the spacecraft.

There are at least three factors limiting interstellar application of fission-electric propulsion. One is propellant availability. To reduce thruster erosion, the inert gas xenon is used as propellant in most current solar-electric drives. Applying this approach to the much more massive fuel requirement of an interstellar probe would likely far exceed the annual terrestrial production rate of xenon. Alternative propellants should be investigated.

Then there is the matter of geopolitics. Many citizens of our planet would be somewhat unnerved if one of the major space powers began to store the large amount of fissionable material required in Low Earth Orbit during construction of the massive probe. One way around this is to construct the probe as an international project, similar to that applied to creation and operation of the International Space Station.

Technology is another limitation. Present day ion thrusters are limited to exhaust velocities of about 100 kilometers per second. So a nuclear-electric rocket launched using current technology might require 10,000 years to reach Alpha/Proxima Centauri.

Exhaust velocity must be raised to at least 1000 kilometers/second to propel a “1000-year ark”, as discussed by Les Shepherd in his 1952-vintage JBIS paper on interstellar travel. To reduce probe flight time to 100 years or so, the ion-exhaust velocity must be increased by another order of magnitude.

Another required improvement to implement ion-propelled interstellar travel is the reduction of the propulsion system’s specific mass (kilograms/kilowatts). As my late friend, the UK propulsion expert Dr. David Fearn once told me, such a reduction is challenging but ultimately not impossible.

Thermonuclear Fusion Rockets

There are two major types of fusion under development. Magnetic fusion, which confines the reacting plasma in EM fields, seems to always be a few decades in the future. Some have quipped that it is the energy source and the propulsion system of the future and always will be.

Small scale inertial fusion confines and compresses micropellets using crossed electron or laser beams. Large scale inertial fusion—the hydrogen bomb—accomplishes confinement and heating reactants using fission charges, and has of course been operational for more than 60 years.

Large scale inertial-fusion propulsion was first investigated during the early space age by NASA and the US Department of Defense in the original Project Orion. The first demonstration in a scientific journal of the near-term feasibility of large-scale interstellar travel was Freeman Dyson’s original paper on an interstellar Orion in the October 1968 issue of Physics Today. Assuming propulsion by exploding hydrogen bombs, Dyson demonstrated that the US and USSR Cold War nuclear arsenals were sufficient to dispatch thousands of migrants on colonization ships. The estimated duration of one-way voyages to Alpha/Proxima Centauri was 130-1,300 years.

In an ideal world, the former Cold War adversaries would be glad to donate their now-obsolete thermonuclear arsenals to the worthy cause of promoting an interstellar diaspora. Sadly, we do not live in such a world.

Even if nuclear “devices” would be donated to the worthy cause of interstellar exploration/colonization, there are a few technical difficulties to contend with. Unless we can master aneutronic fusion reactions such as the boron-proton scheme, it must be demonstrated that spacecraft structures can survive periodic high-energy thermal-neutron doses.

Application of fusion micro pellets also has a number of technical issues. First, there is the problem of fuel availability. To reduce neutron irradiation on ship structures, the Daedalus study of the British Interplanetary Society (BIS) considered a Deuterium-Helium3 fusion fuel cycle. The problem is that Helium3 is very rare on Earth. To construct a Daedalus craft, cosmic helium sources must be tapped—perhaps the lunar regolith, atmospheres of giant planets or the solar wind.

The BIS follow-up to Daedalus, called Icarus, uses a Deuterium-Tritium fuel cycle. Here, it might be necessary to breed Tritium in nuclear fission reactors.

Some engineering issues must be addressed before Daedalus/Icarus-type pulsed fusion ships can become operational. What are the acoustic effects of repeated fusion ignitions within the reaction chamber? Will the walls of the reaction chamber be damaged if laser- or electron-beams miss a fuel pellet?

Another significant issue is the enormous size of inertial fusion ships. Even if payload mass can be drastically reduced, the beam projectors, reaction chamber and associated gear are massive.

One suggestion to reduce the mass of an inertial-fusion propelled spacecraft is worthy of future study. That is Johndale Solem’s Medusa concept. In Medusa, the massive reaction chamber is replaced by a hyper-thin, high-melting-point, radiation-tolerant sail. Fusion charges are ignited within this flexible canopy, which is connected to the payload by strong cables.

The Solar-Photon Sail

There are several reasons why photon sails have emerged as the near-term interstellar propulsion system of choice. First, small photon sails have been unfurled and operated in Earth orbit and interplanetary space.

Second, the photon sail can be scaled with the payload. A payload-on-a-chip requires a small sail. If the payload is small enough, sail and payload can be deployed from a small cubesat. Sail deployment and integration with payload can therefore be based upon current operational experience.

But today’s multi-layer solar-photon sails are not really capable of interstellar travel. Even if sail acceleration is combined with giant-planet gravity assists, it seems clear that Alpha/Proxima Centauri travel times less than 10,000 years will be difficult to achieve.

The best we can expect from current solar-photon sails is exploration of the heliopause at around 550 AU, the Sun’s gravity focus at >550 AU, and the inner reaches of the Sun’s Oort Comet Cloud.

In all likelihood, interstellar probes launched by solar-photon sails will never be as fast as those launched by laser-photon or maser-photon sails. The reason for this is that solar irradiance is an inverse square phenomenon—acceleration at Jupiter is 1/25 that at Earth’s solar orbit. A collimated and accurately aimed beam could maintain sail acceleration over much greater distances.

But the advantage of solar-photon over beam-photon sails is that mission designers need not concern themselves with the beam-projection system. The solar constant should not vary too much for the foreseeable future.

So a number of researchers have evaluated the possibility of all-metal sails, dielectric sails, carbon nanotube sails and mesh sails. But the ultimate sail material might be a molecular monolayer such as graphene.

Graphene is a hyper-strong layer of carbon, one molecule thick. Its melting point is in excess of 4,000 K and it is impermeable to many gases. In the visible spectral range, graphene is essentially transparent. Its fractional visible absorption is 0.023. As I describe in a 2012 JBIS paper, combination with other materials can increase reflectivity to about 0.05 and absorption to ~0.4. Graphene sails carrying robotic payloads and unfurled near the Sun seem capable of reaching Alpha/Proxima Centauri in a few centuries. Because human-carrying arks are limited to ~3g accelerations, these larger ships require about 1,000 years to reach these stars if they are propelled by graphene sails.

But here is where Project Starshot can play a very major role. In order to reach ~0.2c in a ~50 GW laser beam without melting, the sail reflectivity to laser light must be very high. Perhaps this can be achieved with an appropriate mesh-like meta material. Or perhaps the reflectivity of molecular monolayers such as graphene can be greatly increased.

After the Project Starshot workshop last August, participants produced draft Requests For Proposals (RFPs). I have discussed the possibility of increasing graphene reflectivity with theoretical condensed-matter researchers at my home institution (CUNY). It is quite possible that they will submit a proposal in response to the RFP when it is issued.

If monolayer reflectivity can be greatly increased, it will be necessary to demonstrate that this action does not adversely affect monolayer tensile strength so that the wafer sail is strong enough to support the payload during a very close solar approach. It will also be necessary to demonstrate that sail and payload can survive the very hostile environment encountered near the Sun.

A solar-photon sail will likely never achieve the ~0.2c interstellar velocity of the laser-boosted Project Starshot sail. But, just possibly, solar-photon-sail terminal velocities capable of making the journey to Alpha/Proxima Centauri in a century or so may not be totally infeasible.

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Comments on this entry are closed.

  • Daniel de França Diniz Rocha May 26, 2017, 10:25

    I have a more modest suggestion. Sending hundreds or thousands of cheap mini satellites, with 300g – 1kg, towards asteroids and dwarf planets, using ion propulsion. Much of the price is due hardened pieces to thrive in space situations. But if the science measuring precision were lowered, and in higher quantities. It should compensate.

    I wonder if we could do a lot more science by sending thousands of cheap “nano”-Voyagers, with all miniaturized components, due the natural evolution of instruments.

    • Greg Matloff May 26, 2017, 18:58

      Dear Daniel

      Perhaps we could develop nano-probes for solar system exploration that could evolve over time into star probes.

      Greg

      • Daniel de França Diniz Rocha May 27, 2017, 20:00

        Dear Greg,

        Thank you for taking your time to answer my question. That’s what I had in mind. So, to convince the scientific community, we have to make a deep impression, similar to the revolution in DNA sequencing in the last 15 year. We are advancing to the point that the vast majority bacteria known nowadays is not cultivated in lab, but merely constituted by sequences diluted in water. The behavior is deduced by comparison of homologous genes.

        So, lowering the price of exploration to absurdly low levels, is perhaps a way to convince people. Get rid of of nuclear weapons is a farfetched thing in the mind of great military powers, and more over, use them in space, is truly scary to the general people. So, we have to considerably change the minds and hearts of people otherwise, that will be an utopia.

        The use of solar sails might be expensive too, despite being not that a utopia. It would be seen as a waste of money by the public. So a similar approach would be valid.

        Would you be willing to exchange some ideas with me? I have some uses for graphene, amazing ones, I’d like though to discuss them more in private. It would truly impress the public and raise awareness.

        My email is danieldiniz at gmail dot com

        • Greg Matloff May 29, 2017, 7:56

          Dear Daniel

          I will e-mail you.

          Regards, Greg

        • Michael May 29, 2017, 11:11

          If you are interested in graphene you will not go wrong reading this article, it is large but sets some solid foundations for the future of a remarkable material.

          http://pubs.rsc.org/en/content/articlehtml/2015/nr/c4nr01600a

          Bon appétit

  • Winchell Chung May 26, 2017, 11:07

    Is there a paper discussing Brice Cassenti’s toroidal scoop?
    The only reference I can find is in chapter 2 of Frontiers of Propulsion Science, a brief mention of a personal communication between B. Cassenti and R. H. Frisbee

    • Greg Matloff May 26, 2017, 18:56

      Dear Winchell

      Brice’s EM scoop is discussed in B. Cassenti, “Design Concepts for Interstellar Ramjets”, JBIS, 46, 151-160 (1993). It is also published as AIAA 91-2537 (1991) and is included in Chap. 8 of my book Deep=Space Probes.

      Gregf

  • Alex Tolley May 26, 2017, 12:21

    1.

    But such a long travel time for a robotic probe would be difficult to sell to the scientific community since most research participants would prefer to see some results within their lifetimes.

    2.

    So any NASA-funded interstellar initiative intended for possible implementation within the next few decades should not attempt to duplicate the goals of Project Starshot. Rather than a 20-year travel duration, a 100-year flight time might be more realizable in the near term. Mission planners need to realize that even this is quite a challenge. A 100-200 year travel duration might be a reasonable goal.

    1 & 2 seems contradictory to me. If travel times need to be within human lifetimes for scientific career reasons, then why should NASA implement 2? The only way out is to increase researcher lifetimes. Either through biology for humans (or possibly hibernation), or by the researchers being machines.

    The high energy constraints are very much a function of our relatively mayfly-life biological life spans. Remove that constraint and velocities and energies can be reduced to realistic levels using much more conventional technologies, like solar-photon sails. To me, this means that the focus should not be the technology of the propulsion (an advanced solar sail is fine, with or without boosts from beams), but rather how we extend the lifetime of agents managing the projects. Biology is rapidly homing in on aging and may achieve the possibility of “immortality”. Or biology will solve the hibernation/cold sleep problem, perhaps with humans+. The last solution is simply that machines with AGI will do the long-term projects when they wrest control of our economic and technological resources from us. While not a palatable solution for us, this may be the solution that arrives first.

    • Greg Matloff May 26, 2017, 19:01

      Dear Alex

      I have absolutely no trouble with 300-, 500-, or 1000-year probes. Neither do many others. But my discussions with some scientists indicates that many people disagree and want results in their lifetimes. A 100- or 200- year trip time seems a bit more reasonable to me.

      Greg

      • NS May 30, 2017, 1:14

        There’s also the issue that science itself will have changed so much in 300, 500, or 1000 years that it will be merest chance if any data the probe returns will still be of interest that far in the future.

        • ljk May 31, 2017, 13:55

          Such “antiques” may contain data from particular periods of time that cannot be reproduced with a fancy future probe no matter how advanced it may be otherwise.

          Plus the vessel could be of interest in itself as a historic artifact.

          If we assume that some ambiguous person or group in the undetermined future will automatically make something better and faster than we can now, we may find out the hard way that such descendants did not feel obligated to bow to our wishes. I for one do not want to wait for the future of either supposedly smarter humans or advanced, altruistic aliens to do our work for us.

          • NS June 1, 2017, 4:23

            I agree. Let’s do our work and let the people of the future do theirs. These 300, 500, 1000 year missions amount to us trying to do the work of the future when we don’t even know what the work will be, and when we can’t even come up with the money to do the work we know about.

  • Royce Jones May 26, 2017, 12:45

    You seemed to have missed what is most likely the best option for starship propulsion – Fission Fragment Rockets. Potentially available within a decade or two and with an isp of 500,000 – 1,000,000 they have tremendous potential to achieve interstellar travel by a manned vehicle in less than a human lifetime.

    • Greg Matloff May 26, 2017, 19:02

      Dear Royce

      I am aware of this technology. But I am also very aware that it will be difficult to sell on geopolitical grounds.

      Greg

    • Petar Popov May 27, 2017, 6:38

      I was about to post the same comment ☺you have also missed inertial electrostatic confinement fusion. A question: can you use an IEC device as neutron source to drive fission in thorium or uranium which is not suitable for chain reaction bombs? This way the political argument against it would disappear?

      The geopolitical argument applies to any system capable of moving a kilogram sized payload to the nearest star in less than a century. The energies involved will mean that the system, whatever it is can be used as a weapon.

      • Greg Matloff May 28, 2017, 6:49

        Dear Peter

        Yes, inertial electrostatic confinement fusion is a possibility. I don’t have any idea regarding its associated politics.

        Even though sails move at a very high velocity, whether Sun-launched, or beam-launched, they would likely burn-up in the atmosphere if directed towards Earth. A very poor weapon.

        Greg

  • Dave May 26, 2017, 12:54

    A combination of concepts that seems attractive to me is an Orion type nuclear pulse rocket activated deep within the solar gravity field, inside the orbit of Mercury and as close to the solar photosphere as prudently possible.

    Such rocket would be transported by prior chemical rockets and planetary gravity assists from Earth or near Earth orbit to the Orion launch site. The speed of the rocket before nuclear launch would be high, due to the gravity assist of falling towards the sun, then the nuclear pulse propulsion would minimize the time to achieve significant distance from the sun, minimizing the slowing to the suns gravity.

    After launch from the solar system by the nuclear pulse stage, additional velocity and course adjustment could be accomplished by atomic powered ion propulsion to speed it on its way.

    While I don’t have the technical skills needed to calculate or model the planetary gravity assists, solar “gravity well maneuver, pulse propulsion or the benefit of ion propulsion final boost out of the solar system, all of the propulsive efforts are highly efficient, and any nuclear waste would either fall into the sun or leave the solar system entirely.

    • Robert Feyerharm May 26, 2017, 13:58

      An interesting idea, but any variant on the Project Orion concept of nuclear rocket propulsion is already dead on arrival – it would violate the Partial Test Ban Treaty of 1963.

      • Greg Matloff May 26, 2017, 19:05

        Dear Robert

        Agreed!

        Greg

        • ljk May 31, 2017, 13:59

          If China wanted to build an Orion, would anyone really try to stop them? More likely we and the Russians would be trying to get a piece of that action, especially since China would have done all the hard work and taken the brunt of criticism first.

          Same thing with METI: China has their huge FAST radio telescope working, which they have declared officially they want to be the first nation to have a successful SETI project. If China also decides to use FAST for METI, who is going to stop them? Sure lots of people have concerns about sending signals into the galaxy to get the attention of advanced aliens, but all I ever see about this are complaints.

          • Michael May 31, 2017, 15:54

            The radio transmitter can easily be blinded.

            • ljk May 31, 2017, 16:48

              And I can just imagine China’s reaction to having their brand new 500-meter wide radio telescope being interfered with.

    • Greg Matloff May 26, 2017, 19:04

      Dear Dave

      A nuclear-pulse or thermonuclear-pulse rocket using this Oberth maneuver is certainly feasible on technical grounds and would reduce propulsion requirements. But like all nuclear “device” applications in space, it will be hard to sell.

      Greg

      • Chris S June 2, 2017, 16:41

        The Oberth Effect only helps you to around the escape velocity of the body in question. Even on the surface of the Sun, the escape velocity is 618km/s. That’s all the boost you’re going to get, and I suggest not going quite that close. Going 4 light years at 600km/s takes 2000 years, so you’ve not made much of a dent, probably not enough to be worth the hassle.

        However, Oberth Effect of a powered Sun swingby IS a very good approach to heliopause and 500-1000AU. A multi-stage, high-thrust chemical rocket would be sufficient to get you going to 100km/s, so you could reach the heliopause in ~5 years (from deep burn) to gravitational focus in like ~25 years and the inner Oort Cloud in ~50ish years.

    • Michael May 27, 2017, 9:18

      The mini fission implosion drive where a fission material is imploded by powerful lasers to critical density would not run foul of the nuclear test ban treaty because it is not a weapon, it is a fuel. This technology has much merit and it can be improved a fair amount with nanotube neutron channel reflectors. Perhaps we could have fission implosion drive and a sail hybrid, the sail has the fissionable fuel on it and is accelerated by lasers and the fission fuel is then consumed by the main craft.

  • Patrick Underwood May 26, 2017, 13:54

    “But there is a geopolitical obstacle to the construction of a ~gigawatt laser-, maser-, or particle-beam projector in space. Such a device could be applied to accelerating a starship or diverting an Earth-threatening asteroid; it could also be construed as a weapon.”

    Build it on the Lunar far side. Unlimited supplies of building materials, optically perfect conditions for beaming, and impossible to target the Earth directly.

    There are two other factors that favor the Lunar surface:

    1. Something I believe has been seriously overlooked in the debate over asteroidal vs lunar resources: gravity. Humans know how to use tools and build things under constant acceleration. We’ve been doing it for millennia; we’re designed for it. Look at the history of orbital EVA to see just what an elaborate exercise it is for humans merely to move around outside a spacecraft, never mind performing work with tools; then watch, say, In the Shadow of the Moon to see humans bounding around on the Lunar surface like they were born there. I predict robots will have similar advantages in gravity, not least because they’re designed by humans. Then consider all the civil and architectural engineering knowledge base of our civilization. Most of it is directly applicable to work on the Moon, with the adjustment of a single constant.

    2. Jeff Bezos. Unlike my hero Elon Musk, Besos is focused on establishing human habitation and industry in cislunar space. By his own admission, this man is spending $1B/yr of his own fortune on human spaceflight. That’s already on the order of (perhaps a quarter) of NASA’s HSF budget; and each “Bezos dollar” is probably worth 10 of NASA’s, considering the political and bureaucratic conditions imposed on that agency from within and without.

    • Greg Matloff May 26, 2017, 19:09

      Dear Patrick

      Jeff Bezos and Elon Musk are doing great work. They are certainly favoring human expansion into the cosmos. Placing a beamer on lunar fared is nice, but it will only get solar energy for half a lunar day. Also, lunar and Earth motions must be compensated for–a problem in a ~trillion-km beam collimation length.

      Greg

  • Douglas Loss May 26, 2017, 14:15

    Dr. Matloff’s attendance at the Tennessee Valley Interstellar Workshop will be sorely missed, but anyone else interested is welcome to attend. The final lineup of papers to be presented will be online within the next week, and will include detailed information from the Breakthrough Initiative and from Tau Zero. Early registration extends until June 30. Please visit the website at https://tviw/us and consider attending! Our host here, Paul Gilster, will be there.

    • Greg Matloff May 26, 2017, 19:11

      Dear Douglas

      I would very much love to attend. But there are simply too many interstellar conferences and I am committed to the Adelaide IAC shortly before Tenn. All. since my wife’s sister lives in Australia and this is a rare opportunity to see her. I hope that the Huntsville event is a great success.

      Regards, Greg

      • Michael May 29, 2017, 2:45

        ‘But there are simply too many interstellar conferences…’

        A decade ago people would have laughed at that suggestion :)

  • Eric May 26, 2017, 14:38

    What about the work of Positron Dynamics to develop an antimatter drive that does not require antimatter storage?

    • Greg Matloff May 26, 2017, 19:13

      Dear Eric

      I am not aware of this concept. But a huge amount of antimatter is required to accelerate even a small probe to high relativistic velocities. I don’t see how it is possible to do this without producing the stuff in advance and storing it on-board.

      Greg

  • Jeff Wright May 26, 2017, 15:10

    I want to see more folks here also develop an interest in calling for launch vehicle growth.

    One of the things I’d like to see is advocacy for vehicles like Sea Dragon
    https://www.youtube.com/watch?v=6e5B7EKVg48

    Recently, it was shown that AUSTAL, a shipyard in my state of Alabama, won a huge contract:

    http://www.nextbigfuture.com/2017/05/single-largest-arms-deal-in-us-history-signed-with-saudis-for-110-billion-and-will-be-worth-350-billion-over-ten-years.html

    With ITS tank troubles–as SLS having pproblems with pin-welds and such–it is time to replace white-coats with hard-hats. Scale things up..dumb things down.

    I can imagine a 500 ton solar sail mission, with half the payload being an insertion stage to move the sail to Jupiter and do a sundiver mission–you can have a precursor mission right now.

    Next–I think folks need to work on the Nuclear Salt Water Rocket.

    Such a payload atop Sea Dragon would be perfect.

    The payload can get water from the ocean–where the Sea Dragon also gets most of its propellant.

    Uranium salt in the nose–a bit of kerosene in the tail.

    Instant starship–just add water.

  • Geoffrey Hillend May 26, 2017, 17:37

    The problem with the fusion ramjet is that the interstellar hydrogen is too scarce for it to work, Parise, “Nuclear Fusion Ramjets,” article on web.

    Quote by Dr. Greg Matloff : “Conservation of Linear Momentum. Although excess unidirectional thrust seems to be generated by the EM Drive, Marc Millis has described in this blog numerous possible causes for this effect that do not violate this law. ” Do you know the email address for this blog? I like to see physicist Mark Millis possible classical explanations for the EM drive. I am also skeptical of the EM drive since they claim it violates a classical physical law especially when they give the pilot wave theory as an explanation which has long been made obsolete by the Copenhagen interpretation.

    As for the anti matter drive we don’t have to limit them to Penning Traps which could be modified with better shielding. If we could make antimatter at a much faster rate, it would not have to be stored for weeks. Maybe a combination of Fusion reactor or bremsstrahlung machine which makes gamma rays and combines them into positrons so the anti matter drive can make positrons and antimatter at a faster rate. If the electromagnetic anti-matter containment field can be filled very quickly, then the antimatter fuel can be annihilated all at once or burned faster with a variable thrust or higher thrust. We have a fusion rocket with an anti-matter boost, an antimatter fusion rocket combination. Making one’s anti matter fuel along the way would need a very large spacecraft though which is not practicable in the near future.

  • DJ Kaplan May 26, 2017, 18:09

    Instead of expecting this postage stamp of an interstellar probe to radio clear data back to Earth, would it be simpler to turn it around and shoot it back to Earth using solar pressure from the (double) Alpha system? Absent a laser array over there (unless our alien friends prove helpful), how much acceleration would that provide?

    • Greg Matloff May 27, 2017, 6:46

      Dear DJ

      An awful lot less acceleration than the laser!

      Greg

      • DJ Kaplan May 30, 2017, 12:07

        Yep. You betcha. Would that be a preferable (i.e., more realistic) alternative to designing a radio unit that would be at once feather light and capable of sending a detectable signal back to planet Earth?

  • Marshall Eubanks May 26, 2017, 18:31

    I regard it as pretty definite that Proxima and Alpha Centauri are gravitationally bound – see https://arxiv.org/abs/1611.03495

    • Greg Matloff May 27, 2017, 6:47

      Dear Marshall

      I agree. But not all observers do.

      Greg

  • Greg Matloff May 26, 2017, 19:15

    Dear Jeff

    I agree that launch vehicle development is significant. I don;t think that the exhaust velocity of the nuclear salt water rocket is sufficient for interstellar application. But I admit that I may be wrong about this.

    Greg

  • Michael C. Fidler May 26, 2017, 23:07

    What about a grazing incidence X-ray telescope in reverse with the X-ray lasers on earth lined up and beaming down the barrels. Hmm, Molecular monolayer graphene grazing incidence mirrors with 100% reflectivity in a concentric pattern with a large surface area for thermal release. The X-rays could even be focused into the next X-ray beam rocket ahead of the first one so more power could be retrieved to accelerate it, over and over again! They are basically concentric light weight tubes and you maybe able to do a reverse thrust when arriving at destination. The biggest problem is much smaller aiming area but with higher freqs of X-rays less diffraction and dispersion

    Biggest X-ray laser in the world generates its first laser light.
    https://phys.org/news/2017-05-world-biggest-x-ray-laser.html

    X-ray Telescopes.
    https://imagine.gsfc.nasa.gov/science/toolbox/xray_telescopes1.html

    • Greg Matloff May 28, 2017, 6:51

      Dear Michael

      Nice. But first we must have a high-graphene reflectivity.

      Greg

      • Michael Fidler May 28, 2017, 20:06

        Grazing incidence is a high reflectivity mirror – optics. Check the link for x-ray telescope.

        • Michael May 29, 2017, 1:18

          The angle of reflection may be high for shallow angle reflection but there are two problems that I can see. For one the reflection is not back towards the source and so little transfer of momentum takes place, the other is that the thickness of material and therefore mass increases to get enough reflection at shallow angles.

  • Al Jackson May 27, 2017, 7:48

    I have been convinced of the merits of star faring for more than 50 years now, for me the value of Breakthrough Starshot is meetings and papers. The history of published research in interstellar flight is that up until recently it has been a hobby! I mean nobody paid for it.
    I remember my friend and co-author Dan Whitmire published, while we were graduate students at University of Texas, his paper on the Catalytic Nuclear Ramjet, which goes a ways towards solving the p-p reaction cross section problem. (Alas this paper has faded to the background, gets little mention these days.) Since Bussard had published the Interstellar Ramjet in Acta Astronautica ,Dan submitted there. The paper was accepted. The journal did not make it clear there would be page charges. Dan got a bill for that after the paper was published; he asked the University of Texas if they wished to pay, they declined. Being a poor grad student Dan threw the bill in the trash. I have no idea what would have happened if he had submitted to the same journal again.
    (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.492.6775&rep=rep1&type=pdf)
    The journals of the American Institute of Aeronautics and Astronautics, the most prestigious astronautics publisher in the world, has been, I think it has changed, reluctant to publish interstellar flight papers. They did publish several of Bob Forward’s. There is a steep page charge; I think Bob had money from his employer in those days for such. The Journal of the British Interplanetary Society has been a savior, very enthusiastic about interstellar flight with no page charges. Alas the JBIS struggles these days. I know one can publish on line but there is no refereed interstellar flight E-journal that I know of. (Astrophysical Journal has published some lately, not sure everyone would know to look there.)
    There had been spotty SETI meetings with interstellar flight as a side bar, but until the 100 Year Starship meeting in 2011 Orlando I don’t really remember another like it (there may have been). Now there are sometimes 4 open interstellar flight meetings a year! Though there are no AIAA or IAF sponsored interstellar flight meetings that I know of. Still, at least, researchers and interstellar travel comrades can do some schmoozing tougher now.

    • Greg Matloff May 28, 2017, 6:55

      Dear Al

      Nice comments. Page charges are always a bummer, especially to grad students. I hope that JBIS gets better under its new management and that the new on-line publications of the Institute for Interstellar Studies improves the situation. See you in a few weeks at the Citytech Interstellar workshop.
      Regards to all, GREG

  • Michael May 27, 2017, 9:05

    Hi Greg

    Always good to see your work on here.

    Apologies beforehand for the long response though.

    ‘Another possible function of such a probe is extra-galactic astrometry. If the probe carries a telescope, the very-long baseline observations possible when pairing with solar-system instruments during interstellar cruise should yield valuable data regarding distances and kinematics of extra-galactic objects. ‘

    Having the sail flying edge on allows valuable observation time using thin lens technologies to see a huge amount of the universe on the journey, indeed the journey may be more interesting than the arrival. Many thin lens discs could be on the surface of the sail to make these observations and they are very light weight. These observations will give us enormous amounts of data about the regions of dark matter by observing light distortions of distance sources.

    http://wccftech.com/ultrathin-lens-could-change-future-devices/

    ‘During the interstellar transfer after the probe’s distance from the Sun exceeds 550 AU, the Sun’s Gravitational Focus can be applied to obtain greatly amplified images of astrophysical objects occulted by the Sun. ‘

    It should allow a gravity wave observations as well as they should be concentrated as well, this technique could be used to observe the CBH or CWB.

    ‘Our early probes should almost certainly be directed towards the nearest stars—the Proxima/Alpha Centauri triple star system.’

    It may be better to first go in the opposite direction to use the solar lens effect.

    ‘But such a long travel time for a robotic probe would be difficult to sell to the scientific community since most research participants would prefer to see some results within their lifetimes.’

    The journey may well keep them busy for decades.

    ‘It must have near perfect reflectivity, high emissivity, low areal mass thickness and very high melting point.’

    It must also have very low absorption, Silicon looks to be a very good candidate.

    ‘Adaptive optics must be used not only to compensate for the effects of Earth’s atmosphere but to insure that the beam completely fills the sail during the acceleration process at distances measured in millions of kilometers. ‘

    There are technics that allow us to adaptively adjust the lasers quite fast, micro to 10s of nano seconds.

    ‘Assuming that the sail survives the acceleration process, it must possess ample on-board intelligence to perform several tasks independent of Mission Control.’

    Perhaps we could have a distributed e-brain design so that if the sail does break apart each of the smaller part of the sail can carry out data collection even though the main sail has been disrupted, a sort of hydra concept.

    ‘But we’re not done yet. Approaching Proxima/Alpha Centauri, the sail must reorient itself once again to allow its instrument suite to survey the environment of the destination stars and to send the results towards Earth. A very tall order indeed for a ~gram-massed nano payload.’

    We may not need to change the edge on direction as we will have the sail going towards the central star to avoid ions coming from the alien sun, we then make observations as we pass.

    ‘None of the above challenges present physical impossibilities. The question is whether they can all be achieved in a single nano-spacecraft within the next few decades.’

    I believe we have the tech now but getting it all together on such a grand scale will be difficult.

    ‘One way around this is to consider lower performance ramjet alternatives such as the ram-augmented interstellar rocket (RAIR) that carries on-board fusion fuel and uses scooped protons as additional reaction mass.’

    We could send a stream of better fuels out first so they an be picked up by the craft as it moves towards its target.

    ‘Any funded consideration of interstellar probes would be wise, however, to investigate terrestrial and in-space experiments to demonstrate the utility of beamed propulsion.’

    The beauty of the concept is the smallness of it, these techs could be tested by simply placing them at little to no cost on the surface of satellites that are put into orbit.

    ‘Theoretical researchers might also expand the concept of particle-beam propulsion. Because electrically charged sub-atomic particles carry significantly more linear momentum than photons, it would be interesting to develop an understanding of particle-beam collimation over interplanetary and interstellar distances.’

    Particle beams have much merit but our atmosphere is a bit of a pain sometimes, once in space they will be very useful.

    ‘If such an enormous beam projector could be constructed in space and could maintain its aim for decades, a hybrid interstellar propulsion system might ultimately become feasible. This is the laser ramjet. In such a vehicle, interstellar ions collected by a Cassenti EM scoop could be accelerated by energy beamed from the solar system.’

    This is possible at the Luna poles, truly huge lasers can be built there and can hold their aim for a very long time, but only target stars in that narrow plane.

    ‘Graphene is a hyper-strong layer of carbon, one molecule thick. Its melting point is in excess of 4,000 K and it is impermeable to many gases…’

    Although it is light weight it absorbs light which would destroy other sensitive electronic instruments, Silicon has a lot going for it.

    ‘…Because human-carrying arks are limited to ~3g accelerations, these larger ships require about 1,000 years to reach these stars if they are propelled by graphene sails.’

    Encased in liquid a human can take a lot more ‘g’s into the 10s of ‘g’s.

    ‘But here is where Project Starshot can play a very major role. In order to reach ~0.2c in a ~50 GW laser beam without melting, the sail reflectivity to laser light must be very high. Perhaps this can be achieved with an appropriate mesh-like meta material. Or perhaps the reflectivity of molecular monolayers such as graphene can be greatly increased.’

    Graphene’s reflectivity can be increased a lot by say Calcium or Lithium addition, I don’t know the effect on the absorption though. Silicon is a remarkable material which can be made very reflective a light weight. Combine these two technics (links) and we have a very light weight and highly reflective material, perhaps coupled with total internal or retroflection and we could have a very viable material.

    http://nanophotonics.eecs.berkeley.edu/research/sweepr/sweepr.htm

    http://www.nanowerk.com/news/newsid=23296.php

    • Greg Matloff May 28, 2017, 7:00

      Dear Michael

      Wow, what a long comment! Essentially, I agree with you. I did not wish to discuss methods of increasing graphene reflection because I don’t want to jeopardize chances of those who submit. I have heard about the concept of immersing humans in liquids to take 10’s of gs. BuI also heard that breathing was a problem. Perhaps humans and other on-board life could be placed inn torpor during the hours-duration acceleration process.
      Greg

      • Michael May 29, 2017, 2:22

        ‘BuI also heard that breathing was a problem.’

        For the period of acceleration forced breathing in a high oxygen atmosphere may do.

        ‘Perhaps humans and other on-board life could be placed inn torpor during the hours-duration acceleration process.’

        Might be best to have our wits about us and the animals in a torpor during the acceleration period.

      • Michael May 29, 2017, 10:15

        For what I can see doped graphene shows great promise, my worry is absorption and heating. This could be offset by protecting the much smaller electronics with extra reflective higher massed materials and use the graphene combination as the light weight sail material. I hope much good work comes out from these studies.

        • J. Jason Wentworth May 29, 2017, 22:10

          Michael, there is good news on the long-life, radiation-resistant electronics front, which is directly applicable to long-lived spacecraft of all kinds (including interstellar probes–even “immortal” Bracewell probes):

          NASA and the Korea Advanced Institute of Science and Technology (KAIST) is developing—and even has prototypes of—self-healing electronics for “spacecraft on a chip” vehicles (see: http://spectrum.ieee.org/semiconductors/devices/selfhealing-transistors-for-chipscale-starships , http://www.google.com/#q=StarShot+probe+self-healing+electronics , and http://www.google.com/#q=StarShips+probe+self-healing+electronics ), which can repair radiation damage (and can do so repeatedly—up to approximately 10,000 times thus far, at this stage of the R & D work). This capability would make long-life Bracewell interstellar messenger probes practical, and it would—perhaps even at the current state of the art of this technology—ensure that the StarShot probes would survive the radiation exposure during their long passages to the Alpha Centauri system. Earth satellites and other solar system spacecraft (including Venus, Mercury, Sun, Jupiter, Io, Europa, and Ganymede probes) would also benefit from—*or be made possible by*—self-healing electronics.

      • Larry Kennedy May 29, 2017, 11:29

        The problem of breathing under high gee immersion would seem to be more easily solved than many problems discussed here. Breathing liquid is considered viable and has been demonstrated in mammals. In fact respiration is not necessary at all, only gas exchange.
        On the other hand, circulation of blood might be a major problem due to pooling etc.

      • Winchell Chung May 29, 2017, 12:58

        According to
        Guyton, Arthur C. (1986). “Aviation, Space, and Deep Sea Diving Physiology”. Textbook of Medical Physiology (7th ed.). W. B. Saunders Company. p. 533.
        acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15 to 20 G. Extending acceleration protection beyond 20 G requires filling the lungs with fluid of density similar to water.

        According to this reference
        http://journals.lww.com/asaiojournal/Abstract/1996/42060/Quantitative_Structure_Activity_Relationships_of.9.aspx
        the problem is finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, so it won’t work.

        References are courtesy of the notoriously inaccurate Wikipedia, which is why primary references were supplied.

        • Michael May 30, 2017, 2:58

          ‘Perfluorocarbon fluids are twice as dense as water, so it won’t work.’

          If the whole body is immersed in the same dense liquid as in the lungs it would not mater as the fluid in our bodies is practically incompressible, perfluorocarbon fluids with forced ventilation could work.

          • torque_xtr May 31, 2017, 4:27

            If it is possible to fill lungs with 1.03…1.06g/cm3 liquid, then the range extends towards ~100g. The next big differences are bones and tendons, which are the strongest parts, and eye structures, which are small, but also weak, and this may be concerning.

            Made me think about water, which can be saturated with oxygen at elevated pressures. 40 bars of oxygen are enough to raise O2 concentration in moles per liter to atmospheric levels at 37 oC and physiological salinity. The question is – will it react with human tissues as it is gaseous oxygen at 40 bars, or activity coefficient is substantially low when dissolved in water? If it is possible to breathe water with high-bar dissolved oxygen, then high-gee problem is solved at the cost of 40bar-pressurized environments.

          • Brett Bellmore May 31, 2017, 5:57

            Technically, the problem (At moderate Gs.) with the density is mostly that the diaphram muscles are intended to move gas, and just aren’t strong enough to move enough perfluorocarbon. So, as you say, forced ventilation is necessary.

            But it’s also a problem, because as the acceleration goes up, the differences in density start to matter more. Eventually you reach the point where your bones fall through your body because the tissue around them isn’t strong enough to support them, the fat rises to the top, and everything sorts itself out in terms of density like an oil and water mix separating.

            Which is pretty hard to survive.

          • Alex Tolley May 31, 2017, 13:31

            I think humans are going to need lowers g’s and longer acceleration times, unless we can control inertia with some fancy new physics (which would magically solve a lot of other problems too.)
            The difficulties of current imagined technology and human star flight reinforces the idea that AIs are the ones going to make these trips first, possibly even the only voyagers. If there was some way to allow instant communication across interstellar distances, then humans could participate vicariously, using robot avatars, but again, this is “new physics”.

    • Al Jackson May 29, 2017, 7:58

      Joe Haldeman had g-mediation ‘pods’ in The Forever War (1974) , that is a method with fluids was used for high acceleration. I don’t remember it’s use in prose science fiction before 1974 but it could have been.
      I do have the vague recollection that this kind of technology was being studied in the 1960s , maybe even before.

      • Winchell Chung May 29, 2017, 13:03

        I have not managed to find an earlier prose science fiction use of liquid breathing IN AN ANTI-ACCELERATION MODE than Haldeman’s The Forever War.
        There are quite a few science fiction references of earlier liquid-breathing, but they appear to be all for controlling the effects of water pressure when undergoing deep ocean diving.

        • J. Jason Wentworth May 29, 2017, 22:29

          In Arthur C. Clarke’s 1962 short story “Maelstrom II” (see: http://www.google.com/#q=Maelstrom+II+by+Arthur+C.+Clarke ) his financially-challenged lunar hydroponic engineer, Cliff Leyland, elected to take the cheapest way back to his family in Africa–a freight capsule slung into space by a “lunatron” electromagnetic launcher. During the high acceleration of the launch, he was cushioned by being surrounded (in his suit) by water filling the tiny cabin, which was drained away after the launch (he didn’t breathe a liquid containing dissolved oxygen during the acceleration, however). I won’t spoil the story in case you haven’t read it, but I will say this: Clarke did not disappoint!

  • Brett Bellmore May 27, 2017, 10:54

    Mass beam propulsion using macroscopic particles.

    You start with Starshot, but instead of sails meters wide expected to survive the entire trip to the destination, use smaller, shorter lived sails. Instead of expecting them to reach the destination, they impact a pusher plate on a larger, more conventional probe. (The pusher plate doesn’t have to be solid, it could be a dense plasma contained by a magnetic field.)

    The smaller sails can sill be capable of course control in the diverging beam, and so can home on the probe. They’ll be simpler, and be mass produced by the millions.

    This doesn’t solve the problem of slowing down at the other end of the trip, of course, but a mag-sail may be able to do that.

    As far as the political problems of near term use of nuclear propulsion in space go, it seems likely to me that we won’t really be launching a serious effort to cross the interstellar void until after we have a substantial presence in space, including colonies.

    Once those colonies have some degree of political and economic independence, they’re unlikely to share this absurd phobia, and will go with the best technical solution unconstrained by Earthly politics.

    • Michael May 27, 2017, 13:17

      The sails need not impact a pusher plate, a powerful magnetic field could induce huge currents in them that will vaporise them and the ions interact with the mag field to push your craft. The acceleration will have to be slower as the field could collapse under the pressure. If we had slower sails sent earlier perhaps we could intercept them and use them to slow down.

      • Greg Matloff May 28, 2017, 7:06

        Dear Michael

        Magnetic fields work if most of the reaction energy is associated with charged particles. In today’s fission and fusion reactions, something like 70% of the energy is associated with neutrons, gamma rays, X-rays. I (and others) wonder what repeated thermal-neutron assaults do to structural integrity. If we can tame Boron-Proton fusion, things get a lot neater.

        Regards, Greg

        • Michael May 28, 2017, 13:13

          We have a lot of experience with nuclear reactor cladding (zirconium) on earth, annealing is a good way to re-crystallise the material. Perhaps the engine is switched off for maintenance and a heavy electrical current passed through it to heat it could do. As for boron-proton fusion the Lawson criteria is formidable He3-He3 is more likely but even it is far from easy.

    • Greg Matloff May 28, 2017, 7:02

      Dear Brett
      Yes, inhabitants of in-space habitats may have different concepts regarding nuclear applications than terrestrials. They may also feel better about the concept of millennial interstellar journeys. I feel that no terrestrial lifeforms will leave the solar system. But our in-space descendants might.
      Regards, Greg

  • JohnHunt May 27, 2017, 23:18

    So, the article concludes by suggesting the SunDiver concept. Can we please use the name?

    • Greg Matloff May 29, 2017, 8:03

      Dear John

      Sure. I have no copyright no the name. In fact, although I researched the trajectory, I did not name it. According to my friend and colleague Dr. Greg Benford, sci-fi author David Brin is the originator of the name. Another nice name for this trap., from Ray Bradbury, is Rainbow trajectory.

      Regards, Greg

  • Adam Crowl May 28, 2017, 4:23

    Hi Greg
    There’s a lot to be said for Marshall Eubanks’ suggestion of quark matter inside asteroids as a potential antimatter source. If we can’t store antimatter in a meaningful way a starship might need to haul a quark nugget along for an energy source. Likely a very big starship since quark nuggets would mass in the ~megaton range.

    • Michael May 28, 2017, 9:46

      It may be better to store anti matter on the outside of a ring open to space with electro/mag containment, in the event of a uncontrolled event occurring a laser is shot out pushing the anti matter into deep space.

    • Greg Matloff May 29, 2017, 8:06

      Dear Adam

      Hello! Although I was not aware of Marshall Eubanks’ suggestion, it should certainly be investigated by asteroid miners. This might prove to be a nice on-board power source for large arks or worldships.

      Regards, Greg

  • Sean Robert Meaney May 28, 2017, 8:24

    Why do we need to store antimatter? Last I heard they could make it by exciting gold with a laser. So storage of a gold mass as half the fuel required.

    • Brett Bellmore May 28, 2017, 13:26

      Yes, you need to store antimatter. Because if you’re making it on the ship, you’d always be much, much better off just using that energy directly for propulsion, instead. Unless maybe you were just using it to trigger fusion reactions, of course.

      Antimatter is mostly useful as the densest possible way to store energy, that’s all.

      • Michael May 29, 2017, 5:28

        That is not strictly true, particle accelerators can impart energy in the form of kinetic energy much higher than that, each atom can hold thousands of times more energy than an anti-matter particle. It would be very useful in starting a nuclear reaction as you have stated though.

  • Greg Matloff May 29, 2017, 8:07

    Dear Brett

    I agree.

    Greg

  • J. Jason Wentworth May 29, 2017, 8:52

    The launch of The Planetary Society’s LightSail 2, aboard the first SpaceX Falcon Heavy rocket, is coming up later this year. If all goes well and the solar sail deploys in orbit, it (as was hoped for Cosmos 1) could be used to test microwave-pushed sail propulsion by “beaming” it with a radio telescope.
    Both microwave-pressure acceleration and “tacking” (braking the sail so that it would move in a lower, faster orbit) could be evaluated. If the sail’s CubeSat payload functions long enough, the sail could conceivably be “beamed” enough–by radio telescopes’ microwave transmitters as well as sunlight pressure–to eventually reach escape velocity by spiraling outward from Earth.

    A way around the geopolitical problem of nuclear *anything* in space might be provided by Russia. Joint U.S./Russian (and perhaps ESA and/or ISRO, too) fission-ion propulsion probes could use U.S. ion thrusters and instruments, a Russian Topaz space reactor, and be launched from Russia aboard a Russian launch vehicle.

    The “along the way” observations that Greg listed are all “meaty” ones that would likely make even interstellar precursor probes (“ultraplanetary” probes, to use Robert M. Powers’ term in his book “Planetary Encounters”) attractive. To his list I can add another possible one, which–if it would work–would “sweeten the pot” for scientists who wouldn’t live to see the probe(s) arrive in the Alpha Centauri system after a century or so of traveling:

    If two or more probes were sent, they could spread out along a wide baseline (or baselines) that were perpendicular to their line of flight toward Alpha Centauri. This should enable them to use interferometry (as is done between two widely-separated optical or radio telescopes, to give them the resolving power of one huge instrument as wide as the distance between them) to obtain close-up multi-spectral images–and radio “images”–of the Alpha Centauri stars, and of any planets (besides Proxima b) that may be orbiting them. It might be possible to resolve starspots on the three suns and continents on the planet(s). Light polarization measurements might reveal dust and/or asteroid belts, provide hints about the three stars’ magnetic fields and their interactions, and perhaps even show signs of photosynthesizing plants–if any exist–on one or more of the planets.

  • Marshall Eubanks May 29, 2017, 12:44

    Unfortunately, this interesting idea probably won’t work with the technology we have readily available.

    Let’s assume we have the Goldstone Planetary Radar available – 500 kW at 8500 MHz (3.5 cm) on a 70 meter dish, and ignore any losses. That means that at 400 km the radio flux would be confined to a spot about 200 meters in diameter, with a flux of ~ (500 Kw / (30,000 m^2)) or about 15 W / m^2). As radiation pressure goes as E/c, and as this flux is about 1/100th of the Solar Constant, this says that this radiation pressure will be about 1/100th the solar radiation pressure at 1 AU, or about 90 nanoPascals (or twice that for a perfectly reflecting sail). With a 100 meter sail weighing 1 kg, that would amount a thrust of order 10^-3 m/sec^2. As the most a local antenna could track an orbiting solar sail is maybe 15 minutes, this would amount to a velocity change of order 1 m/sec, which sounds possibly detectable. However, at 400 km the atmospheric drag is > solar radiation pressure, and we could never separate drag from the pressure from Goldstone in the time available. At 2000 km (more reasonable with respect to drag) the force is down by a factor of 25, but the time on source is longer, so you might get 10 cm/sec velocity shift out of the test. If your sail has good GPS receivers capable of doing connected phase, you might be able to pull this out of the noise.

    • J. Jason Wentworth May 29, 2017, 22:47

      No matter–the subsequent LightSail vehicles are, if memory serves, intended to be unfurled in higher orbits (perhaps, at least partially, for the very reason you mentioned). But even the lower-orbiting LightSail 2’s changes of velocity in response to microwave illumination might be “teased out” from the other, natural influences on its motion, if the microwave “beaming” was done in a systematic way (so that tiny “jumps” in the sail’s velocity, when plotted, coincided with the “beaming times”). This might be easier to detect if such microwave thrusting attempts were conducted over the night side of the Earth, when the upper atmosphere is cooler and less expanded due to the absence of direct solar heating and excitation.

  • Nick May 29, 2017, 14:51

    I’m a believer that the first probe to reach another star will be a laser-pushed lightsail.

    That said, I might be kind of biased against the idea of wafer-sized spacecraft. The communication problems alone make me think that it might actually be easier to send a sailcraft that’s more comparable in mass to our current space probes.

    But with a larger sailcraft, you’re going to need much more acceleration time, and a much longer acceleration path – perhaps billions of kilometers long. That’s why I’ve always thought that the single biggest obstacle to overcome is the beam divergence problem. Most of the concepts I’ve read about use a single laser, or a laser array stationed at a single location.

    How small of a laser “spot size” could you realistically hope to achieve over those extreme distances?

    I talked to a few people on a physics forum to try to figure this out, and eventually I came across the Gaussian equation for evolving beam width:
    https://en.wikipedia.org/wiki/Gaussian_beam#Evolving_beam_width

    Assuming an ideal Gaussian beam, I believe you can use this equation to figure out the optimal beam waist size and spot size for a given distance and wavelength. I plugged the equation into a spreadsheet and fiddled with the numbers until I got what looks to be the optimums for a few different wavelengths and a total distance of 1 billion kilometers. Below are the results I got for a couple different wavelengths. The beam waist is in the middle at the 500 million km mark along the beam. The spot sizes and beam diameters shown are the maximums over that range – this is the spot size and beam diameter at the source and at the 1 billion km mark:

    Wavelength: 400 nm (Visible Violet)
    ——————————————–
    Waist Size (m): 253 m
    Spot Size (m): 356.83 m
    Beam Diameter (m): 713.65 m

    Wavelength: 10 nm (Extreme Ultraviolet)
    ————————————————–
    Waist Size (m): 40 m
    Spot Size (m): 56.42 m
    Beam Diameter (m): 112.84 m

    So for the Visible Violet example, I think this means that you would have to project the laser through a telescope with a 713.65-meter aperture.

    Can anyone tell me if I’m using the evolving beam width equation correctly, and if I’m drawing the right conclusions from it? I have no idea how accurate it is when you scale things up this much.

  • Larry Kennedy May 29, 2017, 16:01

    Thanks for the good info!

  • Gregory Benford May 29, 2017, 16:49

    Sky Lift by Heinlein 1953 uses water pods to counter acceleration.
    I think I named David Brin’s novel SUNDIVER. I know my brother and I used it specifically for sails getting accelerations near the sun in a paper around year 2002.

  • James Benford May 29, 2017, 17:43

    The reference Greg is referring to is “Near-Term Beamed Sail Propulsion Missions: Cosmos-1 and Sun-Diver”, James Benford and Gregory Benford, Beamed Energy Propulsion, AIP Conf. Proc. 664, pg. 358, A. Pakhomov, ed., 2003.

  • ole burde May 29, 2017, 17:48

    One of the main problems with beamed propulsion is the need to forcus an earthbased beam over great distances , and the resulting need for almost impossibly big acelerations . Nobody else seem to suggest that this aspect of our problem might have a relatively cheap solution , so here it comes again : imagine a series of perhaps 30 mass-produced automatic beam-units somehow lined up in space , each capable of delivering beamed acceleration for a few seconds to a sailcraft as it passes by . These units would not need a gigantic powersupply , but instead store energy which was produced (or recieved) over a much longer period , and then releasing it in a few seconds .
    This plan would demand a major advance in the ability to store energy in a lightweight system , but all the plans put forward here demands some kind of breaktrough , so perhaps energy density of storage in a vacum might be a relevant subject for research . One of the great advantages of such a scheme is that it would be fully scalable , starting perhaps as an additional acceleration source for the sail units planned for Breakthroug Starshot

    • J. Jason Wentworth May 29, 2017, 23:44

      Sun-stationary *statites* (“STATionary satellITES” [or “STATic satellITES,” see: http://www.google.com/#q=statite+solar+sail ]–Robert Forward’s name for high-performance solar sails that can hover over one spot on a star–or even above the poles of a planet, without orbiting; he and/or Colin McInnes envisioned entire space cities hovering above the Sun, with the help of huge statites) might serve as such “lined-up” automatic beam-units. (There is also at least one “Centauri Dreams” article about statites, see: https://www.centauri-dreams.org/?p=13631 .) Also:

      The statite beam-units could be “stacked” over either pole of the Sun (for an Alpha Centauri mission, using the Sun’s south pole would be more straightforward), and their sail-propelling beams could be directed toward the passing sail probes using movable mirrors (thrust-compensating beams could be projected in the opposite directions from the sail-pushing beams, to avoid pushing the statites off-station), and:

      If statites can be “hung above the Sun” over locations other than its poles, the beam-unit “stack” could be lined up to point directly at the Alpha Centauri system, which would allow direct beaming toward it (and, depending on the actual hardware arrangement, might make the thrust-compensating beam unnecessary). At the very worst (to maintain balanced photon thrust on the probes’ sails as they passed by the beam-units), two or three parallel “stacks” of statite-supported beam-units might be needed (three such parallel “stacks” could be arranged in a triangular pattern), and the sail probes would pass up between them. In addition:

      A probe-launching statite could hover between the parallel beam-unit “stacks,” at the “bottom” (closest to the Sun, that is). This probe-launching statite would also double as an occulter, so that the sail probes could be launched upward–perhaps electromagnetically–with their sails fully unfurled. If desired (or necessary), the probe-launching/occulting statite could be located close enough (in altitude above the Sun’s surface) to the bottoms of the beam-unit “stacks” that no direct sunlight would illuminate the probes’ sails until they had passed the lowest beam-units in the “stacks.”

      • ole burde May 30, 2017, 9:49

        Right ,to make our beam-units line up , they will have to be close to Forwards idea of ‘Statites’ . Their solar sails will have two funktions , stationkeepeing and produktion of electricity to be stored for the beam-unit . In order to make this plan economicly possible , the energy have to be stored with a high density , and it also have to be relased in a very short period of time . In the case of Microwave propulsion this demand becomes more critical because of the more limited range …another idea would be to have some kind of micro-nuclear explosions supply the shortlived beam , but that could easily become a onetime afair !

    • Brett Bellmore May 30, 2017, 6:04

      How about this: A neutral particle beam for propulsion. You could string out along the beam a series of beam reshaping stations, which would compensate for dispersion using a non-mechanical means such as laser cooling, and, importantly, have big holes through them. (Laser cooling only requires light and magnetic fields in the beam, not physical structures.)

      They’d interact with the beam a little, and thus be gradually accelerated along, and supplemented by new stations being added. After a while, you’d have a very long beam established with constant power density until the string of stations was passed.

      You then drop your probe into it, and it constantly accelerates, passing through the holes in the stations.

      How far can the stations be apart? Laser cooling can take atoms into the micro-Kelvin range, or below, at which point their velocity amounts to a couple inches an hour. The beam, of course, would be traveling at something just under the speed of light. Assume you permit a tenth of a meter dispersion between stations, about every couple of light hours. (Obviously the beam reshaping stations would have to be closer near the transmitter.)

      A couple thousand beam reshaping stations would allow your probe to get up to half light speed, even assuming only 1G acceleration. So, it even has the potential to launch manned star ships.

      Can you do laser cooling based beam reshaping on a relativistic neutral particle beam? I can’t see any reason why not.

      • J. Jason Wentworth May 30, 2017, 9:52

        I’m afraid I’m mostly, or entirely (in the case of laser cooling) unfamiliar with the concepts you mentioned–but anything that can launch crewed starships sounds good to me!

        • Brett Bellmore May 30, 2017, 13:17

          It’s fairly straightforward: Laser light just below a key frequency is shined through the gas, (Or in this case, the beam.) and is preferentially absorbed by atoms which are moving towards the source, and thus see the light blue shifted. The recoil slows them down.

          If done right, you can lower the speed of the atoms to meters a day, in the beam direction. While leaving it unaffected in any other direction.

          https://en.wikipedia.org/wiki/Doppler_cooling

          The result should be a neutral beam carrying momentum, which remains usefully focused for many light days.

          • ole burde June 1, 2017, 16:57

            I tried to read up on these subjects , but if you try to get deeper it’s either classified or it was part of a ‘starwars’ program from the 80’es that just got lost somewhere ….perhaps NPB is a serious weapons technology still under development

  • Astronist May 29, 2017, 17:51

    “Application of fusion micro pellets also has a number of technical issues. First, there is the problem of fuel availability. To reduce neutron irradiation on ship structures, the Daedalus study of the British Interplanetary Society (BIS) considered a Deuterium-Helium3 fusion fuel cycle. […] The BIS follow-up to Daedalus, called Icarus, uses a Deuterium-Tritium fuel cycle. Here, it might be necessary to breed Tritium in nuclear fission reactors.”

    This problem seems to have been solved. The fuel will be lithium deuteride. See R.M. Freeland II, “Project Icarus: Fission-Fusion Hybrid Fuel for Interstellar Propulsion”, JBIS, vol.66, pp.290-296 (2013).

  • Paul Titze May 29, 2017, 22:00

    Hi Greg,

    Thanks for the interesting article, although I have my serious doubts about an effective interstellar mission using gram size starships. Until a means is found to protect the payload from space radiation for eg (both via active and passive means) interstellar travel will remain a dangerous prospect and lead to a high probability of failure. For a starship travelling at 0.3c for eg, energetic protons from the interstellar medium carry about 50 MeV and are very dangerous. At 0.1c, 5 MeV is still bad for circuits.

    On the other side of mission scale sizes, one of the other biggest hurdles for a realistic interstellar mission is getting large amounts of hardware into orbit 120Km from the ground without relying on chemical rocketry. Until this happens, it will probably be way too expensive proposition for tax payers.

    http://vixra.org/abs/1506.0194

    Cheers, Paul Titze.

  • Michael May 30, 2017, 3:10

    Hi Greg

    Will the foundation go as far as looking into how the ‘Starship’ concept can be integrated into a space roadmap. I mean the concept could be used for many different ideas such as been used to launch larger rockets by direct heating for instance. I see that the more of these other projects can be integrated into the concept the lower the cost to any individual entity will be.

  • Michael C. Fidler May 30, 2017, 11:23

    Fluidic Electrodynamics: On parallels between electromagnetic and fluidic inertia.
    AlanCo on emdrive-forum.com relayed me an article by Alexander A Martin ,

    arxiv.org/ftp/arxiv/papers/1202/1202.4611.pdf

    Alexander A Martins wrote:
    The purpose of the present work is to trace parallels between the known inertia forces in fluiddynamics with the inertia forces in electromagnetism that are known to induce resistance forces onmasses both due to acceleration and at constant velocity. It is shown that the force exerted on aparticle by an ideal fluid produces two effects: i) resistance to acceleration and, ii) an increase ofmass with velocity. These resistance forces arise due to the fluid dragged by the particle, where thebare mass of the particle at rest changes when in motion (“dressed” particle). It is demonstratedthat the vector potential created by a charged particle in motion acts as an ideal space flow thatsurrounds the particle. The interaction between the particle and the entrained space flow gives riseto the observed properties of inertia and the relativistic increase of mass. Parallels are madebetween the inertia property of matter, electromagnetism and the hydrodynamic drag in potentialflow. Accordingly, in this framework the non resistance of a particle in uniform motion through anideal fluid (D’Alembert’s paradox) corresponds to Newton’s first law. The law of inertia suggeststhat the physical vacuum can be modeled as an ideal fluid, agreeing with the space-time ideal fluidapproach from general relativity.

    and an associated patent

    patentscope.wipo.int/search/do…77/APBDY/WO2010151161.pdf
    google.com/patents/WO2010151161A3?cl=en
    patentscope.wipo.int/search/en…&sortOption=&queryString=

    Alexandre Tiago Baptista De Alves Martinq wrote:
    The present invention relates to a new form of aerial, terrestrial, underwater or space propulsion, achieved through the manipulation (or engineering) of the vacuum with the proper electromagnetic interactions. This vacuum manipulation will allow the use of a new form of propulsion, and has applications in energy production and on the change of the time decay of radioactive elements. Opposing magnetic or electric fields create a mass repelling force, while attracting magnetic or electric fields create a mass attracting force. In particular, this vacuum manipulation process can be used to propel a mass (6) that contains the field sources that perturb the vacuum. One possible application is the creation of a repulsion point (48) in space through the interference of two or more longitudinal electrodynamic wave beams (46), which cause a repulsion force on mass (6).

    Claims

    1. Device for vacuum manipulation (antigravity propulsion through the repulsion or attraction of a mass like planet Earth (7) or a general mass (6)), characterized by a geometric (or not geometric) arrangement of at least one pair (two magnets or any other number) of magnets (20) with the north pole or the south pole in opposition (disposed in the same parallel plane, or at a perpendicular plane, or at any desired angle or angles) ; or were the magnets are arranged symmetrically (circular, hexagonal or any other) or asymmetrically (semicircular, conic, pyramidal, or any other) ; or by the magnets being disposed in a way to oppose the magnetic vector potential component (with magnetic poles face to face, or side to side, or at any other angle) in order to stress the vacuum and create a gravitational repulsive force on nearby masses (if the mentioned magnets are disposed with the magnetic vector potentials in attraction, with their vectors in the same direction, then they would generate an attractive gravitational force instead) ; or by the possibility of using a grid of multiple repelling (or attracting) magnets (20) with (or without) coil or coils (14) wrapped around them (each magnet or the whole or partial assembly of magnets) ; or by one or more magnets being wrapped by one or more coils (14)

  • Michael C. Fidler May 30, 2017, 11:23
  • J. Jason Wentworth May 30, 2017, 11:23

    Please pardon the unusual formatting; the messages below only appeared in one of my e-mailed comment posting notices, *not* on the blog itself (below Greg Matloff’s article) on my computer, so I had to copy them from my e-mailed posting notice:
    ***********************
    NS commented on Near-Term Interstellar Probes: Some Gentle Suggestions.

    in response to Greg Matloff (who wrote):

    Dear Alex I have absolutely no trouble with 300-, 500-, or 1000-year probes. Neither do many others. But my discussions with some scientists indicates that many people disagree and want results in their lifetimes. A 100- or 200- year trip time seems a bit more reasonable to me. Greg

    NS wrote:

    There’s also the issue that science itself will have changed so much in 300, 500, or 1000 years that it will be merest chance if any data the probe returns will still be of interest that far in the future.
    ***********************

    I have seen this statement made by several people (including authors), and I have never understood why those who say this (that an old interstellar probe’s data would be of no interest to people in the future)–here is why:

    No matter how science might change, the planetary (and stellar) parameters that are of interest will not change. Regardless of how fancy future instruments will be, they’ll still report on planets’ atmospheric constituents, densities, temperatures, and wind velocities, surface compositions, morphology, and geological processes, as well as magnetosphere parameters (ditto for the various stellar parameters). Moreover, time-variant data on these and other parameters are valuable because they indicate changes that take place over time (and whether they’re cyclical or not), and:

    For these reasons, data and images from old interstellar probes will not only *not* be ^uninteresting^ to future astronomers and planetary scientists, but they will welcome and cherish such old observations, and they will be thankful that their ancestors launched the starprobes. Even now, the magnetic field and plasma data that are still being returned by the two Voyager spacecraft (plus the decades’ worth of such data that Pioneer 10 and Pioneer 11 returned) are greatly valued because they illustrate how the Sun’s magnetic and solar wind activity have changed over long periods of time and great distances in space–and because the Sun is a dynamic body, those data are unique and irreplaceable because those solar parameters will never be quite the same during any similar period of time in the future. Likewise, the long-lived (1973 – 2006) Explorer 50 (Interplanetary Monitoring Platform, IMP-8) distantly-orbiting Earth satellite was greatly appreciated because it provided decades of data on variations in the interplanetary magnetic field and interplanetary plasma in Earth’s vicinity. But even older, longer-term (comparable to the time periods involved with interstellar probe missions) collections of observations are prized by scientists. To give just one example among myriads of others in the various sciences:

    Astronomical observatories preserve–or copy in digital-image form–old telescopic photographs (which often date back well over a century), because they are so often useful for refining or “back-tracking” the orbits of recently-discovered objects which appear in the old images; they also show surface and/or atmospheric changes on the Moon and other planets over many decades. Even Galileo’s observations of Neptune proved useful, over two centuries after he recorded them. (He didn’t know what it was, of course, having only his tiny, relatively crude telescopes. He did, however, note the location of the “star” in his notebook, and the fact that it had moved between two nights when he had observed it.)

    How wonderful it would be, if we (somehow) were able to receive data and images from an old–even centuries or millennia old–interstellar probe in the vicinity of one of the relatively nearby stars! (If we happened to pick up radio or laser transmissions from an alien starprobe in such a location–which, while extremely unlikely, isn’t impossible–we would have the benefits of gleaning new knowledge from its observations, no matter how old they might be.)

  • xcalibur May 31, 2017, 16:25

    I think the abstract is that we have several viable methods, but each requires improvements in technology and/or infrastructure before they can be executed. In addition to this, the most promising methods are burdened with thorny geopolitical concerns which must be addressed or worked around.

    As I’ve stated before, we will need to develop a solar-system-wide economy before we can travel to the stars. Developing nearby resources will naturally lead to the improvements in technology & infrastructure needed to make interstellar trips possible. It will also have the cultural effect of altering our perspective, so that space becomes a human place rather than a mysterious frontier. In addition, new generations born in space colonies will be much more willing to colonize other stars than those of us who remain earthbound.

    Finally, while these developments will most likely take longer than a given scientists career or lifetime, that doesn’t make them untenable. To cite a historical example, medieval cathedrals often took over a century to build, with multiple generations contributing to the project. This proves that we are capable of thinking, planning, and acting in long time-scales — a paradigm which will be important for reaching the stars. Admittedly, our present culture is very near-sighted, which makes matters difficult, but that doesn’t mean it’s impossible.

  • J. Jason Wentworth June 1, 2017, 14:47

    There are differences, xcalibur–and perhaps crucial ones–between the motivations of the people who built cathedrals that took multiple lifetimes to complete, and the motivations for developing interstellar travel:

    The people who built the cathedrals did it as an act of worship–to honor, and to demonstrate their gratitude toward–God, someone who by definition is eternal and far above Man. Also, their societies were very spiritually homogenous (that is, nearly everyone was a member of the Church that was involved in the project), and the people were, with few exceptions, strong believers.

    This strong and nearly-universal transcendental motivation is absent from the proposed colonization of the solar system and the development of interstellar travel. Some people do view these things as being a Divinely-appointed destiny for Man, but this view is a minority one. Even doing these things for purely secular reasons–for the adventure, to gain new scientific knowledge, to ensure the survival of humanity and terrestrial life–is very far from enjoying majority support, and even those who do support them seldom desire to move off-world themselves. (I am in this camp myself; while I would love to visit other worlds–even those around other stars, but only if warp drive or an equivalent was available, so that I could return home–I would not want to *live* on another world, or in a space colony, because Earth is my home.)

    Only if the Earth was soon going to become uninhabitable would these attitudes change. But even then, I suspect that many people–especially those who were middle-aged or older–would prefer to stay on Earth and die with it rather than take their chances in the vast interstellar night, especially if the only available method of travel was multi-generation starships. If the destination world(s) was/were very Earth-like, and fast interstellar travel–or reasonably reliable hibernation starships, which would effectively be the same thing as far as the travelers were concerned–was/were available, more people would probably find such an extraterrestrial exodus palatable.

    • ljk June 1, 2017, 21:43

      If the only “doable” reason we can come up with for manned interstellar travel is that our species and our planet are in deep, cosmic trouble, then there is going to be a lot of trouble for the concept ever literally taking off.

      Not only is it against the spirit of science, but if we are in terminal danger and it involves escaping not only Earth but our Sol system, then in all likelihood there will not be enough time or resources to accomplish even one WorldShip. Also humanity will likely be in such a state of chaos that something so complex and critical as a massive interstellar multigenerational vessel could not be built properly, if it all.

      Ever see the 1951 SF film, When Worlds Collide? I am pretty sure the laborers and others who know they won’t be able to go on the rocket that can save only some humans to another planet will wait until nearly launch time as they did in the film:

      http://www.dailymotion.com/video/x24k211_when-worlds-collide-1951-feature_shortfilms

      So if there is enough time what this means is that whoever is building such a ship will do so in complete secrecy and in a place that is as isolated as possible. Which means that the survivors of a doomed Earth will very likely be the only ones who already have everything and now get to carry on the species as well.

      I still predict the first humans to leave the Sol system will be aboard a hollowed out planetoid or comet made livable as a WorldShip by the Superich, who of course will already be living in isolated luxury in interplanetary space. They may leave our planetary neighborhood simply because they want a new and virgin solar system to colonize and call their own. It has happened before many times on Earth.

      • J. Jason Wentworth June 2, 2017, 7:52

        Yes, I’ve seen the film “When Worlds Collide” (its sequel–based on the sequel novel “After Worlds Collide”–was also intended to be made, but the relatively poor box office performance of the first movie ended the sequel movie plans).

        I might be wrong, but I find it hard to picture super-wealthy people enjoying living aboard a worldship when any experiences and luxuries it could offer (except–on a limited basis–weightlessness) would pale in comparison with what our Earth provides, and far more cheaply (and free, in many cases). Also, I agree that chaotic circumstances could easily preclude such a exodus from Earth ever happening.

        • ljk June 2, 2017, 10:43

          As Earth becomes more crowded and less livable, the very wealthy will do what it takes to find better accommodations. They will certainly have the means for it.

          Already some of them are planning to survive all sorts of potential disasters, in luxury and comfort, of course:

          http://www.nextbigfuture.com/2017/01/survivalism-for-rich-and-famous.html

          Turning a planetoid or some plot of land on the Moon or Mars into their new home seems like a logical extension of the Superichs’ behavior to me. Especially once someone figures out how to profit from all that room and vast resources in space. Why do you think the private sector is taking the lead in the Final Frontier, fronted by billionaires?

          • J. Jason Wentworth June 8, 2017, 0:45

            Revealing the existence of their “SHTF shelters” (the “F” stands for “Fan…”) seems unwise; while they might survive anyway, why reduce their odds unnecessarily? According to that article, those self-made wealthy folks anticipate a violent revolt after Artificial Intelligence makes most jobs obsolete.

            While preparing to survive earthquakes, hurricanes, and other natural disasters makes perfect sense, “prepping” for such apocalyptic events as anti-AI revolts seems futile to me. Since such an event would be human-made, it could be human-prevented–just because something *can* be done doesn’t necessarily mean that it ^should^ be done.

            Instead of the super-rich building new “bug-out compounds” in space where they would survive–at least for a while–while their fellow tellurians slid into ruin and starvation back home, I’d much rather see them put their efforts into creating a better world, in which space technology would play an important role. Otherwise (despite what they think) they’ll just be the last to die–only in greater comfort than their fellow human beings–an “advantage” that isn’t worth having, much less worth spending fortunes on.

    • xcalibur June 6, 2017, 10:20

      I agree, cathedrals were built by socio-religious drives which are absent from our society. But that doesn’t necessarily mean that there’s no other way of undertaking long-term projects. A better historical example might be Manifest Destiny. This was not an organized, articulated plan or idea, but rather an overall sense of self-confidence, capability and enthusiasm for consolidating US territory from east coast to west. I think the drive for space colonization will most likely take a similar form. Once infrastructure and investment are in place, a new frontier will open up; and frontiers inspire exploration, adventure, völkswanderung, and conquest.

      • J. Jason Wentworth June 6, 2017, 23:06

        I won’t say that it ^can’t^ happen, but as time goes on, our experiences with living in space reveal more and more difficulties that make it less attractive. None of them are show-stoppers, but in the aggregate, they make living in space an endurance contest. Just two examples are toothpaste and shampoo–aboard the ISS, the crew members have to use swallow-able toothpaste and shampoo that can be left in their hair (it’s rather like the water-less “skip-bath cleansers” that are made for dogs). Also:

        Nearly every little detail of daily life on Earth has to be modified (or done without) in space, and I doubt if most people would care to live that way on a permanent basis. This is where the comparisons with past exploration and colonization on Earth break down.

        Even the most extreme environments on Earth (in polar and desert locales) provide opportunities for basic comforts as well as sources of food and water, plus open spaces. Unless we are so fortunate as to find an exoplanet (and develop the means to get there!) where people can live as we do on Earth, every place where people might live in space will be inside a tin (or stone) can, or underground or inside a lava tube, or under a dome. While some people will move to such places to live (as opposed to going there to visit them), I don’t think most people would care to trade their Earthly homes for such extraterrestrial abodes.

        • Alexander Tolley June 7, 2017, 13:07

          The ISS bathing issues you mention are due to 2 factors – zero-g and a tight water ration. Neither of these need apply in a large, centrifugal-g colony.

          (…) every place where people might live in space will be inside a tin (or stone) can, or underground or inside a lava tube, or under a dome. While some people will move to such places to live (as opposed to going there to visit them),(…)

          In a sense we already do, and increasingly so. Cities are very much like living in stone tubes, even if the sky is sometimes visible. Montreal has a pretty good underground mall that keeps shoppers warm in winter. Other cities are A/C conditioned – like parts of Hong Kong where you really don’t want to go outside in the sweltering summers. It was joked about even when I was a lad that kids in inner London had to take a bus to find green spaces. Brief forays outside our city buildings might just as well be under domes. If they were lava tubes, they light look like the Venetian in Las Vegas, with false skies. If we ever build arcologies, going outside might become as unusual as Elijah bailey’s trips outside the “caves of steel” cities of Asimov’s robot stories.

          I don’t think it will be that much of a stretch even for current and near future city dwellers to adapt to a life in a space colony as long as it is large and has spaces to explore.

          From personal experience, when I lived in Bermuda for 2 years, initially I explored the whole island as it was so small, but gradually restricted myself to the main town, an unusual way to live. I did get over the “rock fever” by taking boat rides offshore. Being on the water in a small boat may not be so different than taking a small spacecraft for a spin outside the colony. Bermuda is about the same scale as an O’Neill Island 3 colony, and many people are quite happy to live their lives on that island without traveling elsewhere. While I needed to travel off the island, I could imagine that if there were swarms of such space colonies, the need to see and explore new places would have been satisfied for me.

          As global warming results in more extreme weather, I can quite imagine that living under a false sky might be quite comforting for us grounders.

          While I am not so sure how we might adapt to different gravity levels on planetary surfaces, I do think the other hazards are overblown. For example, the radiation exposure when traveling on the surface of the Moon or Mars might be irrelevant if we use avatars in our stead. There is no reason to believe that we must be bodily present to explore, as long as the communication latency is very low as to be almost unnoticeable. We will probably be doing something very similar on Earth as we improve our underwater ROVs so that they become external embodiments of our minds. The technology we are developing will get us there quite soon, and I think will make living in artificial environments quite safe and comfortable.

          • J. Jason Wentworth June 7, 2017, 23:33

            I suspect it will be a long time before such large, centrifugal-gravity colonies become economically feasible (and lunar and planetary settlements are stuck with the fractional-g gravity “that they come with.” Radiation is also a more serious problem than was once thought, and the toxic perchlorates on Mars’ surface were unexpected.

            Isaac Asimov, by his own admission, lived a “technologically cloistered” life in his New York City apartment, but he knew that Central Park (and other outdoor places) were always available, as such places always are for city dwellers. Knowing that they’re there–even if one never avails oneself of them–is very different from knowing that cramped quarters *are* one’s physical world.

            Even the astronauts and cosmonauts–many if not most of whom had never previously been interested in gardening–so often become interested in growing plants in space because they fulfill an emotional need; ditto for their great interest in spending as much time as they can watching the Earth below.

            I’m not expressing an objection, but an observation. All of the engineering problems of living in space and on other worlds are solvable, but I’m not confident that most people would care to pay the intangible prices of moving off-world; that is an issue which transcends technology. I know many people, from all different kinds of backgrounds and with different interests and levels of education, and not one of them has the slightest interest in living off-Earth. While I’m sure that there are people who would gladly make such moves, I don’t think they would have to fight crowds for empty seats on the spaceships.

            • xcalibur June 8, 2017, 11:13

              I suspect it will be a long time before such large, centrifugal-gravity colonies become economically feasible

              Of course, nothing is built in a day. It will take long-term development of a space economy before we start building large-scale colonies. However, I believe it is an attainable goal. With the proper resources and infrastructure, we could build colonies that avoid the social problems you’re describing. Lots of people living in a large submarine would cause psychological/societal issues; lots of people living in a worldship with ecological parks would not. Regular trade and traffic to/from such a colony would also help.

              Radiation is also a more serious problem than was once thought

              Radiation is a serious challenge, but it’s one that can be met with engineering.

              not one of them has the slightest interest in living off-Earth.

              It is important to note that space is currently a hostile wilderness, more foreboding than any other environment we’ve visited (with the possible exception of the deep ocean). Not many people would want to permanently live in the ISS. But again, once a significant infrastructure and economy is built up, that will change the equation.

              • J. Jason Wentworth June 9, 2017, 2:33

                I think that eventually some version of yesterday’s (and/or today’s) visions of space settlement will come to pass, but not necessarily soon or on a large scale. I won’t be surprised if it doesn’t occur–or even begin to–in my lifetime.

              • Michael June 9, 2017, 3:00

                ‘I suspect it will be a long time before such large, centrifugal-gravity colonies become economically feasible (and lunar and planetary settlements are stuck with the fractional-g gravity “that they come with.”’

                On the moon we can build large 1 g torus’s supplemented with the worlds free g. We could build very large torus on the moon because they can be supported by the physical mass of the world. Eventually the whole moon could be dismantled to form huge off world colonies.

        • Alexander Tolley June 7, 2017, 13:25

          Another thought. Humans seem to even adapt to prison conditions – no sky except for perhaps an hour or 2 per day, no freedom, no greenery. If humans can manage to adapt to those conditions, I cannot see a space colony being worse. Even the food is likely to be much better than prison fare.

          • J. Jason Wentworth June 7, 2017, 23:59

            For health reasons, I live just this sort of life, and I have done so for about eight years. While I have adapted to it, it is only “positive” by comparison with the alternative–so far; I’m at the bleeding edge where it is just barely preferable (everyone has his or her personal criteria for that). Living and traveling in space have one feature that makes prison life preferable by comparison, and this was known long before long-duration interplanetary space travel was envisioned as being–or becoming–possible:

            Unless a space vehicle or a colony is very large, there is no possibility for real solitude, and being crowded together for long periods has long been known to cause psychological and physical problems in people (depression is just one of the more noticeable ones). At least in prison, one can break a rule and “earn” some time in solitary confinement, to escape the crowding for a while (by comparison, every member of a space crew is necessary, so that only becoming ill or injured “earns” one a respite from the crowding–but who would want that?).

  • J. Jason Wentworth June 1, 2017, 23:21

    Xcalibur, your posting (and my reply to it) reminded me of a 1950s or early 1960s school reading book I used to have, which contained a science fiction story that expressed the feelings that most people have about living on Earth. (It contained a surprising amount of space-related material–a story about the first [USAF] manned landing on the Moon in 1970 that had unexpected twist, an equally “unexpected ending” story about what first Mars explorers found, and a short poem titled “The Astronaut” that goes: ‘Far, far I have flown / Through space and the unknown / The wonders of God spread before me’.) Now:

    The title character of “The Last Martian’s Story” expressed what most people feel about living on Earth. His body was found aboard a huge spaceship that early human explorers found on Mars. It was inside a building in one of the several small, ancient towns whose crumbling ruins were discovered by the settlers from Earth. An archaeologist–a Dr. Pablo, who had deciphered the writing left behind on some of the buildings (which was very similar to the writing of an isolated tribe of primitive red-skinned people of quite unusual physique, who had been found on Earth some years before), read a message that the humanoid being in the spaceship had left. He had scratched it onto a sheet of metal before he died (by pumping the air out of his compartment, as he was the last of his kind on Mars and wanted his message to be preserved by the vacuum, just in case someone might discover it in the far future), and his message–in part–was this:

    Being from Astra, a world that once orbited the Sun between Mars and Jupiter before the two rival factions there caused it to break up (even popular astronomy books back then discussed the possibility that Jupiter’s gravity could have shattered a too closely-wandering planet that might once have existed beyond Mars, which resulted in the asteroid belt), he was no native of Mars either, which was why he wrote: “A few simple towns we built, but our hearts were not in it, and more of us died than were born.” Also:

    Even if we do colonize all of the planets and satellites in the solar system that can be settled, the Earth will always be the emotional “heart” of the solar world, because only here can people stand under open skies and breathe without being confined under a dome or inside a lava tube, and without their physical existence being made possible by pumps, filters, fans, heaters, and electrical power supplies to run such collections of sophisticated apparatus. If the Earth became uninhabitable, no number of O’Neill-type space colonies could ever fill the place in peoples’ hearts that our Earth occupies. While it is not impossible that a perfect–or even acceptable–possible “second home” might be found orbiting another star, it is so unlikely that it would be the height of folly to count on it, and:

    I strongly support the development of interstellar space flight because of the sheer adventure–even from robotic probe missions–and the new knowledge that such voyages will return, but I have no illusions about galactic settlement. Indeed, images of exoplanets (even the most Earth-like ones) sent back by interstellar missions will forcefully drive home–even more emphatically than the photographs we’ve seen of it hanging in the void–just how unique our Earth is.

    • ljk June 2, 2017, 8:39

      That might be true for a generation or two that can recall Earth, but once you have descendants born and raised on other worlds or space facilities, the third rock from Sol will just be a story they heard about from their ancestors. Home will be where they grew up.

      Do you miss the country your ancestors migrated from? Do you consider that your true home, or the place you actually grew up in?

      Earth has a strong grip on us only because so few have ever left it, and even then the periods have been too short and the planet was never out of sight, so therefore not out of mind.

      Isaac Asimov’s Foundation Series has a galaxy-spanning human civilization for which Earth is only legend and myth. Some folks do find it later on, but it turns out the planet is a radioactive wasteland which is why humanity spread out into space to begin with.

      • Alex Tolley June 2, 2017, 11:19

        Isaac Asimov’s Foundation Series has a galaxy-spanning human civilization for which Earth is only legend and myth.

        Also his “Pebble in the Sky”.

        Do you miss the country your ancestors migrated from? Do you consider that your true home, or the place you actually grew up in?

        I don’t even miss the country I grew up in. Perhaps more relevant is whether Polynesians miss the S E Asian area their ancestors came from. The answer is clearly no. But the circumstances may be different. Some people seem to want to return to that biblical Eden. I suspect that unless colonists live in a world (or in an environment) as attractive as Earth, there will be a longing for those “Green Hills of Earth”. If however, their new worlds are as attractive as Earth or even more so, then Earth will not be anything more than a curiosity. I hope we can indeed find, or more probably construct, those Edens.

        • ljk June 2, 2017, 14:17

          Do kids even appreciate going outdoors any more? This is an only half-facetious comment.

          • xcalibur June 6, 2017, 10:04

            Young people still go outside, I’m still young and I’ve gone camping in the Catskills. But it’s true, there are more glowing screens of distraction than ever before.

      • J. Jason Wentworth June 2, 2017, 19:33

        “Do you miss the country your ancestors migrated from? Do you consider that your true home, or the place you actually grew up in?”

        Yes. I was born in Miami, but it never felt like home. I always felt drawn to the British Isles, even though no living relatives had ever been back there or told me about them. When I visited England (rural Lincolnshire) in 2010, everything–from the lower cloud level to the color of the sunsets to the different (from what I’d seen) topography, birds, and foliage–was just as I’d pictured it (and I had deliberately *not* pored over photographs and books about the place). Had my health, age, and finances had permitted it, I would gladly have moved there.

    • xcalibur June 6, 2017, 10:41

      I agree that Old Earth will be the center of human civilization for centuries to come. Even after the solar system is extensively colonized, Earth will be the capital and the mecca.

      However, I think you underestimate the willingness of individuals and societies to leave the Earth and live in parts unknown. It would not take catastrophe to convince people to leave our home planet. At first, only a minority would strike out into space, but these would germinate into space-based cultures, existing comfortably in a cylinder, torus, or dome. They may even take pride in this way of life and prefer it to being ‘earthbound’. We humans are adaptable, and that will be a boon to space colonization.

      • J. Jason Wentworth June 6, 2017, 23:24

        We shall see. One motivator for an “Exodus from Earth” that I can foresee–which Poul Anderson described in his novel “Tales of the Flying Mountains” (planetary scientist John Lewis also referred to this possible motivation in his non-fiction book “Mining the Sky”)–is an escape from political, economic, and/or even religious persecution on an Earth whose societies might become too rigid and demanding. (Anderson’s fictional “geegee” [gyro-gravtic] technology, as well as enabling fast space transportation, also made the worlds of his Asteroid Republic very attractive places, which could hold down breathable atmospheres!)