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Building a True Interstellar Probe

Imagine yourself aboard a spacecraft pushing into interstellar space. At what point would the Sun cease to be the brightest object in your sky? We’re already looking at missions designed to study the local interstellar medium (LISM), with the goal of reaching anywhere from 300 to 400 AU, a region believed to be undisturbed by the Sun. From that range, the Sun still shows an apparent visual magnitude of -13.7, making it brighter than any other star we see from Earth (Sirius comes in at magnitude -1.46).

So it’s a long push. In fact, an early interstellar probe moving at 75 kilometers per second would have to travel six thousand years to reach the point where the Sun is no longer the brightest star. At 100,000 AU, which is 1.61 light years, our imaginary probe occupant would finally see a sky where the Sun was just another bright star.

I get this information from a fascinating paper by Mike Gruntman (USC), who was kind enough to forward links not only to it but several other papers we’ll look at in the near future. This one is a 2004 examination of the instrumentation we’ll want to have aboard the first true interstellar probe. Note the word ‘true.’ We do have two functional probes that are pushing toward interstellar space right now, but neither Voyager craft was designed from the start with an interstellar science package in mind. Nor was New Horizons, even though it may tell us much not only about Pluto/Charon but also the Edgeworth/Kuiper Belt.

No, Gruntman is talking about a specific mission indeed. You could say that such a mission is modest compared to what we hope to achieve in interstellar flight. But pushing out to 400 AU is no easy business, especially if you want to do it within the working lifetime of the people who built it. Chemical propulsion is out — even the most efficient chemical systems (specific impulse 450 seconds) demand a mass ratio of 340,000 to reach the necessary velocity. Nuclear electric propulsion and solar sail technologies, however, are in the mix, and both are not that far from the technology readiness level demanded of this mission.

The Innovative Interstellar Explorer concept shows how the idea of a dedicated interstellar probe has evolved. Gruntman is a member of this team, and much of what he brings to IIE in terms of instrumentation and methods of measurement can be seen in this paper. IIE re-examines the concept in significant ways, as can be seen from this quote on the site:

We have provided a first cut of a self-consistent design for such a mission using Radioisotope Electric Propulsion (REP), existing launch vehicle hardware, and a Jupiter gravity assist. While the final speed of the probe is not as high as might be wished, it is sufficiently high to provided new – and potentially transformational – knowledge of our surroundings in interstellar space. More importantly, the required technology advances are evolutionary such that the probe could be built – and launched – as soon as the next launch window opens in late 2014.

Whatever the final design, the thinking within NASA on mission concepts like this goes back to a 1976 conference at the Jet Propulsion Laboratory that looked at the engineering problems it would face and the scientific gains it might accomplish. Ever since then, precursor missions pointing to the current Innovative Interstellar Explorer and beyond have continued to be discussed and refined both within NASA and without.

But just what is the mission? The primary goal is to reach the true interstellar medium and examine its properties. But ponder this: The Sun moves at a velocity of 26 km/s with respect to the surrounding interstellar medium. The motion is called the ‘interstellar wind,’ and as Gruntman points out, its direction is close to the plane of the ecliptic. We consider the heliosphere the place where the Sun controls the local plasma environment, but our data on its interaction with the LISM are scarce, despite the intriguing exploits of the Voyagers. So along with studies of the LISM itself, the second goal is to put the best instruments to work on what happens as the Sun’s influence effectively ends.

Gruntman sees a dedicated probe to this area as a natural next step as we go about the lengthy process of building an interstellar future. From the paper:

The lack of the direct experimental data and the resulting uncertainties in understanding of fundamental processes of the sun–LISM interaction severely limit our ability to develop a self-consistent concept of the heliosphere. Exploring in situ the nearby galactic environment and the region of the solar system frontier is thus an essential, logical, and unavoidable step in our quest for understanding our star, its interaction with the Galaxy, and laying out the foundation for the truly interstellar flight of the distant future.

The Voyagers, of course, are immensely helpful already as they push through the termination shock, and their expected lifetimes for transmitting data may see them through the heliopause, that region where the Sun’s influence becomes negligible. And as you can see, the powers that be now speak of the Voyager Interstellar Mission. But Voyager was never designed for this kind of work, and a dedicated mission to explore the heliosphere and its interactions with what lies beyond, as well as evaluating the nature of the interstellar medium itself, would clearly upgrade our fundamental knowledge.

Figuring out what kind of instrumentation such a probe will need is crucial. Interstellar plasma flows around the heliopause and is thus inaccessible to us. Large interstellar dust grains make it to the inner heliosphere but small grains are pushed out by the solar wind. Even the larger grains are heated by the Sun, with the possibility of destroying organic molecules of the kind we need to study. For these and other reasons, the local interstellar medium is largely unknown to us. We’ll talk tomorrow about how a fast moving interstellar probe will examine that medium, and what other opportunities a mission like this might open for wide-ranging study.

The paper is Gruntman, “Instrumentation for Interstellar Exploration,” Advances in Space Research 34 (2004), pp. 204-212 (available online).

Comments on this entry are closed.

  • ljk January 10, 2008, 15:42

    Implications of Interstellar Dust and Magnetic Field at the Heliosphere

    Authors: P.C. Frisch

    (Submitted on 19 Jul 2007 (v1), last revised 9 Jan 2008 (this version, v3))

    Abstract: Tiny interstellar dust grains causing the polarization of light from the nearest stars are deflected sideways in the outer heliosheath regions, along with the interstellar magnetic field. Observations of optical polarization of stars beyond the heliosphere nose, suggest the direction of the upwind interstellar magnetic field is relatively constant. The polarizations of nearby stars and offset angle between HeI and HeI flowing into the heliosphere have position angles in galactic coordinates of 30- 40 deg, indicating a local magnetic field direction inclined by ~55 deg and ~65 deg, respectively, with respect to the galactic and ecliptic planes. The hot and cold poles of the measured Cosmic Microwave Background (CMB) dipole moment are nearly symmetric around the heliosphere nose direction, and the v^{22} quadrupole vector is directed towards the heliosphere nose. The area vectors of the CMB quadrupole and ocotopole moments are directed towards the band perpendicular to the ecliptic plane formed by the alternate locations for the 3 kHz emissions detected by Voyagers 1 and 2. In the upwind direction, the position angle of the null plane separating the CMB dipole hot and cold poles is nearly aligned with the interstellar magnetic field direction at the Sun. Heliospheric foreground contamination of the low-$\ell$ CMB modes now requires detailed study.

    Comments: Submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.2970v3 [astro-ph]

    Submission history

    From: Priscilla Chapman Frisch [view email]

    [v1] Thu, 19 Jul 2007 21:14:53 GMT (266kb)

    [v2] Mon, 23 Jul 2007 04:17:20 GMT (267kb)

    [v3] Wed, 9 Jan 2008 04:48:30 GMT (446kb)


  • dad2059 January 10, 2008, 19:49

    Interesting coincidence about the subject Paul. I was just thinking about Larry Niven’s Known Space Series (yeah I know I’m dating myself here) today and his unmanned interstellar “ram-robots” (Bussard ramjet probes) that were launched using a massive linear accelerator and using Jupiter as a gravity assist to bring it up to ramjet “light-up” velocity.

    It’s not a far-fetched idea. I don’t know about slinging the probe to that high a speed to light a ramjet, but the gravity assist can be useful. Especially if your goal is to get a probe up to 100 kps or faster.

  • Adam January 11, 2008, 2:36

    Hi dad2059

    I’m not that old and I have fond memories of “A Gift from Earth” ;-)

    As for your idea it’s very useful to fling vehicles out to Jupiter to launch them into a near-Solar bypass. Close to the Sun and a minor velocity boost translates into a huge hyperbolic excess. For example Vernor Vinge’s story “Longshot” sends a vehicle Sunwards to do that maneuver and boosts it to 110 km/s.

  • Michael Spencer January 11, 2008, 7:11

    The on-going quality of this blog is astonishing.

  • Administrator January 11, 2008, 9:09

    dad2059, we have both flagged our age, because I’m a great fan of the Known Space series. Adam’s follow-up comment about Vinge’s ‘Longshot’ (I haven’t read that one) gets my attention with its boost to 110 km/s. Geoff Landis worked out a conceivable 500 km/s with a maximum sun-diver maneuver when we were discussing sail concepts in Cleveland (this was when I was doing research on my book). So yes, it’s amazing what these various kinds of maneuvers can do.

  • Administrator January 11, 2008, 9:11

    Michael, your comment is deeply appreciated, and I never forget that a large part of the value here lies in the insights the readers bring to the table. It’s wonderful to be involved in this kind of collaboration.

  • Adam January 11, 2008, 16:34

    Hi Paul

    Was that 500 km/s from a rocket boost or solar-sail? Claudio Maccone and Greg Matloff have computed up to 1,500 km/s for solar fry-by solar-sail missions – “The Starflight Handbook” mentions a probe design for getting up to 4500 km/s.

    Utterly ridiculous star-sails could reach ~ 0.12c according to a NIAC study by one of Bob Zubrin’s colleagues.

    I think I need a lie down, these speeds are getting to me ;-)

  • Administrator January 11, 2008, 18:02

    Adam, as I recall that conversation, Geoff was talking about a pure solar sail Sun-diver maneuver — no rocket boost. I’ll double check that in my notes.

  • James M. Essig January 12, 2008, 0:10

    Hi Adam and Paul;

    If carbon nanotubes can be developed that are very higly conducting and perhaps highly reflective with a very shallow optical skin depth for visible, near visible IR, and UV-A and UV-B solar radiation, perhaps a very large solar diving solar sail that has the form of a reflective grid with spaceings between the carbon nanotube constituent treads is on the order of 0.1 micrometers would make an extremely low mass per unit area, highly refractive solar sail. If the sail could be composed of carbon nanotubes on the order of one nanometer in diameter, the sail could obtain a mass savings per unit of area of perhaps roughly two orders of magnitude compared to that of a monolithic higly reflective nanometer thick sail. Recall that carbon nanotubes may be as much as 60 times stronger than many high strength steels. Since many solar sail materials being experimented with are on the order of 2 to 5 microns thick, the potential mass savings per unit of area may be as high as 5 1/2 orders of magnitude compared to the 2 to 5 micron thick monolithic sails. Assumming a better part than 50% for the reflectance for the carbon nanotube sail and the possibility of deploying vary large area high refractive material carbon nanotube sails, I can see how a solar dive and fry space craft might even be able to approach velocities greater than 0.12 C. With a large enough sail, this velocity might apply to large manned local interstellar star bound space craft with adequate internal G-force cancelling measures.



  • Starfleet commander January 12, 2008, 6:00

    Do we know anything about how measurement systems or communications function on a craft doing thousands of km per sec in vaccum? I mean post- acceleration.

    Also if you start with an initial boost to an intertellar probe that gets it speeding along at a few thousand km per sec; could it then use a longer lasting propulsion mechanism to keep accelerating for a few years? So if a craft is travelling say 4000 km/sec and it turns on an ion propulsion engine would even a little thrust give it extra speed, or does one have to take into account that it needs the equivalent of 4000km/sec thrust and that bit extra to make it go faster?

  • Starfleet commander January 12, 2008, 6:02

    Just one more point. Surely we could do very neat relativity experiments remotely with a craft moving at those speeds?

  • dad2059 January 12, 2008, 11:40

    Starfleet commander: Sure. Build a linear accelerator in the outer asteroid belt or a Jupiter Lagrange point. Shoot the probe at Jupiter for a gravity assist shot to the Sun. Engage some form of ion motor for a little extra “oomph”. Perform the Sundiver/fryer manuever for that last kick, deploy Mr. Essig’s carbon nanotube solar-sail, and off you go! Easily >.10c on the first shot if your figures are right.

    And the probe can be small also, 1 kg. with canned nanoprobes if the primary mission is testing functionality during relativistic flight.

  • Administrator January 12, 2008, 13:20

    Re the ion boost, I think I’d forgo it. My thought is that rather than carrying the extra weight, I’d rather put everything I have into the primary propulsion system, especially if I had figured out a way to reach some of the speeds we’ve been talking about. As things stand right now, we’re having all kinds of trouble working out a way to get to Gruntman’s 75 km/s — of course, that’s with existing or very near term technology. Weight is a huge constraint even there.

    But nanoprobes certainly make sense, and are doubtless the way we’ll hold down payloads as we do start pushing into higher speed ranges. It’s one thing to push a five ton probe to relativistic velocities, but what about Robert Freitas’ idea of probes the size of sewing needles equipped with assemblers that can extract local materials and build macro-scale research stations, etc.? Love it.

  • Administrator January 12, 2008, 13:26

    Starfleet, you’re right about the experimental possibilities with a fast-moving probe, and even Gruntman’s 75 km/s spacecraft offers us possibilities in that direction. We’ll talk more about this on Monday, along with some discussion of what increasing speeds mean for measurement accuracy, etc.

  • dad2059 January 12, 2008, 15:17

    I was thinking of the ion engine as a stage to be jettisoned before the sundiver slingshot, but hey if Freitas’ nano-needles can be incorporated into the nanotube sail itself, your weight issue almost solves itself because essentially you’re just launching a sail.

    That in itself might be limiting because of drag, but I’m guessing here.

  • James M. Essig January 12, 2008, 22:12

    One can imagine the velocity obtainable by a nanometer thick grid like sail that is 99 % open space doing a dive and fry manovuer around the sun. The possibility of doing a dive and fry manuvuer around a much more luminous blue supergiant or red giant star which have up to 10 million times the luminousity of the Sun is phenomenal perhaps yielding terminal velocities close to C and high gamma factors. Note that the radiation pressure produced on a sail by a blue supergiant star with a surface temperature of 60, 000 K would be 4 orders of magnitude greater than that of the sun wherein the craft was located at a distance from either star at which the spherical angle subtended by the two stars would be the same. No doubt, it would take some form of exotic yet to be developed sail material to with stand the thermal radiation at a location of a few stellar radii away from a blue supergiant. But since the total luminosity and total irradiance of a blue supergiant is as high as 10 million times that of the Sun, the useful range of the associated solar sailing is, by the inverse square law, 3 1/2 orders of magnitude greater than that of that of the Sun.

    Assuming that at some distant future date, some very exotic form of low density sail material that is highly reflective wherein the thickness of a mesh type sail could be made a fraction of an atomic diameter would allow solar salling velocities or stellar sailing velocities with much greater gamma factors and would extend the useful range of the stellar sail dramatically by reducing the mass per unit area of the sail.

    It is interesting to note that just as we sailed the ocean blue during the precolonial and colonial eras of past centuries, we might end up salling the ocean black on solar and stellar sails to perhaps ply the depths of the Milky Way Galaxy. At least the huge energy sources involved would give off free renewable energy.



  • James M. Essig January 13, 2008, 3:12

    Hi Folks;

    I could not resist the urge to mention a sail concept that I once heard about and which I believed I have mentioned at least a few times on the Practical Positron Rocket, the Practical Positron Rocket II, and /or the Practical Positron Rcoket II Overflow Thread. I believe I first became aware of this concept in its most basic form after reading an article in good old “Popular Science Magazine” in which the respective concept was mentioned briefly.

    Basically, the concept involves the extention of a vast cosmic microwave background radiation reflecting membrane that would reflect CMBR incident on one side and transmit CMBR incident on the reverse side. Now granted that the radiation pressure exerted by the CMBR per square meter is tiny, thus a very large sail would have to be deployed to produce large forces. A very large monolithic sail would be very heavy even using 2 micron thick material. Therefor, such a sail might be best used to send out space probes or to power huge many, many human generation space arks into interstellar space where very small but gradual accellerations are acceptable.

    To greatly decrease the mass per unit area of the CMBR sails, perhaps selectively transmissive nanotube material could be used in the fabrication of the sheets wherein only an approximate 10 millionth of the area of the sheet would actually be non-empty space. The spaceing between the nanometer scale threads composing the sheet could be on the order of 1 millimeter to 10 millimeters inorder to reflect yhe sizable portion of the CMBR wavelengths that are larger than these grid line spacings. If some sort of extremely conducting and perhaps even superconducting selectively transmissive threads were used, then perhaps the radiofrequency skin depth on the reflective side of the sheets could be reduced to an effective nanometer or less.

    The beautiful aspect of using CMBR sails is that CMBR is essentially homogeneously distributed just about everywhere within galactic and intergalactic space. As the readership of this site is most probalbly well aware, the flux densities of the CMBR as measured among differing angular directions by the Cosmic Microwave Anisotropy Probe deviates by no more than roughly one part in 100,000. As a result, such a sail could in theory accellerate essentially forever providing that the foward siide of the sail could maintain its transparency.

    If the forward side of the sail could some how have negative index of refraction properties for the incident greatly relativistically dopplar blueshifted CMBR, the sail might be pulled along to accellerate in its direction of travel by the incident blueshifted CMBR while at the same time being repulsed by the reflected CMBR from behind thus leading to a perpetually accellerating CMBR sailing intergalactic space ark. Note that the drag induced by baryonic matter gas would be minimized in the depths of intergalactic space because the intergalactic atomic, ionic, and electronic matter concentration is much lower in intergalactic space, but this is generally not the case with the isotropic

    The reader is directed to the Duke University (of Chapel Hill, North Carolina) website and/or related links to learn more of the fascinating research being done on negative index of refraction materials and their bazaar electromagnetic/optical properties.



  • Starfleet commander January 13, 2008, 5:49

    Thanks for all the information guys.

    One more question if i may:

    I think I read in a Paul Davies book that a probe/craft moving at 87% speed of light, would have a kinetic mass of over 2x the rest mass of the probe.
    And that after 87% speed of light the mass increases at a rate which makes it practically futile to push it any harder because of the acceleration required. So i am assuming that means that if your ship had a mass of 1 ton at rest it would only weigh just over 2 tons at those speeds?

    Is that right or did I misunderstand the book?

  • James M. Essig January 13, 2008, 10:49

    Hi Starfleet commander;

    You are right. The formula for relativistic mass increase at velocities that close to C is Mrel = Mrest/{[1 – [(V EXP 2)/(C EXP 2)]] EXP 1/2}. The value of ship velocity at which the relativistic or total mass of the ship would be twice its rest mass is (86 +2/3)%C or 86.66666…% the speed of light. Even though the incremental rate of mass increase increases sharply as V approaches C, so does the relativistic time dilation and so one can, according to Enstein’s theory of relativistic time dilation, potentially travel an infinite distance in space while only a small fraction of a second passes by on the ship based clock as one approaches the limiting value of C exactly. Accordingly, accelerating to C would take an infinite amount of energy and so practically speaking, it is hard to see how inertial travel through space could ever reach C. Note however, that one can approach C arbitrarilly closely by pumping more and more kenetic energy into the craft. Note that the formula for time dilation is delta T1 = delta T2/{[1 – [(V EXP 2)/(C EXP 2)]] EXP 1/2} which of simmilar form to the relativistic mass increase formula. At 86.666 % C, the time dilation formula would yield delta T1 = delta T2/(0.5) or delta T1 = 2(delta T2). If delta T2 equals ship based time is equal to one year, delta T1 or Earth time equals 2 years for a ship that has accelerated to .8666 C and maintained this velocity for one year or delta T2 ship time.

    Thanks for asking.


  • Starfleet commander January 14, 2008, 7:30


    Thanks for the answer – and how exciting!

    It appears we can push a craft up to ridiculous speeds and suffer very few consequences because i would have thought if you’ve only doubled your mass at that speed it seems a relatively light penalty to pay.

    If we can sort out the right propulsion it seems we could be doing this sooner than later. Unmanned obviously.

  • jst-a-humanoid October 11, 2008, 21:03

    Hi there guys, verry interesting stuff, just one issue if i may,
    any ideas for a deceleration process?
    I mean maybe you want to go to Alpha Centaury let’s say, when you get there you need to stop:)

  • Administrator October 11, 2008, 21:30

    One interesting possibility is to use a magsail, which would provide deceleration into a destination star system by creating drag with its solar wind. This idea grew out of studies of the Bussard ramscoop concept, which turns out to create so much drag as to make it likely impractical as a propulsion solution, but interesting as a way to brake upon arrival.

  • tim November 22, 2008, 18:18

    if a light sail massing 1 gram per sqaure meter of the light sail area passes within 100,000 km of the surface of the sun and it survives the heat it may push its payload up to about 12 % of light velocity. It would also produce that close to the sun a very high acceleration rate that only a robot probe can with stand.

  • Dmitri Gorskin December 21, 2010, 17:12

    We can create very small interstellar microzond today.
    It will be look likes a spy machine.
    We can use a lazer, sun sail from grafen for acceleration.We can create 1000 this zonds and send them after small period time.