Interstellar Journey: Shrinking the Probe

by Paul Gilster on July 14, 2014

We’ve all imagined huge starships jammed with human crews, inspired by many a science fiction novel or movie. But a number of trends point in a different direction. As we look at what it would take to get even a robotic payload to another star, we confront the fact that tens of thousands of tons of spacecraft can deliver only the smallest of payloads. Lowering the mass requirement by miniaturizing and leaving propellant behind looks like a powerful option.

Centauri Dreams regular Alex Tolley pointed to this trend in relation to The Planetary Society’s LightSail-1 project. In a scant ten years, we have gone from the earlier Cosmos 1 sail with an area of 600 square meters to LightSail-1, with 32 square meters, but at no significant cost in scientific return because of continuing miniaturization of sensors and components. We can translate that readily into interstellar terms by thinking about future miniature craft that can be sent out swarm-style to reach their targets. Significant attrition along the way? Sure, but when you’re building tiny, cheap craft, you can lose some and count on the remainder to arrive.

The Emergence of SailBeam

I inevitably think about Jordin Kare’s SailBeam concepts when I hear thinking like this. Kare, a space systems consultant, had been thinking in terms of pellet propulsion of the kind that Clifford Singer and, later, Gerald Nordley have examined. The idea here was to replace a beam of photons from a laser with a stream of pellets fired by an accelerator — the pellets (a few grams in size) would be vaporized into plasma when they reached the spacecraft and directed back as plasma exhaust. Nordley then considered lighter ‘smart’ pellets with onboard course correction.

I’m long overdue for a re-visit to both Singer and Nordley, but this morning I’m thinking about Kare’s idea of substituting tiny sails for the pellets, creating a more efficient optical system because a stream of small sails can be accelerated much faster close to the power source. Think of a solar sail, as Kare did, divided into a million pieces, each made of diamond film and being accelerated along a 30,000 kilometer acceleration path, all of them shot off to drive a larger interstellar probe by being turned into a hot plasma and pushing the probe’s magnetic sail.

sailbeam

Image: Jordin Kare’s ‘SailBeam’ concept. Credit: Jordin Kare/Dana G. Andrews.

Kare, of course, was using his micro-sails for propulsion, but between Nordley and Kare, the elements are all here for tiny smart-probes that can be pushed to a substantial fraction of the speed of light while carrying onboard sensors shrunk through the tools of future nanotechnology. Kare’s sails, in some designs, get up to a high percentage of c within seconds, pushed by a multi-billion watt orbiting laser. Will we reach the point where we can make Kare’s sails and Nordley’s smart pellets not the propulsion method but the probes themselves?

In that case, the idea of a single probe gives way to fleets of tiny, cheap spacecraft sent out at much lower cost. It’s a long way from LightSail-1, of course, but the principle is intact. LightSail-1 is a way of taking off-the-shelf Cubesat technology and giving it a propulsion system. Cubesats are cheap and modular. Equipped with sails, they can become interplanetary exploration tools, sent out in large numbers, communicating among themselves and returning data to Earth. LightSail’s cubesats compel anyone thinking long-term to ask where this trend might lead.

A Gravitational Lensing Swarm

In Existence, which I think is his best novel, David Brin looks at numerous scenarios involving miniaturization. When I wrote about the book in Small Town Among the Stars, I was fascinated with what Brin does with intelligence and nanotechnology, and dwelled upon the creation of a community of beings simulating environments aboard a starship. But Brin also talks about a concept that is much closer to home, the possibility of sending swarms of spacecraft to the Sun’s gravitational focus for observation prior to any star mission.

We normally speak about the distance at which the Sun’s gravity bends light from objects on the other side of it as being roughly 550 AU, but effects begin closer than this if we’re talking about gravitons and neutrinos, and in Brin’s book, early probes go out here, between Uranus and Neptune, to test the concept. But get to 550 AU and beyond and photon lensing effects begin and continue, for the focal line goes to infinity. We have coronal distortion to cope with at 550 AU, but the spacecraft doesn’t stop, and as it continues ever further from the Sun, we can be sampling different wavelengths of light to make observations assisted by this hypothesized lensing.

Before committing resources to any interstellar mission, we want to know what targets are the most likely to reward our efforts. Why not, then, send a swarm of probes. Claudio Maccone, who has studied gravitational lensing more than any other physicist, calls his design the FOCAL probe, but I’m talking about its nanotech counterpart. Imagine millions of these sent out to use the Sun’s natural lens, each with an individual nearby target of interest. Use the tools of future nanotech and couple them with advances in AI and emulation and you open the way for deep study of planets and perhaps civilizations long before you visit them.

The possibilities are fascinating, and one of the energizing things about them is that while they stretch our own technology and engineering well beyond the breaking point, they exceed no physical laws and offer solutions to the vast problems posed by the rocket equation. Perhaps we’ll build probes massing tens of thousands of tons to deliver a 100 kilogram package to Alpha Centauri one day, but a simultaneous track researching what we can do at the level of the very small could pay off as our cheapest, most effective way to reach a neighboring star.

More on this tomorrow, as I take a longer look at Clifford Singer and Gerald Nordley’s ideas on pellet propulsion. I want to use that discussion as a segue into a near term concept, Mason Peck’s ideas on spacecraft the size of computer chips operating in our Solar System.

And today’s references: Cliff Singer’s first pellet paper is “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107–115. Gerald Nordley’s ideas can be found in “Beamriders,” Analog Vol. 119, No. 6 (July/August, 1999). Jordin Kare’s NIAC report “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion,” (Final Report, NIAC Research Grant #07600-070, revised February 15, 2002) can be found on the NIAC site.

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{ 32 comments… read them below or add one }

ljk July 14, 2014 at 11:05

Riding on Interstellar Winds: New Solar Outburst Confirms NASA’s Voyager 1 Spacecraft Has Exited Heliosphere

By Leonidas Papadopoulos

“Sailors on a becalmed sea, we sense the stirring of a breeze.”

— Carl Sagan, “Pale Blue Dot” (1994)

In the last couple of years, there had been considerable uncertainty as to whether NASA’s Voyager 1 spacecraft had crossed the boundary separating the Sun’s magnetic sphere of influence, better known as the heliosphere, from interstellar space. Previous data from Voyager’s onboard instruments provided the first compelling evidence that the spacecraft had crossed that boundary in August 2012, signaling perhaps one of the greatest achievements in the history of our species.

New readings of the spacecraft’s surroundings, taken earlier this year, have now come to confirm that the venerable robotic explorer has indeed exited the vast region around the Sun that is dominated by the solar wind, and is now guided by the interstellar winds in the uncharted open sea of interstellar space.

http://www.americaspace.com/?p=63989

To quote:

As monumental as the ongoing mission of Voyager 1 is, the emblematic robotic spacecraft hasn’t escaped the Solar System and the complete grasp of the Sun yet. Although now outside of the realm dominated by our home star’s magnetic field, Voyager 1 will still spend its next 30,000 years in the Sun’s gravitational grip, slowly crossing the vast reservoir of trillions of comets that envelopes the Solar System, known as the Oort cloud, at a speed of 17 km/s, or 3.5 Astronomical Units per year. By then, the spacecraft will be long silent, unable to record and transmit any scientific observations of any possible wonders it might fly by. Currently operating on a feeble 23 watts of electrical power down from their full capacity of 470 watts at the time of launch, Voyager’s radioisotope thermoelectric generators will have depleted their plutonium-238 power sources by the end of the next decade, leading to the spacecraft’s inevitable end of operations, after having long completed their roles as both space explorers and sources of inspiration for society and art. Even then, Voyager 1 and its twin Voyager 2, far from being worthless heaps of junk, will continue on circling the Mikly Way galaxy far from their place of origin, while silently carrying their onboard Golden Records, loaded with humanity’s sights and sounds of a bygone era, finally taking on their ultimate role as interstellar robotic ambassadors of a species called Homo sapiens, in the Universe’s infinity of space and time.

jamesmessig July 14, 2014 at 11:29

Another interesting take on all of this Paul would involve quantum computers.

Accordingly, a quantum computer would calculate from deterministic principles the precise energy state of a location on a planet billions of light-years away. The energy state of that location would be replicated back here around o’l Sol.

Next, the duplicated energy state would be made indistinguishable from the remote version. Alternatively, both versions would become quantum-mechanically enmeshed so that they evolve identically in time.

The end result would be a superluminal teleportation to the remote location, or a real-time surveillance of the remote system back here in our solar system.

It may be possible that a classical state evolution simulator could make a very close approximate duplication. Perhaps the degree of enmeshment would be a function of the fidelity of the duplication. Lower fidelity would result in a cloud like or “mist-like” presence, and perfect fidelity would actually cause the person doing the viewing to arrive at the distant location.

I’m not saying any of this is possible, but we should be open to all kinds of possibilities for enabling true interstellar travel.

Einstein would have held that the entire past, present, and future history of the universe in all of its details could be computed and predicted retroactively and pro-actively. I also tend toward being a “God does not play dice.” kind of guy even amongst the EPR paradox phenomenon. But my thoughts on that are a different topic altogether.

Alex Tolley July 14, 2014 at 11:38

Technology moves on rapidly since the Kare paper. Watch Dr. Philip Lubin’s presentation at last years Starship Century conference (Day 1 pt 3). He already has sail material at greater than 99.99% reflectivity, higher than Kare’s best proposed films. His presentation was on phased laser arrays. His proposed De-Star class 4 arrays would meet Kare’s 50 GW sail beam power requirements. I think we are talking a few decades away for such a device. He was talking about pushing payloads with sails to 2% c at the edge of the solar system using existing 10uM sail (LightSail-1 uses 4.5 uM mylar).
With nano scale technology sails, he was offering 20%c velocities.
My take home is that we could be a lot closer than we think for Paul’s proposal for both swarms of sensors for gravitational telescopes and even flyby probes to the nearer stars.
His 15 days to 25AU using 10uM sails would get you to FOCAL ranges in just one year (fortunately the focal length is infinite!), and a nanotech 0.1uM sail in a month! Forget the decades using conventional rockets. An Alpha Centauri flyby with a flight time of 20+ years!

Lubin’s main focus appears to be planetary defense so I suspect his propulsion slides for the conference were put together specifically for that track. But it is clear that his phased laser’s would provide the power for the sails, and that this likely could be achieved sooner, rather than later.

To address ljk’s worries about their use as a weapon (after all they are being built to vaporize asteroids), I suspect that safeguards to de-phase the lasers would help obviate hijacking the array. However I cannot disagree that one motivation of DARPA funding for military use would be for their use turned towards Earth. Their focus is small enough that ground targets could be quickly vaporized. But they might also find a role in weather control, e.g. disrupting hurricanes. Perhaps one of the Benford’s could weigh in on Lubin’s work in reards to beamed sails?

RobFlores July 14, 2014 at 11:54

These smaller probes are good near term projects. But the same
problem of obsolescence with propulsive engines speed during the mission time , you will have with the attempt to miniaturize the payload.
For example: You send a probe to Alpha Centauri. Lets say that
our light-sail can get the probe there in 100 yrs. It weighs 100 Kilos.
fifty years after launch a much more advanced probe weighing
3 Kilos is developed and launched . With a lower mass and a slight improvement to the light sail propulsion. The new probe can beat the old probe to Alpha C.

It makes more sense in thinking about probes that have reasonable timelines for nearby targets. Something like 30Ly and 60 years of travel to the farthest range. I know that is fast, but if you make your probe as light as possible (under 3 Kilos) I think it brings such a project into what is plausible, especially if you use a flight path that takes a probe PAST a target solar system at a few AU above the Primary Stars’ rotational plane. This
would be a fly-by only, which is fine, the goal of such fast probes should be
quick survey not deep analysis you’d get from attempting to brake and enter into orbit about the target star.

Al Jackson July 14, 2014 at 12:03

I have always loved this idea since read it in Galaxy in 1955 when I was 15…. Phil Dick’s Autofac, and you wrote about it sometime back, so I may be repeating the link:

https://archive.org/stream/galaxymagazine-1955-11/Galaxy_1955_11#page/n71/mode/2up

by the by, those are beautiful black and white illos are by the artist Ed (Emsh) Emshwiller , the equal of Kelly Freas in the 1950′s and 1960.s.
(Emsh is still under appreciated!)

Anyway , as Feynman said there is Plenty of Room at the Bottom

http://www.zyvex.com/nanotech/feynman.html

ad astra per minutias

Alex Tolley July 14, 2014 at 13:27

A FOCAL mission using swarms of small sensors would be a very interesting project, especially if the short flight times make this a reasonable undertaking for current project times for scientists. One possible approach to the sensors is to use single pixel imaging. I would envisage the “sail” providing the random sampling. This would allow good data compression of the signals back to Earth to reconstruct the target. Because the image is reconstructed from different time slices, it should also help remove noise.

As we push the limits of engineering to the atomic (sub-atomic?) scale, we should be able to devise ever smaller sensors as payloads, allowing more to be packed into a fixed payload, or to drive down the payload mass until it becomes a small fraction of any sail, possibly even part of the sail itself.

Which of course gets us back to the Fermi paradox. As a number of us have argued, are these tiny probes already in the solar system, beaming their signals to a hidden transmitter somewhere in the solar system?

And for Heath Rezabek, what information could be packed into such small sails (“nano-vessels”) if we spread them out in the solar system or even the stars like seeds?

ljk July 14, 2014 at 13:33

Alex Tolley said on July 14, 2014 at 11:38:

“To address ljk’s worries about their use as a weapon (after all they are being built to vaporize asteroids), I suspect that safeguards to de-phase the lasers would help obviate hijacking the array. However I cannot disagree that one motivation of DARPA funding for military use would be for their use turned towards Earth. Their focus is small enough that ground targets could be quickly vaporized. But they might also find a role in weather control, e.g. disrupting hurricanes. Perhaps one of the Benford’s could weigh in on Lubin’s work in reards to beamed sails?”

Why is DARPA interested in forming an interstellar vessel organization with the 100-Year Starship? Is it because they want to explore and colonize Alpha Centauri? Doubtful, at least for now. Should an alien race pose a threat someday, that attitude might change. No planning fake alien invasions to get space funding now, folks.

What the military most likely wants is to see what kind of ideas and technologies a collection of space geeks can come up with and give to them for free in their excitement that a group like the Department of Defense seems interested in starships.

BTW, the $100K they threw at the 100-Year Starship project is pocket change to an organization that gets about $700 billion annually (and has over $8 trillion unaccounted for since 1996).

The military’s primary function is to uphold and defend the government they serve. Anything else that comes from it is a bonus, crumbs falling from their dinner table. If they end up being responsible for making space-based laser systems, you can bet your farm their primary purpose is for threatening and taking out the enemy. At best they may allow a few science projects along the way; it makes good PR.

I know, I sound terribly cynical and I really don’t want to be. I say what I do say to save space fans decades of disappointment in our space program and the realities of political whims. But look yet again at Apollo or even the Space Shuttle. Both were designed to serve geopolitical needs and probably never would have existed if science were their only goals. The Space Shuttle was supposed to loft really big spysats, but that idea went away after the Challenger disaster in 1986.

Check out this NASA book on Apollo from 1989:

http://history.nasa.gov/SP-4214/cover.html

You will see how much it was an engineering project over a lunar science one in the eyes of many at NASA and beyond. I still recall the day when I realized that most of the astronauts were far more interested in flying big fast space machines than exploring the Final Frontier for science. Deke Slayton told the science astronauts chosen by NASA that they would never get a space flight so long as he was in charge of the astronaut program. It was definitely a good ol’ boys network back then.

Please read that book I linked to above. Throughout much of it the author’s attitude is that he wished the scientists would stop whining and just be grateful that NASA let them do any science at all on the Moon. I doubt his attitude was an isolated one. The fact that NASA allowed him to say as much in one of their own authorized publications says volumes. I would like to think these are past attitudes from an era where women were still not considered good enough to be astronauts and everyone smoked because it was cool, but we shall see when those first space-based laser platforms get built and by whom.

ljk July 14, 2014 at 13:50

Alex Tolley said on July 14, 2014 at 13:27:

“As we push the limits of engineering to the atomic (sub-atomic?) scale, we should be able to devise ever smaller sensors as payloads, allowing more to be packed into a fixed payload, or to drive down the payload mass until it becomes a small fraction of any sail, possibly even part of the sail itself.”

Good idea making the sail the probe. Just had to emphasize that.

“Which of course gets us back to the Fermi paradox. As a number of us have argued, are these tiny probes already in the solar system, beaming their signals to a hidden transmitter somewhere in the solar system?”

You know that dust you sweep from your coffee table once a week or so? Well they are smaller than that and even more numerous…

http://www.centauri-dreams.org/?p=29963\

This is why our current ideas about SETI will soon be seen as really retro thinking when it comes to highly advanced alien tech. Radio messages beamed from a planet’s surface? Puleeeze.

“And for Heath Rezabek, what information could be packed into such small sails (“nano-vessels”) if we spread them out in the solar system or even the stars like seeds?”

See the link to the February 2014 Centauri Dream article I supplied above. The answer is plenty as we continue to improve miniaturization technology.

Michael July 14, 2014 at 13:54

I and other have mentioned here before, the smaller you go the more radiation has an effect on the electronics. Although small is a simple solution to the ‘mass’ problem there is a minimum size (mass) required to protect the electronics of the craft. Engineering the dumb components such as the optics around them would do much to reduce the problem. The sail after use could be rolled around it which would help as well.

ljk July 14, 2014 at 13:57

About the Lubin De-Star concept: Not only could it be used to vaporize planetoids and other objects, but one could also fine tune the system to just nudge planetoids in a particular direction – say at an enemy target. Oops, so sorry! The system must have had a glitch. Darn celestial mechanics!

Randy Chung July 14, 2014 at 16:08

If you could “tack” the micro-sails across the beam, a pair of micro-sails might be able to adjust their relative speeds and rendezvous. For instance, a micro-sail in front of an overtaking micro-sail might reflect the beam backward both to accelerate itself and to slow down the overtaking micro-sail.
It might be possible to build up more functionality by agglomerating micro-sails. It would be great to build up a fission reactor from micro-sail components, and power it with micro-sail fission fuel. If you could reach that level of complexity, you should be able to build an ion engine and use it as a retro-rocket to slow down and stop at the destination.

Alex Tolley July 14, 2014 at 16:53

We know the DoD has long been interested in the use of powerful lasers – from H-bomb detonated x-ray lasers to the ongoing use to shoot down missiles during boost phase. There was also the idea of using the high ground of space to shoot “brilliant pebbles” down onto the surface to destroy armored vehicles and facilities. In his talk, Lubin mentions that the lasers are independent and targetable, so no doubt there is interest in using them for a number of military applications while awaiting that stray asteroid. The DoD also has suggested using solar power sats to provide power for battlefield operations, removing the need for fuel supply logistics.

So I don’t think we need be cynical in believing the DARPA funding for lasers is primarily for military purposes, secondarily for planetary defense, and lastly for civilian use.

But, as we have seen from history, once the technology is created, eventually it becomes accessible for civilian use. More importantly, once it gets into private hands, it can be used for any ends. If powerful space lasers are built, then eventually cheaper versions will be built that are private and used for private aims. I’m optimistic those aims will mainly be benevolent, and not run by someone with a fondness for white, Persian cats. So let DARPA provide the development funds for military toys, because once access to space becomes low cost, civilians will be able to adapt those toys for non-military uses and we will be able to power our sails at a lower cost than we ever thought possible.

Sara July 14, 2014 at 16:54

Why must payloads be small to be transportable? Why not use common sense and build the ships and their payloads in space, in orbit around the home world? A space-based refining and fabrication plant is not such an outrageous idea, either. And why is it automatically assumed that all materials for construction of a ship and payloads have to come from Earth, instead of being created in space, where zero gravity creates specific properties to exist? The point to all of those experiments on the space station has been to discover previously unknown physical properties of common objects, as in the flameball experiments, which not only demonstrated a previously unknown property of open flame.

Technology is progressing at exponential rates. Build robots in space that become part of a spaceship when they are done. In other words, let the starship build itself, with the ability to upgrade its intelligence system (NOT artificial intelligence) as it progresses through the building process and after the construction is complete.

I’m just saying stop limiting this business of space travel to the corporate groundhog level, because the lack of imagination down there will lower the rate of utilizing space to the snail’s pace it already occupies.

Arthur July 14, 2014 at 19:39

Of course you could go to the opposite extreme; building 500 metric tonne spacecraft in factories on Earth, doing a complete checkout then boosting them into orbit with a Sea Dragon.

While this may be equally fanciful, it is actually both doable and more in line with the idea of sparking a large human presence in space. Micro and nano spacecraft, as envisioned above, are very interesting and clever solutions to various problems, but won’t really appeal to the general public, or excite much new interest or support for space. A Sea Dragon launch is both spectacular in of itself, and a fully operational spacecraft or space station boosted into orbit and then married up with its crew (and possibly fuel or supplies you don’t want to boost with a Sea Dragon) will have a certain level of excitement all of its own.

It seems that to really get ahead in space, you have to develop technological extremes at both ends of the scale.

Eniac July 14, 2014 at 21:59

Sailbeams are a very interesting concept, and Paul is right on with the idea of using the sails for surveillance instead of propulsion. See the insightful comment of an obviously super-smart fellow back in 2010:

I am intrigued by the possibility that you could do away with the starship at the far end and use a steady stream of microsails like a “probe-beam” which acts as a sensor array and communication relay chain to make observations and send the data back home.

http://www.centauri-dreams.org/?p=10982#comment-78013

The key here is that a matter stream must be actively focussed, i.e. each “pellet” must have the ability to home in on its target using a guidance and propulsion system. Otherwise, dispersion is a complete show-stopper. Kare realizes this, many others don’t.

Eniac July 14, 2014 at 22:08

Alex Tolley:Technology moves on rapidly since the Kare paper. Watch Dr. Philip Lubin’s presentation at last years Starship Century conference (Day 1 pt 3). He already has sail material at greater than 99.99% reflectivity, higher than Kare’s best proposed films.I would be interested what this wonderous material is that Lubin has. I was not able to find a reference to it. Could you elaborate, please?

Eniac July 14, 2014 at 22:09

Alex Tolley:

Technology moves on rapidly since the Kare paper. Watch Dr. Philip Lubin’s presentation at last years Starship Century conference (Day 1 pt 3). He already has sail material at greater than 99.99% reflectivity, higher than Kare’s best proposed films.

I would be interested what this wonderous material is that Lubin has. I was not able to find a reference to it. Could you elaborate, please?

Alex Tolley July 14, 2014 at 22:29

@sara And why is it automatically assumed that all materials for construction of a ship and payloads have to come from Earth, instead of being created in space, where zero gravity creates specific properties to exist?

It isn’t. But the value of space manufacturing will mostly be for bulk, low complexity materials, e.g. lunar regolith for lunar bases. Many components of spacecraft will be far more difficult to manufacture, and so it is better to manufacture those on earth where all the facilities, labor and materials exist. Therefore one key to space is reducing launch costs. But inevitably manufacturing and local resources will become much more important. If you intend to build a worldship, I expect much of its mass will be sourced extra-terrestrially.

@Arthur – sea launch was never built so we will never know if it could have been both successful and cost effective. It’s payload would have been 3-4x that of the SLS, so you are talking a very big launcher.

I note the 2 competing ideas between Sara and Arthur. Both recognize size, especially for human missions needs to be large. Sara plumps for space manufacture, like the original starfleet Enterprise, while Arthur wants Earth manufacture and a way to place the finished product in orbit. No doubt practicality and economics will show the way.

My preferred approach borrows from both. Use advanced technology to manufacture strong, lightweight, foldable structures than can be easily launched. Then use space resources, like water, to provide fuel, consumables and structural mass. This minimizes costs for launch for whatever is the prevailing launch system (no waiting for special vehicles), yet also uses cheaper, simple bulk material readily extractable from space resources.

Matt July 14, 2014 at 23:07

@Sara the advantage to building very small, and very low mass space craft is that it is much easier to accelerate them to high speeds. Of course the high speeds help to reduce trip times to distant stars. That is true whether they are built on earth or in space.

I assume space craft used in star travel will be built in space. In any case the infrastructure to work and build in orbit will be needed for settling our solar system which I think is worth doing even if we never go to the stars.

Louis Friedman July 15, 2014 at 0:21

Very good. We wrote about this for the 100 Year Starship and the JBIS a couple of years ago — http://adsabs.harvard.edu/abs/2013JBIS…66..252F

Randy Chung July 15, 2014 at 0:39

@Sara: I agree that near space construction is the way to go. I believe that’s the only way to build starships affordably, for perhaps a few billion dollars a year.
There was a question on Quora.com, “Engineering: If you had a dream team of engineers and were given an unlimited budget and were told to build something awesome (it could be anything at all), what would you propose the team build?” My answer: build a self replicating factory that could build just about anything manufacturable, using only sunlight as a power source and dirt (asteroids) as the raw material. The first factory could build a bigger one, then that one builds yet bigger, and so on.
In our solar system, we could then build a large power beamer to propel a ship outward, which could have a fission reactor and ion engine to slow down and explore the destination system. If the ship also brought along a small self replicating factory, it could eventually build a power beamer at the destination system, along with other infrastructure. Then there’d be two way power beaming.

Adam July 15, 2014 at 7:11

Alex,
Speaking for Jim Benford, based on my discussions with him, I think he’s more inclined towards microwaves, since we can make them at high power, high efficiency and low cost. Diffraction does mean the microwave projector banks for pushing star-sails are very large. The military likes lasers because they’re compact, but in space applications with required ranges of thousands to millions of km, that’s irrelevant. The optics are huge, either way.

Whether we use microwaves or lasers is only half the problem, power supply being the other side of the equation. Only self-replicating solar power satellites can supply it cheap enough IMO.

Adam

Jim Early July 15, 2014 at 8:32

One challenge of the high velocity, small mass fly-bye mission has always been the very low observation time. A small mass probe might be limited to a 10 cm observation optic. At velocities of 0.1c the time it could resolve a earth sized planet would only be several hours. The time at 10 km resolution would be around 10 seconds.
This architecture needs either very low mass large optics for observations or a method to decelerate the probe.

Alex Tolley July 15, 2014 at 10:12

@Eniac – Here is a link to Lubin’s talk. he’s first on. Best watch his presentation and go from there. Because the presentation was just recorded, you need to watch full screen and pause to try to decipher the slide content. It would have been nice if the the slides were the main feature with a sub picture of the presentation, but that is all that is available, AFAIK.

Alex Tolley July 15, 2014 at 10:39

@Adam – I agree about the power issue. However there seems to be a lot of work in reducing mass/Kw, with thinner arrays and higher conversion rations. I don’t know that self replication is a requirement. And when has the military let costs get in the way – c.f. F-35 Lightening.
In the short term, you could use terrestrial power, beam it to space and convert it back to electrical power. Inefficient yes, but it allows experimentation without committing resources in advance.
I do not know the relative merits of lasers vs microwaves. One advantage with microwaves is that the sail can be perforated or even a mesh, rather than a solid sheet, reducing mass and improving performance when under power. Perforations work even for solar sails if the hole size small enough.

Speaking of experiments, Jim Benford was going to try to push Cosmos-1 with microwaves. I note that one of the Planetary Society’s Lightsail’s due to go up next year is going to fall back to Earth. Is there any thought to do the beam experiment on this Lightsail. or would be be too messy to distinguish the effect due to the variability of the atmospheric drag? What about the 2016 mission?

Alex Tolley July 15, 2014 at 11:26

Building lightweight vehicles to reduce launch mass.

Way back in the last century, Arthur Clarke noted that ships that only travel in space can be made extremely fragile. We see that in the Apollo program where the LM ascent stage was less than 40% of the mass of the command module. In some places the LM was very thin walled. Imagine a structure like the Echo 1 satellite, made of 12.7 uM Mylar, having a 30 m diameter yet weighing just 180 Kg. Such inflatable forms are easily packed in small dimensions for launch, yet could be used as propellant tanks and pressure vessels. I could imagine such lightweight materials, strengthened with graphene as the basis of very lightweight structures. A 10m. 10uM sphere with 500 m^3 of internal volume would mass just 3 Kg. Other structural components could be additively manufactured in situ, from plastics and metal. Add water from Ceres as ice to reinforce the hull for micrometeoroid damage and radiation protection. This substantially reduces the size and mass of components, and allows existing launchers to be used, reducing costs.

To my mind, lightweight structures also benefit human exploration. Solar sails, inflatable habs, etc will give us experience in manufacturing, deploying and testing these structures and determining their usefulmess for other purposes. While space manufactured materials may be advantageous, additive manufacturing may prove even better in creating strong, lightweight materials, with multiple functional components embedded with them, much like living structures.

Economics will be important considerations on any spaceflight. We’ve seen how Lightsail’s reduced size also reduced costs, especially launch costs (the 2016 flight will be gratis). The more we can reduce size and mass of large structures, the cheaper they will be. Adam notes above that the solar PV costs for space beaming will be very high unless such components can be made in situ. My view is that thin PV and lightweight concentrators will make these components much less massive and thus costly to launch. These components will be made on Earth where we have the infrastructure to make these high tech materials and components. So reducing component mass and lower cost space access will become a virtuous circle making space much more affordable, whether we are talking tiny robotic probes or manned space exploration and eventual colonization.

If humans are to go to the stars, I don’t believe that we can assume that GDP will exponentially grow to meet the costs of such endeavors. Better to assume limits and plan accordingly, and be pleasantly surprised if that plan happens to extend that growth by making a solar system economy more likely.

Eniac July 15, 2014 at 20:13

Alex Tolley: I saw the mention, but there isn’t anything about what this material might be, or where a paper describing it could be found. The number is unbelievable. I will consider it a theoretical assumption for modelling until I find out otherwise.

Michael July 16, 2014 at 2:48

@Randy Chung July 14, 2014 at 16:08

‘It might be possible to build up more functionality by agglomerating micro-sails. It would be great to build up a fission reactor from micro-sail components, and power it with micro-sail fission fuel. If you could reach that level of complexity, you should be able to build an ion engine and use it as a retro-rocket to slow down and stop at the destination.’

There is the possiblity of activating a plutonium 239 source at near the end of the trip and allowing the volatile fission decay products to leak onto the target stars side of the sail, they will be emitted slowing down the craft over time. Fission fragment propulsion is very mass efficient and would form a very effective braking mechanism for a large sail.

Alex Tolley July 16, 2014 at 10:33

@Eniac – a quite Google suggests that 99.99% reflectance is claimed. This company is one example. There are some scholarly papers too. So I am inclined to accept Lubin’s claim that such high reflectance materials exist, especially when tuned to a narrow laser wavelength.

ljk July 16, 2014 at 12:32

I wonder what we could do interstellar vessel-wise with this incredibly non-reflective material…

http://www.extremetech.com/extreme/186229-its-like-staring-into-a-black-hole-worlds-darkest-material-will-be-used-to-make-very-stealthy-aircraft-better-telescopes

Just trying to think outside the box here.

Michael July 17, 2014 at 2:24

@ljk July 16, 2014 at 12:32

‘I wonder what we could do interstellar vessel-wise with this incredibly non-reflective material…

http://www.extremetech.com/extreme/186229-its-like-staring-into-a-black-hole-worlds-darkest-material-will-be-used-to-make-very-stealthy-aircraft-better-telescopes

I wonder if it would find use in a deflection shield, the vertical nanotubes if slightly angled to the normal will tend to bend oncoming radiation particles to the side through shallow angle deflections as they move down the tubes.

They could be used to maybe form the backbone of a sail by covering one side with vapourised metal, it would form a material which is very flexiable in one direction than in the other allowing it to be rolled up tightly. Or as neutron focuses for a fission drive using a central neutron generator.

Eniac July 20, 2014 at 22:33

Alex Tolley: It looks like you are right, you can actually buy stuff with 99.99% reflectivity. I would still like to know what it is, though, and it seems at Artemis they don’t say. It is also not clear if this would work as a thin film sail, but one can hope.

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