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Project Longshot: Fast Probe to Centauri

Project Daedalus, discussed frequently in these pages, was the first in-depth design study of an interstellar probe. Its projected fifty-year flyby mission to Barnard’s Star at 12 percent of the speed of light was beyond contemporary technology (and certainly engineering!), but not so far beyond as to render the design purely an intellectual exercise. I bring up Daedalus again because I keep getting asked about Project Longshot, which some have mistakenly seen as a successor to Daedalus with a NASA pedigree. And wasn’t Longshot a far more advanced design?

Actually, no. But the other day I again ran into Longshot in the form of an online post describing it as a hundred-year mission to Alpha Centauri (true enough), evidence that NASA had the technology right now (not true) to get us to the nearest stellar system in a century, which would be faster by far than the thousand years I’ve always used as an absolute minimum for getting there with the technology we have today. Even that 1000 years is deeply problematic. I mentioned it several years back to Les Johnson at NASA’s Marshall Space Flight Center and he said, “If we can get to Alpha Centauri in a thousand years, I’ll take it!” Meaning we were, in his view, not even that far along.

So what is this Project Longshot? The first thing to do is to untangle its origins. This design for an unmanned interstellar probe grew out of the NASA/USRA University Advanced Design Program, which ran between 1984 and 1994. The idea of the program was to help integrate future NASA design projects into university curricula. Engineers from the agency would work with students and faculty from US engineering schools, thus fostering engineering design education and adding synergies to NASA’s own efforts in the area of advanced space design. Project Longshot was a concept that grew out of this program’s involvement with the U.S. Naval Academy, including seven first class midshipmen, faculty advisors and two visiting professors, one of whom was NASA representative Stephen Paddack, a visiting professor based at Goddard Space Flight Center.

As for using current technologies, the Longshot team made no such claim. This is what they had to say:

Our probe will be a completely autonomous design based upon a combination of current technology and technological advances which can be reasonably expected to be developed over the next 20 to 30 years. The expected launch date is in the beginning of the next century with a transit time of 100 years.

The expected launch date, in other words, would have been about now, but the technologies anticipated for it to occur still have a long way to go. Longshot was conceived as being built with modular components on the ground and then launched to low-Earth orbit for assembly at the space station presumed to be operational there. The enabling technologies included a “pulsed fusion micro-explosion drive” (I’m quoting from the Project Longshot report) with a specific impulse of 1 million seconds, along with a long-life fission reactor with 300 kilowatts power output.

The differences between this concept and Project Daedalus are profound in both emphasis and execution. Daedalus was to be a fast flyby of Barnard’s Star, scattering smaller probes as it entered the system to explore any planets found there. Longshot, audaciously enough, was intended to carry enough fuel to actually brake as it entered the Centauri system and go into orbit around Centauri B, which the report erroneously calls Beta Centauri (Beta Centauri is another star altogether, the components of Alpha Centauri being Centauri A, B and Proxima Centauri). That last just reminds us that the pooled light of the three Alpha Centauri stars makes it appear to be a single star, so that the second brightest object in Centaurus came to be known as Beta Centauri.

Needless to say, including enough fusion fuel to slow an object traveling at these speeds to brake into orbit around Centauri B would require an engine far more efficient and powerful than anything envisioned for Daedalus. That’s because you’re carrying, when you begin the journey, not just the fuel to get you up to cruising speed, but all the fuel needed for the deceleration. The numbers quickly start running away with you here — while Daedalus offered a first-step flyby that strained every technological resource we would possess in the near future (including the need to mine for helium-3 in places like the atmosphere of Jupiter), Longshot pushed credibility to the max by insisting that a similar design could stay in the Centauri system and do useful science, reporting the results via a laser beam communications system that seems workable.

Where Longshot was perhaps closest to technological realization was in the area of autonomy. Here’s what the report says on this subject:

Due to the great distance at which the probe will operate, positive control from earth will be impossible due to the great time delays involved. This fact necessitates that the probe be able to think for itself. In order to accomplish this, advances will be required in two related but separate fields, artificial intelligence and computer hardware. AI research is advancing at a tremendous rate. Progress during the last decade has been phenomenal and there is no reason to expect it to slow any time soon. Therefore, it should possible to design a system with the required intelligence by the time that this mission is expected to be 1aunched.

All of which seems reasonable enough. The Longshot report was compiled between 1987 and 1988, and we have certainly seen our share of computer advances in the time since. Indeed, I am now and again told by its partisans that the ‘Singularity’ event could happen any time now, but certainly within the next few decades, in which case AI systems to run such a probe would be plentiful, one assumes, although whether intelligent hardware would not want to re-design the whole spacecraft remains an unanswered question.

I, for one, appreciate the report’s attention to long-term thinking. In discussing the “human side of the infrastructure” supporting Longshot, the authors note that given the time for design, procurement, in-orbit assembly and transit, the likely time before return of data would be more on the order of two centuries than one. And they go on to say this:

…the greatest challenge comes with the caretaker portion of the mission — the century of travel time when very little data will be transmitted. The problem here is not the number of people required, since it will be small, but rather the time involved. There has never been a similar project in modern history carried out over such a long time scale. However, there have been organizations which have lasted for such a time. In fact, some have lasted longer! Some examples include: the militaries of nations such as the U.S. and the U.K., various research institutions like the National Geographic Society and the Smithsonian Institution, and private organizations such as the Red Cross and the Explorer’s Club.

Robert Forward used to worry about precisely this point. In considering one of his mind-boggling lightsail designs, he wondered what political will might be needed to keep the power supplied to the huge lasers that drove the lightsail over spans of a century or more. You can see the subject entertainingly explored in his novel Rocheworld (1990), expanded from his 1984 work The Flight of the Dragonfly. We’ve clearly got the patience to work with probes that are thirty years old and more, as witness our Voyagers and Pioneers, but a century or longer imposes more challenges, especially given the political changes that might take place in the interim.

The Longshot team pondered the possibility of laser lightsails for its work as well, but ended up with pulsed fusion. And again, the report points out that such a drive “…is not a current, but rather an enabling technology.” The concept is to fire high energy particle beams at small, fusion-able pellets whose implosion and subsequent channeling out the nozzle would drive the vehicle. Helium-3 is deemed necessary, as with Daedalus, with atmospheric mining of Jupiter being just one of the methods discussed for gathering sufficient quantities. “…[T]he collection of fuel will be the most difficult and time consuming portion of the building,” says the report, and that’s something of an understatement.

Project Longshot, then, should be seen as a gutsy academic exercise that never proceeded to the intricate analysis given to Daedalus, lacking the resources of time and expertise that the British Interplanetary Society was able to deliver to the latter. Even so, the Longshot report is a fun read that places many of our current interstellar concepts in context. The rough sketch of an interstellar probe called “Project Longshot: An Unmanned Probe to Alpha Centauri” can be downloaded here.

Comments on this entry are closed.

  • dad2059 February 6, 2008, 15:53

    That was always the problem with laser-propelled light sails, “will they keep the lights on”?

    Karl Schroeder tackled this with his “interstellar cycler” idea: http://www.kschroeder.com/my-books/permanence/interstellar-cyclers
    Pretty good idea I’d say.

    As for Project Longshot, given the political and economic climates, unfortunately it’ll remain an intellectual exercise only.

  • ljk February 6, 2008, 16:34

    Space magnetism may solve secrets of fusion energy

    USA Today Online


    By Jeremy Hsu, space.com

    February 6, 2008

    New discoveries about magnetic field lines and the first-ever
    direct observation of their reconnection in space are offering
    hope that scientists will learn how to unlock fusion power as
    an energy source in the future.

    “The reconnection processes in the [Earth’s] magnetosphere
    and in fusion devices are the same animal,” said JAMES DRAKE,
    a University of Maryland physicist….

    During a sabbatical at the UNIVERSITY OF CALIFORNIA-
    in the same office as TAI PHAN, AN OBSERVATIONAL
    PHYSICIST who was looking at magnetic field data from
    the European Space Agency’s Cluster satellites.

    “I was doing theory, Tai was doing data and we suddenly
    saw this correspondence,” Drake marveled. “It was purely

  • Bob Shaw February 6, 2008, 17:33

    Vernor Vinge famously described a fictional, centuries old SF magazine publishing house – but there are real, commercial entities which trade today and go back to the voyages of Columbus. One such is the Aberdeen Shore Porters Society, which has been in the transport business for over 500 years! It was founded in 1498, and is the oldest company still in business in the UK. It’s strange going past their trucks on the motorway and seeing ‘Established 1498’ on the side.

    I’m sure they’re matched elsewhere – and not just by the various religious organisations out there, which are not always the best of models. I wonder whether any organisations survived the fall of Constantinople, and perhaps go back to the days of the eastern roman Empire?

    So an organisation *can *survive for half a millenium…

    Bob Shaw

  • James M. Essig February 6, 2008, 18:16

    Hi ljk;

    Thanks for providing the link on solar magnetic field lines reconnection. The fact that the basic physics behind such reconnection remains a mystery shows that there is still lots of room for us to grow in our understanding of classical electrodynamics at a macroscopic scale and at a fundamental level. The prospect of electromagnetic theory making new territory beyond Maxwell’s equations intrigues me at least in terms of the traditional understanding of the scope and field of such equations.

    What if somehow magnetic fields could be produced over a macroscopic volume having an average field strength of 10 EXP 55 Tesla which is according to some theories the ultimate limit which when exceeded, would result in the breakdown of a pure vacuum in which the field exists into monopole particles and perhaps other bazaar entities. The energy within a cubic meter of space permeated by a 10 EXP 55 Tesla magnetic field would be 10 EXP 30 times greater than that of the entire real mattergy content within our observable universe at about 10 EXP 23 to 10 EXP 24 solar masses.
    Causing magnetic energy releases based on extreme magnetic field intensity permeated regions of space might have some use in interstellar travel if the field energy could somehow be harnessed and safely channeled

    One a more down to Earth note, I hope this discovery can lead to effiecient fusion rockets to send us to our nearest stellar nieghboors.


    Your Friend Jim

  • Ronald February 6, 2008, 19:29

    Helium-3 procurement shouldn’t be a major problem (once we master nuclear fusion) given its enormous abundance in the atmospheres of the giant gas planets. Though I would rather get it from Saturn or even better Uranus, because of lower gravity. See Zubrin’s ‘Entering Space’.

  • Administrator February 6, 2008, 20:23

    Re Bob Shaw’s comment, not only can an organization survive for half a millennium, but so can specific buildings be maintained for that long a period. The Hagia Sophia in Constantinople and the Pantheon in Rome are examples, or consider the cathedral of Chartres, or the Shinto temple complex in Japan known as the Ise Shrine. All maintained continuously for century after century, good examples all for our future probes.

  • jh2001 February 6, 2008, 23:32

    Two questions: one basic & one more involved.

    1) If helium-3 is so difficult to obtain, why does it seem to be a requirement for both missions? Why not use some other fusion fuel?

    2) If we might want to “leave the fuel behind” why not do the same with fusion explosions and avoid the rocket equation entirely yet use a very powerful energy source (i.e. fusion) which is within out abilities today. I’m thinking about an acceleration lane of small but ordinary nukes.

    I read somewhere where the rate of expansion of the products of explosion from fission is something like 100,000 mph which is far less than the fraction of light which we would like. I presume that fusion would yield a faster expansion of explosive product but how fast? Also, could there be a way to reflect the explosion of a fusion bomb so as to increase the percentage of explosive product in one direction and possibly also increase it’s speed?

  • Adam February 7, 2008, 3:10

    Hi All

    Long-term planning takes a different outlook on one’s place in the wider fabric. An Aboriginal friend of mine is a traditional owner of Stradbroke Island in Moreton Bay, Brisbane. In discussions with our State Government his people were talking of 500-year management plans, rather than the typical 25 year development plans Governments are used to.

    Aboriginal collective wisdom views each person as an expression of an immortal “totem” animal, a Dreaming being – and the duties and rights of each person are determined by that Dreaming entity. Individuals come and go, but their place and role persist as characters in Aboriginal lore. Doubtless a lot of unrecorded change occurred in Aboriginal culture over time, but some groups can still “read” the millennia old rock paintings of their people, while others maintain fish traps and bush-tracks established over similar timespans. Old stories of tsunamis and other events have oftentimes been corroborated by later geological studies – it’s surprising what can persist in “primitive” cultures, via oral history and collective ritual.

    Whether that’s applicable to interstellar travel, who can say?

  • Enzo February 7, 2008, 5:31

    The catholic church is an organization whose structure has not substantially changed since at least the fourth century AD :-) Hate or love them you can’t deny that they are still fairly successfully run.

  • Bob Shaw February 7, 2008, 8:25

    The problem with using religious organisations as models is that times change, paradigms shift, and so on. The Hagia Sophia survived the fall of Constantinople largely because it was built to last by the Eastern Roman Empire, and last it did – the thing was built well, and the structure was repurposed as a mosque by the conquerers. But what if your model is Stonehenge, or Silbury Hill, or the pyramids at Giza (although built to last, the pyramids were looted within generations and just as at Stonehenge we see but a pale memory of what was once there).

    I like the enduring ideas of human self-interest and business acumen far more than passing religious fads…

    Bob Shaw

  • ljk February 7, 2008, 8:59

    Someone on another forum noted that to focus a laser
    beam on a lightsail starcraft at one light year distant would
    require an aperture larger than the entire Sol system.

    Assuming the physicists and engineers who have designed
    such starships are aware of this, what have they done to
    resolve this issue? Politics and society are one thing, but
    physics is physics.

    It would be nice to think that a mission to another star system
    would unite humanity and solve many of our current issues,
    but we shall see.

    I think it remains best for a starship to carry its own fuel,
    or have ways of procuring it enroute. And another factor
    to deal with is the AI needed to run things. Can we make
    it smart enough to be independent for the long voyage, but
    not so smart that it decides to go off on its own, or go crazy
    from the decades and centuries the mission might take?

  • Administrator February 7, 2008, 9:54

    jh2001, re helium-3, it turns out that the output of the deuterium/helium-3 reaction—protons and alpha particles—can be manipulated by a magnetic nozzle, something that can’t be said of alternatives like deuterium/tritium. Using deuterium and helium-3 also releases huge amounts of energy for the mass used. One thing that doesn’t apply to an unmanned probe but might come into play with future manned missions is also safety. Deuterium/tritium produces most of its energy in the form of radioactive neutrons, whereas deuterium/helium-3 produces 1/100th the amount of such neutrons; ergo, less radioactive shielding needed. Though again, that’s just a concern for manned missions, not the unmanned probes of Daedalus or Longshot class.

    Fusion runway? You bet. Jordin Kare has written this one up with his ‘Bussard Buzz Bomb’ concept, a spacecraft in the shape of a doughnut that accelerates along a track of fusion pellets on its way out of the Solar System. I want to discuss that Kare idea, along with his ‘SailBeam’ concept, some time soon.

  • Administrator February 7, 2008, 9:58

    Larry, re your comment: “Someone on another forum noted that to focus a laser
    beam on a lightsail starcraft at one light year distant would require an aperture larger than the entire Sol system.” I’ll have to dig around a bit but Forward’s work contradicts that, with a still usable laser working at 10 light years distance. Forward’s Fresnel lens, placed in orbit between Saturn and Uranus, would be huge, but nothing like the comment implies. Let me see if I can find my stack of Forward’s lightsail papers, which is hopelessly buried somewhere in my office.

  • Bob Shaw February 7, 2008, 10:53

    Larry’s right that the comment has been made elsewhere regarding defocussing, but there have indeed been a number of models developed which would put fresnel lenses in the lightpath to solve the problem. However, building, pointing and maintaining those corrector lenses might be as tall an order as building the rest of the starship and laser themselves. So although there are answers, are they remotely practical ones?

    Bob Shaw

  • Mark February 7, 2008, 11:49

    Can someone comment on the applicability of ion propulsion for this application?

  • Hans Bausewein February 7, 2008, 16:17

    “Therefore, it should [be] possible to design a system with the required intelligence by the time that this mission is expected to be 1aunched.”

    If you can upload new software over light years distance you have a human life time to improve it.
    Provided that you can find someone able (and willing) to program that age-old hardware.


  • Adam February 8, 2008, 1:01

    Hi Mark & Hans

    Mark, ion-drives involve putting energy directly into the propellant stream from a power source. Doing so with on-board power is counter-productive for interstellar speeds – a fusion reactor is better used by venting its exhaust as the propellant. “Longshot” and “Daedalus” avoid the energy issue because the propellant is what is fused and vented, avoiding any power conversion efficiency penalties that ion drives involve. “Daedalus” was hoped to power its fusion ignitor guns via magnetic induction drawing power from the propellant stream, but that proved so problematic that the “Longshot” design avoids it entirely. “Daedalus” had substantial reactor power, so powering the ignitors wouldn’t have been a big burden.

    Geoff Landis has demonstrated that an ion-drive powered by a laser-beam in the solar system would be more efficient than a laser-sail for speeds up to 20% of lightspeed, so there might be a role for ion-drives yet.

    Hans, there are people willing to program re-creations of Charles Babbage’s Difference Engine, so I imagine someone might be around ready to re-program a “Longshot” star-probe.

  • Ronald February 8, 2008, 5:21

    @jh2001: in addition to the admin.’s comments, helium-3 is very abundant in our solar system, and obtaining it is not the issue, bu the mastering of the fusion process itself.
    Maybe a combination might work best for interstellar probes: accellerating with laser-lightsail, decellerating with magsail/fusion rocket combination, additional manoevering with fusion rockets.

    The ultimate advantage of (exclusively) laser, however, would always remain, that once you have built the laser installations, you can keep spitting out large numbers of small and relatively inexpensive probes. Disadvantage with this would be the inability to decellerate, hence just zooming by another planetary system. Or would it still be possible to use the stars gravity field for decelleration, even at some 0.12 c? Probably not. In that case better use a laser/magsail combination ?

  • Ronald February 8, 2008, 7:21

    Off-topic, but:

    I just read, that the (Indian?) Austrian physicist Martin Tajmar, who made the news before in relation to research on Heim’s theory and anti-gravity propulsion, has applied for an international patent for “a Process for the generation of a gravitational field and a gravitational field generator”.

    The link is to a scientific news item in an Austrian newspaper, (unfortunately for many) in German:


    See also this site for ref. to Heim, and Dröscher and Häuser, two German physicists who elaborated on Heim’s theory.

    It is of course this kind of break-through propulsion that we are really hoping for, one day.

  • Administrator February 8, 2008, 8:30

    Ronald, I need to qualify this by saying that Martin Tajmar’s work does not involve Heim theory, although sources like the Wikipedia and sometimes the popular press do try to make the connection. While Tajmar mentions Dröscher/Heim theory in passing in at least one paper, it is by way of establishing a context for the broader study of these effects. We’ll have much more to say about Tajmar’s work this spring, but Heim theory is another thing entirely.

  • ljk February 8, 2008, 9:39


    New Streaming Theater Presentation: Robert Goldston, professor of
    astrophysical sciences at Princeton University, explained how
    magnetic fusion can be an abundant, safe, and reliable source of
    energy, making it an attractive, practical alternative for the long
    term. He also described challenges and advances in the field–in the
    United States and abroad–and discussed how and when the use of
    nuclear energy can become a reality.

    Details: http://today.caltech.edu/theater/

  • jh2001 February 8, 2008, 15:28

    Thanks Paul, yes I do recall reading about the products of various fusion reactions and it’s coming back to me now. Clearly helium-3 is the way to go. Now, this mining helium-3 from an outer planet…Isn’t there more than enough on the moon and relatively easily accessible? Is it really more easily mined atmospherically than on the ground?

    Regarding the need for a long-term organization, just how much would it take to monitor a probe that might be doing very little in transit? The launch and arrival activities is where the work is and these projects can be operated by whatever appropriate organization exists at the time. The mission might be like a piece of property that could be passed on as an asset should one organization come to an end. We don’t need to be creating a religious or any other organization to survive for centuries just for this mission!

    Very much look forward to the upcoming articles on the Buzz Bomb & SailBeam. You’ve already addressed them before but I feel that these both are legit contenders for the actual mission and so could use more discussion. Also, add light-powered ion beam to your list. It’s a relatively simple design yet plausible.

  • jh2001 February 8, 2008, 15:43

    Ronald, the combination approch deserves more discussion. There is this suggestion that a large superconducting loop with lots of amps would be adequate for deceleration. It seems to be a simple solution and fairly low weight. If it bears scrutiny then perhaps none of our mission designs need be a fly through.

    I myself am fairly skeptical of internal fusion simply because it seems that we’ve been waiting for it for so long and the equipment to compress the pellets would be rather massive. Yes, external fusion (e.g. pellets runway) gets around the massive lasers but then you’ve got to get the ship up to high speeds some other way. And then there’s the maneuvering to line up with that track. Yipes! Try to get Congress to fund such a risky venture. My guess is that we’ll figure out an easier, less risky approach.

  • Adam February 9, 2008, 17:07

    Hi jh2001

    He3 is only present in the upper layers of the Moon’s soil, impregnated in the crystal structure. A lot of regolith has to be processed to extract it. A nuclear powered ramjet can fly indefinitely in a gas giant atmosphere and extracting He3 from the gas phase is relatively easier than from lunar soil.

    Personally I think using lithium as a fusion fuel makes more sense, but it’s tougher to ignite via beam-implosion I think.

  • Ronald February 10, 2008, 8:11

    JH2001: see http://fti.neep.wisc.edu/fti of the univ. of Wisconsin, about helium-3 fusion and search their publications for moon, helium-3 and mining, quite interesting.

    You’ll find that there is indeed a promising amount on the moon and according to several experts exploitable (in places), but it is absolutely negligible in comparison with the enormous amounts in the atmospheres of the giant planets. I agree with Adam that ultimately He3 extraction from Uranus (maybe Saturn if gravity is not too prohibiting) may prove to be much more feasible, because of abundance and relative ease of extraction.

    Also Google for “Plus Ultra Technologies” for He3 extraction from Uranus and see http://www.asi.org/, the Artemis project for moon mining, among (many others.

  • Adam February 12, 2008, 7:59

    Hi Ron

    If you can find Plus Ultra Tech’s old papers you’ve done better than me. I used to have them, but they got lost a few computers back. Now it seems the company is defunct.

    Shameless self-promotion… check my blog for thoughts on He-3 mining…


  • Ronald February 12, 2008, 12:00


    I will definitely check your site. I still have a (little) article by UPT, pitty it is defunct.

  • ljk February 18, 2008, 15:02

    Laser beam sets record for intensity

    KurzweilAI.net Feb. 18, 2008


    The world’s most intense laser beam
    uses 300 terawatts of power
    concentrated in a 30 femtosecond
    pulse to a 1.3-micron area, or 20
    billion trillion watts per square
    centimeter. University of Michigan
    news release…


  • James M. Essig February 18, 2008, 21:49

    Hi ljk;

    Thanks very much for posting the link for the 300 terawatts laser beam. The flux power density of this device is astounding. The ability to perhaps produce more powerful beams that could “boil the vacuum” of empty space to produce particles offers a potentially new method to produce particles as an alternative to particle accellerators. Perhaps onday, exotic species of undiscovered particles could be produced this way. I was thinking that if the beam could be directed precisely between a set of Casimar plates, the power level of any such future beam could be increased because there would perhaps be less zero point electromagnetic fluctuations to interact with. Reducing the number of zero point electromagnetic fluctuations might result in a reduction of virtual electron and other virtual charged particles production between the plates since these particles at least existentially are coupled to the electromagnetic force. When the beam emerged from the Casimar plates set up, it could interact with the normal vacuum with all of its fury and perhaps produce real particles not yet discovered.

    Now if we can just produce an analogous beam composed of gamma rays with the same or greater total energy and perhaps of a much shorter duration commensurate with the 0.001 to 0.1 angstrom wavelength of typical gamma rays, we could increase the power flux density level by perhaps {[(10 EXP – 7) EXP 2](10 EXP – 7)} EXP -1 times or 10 EXP 21 times assumming 0.001 Angstrom wavelength instead of the approximately 1 micron wavelength used in the experiment and the same total beam energy output. Perhaps to do such would require using matter of density on the order of the atomic nucleous as lasing material, perhaps some form of solid neutronium or quarkonium could suffice.

    Beaming such radiation down fanciful futuristic Casimar Plates made of neutronium seperated by 10 EXP 13 meters or less might be an exellent way to build up tremendous laser power upon which the beam would be liberated to interact with the normal vacuum to produce God knows what! There is a lot of good room for beam power growth and research here.

    Note that the total energy of the beam was only (3 x 10 EXP 14)(3 x 10 EXP – 14) Joules or about 9 Joules however.


    Your Friend Jim

  • Bounty July 17, 2008, 21:36

    civilian rookie questions here….. How do you deal with the interstellar medium with a craft moving that fast.. I think Daedalus was going to have a Beryllium shield and generate a dust cloud. Is that practical? Also to what extent could you gain speed with gravity assits? Maybe a big ion drive working in the inner solar system with gravity assists for a long while would get up to what % of c?

  • Administrator July 17, 2008, 22:00

    No such thing as a rookie question here — we’re all learning new things as we go, especially about a subject as dynamic as this one! You’re right, the interstellar medium itself becomes a problem when a spacecraft is moving at a significant percentage of lightspeed, and that’s something we have to take into account. The Daedalus shield had the disadvantage of adding mass in a situation where every kilogram — heck, every gram — was critical. I’ve also seen concepts using forward-firing laser arrays to sweep out the area in front of an advancing ship, but it’s all tremendously theoretical at this point, and there too you wind up with a mass addition you’d like to avoid.

    Gravity assists are indeed helpful. The classic case is a sundiver maneuver, in which a solar sail-equipped vehicle moves to an exceedingly close solar pass and then deploys the sail. Geoffrey Landis calculated on the back of an envelope one time that you could maybe get 500 kilometers per second out of that maneuver, but of course you’re also pulling huge accelerations doing this. Ralph McNutt at Johns Hopkins Applied Physics lab wrote up a mission concept for NIAC (still available at the site — http://www.niac.usra.edu) that used an engine burn and a sundiver maneuver, the engine being jettisoned after the solar encounter, if I recall right. I can’t remember the specifics, but they’re in the papers at NIAC. This one eventually turned into the Innovative Interstellar Explorer concept that’s still under consideration:


    although much changed from the original.

  • John Jenkins January 1, 2009, 13:44

    I once read an article in Scientific American discussing the possibility that an object could “theoretically” obtain near light speed, or at least a significant percentage of, by using the “slingshot” effect (gravity assist) several times within the solar system to continually increase its speed before ejecting itself from the solar system. Unfortunately, I cannot locate that particular article. The article I recall was a bit “whimsical” in its treatment of the matter but I think a serious study could be done to find out what the theoretical maximum ejection speed from the solar system can be obtained using such a scheme. Heard of any?


  • Administrator January 1, 2009, 16:12

    Interesting question, John. The maximum I’ve heard via close-pass ‘Sundiver’ maneuver is in the range of 500-500 kilometers per second, but I haven’t seen anything about a continuous series of such gravity assists around the planets. Maybe someone else can weigh in.

  • Adam January 11, 2009, 2:12

    There are configurations in which a small mass can shuttle back and forth between larger masses in a ring causing the whole to accelerate to extreme speed – won’t work if it’s a system anchored to a central star. Wish I could remember what it was called, but it was one of those weird many-body results that emerged in the early 1990s as computer codes and the underlying mathematics got quicker.

  • John Jenkins January 28, 2009, 9:05

    Recall that the Galileo Probe used a multiple gravity assist technique between Venus and the Earth to obtain sufficient speed to reach Jupiter. There’s no reason, given time, maneuvers of this nature could not be continued within the solar system to obtain extreme velocities. Without actually doing the math I can envision how the larger planets could be used to increase the speed of a spacecraft and redirect its path toward the sun for a sundiver manuever several times and using propulsion to further increase speed at periapsis of each encounter. At higher velocities it may be necessary to use two or three of the larger planets to redirect the spacecraft back towards the sun for another sundiver manuever. One of the Voyager craft was sent off the plane of the elliptic and such a manuever could be used on the “final” gravity assist manuever to direct a spacecraft towards the Centauri system. Nuke rockets could kick in at this point for one last long blast of speed. Even if a spacecraft could achieve 10 to 15 percent of the speed of light the trip could be made to the Centauri system in a person’s lifetime. The multiple flybys of the planets themselves would be worth the scientific value of the endeavor not to mention what could be learned about interplanetary navigation at relativistic speeds. I personally feel if there is ever to be an interstellar probe attempt this will be the way it will have to go. A voyage lasting centuries, although possible, is certainly not practical.

  • John Jenkins March 14, 2009, 9:43

    This site seems to have gone into hibernation for the long trip to Alpha Centauri. Anybody have any good discussions on the topic?

  • Administrator March 14, 2009, 16:14

    John, most of our discussions re interstellar flight have to do with enabling technologies. For that reason, searching the site is the best way to go — use terms like ‘sail’ to catch solar and lightsail technology, ‘beamed propulsion’ to look for microwave and laser beaming, and son on. I’d also suggest keywords like ‘antimatter’ and ‘fusion.’ Friedwardt Winterberg had some interesting fusion concepts we talked about in an interstellar context not long ago. Another useful search term is ‘daedalus,’ to hunt for the British Interplanetary Society starship design. We’ll have more on that one and its outgrowths in a few days.

  • John Jenkins March 14, 2009, 22:25

    Well, I must say, being a degreed and practiced engineer and quite studied in astrodynamics, physics, etc. I find all these “sci-fi” proposals for instellar flight a bit …. mmmmm…. well let me just say: “Time for a reality check here!!!!” Flights of Fancy will get us nowhere. This site ought to be a serious “Think Tank” for practical methodoligies for what is the most obvious destination of the first interstellar flight. Think about it. The moon program of the 60’s began with the “amateur” rocket societies of Germany in the 1930’s with Wehrner von Braun. Discussion groups such as this can be instrumental in the realization of an interstellar probe. With practical existing technologies we could have a probe in close proximity to the Alpha Centauri system in the same time frame as the Voyager probes. Am I in the wrong website?

  • Administrator March 15, 2009, 10:42

    With practical existing technologies we could have a probe in close proximity to the Alpha Centauri system in the same time frame as the Voyager probes.

    If by ‘existing technologies’ you mean chemical propulsion, I’m afraid this is simply not true. There is no existing technology that can get us into the Centauri system in the time frame you specify. Nor is ion propulsion remotely ready for this. A good source on the subject would be Matloff’s The Starflight Handbook or my own Centauri Dreams book. A Voyager-style probe would take 70,000 years-plus moving at Voyager’s current speeds, and in any case the equations re chemical propulsion simply don’t scale to this kind of mission. A fifty year mission to Centauri using chemical methods would take amounts of propellant that are completely off the charts.

    We’ve discussed this many a time in these pages, and Matloff’s discussion of the ‘rocket equation’ and its ramifications is as good as any I’ve seen.

    The technologies I mentioned are all within the realm of known physics; the problem will be to scale them to future engineering. If you are talking about moving beyond currently understood physics, the recently released Frontiers of Propulsion Science examines what’s being researched and has essays by the major practitioners in the field. I wrote the first chapter of that one, which is an overview of many of the issues involved in pushing the edge of physics and approaches that have been tried. We’re a long way from true breakthroughs, but the key is to keep pushing to find out what’s possible. Both this approach and the continuing development of practical extensions of current technology in the areas I mentioned are what the Tau Zero Foundation is set up to support.

  • John Jenkins March 15, 2009, 15:46

    When I mentioned a time frame such as the voyager probes I meant that we could be seeing results of an Alpha Centauri probe in around 40 years which is about the same age as the voyager probes, give or take. I see now where that may have been misunderstood. My apology. Unless someone learns to bend the laws of physics it all boils down to one of Newton’s laws of motion: For every action there is an opposite and equal reaction. The key is: What action?

  • Administrator March 16, 2009, 13:22

    Re your previous comment:

    This site ought to be a serious “Think Tank” for practical methodoligies for what is the most obvious destination of the first interstellar flight.

    It sounds as though you’re looking for a discussion forum in which to kick around these ideas, but we’re not set up with forum software that makes that possible. I do hope to include a forum at some point in the future, but right now we’re set up as a weblog in which the posts on the main screen draw most of the comments. Older posts do tend to fade with time in terms of comments. We’re also set up so that comments need to be relevant to the original post, which means that as writer of the site, my posts set the topic for each day. You’re welcome to comment on propulsion concepts as they come up, but do be aware that older posts like this one draw fewer eyes just by the nature of the software.

  • R2K October 7, 2009, 13:50

    Given the time and money needed to build these interstellar craft, granted the challenge of it all is very interesting for rocket people who like to explore space, it seems you are generally better off building very very large telescopes. There is no reason why, for less money and time than a mission to a local star, one couldn’t build an array of say 7 – 100 meter optical telescopes (perhaps using gossamer technology) in space. Such an array should be able to make out details on the surface of extra-solar planets. Not to mention serve many other purposes.

  • jballpilot December 15, 2009, 22:30

    star gazers and star chasers … Anyways i guess i’ll throw out some things. One is that we misunderstand time as a constant throughout the cosmos instead of localised to our little gravitational dent in the continuoum. Another, somehow introducing localised pressure differences in front and behind the craft. Ingenuity in simplicity.

  • TampaDude February 27, 2010, 23:46

    A beamed core antimatter drive would have the necessary specific impulse for reaching relativistic speeds with a reasonable amount of fuel. The main obstacle for constructing such a drive is that with our current global rate of antimatter generation, it would take a ridiculously long time to make enough antimatter fuel for a trip to even a nearby star like Alpha Centauri.

  • silgosilgo April 7, 2011, 10:15

    It is very important to remember that this plan doesn’t take into consideration the abundance of He3 on our moon! It was already know back then in the late 80’s that it is possible to strip-mine the regolith of the moon very cost-effectively and much more realistically than getting it from Jupiter, the only possibility mentioned in the sketch, but for some reason this fact is totally ignored . It is estimated that there is over 1 million tones of He3 on the surface of the moon, accumulated over billions of years from the sun’s winds, and that makes it highly plausible to create a probe based on He3 fusion, and it might be able to get there even faster than 100 years, given half-way acceleration and half-way deceleration. It’ll be also useful for arriving to Mars within days.

    Helium 3 mining the moon:
    Part 1 –
    Part 2 –

  • MP February 7, 2012, 3:31


    Yes, I know and agree that the Longhost is just a rough concept. But the main question is: what is the reason to send a multi-billion $ probe to another star system? Except for kind of adventure and satisfying human curiosity, of course …

    I believe that such probe must have true scientific advantage (read: provide more scientific data). I agree with R2K saying that it’s currently much better to build large (even 100-meter) telescopes on Heliocentric orbit. They will not only cost a tiny fraction of interstellar probe, but also provide scientific data immediately (we don’t have to wait 100 years). Using such telescopes would enable us to detect practically all earth-sized planets within range of 50 l.y. (enough for planning future voyages). And actually not only to detect, but to take an image, perform spectroscopic analysis of their atmospheres etc.

    So why to send a probe? Because it can give much more data – about intersellar medium, stars/planets magnetosphers, high-resolution images of planet surfaces, search for life etc.

    And this is the reason why Daedalus looses in comparison with Longshot. Dadealus will perform only a fast fly-by. Out Pluto is located roughly 5 light-hours from the Sun. Giving 10% of speed of light, Daedalus will fly through entire planetary system within 100 hours. How much data can we collect in such a short period of time?

    The Longshot, on the other hand, can circle in another system for YEARS. Just have a look at current martian probes missions (e.g. Curiosity) – they were designed to work for few months only and are working for several years already, providing over 100 times more data than originally planned. YEARS of observations means thousands of photos of all planets and their moons and dozens of gigabytes of data (1000 bits per second and proposed gives us 3.8GB of data in one year).

    That’s why I strongly believe the only reasonable from scientific/economical point of view approach is to send a probe which will decelerate at the target.

  • Damien RS February 7, 2012, 12:19

    “Needless to say, including enough fusion fuel to slow an object traveling at these speeds to brake into orbit around Centauri B would require an engine far more efficient and powerful than anything envisioned for Daedalus.”

    Uh, that criticism seems rather flawed. Going by Wikipedia, Daedalus envisioned a two-stage boost to 12% c — 7% from the first stage alone — and fly-by, while Longshot envisioned a cruise speed of only 4.5% c. Stopping means a total delta-vee of 9% c, a fair bit less than Daedalus. Plus Daedalus planned for a self-contained fusion reactor using D-He3, when we’ve still yet to do that with D-T. Longshot planned to drive the fusions with a fission reactor; I assume making fusion pulses that blast out the back is a lot easier than a self-sustaining fusion reactor. At worst it’s a form of fission-powered ion drive, at best the fusion reactions add net energy to the exhaust.

    Huh, Longshot had payload be 10% of the initial mass, Daedalus only 1%. Some of that’s down to the higher delta-vee for Daedalus.

    Helium-3 can be transmuted from lithium via tritium, though that involves waiting a decade or two for the tritium to decay to He-3.