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Interstellar Flight: The Case for a Probe

Back in May I looked at Jean Schneider’s thoughts on what we might do if we discovered a planet in the habitable zone of a nearby star. In an article for Astrobiology called “The Far Future of Exoplanet Direct Characterization,” Schneider (Paris Observatory) reviewed technologies for getting a direct image of an Earth-like planet and went on to discuss how hard it would be to get actual instrumentation into another solar system. His thoughts resonate given recent findings about Gliese 581g (although the latest data from the HARPS spectrograph evidently show no sign of the planet, a startling development as we investigate this intriguing system).

Whether or not Gl 581g exists and is where we think it is, Schneider’s pessimism about getting an actual payload into another solar system has attracted the attention of Ian Crawford (University of London), who is quick to point out that astronomical remote-sensing, especially for biological follow-up studies of initial biomarker detections, will be inadequate. As we have done with nearby objects like Mars, we will eventually need to send instruments into these systems to study everything from basic biochemistry to evolutionary history there.

Slower Speeds, Improved Technologies

But are such missions possible? Schneider and colleagues assumed 0.3c as a typical velocity for interstellar missions and went on to discuss the huge difficulties in accelerating a payload to such values. But Crawford is skeptical. He sees 0.3c as an overestimate, and reminds us that the better developed proposals in the literature tend to focus on values around 0.1c — as he notes, “0.3c will be an order of magnitude more difficult owing [to] the scaling of kinetic energy with the square of the velocity.” Schneider’s 0.3c is, in Crawford’s view, an arbitrary over-estimate of the speed required.

The original Daedalus study is the most detailed engineering assessment yet available, a fusion-based craft that would require fully 50,000 tons of nuclear fuel and attain 12 percent of lightspeed. Daedalus, in other words, is well beyond our capabilities. But Crawford’s point is to remind us that we’re learning more as we go, and that premature pessimism may overlook useful technological advances. On fusion, then:

…technical advances in a number of fields have occurred which may make fusion-powered vehicles of the Daedalus-type more practical as long-term solutions to the problem of interstellar travel. These include developments in miniaturization and nanotechnology, which would ensure that a much less massive payload would be required than was then assumed, and developments in inertial confinement fusion research for power generation on Earth. Indeed, the National Ignition Facility recently commissioned at the Lawrence Livermore National Laboratory in California (https://lasers.llnl.gov) is, albeit unintentionally, building up technical competencies directly relevant to the development of fusion-based space propulsion systems. For these reasons, there is a strong case for a reassessment of the Daedalus concept in light of updated scientific and technical knowledge, and at least one such study is currently underway…

Surviving Interstellar Dust

That study, of course, is Project Icarus, in which Crawford is an active player. But any vehicle, whether fusion based or using more exotic concepts like antimatter or laser-pushed lightsails, runs into the interstellar dust problem. Drop the speed from 0.3c to 0.1c and the issue is partially mitigated. Dust was a showstopper for Schneider, but Crawford notes that assuming an interstellar dust density of 6.2×10-24 kg m-3, erosion at 0.1c would erode on the order of 5 kg m-2 of shielding material over a six light year flight. So that while we are certainly adding to the mass of the probe with shielding material like beryllium, we have not ruled out the mission.

One of the problems with this discussion is that we know so little about the upper bounds of the size distribution of interstellar dust particles. Crawford sifts through the literature on the subject, discussing the effect of impacts of 100-μm grains, which could be as high as two impacts per square meter over the course of a six light year flight. Collecting the needed data on the interstellar medium will be a priority before we can seriously think about launching such a probe.

And if a spacecraft were large enough, Crawford adds, various methods for sensing potential danger could be employed, using radar, for example, to detect incoming grains and laser or electromagnetic methods to destroy or deflect them before impact. The Daedalus study examined the idea of using a fine cloud of small dust particles ejected from the vehicle that would destroy any incoming large grains before they reached the primary vehicle. The latter approach was conceived for a probe entering a denser interplanetary environment at destination, but there is no reason such measures couldn’t be deployed throughout interstellar cruise.

An Astrobiologically Driven Probe

In his conviction that interstellar flight is difficult but not impossible, Crawford echoes Robert Forward:

Journeys to the nearer stars with travel-times of decades (necessitating velocities of the order of ten percent of the speed of light) will be a considerable technological (as well as economic and political) undertaking. The magnitude of the difficulties should not be underestimated, but neither should they be exaggerated. There is a large technical literature…, which demonstrates that rapid interstellar space travel is not physically impossible and is a legitimate technological goal for the centuries ahead. Ultimately, the development of this capability may be the only way to follow-up any detections of biosignatures that may be made in the atmospheres of Earth-like planets orbiting nearby stars in the coming decades…

A primary driver for interstellar flight is likely to be astrobiology. As we develop a mature space exploration infrastructure in our own Solar System to explore life’s possibilities from Mars to Enceladus to the Kuiper Belt, we will also be creating the necessary technologies that will take us further out. Astronomical observations can only tell us so much, leaving us with the need to get an orbiter or a lander to places like Titan or Europa, and giving us the longer term model of direct probes returning data from astrobiologically interesting planets around other stars.

The paper is Crawford, “A Comment on ‘The Far Future of Exoplanet Direct Characterization’ — the Case for Interstellar Space Probes.” Accepted by Astrobiology (preprint). Jean Schneider has responded to Crawford’s comments and I’ll look at what he has to say tomorrow.


Comments on this entry are closed.

  • Carl Keller October 12, 2010, 11:33

    It’s often been asserted the detection of free atmospheric oxygen would indicate a continuous process which terrestrial life certainly performs. Any transiting spectra showing this in an earthlike exoplanet in the star’s habitable zone would be scrutinized.

    The neighborhood 0.1c seems to be a consensus of late. Sometimes 0.5c to over 0.9c has been cited as goals, and they ought to be given proper study. But we will certainly begin with more modest delta-vees, if modest is an applicable term.

    Remember the ambitious Project Longshot http://en.wikipedia.org/wiki/Project_Longshot which was also featured here https://centauri-dreams.org/?p=1708

  • kurt9 October 12, 2010, 11:44

    There are fusion-based propulsion schemes that can get you 10% light speed. The 30% light speed probably represents the upper limit if the dust particles tend towards the small size and you have a propulsion system that can get you that speed. I think the radiation resulting from collisions with dust particles becomes an issue starting around 30% light speed.

  • Mike October 12, 2010, 12:01

    Considering the immense difficulty of interstellar travel perhaps the only near term (50 to 100 years) practical way to follow up on exoplanet bio-marker detections is to construct some huge space-telescope at the Sun’s gravitational focal point for the direct imaging of exoplanets. I know there are big problems with that approach as well but if it could be made to work one key benefit is not having to wait decades for the results from an interstellar probe. If it even survives the trip.

  • kzb October 12, 2010, 13:22

    Has any cost-benefit analysis been done, of high-velocity (10%c plus) versus massive interferometer systems in solar orbit?

    Probably like anyone on here, I really like the idea of interstellar travel. But what if the bean counters got hold of the options? I actually suspect that large observational platforms within the solar system would be both quicker and more cost effective !

  • Greg October 12, 2010, 13:27

    There are curiosities that could be looked into.


  • Bryan October 12, 2010, 13:49

    Take a look at the power and feature set in something as small as a smartphone. Imagine what kind of power something that size might have in 20 years. With regard to shielding, I think the answer is a fleet of knitting needle shaped probes.

    Their small frontal area reduces the chance of impact, and their duplication increases their odds of success. How does a such a small device get information back to us? If possible, I think the answer might be for them to simply turn around and fly home with the data.

    There is a paper (a bit dated: http://www.engr.psu.edu/antimatter/Papers/NASA_anti.pdf ), which indicates that only micrograms of antimatter are necessary for certain types of hybrid antimatter propulsion systems. It might be possible for such small probes to decelerate at their destination, do something useful, and then turn around and come home.

  • Tulse October 12, 2010, 13:56

    the latest data from the HARPS spectrograph evidently show no sign of the planet, a startling development as we investigate this intriguing system

    Yikes! Can someone more knowledgeable than me explain what the issue is, and how much of a concern this is?

    I mean, how can there not be a planet there, when it is 100% certain it has life?

  • andy October 12, 2010, 14:02

    Dynamics of Cats has a post up about the non-detection of Gliese 581g in the new HARPS measurements, it doesn’t sound good.

    HARPS statement is stronger than “we don’t see it” – they find that if they force a solution they get a negative signal appearing, implying the planet is not there, not just that they are not sensitive to it.

    Oh dear…

  • Ron S October 12, 2010, 14:18

    “Collecting the needed data on the interstellar medium will be a priority before we can seriously think about launching such a probe.”

    I think that launching such a probe is *how* we collect the needed data.

    “…using radar, for example, to detect incoming grains…”

    Think about the size of these microscopic grains and therefore the wavelength of the radar that would get a reflection. Then there’s extreme weakness of the reflection due to the tiny radar cross-section large path length (no point in detecting it a few milliseconds before impact).

    “…using a fine cloud of small dust particles ejected from the vehicle that would destroy any incoming large grains before they reached the primary vehicle.”

    We have to define “destroy” in this context. If it means breaking the grains into smaller grains, the total kinetic energy is much the same. Plus, sending a cloud ahead of the craft will decelerate the craft.

    We may be better off with an ablation shield.

  • Eniac October 12, 2010, 15:43

    If possible, I think the answer might be for them to simply turn around and fly home with the data.

    “simply”? Even with best assumptions about realistic antimatter propulsion, the mass fraction to get to something like 0.3c is on the order of 10. Meaning: to get up to speed, you need 10 times as much fuel as your probe weight. To stop at the target, another 10 times. To head back, again 10 times more, and to stop back here, another factor of 10. That makes a total of 10,000 times as much fuel as the payload. This would be difficult even if antimatter were easy to handle, which it is not.

    We are stretching to even get there for a flyby, with a large mass fraction. Each stop and start will require an extra power on that mass fraction, which very, very quickly gets overwhelming. Such is the curse of the rocket equation.

    The other problem with the miniature approach is the size of the engine. True, at this time we do not know what an antimatter engine will look like. But I think it is safe to say that it will not be microscopic, as it would have to be for those needle probes. The problem with antimatter (or any other nuclear propulsion) is that the charged particles emerge in random directions, and need large magnetic fields to reflect. There is nothing small that can deflect radiation, just as there are no thin radiation shields.

  • Eniac October 12, 2010, 15:59

    I think the best way to start with interstellar travel is to send out a steady stream of probes in a small number of directions, aimed at interesting systems. We would get a steady stream of benefits from them, even if the ultimate benefit of arriving at another system is thousands of years off.

    1) In just years, we will have a powerful communication backbone throughout the solar system
    2) In decades, we have a steady exploratory presence in the outer solar system and Kuiper belt
    3) In a few more decades we reach the sun’s gravitational focus for some high-definition astronomy
    4) All along we get an ever expanding baseline for parallax observation and radio interferometry
    5) It will be fun to mass produce the probes and keep improving them. We may have the newer ones pass their older siblings.
    6) Finally we will get close to the target stars for observations.
    7) The communication problem is less severe because we can relay along the chain

    The trick is to make the probes last long enough to still be of use after thousands of years. We’ll think of something….

  • Greg October 12, 2010, 18:06

    In the case of small probes, they could utilize the Lorentz force and the magnetic field of Jupiter. High enough voltage and a small enough probe could be accelerated.

    Found this from this site.


  • Carl Keller October 12, 2010, 18:40

    I think the Eniac Initiative has merit throughout. Point 5, the production of succeeding models of interstellar probes, would speed our experience which will eventually lead to starships carrying biology.

  • John Freeman October 12, 2010, 19:39

    I suggest that minaturisation isx the way forward, and minaturisation would seem to favour beamed energy techniques, as minaturising the engine and its power plant can be bypassed; this will be ok for a flyby if a means for a very small (1kg ish mass for the sake of argument) probe to return data to earth can be found. For anything more, I really can’t imagine how we’d go about doing that. Well I can imagine it but the imaginings have an overpowering odour of sci-fi to them.

  • Eniac October 12, 2010, 22:45

    @John Freeman: Unfortunately, beamed energy requires large structures to catch the beam, so it seems at least as unsuitable to miniaturization as a nuclear engine. It would be nice if it were otherwise, but I just can’t see it. Maybe a thin linear accelerator (electrostatic?) for the probes themselves, rather than a beam, but for relativistic velocities such accelerators are obscenely long and the polarity switches needed obscenely fast. So obscene, in fact, as to be fantasy more than sci-fi.

  • Andrew W October 13, 2010, 3:46

    Governments are the only organizations likely to have the cash to finance interstellar probes, unfortunately the leaders of democratic governments rarely invest in programs that don’t give a return (political or financial) before they know they leave office.
    It’s almost preposterous to suggest anyone other than the most ardent SETI fans would be prepared see their money put into an enterprise that would return nothing until centuries after they were dead.
    Given that, the only likely paths for exploring the stars are observations from in or near the solar system, and super fast interstellar probes, 0.5c or better.
    Perhaps the latter could be achieved using very long (1,000,000 km) very high g (1,000,000 g) rail guns firing mini probes, that would also give the ability to check out many star systems at little additional cost.
    As a bonus they’d be great for interstellar planetary bombardment, each shot would deliver at least a one hundred kiloton blast on impact.

  • Adam October 13, 2010, 5:37

    Hi All

    Having read Jean’s reply to Ian’s comment I’m finding myself agreeing with them both, but I won’t jump the gun. Let’s see what Paul G thinks tomorrow…

  • Michael Spencer October 13, 2010, 8:01

    Is there anything more stirring to the human heart than interstellar travel?

  • Ronald October 13, 2010, 9:05

    Further to Mike and kzb (with both of whom I agree): has there been a study of the cost of FOCUS (the solar gravitational focus telescope)?

    It seems to me, that any good, even amazingly good, observation platform, such as FOCUS, will always be much (MUCH) cheaper, safer and for most purposes more effective, than even the cheapest conceivable interstellar mission (an ultralightweight flyby mission).

    Further advantages of a space telescope such as FOCUS or a large space interferometer are that it can and will be used on many, many different targets and that greater distance of the target does not pose the same challenges on cost and technology as in the case of a probe. This makes a telescope so cost-effective in comparison with any probe. The only goals for which a probe would ultimately be necessary are close-up images of what that other life actually looks like and physical sampling.

    But first things first: I would argue that before even thinking about sending a probe we should and will get a fairly complete and detailed understanding of planetary systems and their biosignatures in our part of the Milky Way. I believe that this will happen within the next few (4 – 6) decades, long before any interstellar probe becomes feasible (except in the case of a major advanced propulsion breakthrough e.g. Heim’s/Tajmar drive orso).

  • kzb October 13, 2010, 13:26

    Ronald: the other advantage of advanced observational platforms within the solar system is TIME. Data can be obtained straight away (or at least within a few hours). A probe to the nearest stellar system will have a flight time of 40-50 years and then another 4 years for the information to come back.

  • John Freeman October 13, 2010, 15:52

    @Eniac: True, alas, when thinking of interstellar propulsion ideas the question is usually how fantastical is the idea is, rather than of if it is. Still, thats the nature of such long term speculation in general.

  • Eniac October 13, 2010, 17:15

    FOCUS has at least one of the same problems as an interstellar mission: It can only observe one particular target. You have to pick a radial trajectory, and that then determines the observational direction to a very small angle. You cannot turn the “telescope” around. Also, FOCUS is very much in the conceptual stage, it is much too early to meaningfully speak about cost.

    Otherwise I completely agree. FOCUS is not only a great astronomy mission, but also a stepping stone to interstellar travel. A small step, considering the ultimate goal, yet a very large challenge nevertheless.

  • Ronald October 14, 2010, 5:20

    Eniac, you are right and I am completely aware of this limitation of FOCUS. However, ironically, that also makes FOCUS particularly suitable for more remote targets: the more remote the different targets, the smaller the angular difference between them (given a certain distance in ly) and hence also the smaller the distance that the telescope has to be moved.

    Simply put: to aim FOCUS first at one star and then at another star 10 ly from the first one implies a much (MUCH) smaller distance correction if these two stars are, say, in the Andromeda galaxy than if these two stars are in our own local bubble.

    I do not know enough about the technical possibilities and limitations of FOCUS, but theoretically I can imagine that such an instrument would be particularly suitable for detections in neighboring galaxies or remote star clusters in our own MW. Lots of targets within a small angle. However, observation time (time of target within field of view) might be a severe limit then.

  • Paul Gilster October 14, 2010, 7:29

    Just for the record, the gravitational lens mission is actually called FOCAL, not FOCUS, although I’m not sure whether Claudio has settled on just how the acronym is put together :-)

  • Eniac October 14, 2010, 20:47

    Ronald, you are correct, galaxies and star clusters are good targets for FOCAL, in addition to the galactic core and the CMB. Let me point out, though, that observing close objects in more detail can be just interesting as observing many objects. For example, stellar and planetary surfaces could be mapped in great detail for (fairly) nearby stars. Unfortunately only one system per probe, but come to think of it, all of our probes so far had only one system to observe, so this may not be such a serious limitation.

  • Eniac October 19, 2010, 9:22

    I just remembered where I first saw the idea of a slow chain of relay probes discussed, and wanted to reference it here for the record. It was in Caleb Scharf’s fascinating blog post called “Stepping Stones”, a great read if you haven’t, yet:


    I think this proposal is a breakthrough because it gives us a way to go to the stars that is both doable, because of the low speed, and financeable, because of the short term benefits.