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Into the Interstellar Void

We often think about interstellar probes only in the context of what they find at the end of their journeys — astrobiologically interesting planets seem to be the whole story. But not so fast. As Ian Crawford (University of London) notes in a recent paper, there are quite a few fascinating — and indeed critical — things we need to learn about interstellar space itself, in this case what is known as the Local Interstellar Medium (LISM). Crawford, who has been analyzing these matters for the Project Icarus team, notes how much we’ve learned about the LISM since the Daedalus days.

The new paper grows out of Crawford’s presentation at the British Interplanetary Society symposium ‘Project Daedalus – Three Decades On,’ which was held last September in London. It reflects his thinking on interstellar probes in relation to planetary and stellar studies and astrobiology as well as the nature of the medium through which the probe will fly. But today I want to focus on the LISM because what we might erroneously consider to be relatively empty space turns out to be challenging indeed.

Not So Empty Space

After all, at the kinds of speeds we’re discussing for an interstellar probe, somewhere in the range of 10 percent of lightspeed at minimum (Daedalus aimed at 12 percent), what seems empty void can be a minefield. Consider just a few of the issues Crawford raises here:

Direct measurements of the interstellar dust density, and the size distribution of dust particles, will be especially important because sub-micron sized dust particles will erode exposed surfaces (see the analysis performed for Daedalus by Martin [16]), and larger particles (i.e. larger than a few tens of microns) may, if present in the LISM, cause a catastrophic failure of the vehicle. Determining some of the other properties of the LISM will also be important for longer term planning of other interstellar propulsion concepts – for example, determining the LISM hydrogen density, and its ionisation state, will be important for assessing the future practicality of interstellar ramjets…

We’d better, in other words, get up to speed about the interstellar medium before we launch any true interstellar craft, and that means a series of early missions of a precursor nature that can tell us what to expect. Crawford notes that the Sun is currently located close to the boundary of a small low-density interstellar cloud known as the Local Interstellar Cloud (LIC), one of several such clouds within several parsecs of our Solar System — indeed, one study identifies seven such clouds within 5 parsecs of Sol. These are found within the Local Bubble, about which this:

These [clouds] are immersed in the very empty (nH ~ 0.005 cm-3) and probably hot (T ~ 106 K) Local Bubble (LB) in the interstellar medium, which extends for about 60-100 parsecs from the Sun in the galactic plane before denser interstellar clouds are encountered (at high galactic latitudes the LB appears to be open, forming a chimney-like structure in the interstellar medium which extends into the galactic halo…)

We can learn a lot about these matters, including the properties of the Local Interstellar Cloud, by spectroscopic studies of interstellar absorption lines toward nearby stars. Investigations into our Solar System’s interactions with the heliosphere are also useful, but we’ll need to augment these with direct measurements of the interstellar materials just beyond the heliopause, and that means developing the capability to get space probes to distances up to 1000 AU or more.

A Rationale for Probing the Void

Beyond that, and having taken these findings in account in its design, a true interstellar probe would be a priceless tool for measuring everything from dust density and composition to the interstellar magnetic field strength enroute. Crawford points out that a spacecraft moving at 0.1c could do daily measurements 17 AU apart (roughly half the radius of the Solar System) which would offer unprecedented knowledge of the structure of the Local Interstellar Medium.

Centauri A and B evidently lie beyond the Local Interstellar Cloud, although the line of sight from Earth is dominated by another interstellar cloud known as the G cloud. An interstellar mission to this system would tell us whether the Sun is actually embedded in the Local Interstellar Cloud or just outside it in the region where the LIC is interacting with the G cloud. Conceivably a Centauri mission would thus get to sample two different interstellar clouds, along with the low density material between them. As Crawford writes, we would receive a windfall of data:

If the Sun does lie within the LIC, then a mission to α Cen would sample the outer layers of the LIC, an interval of low density LB material, the edge of the G cloud, and the deep interior of the G cloud. This would sample one of the most diverse ranges of interstellar conditions of any mission to another star located with 5 pc of the Sun, as most other potential targets lie within the LIC… Even if the Sun lies just outside the LIC (as argued by Redfield and Linsky [8]), the trajectory to α Cen would still permit detailed observations of the boundary of the G cloud (and its possible interaction with the LIC), and determine how its properties change with increasing depth into the cloud from the boundary.

Shielding Against the Medium

Needless to say, such a pathfinder mission would help in the planning of all follow-up interstellar missions. At 10 percent of lightspeed, a probe would have to be shielded against damage from high speed collisions with dust in the interstellar medium, a topic both the Daedalus and Icarus designers have had to take into consideration. It will be fascinating to see how the shielding options will be modified between Daedalus (1970s) and today’s Icarus design.

As I mentioned above, Crawford’s paper delves deeply into the rationale for an interstellar mission, going beyond the interstellar medium question to address planetary and stellar studies and astrobiology. It’s a fascinating rationale for undertaking these studies and continuing to advocate precursor missions. And note this final caveat re Alpha Centauri as a destination:

The relative proximity of α Cen, together with its interesting interstellar sightline and the presence of stars of three different spectral types, makes it an attractive target for humanity’s first interstellar mission. However, as the bulk of the scientific benefits of interstellar spaceflight pertain to planetary science and astrobiology, a final prioritization must await future developments in the detection of planetary systems around the nearest stars. Fortunately, expected advances in astronomical instrumentation over the next century should ensure that a comprehensive list of prioritized targets is available well before rapid interstellar travel is technically feasible.

The paper is Crawford, “The Astronomical, Astrobiological and Planetary Science Case for Interstellar Spaceflight,” published in the Journal of the British Interplanetary Society Vol. 62 (2009), pp. 415-421 (preprint).


Comments on this entry are closed.

  • Carl Keller September 1, 2010, 11:11

    Recall the very successful Magellan probe, http://www2.jpl.nasa.gov/magellan/ which was budget-cut until its science package was just the imaging radar. A future mission to investigate the IM via the plutoids may benefit from an active radar system opposed to optical devices. Certainly more can be viewed that way than just the sunlit side of larger bodies.

    Such radar should be able to locate anything larger than dust. An apt name for the interstellar precursor mission would have been Magellan, for his voyage was one-way, having been killed in the Philippines. Captain Juan Sebastian de Elcano completed the journey, oddly just a footnote in history http://coloquio.com/famosos/elcano.htm

  • Terry Koberstein September 1, 2010, 11:49

    There is much to be learned about the LISM and the LIC, an exciting prospect. In addition to this if the probe had even a modest telescope with appropriate sensors we would learn a great deal about α Cen long before it actually arrived. Now that would be very exciting!

  • andy September 1, 2010, 14:16

    At present I would still go with Epsilon Eridani as being a more interesting target than Alpha Centauri, but if a planetary system is detected orbiting one of the Alpha Centauri stars that will definitely change things. Epsilon Eridani seems to be a pretty good analogue of the young Solar System, with a fairly close Jupiter-analogue orbiting outside an asteroid belt, with an icy debris disk at large distances from the star. Any planetary system around the Alpha Centauri stars would represent the outcome of a very different set of conditions for planet formation.

  • michael September 1, 2010, 14:59

    I think a laser or particle beam (mostly likely ionised) fired forwards from the craft would remove a significant amount of interseller medium ahead of it reducing the size of the shield needed at the tip of the craft

  • Tulse September 1, 2010, 18:16

    Such a beam would have to be as large across as the ship, and powerful enough to vaporize whatever is there — that would take a lot of power. And even then you’d simply have a fog of smaller particles that would still cause erosion if it hit anything material. I suppose that using a charged beam as you suggest would allow one to charge the medium and thus use a magnetic shield of some sort.

  • Michael September 2, 2010, 1:50

    A laser or particle beam or a combination of both will naturally diverge ahead of the craft easily covering the whole craft and once it encounters material it will most likely ionise it, these particles would tend to repel each other dispersing the material before impact.

    yes there will be a need for a lot of power but the craft will be adding to that power as it gains velocity as well

  • Michael Spencer September 2, 2010, 6:36

    Grounded as they are in science, it is sometimes true that these pages often indulge a certain flight of fancy. And why not? What do we know of the travails of space travel?

    As I child I was fascinated by studies (yes, studies!) wondering if humans would be able to swallow in space. Turned out to be silly, looking back; the point being that very much pure research lay ahead of us. All the better! Let the universe bare secrets one delicious morsel at a time…

    Too, I recall a SF story describing the travails of a modern-day newspaper editor told from the POV of the 1940’s or so. The story, charming as it is, gets nearly nothing right. Why? because the only technology was the telephone, that’s why. Aren’t we in a similar situation?

    (I came across the story as a free download for my iPhone. My gratitude to anyone remembering more about that story!).

  • ljk September 2, 2010, 7:39

    The Philosophy of Astronomy – Simon Schaffer, August 21, 2010

    What is the ideology that propels scientists to go to so much trouble? Think, for example, of the hazards involved in a voyage from Europe to our part of the world in the 18th century. Why would you go to all that effort just to observe the transit of Venus?

    For Science Week, we explore the philosophy of northern astronomy in the Southern Hemisphere with Simon Schaffer, Professor of the History of Science at the University of Cambridge.

    Full article here:


  • Mark Wakely September 2, 2010, 14:32

    Besides the erosion/explosion fears, flying through even a tenuous dust cloud at 0.1c could create enough friction to slow down the craft like an atmospheric reentry. It would then require addition propulsion (and time) to regain 0.1c. The craft could also be thrown off course- perhaps badly off course- requiring a significant course correction and the use of more propellant, also adding time to the mission. (This assumes the craft was designed strictly for the vacuum of space and wasn’t particularly aerodynamic. For example, enormous solar sails as currently envisioned would fare poorly with a dust cloud encounter at significant speed.) Multiply that by the number of dust clouds a craft might encounter and an interstellar mission could take much, much longer than anyone anticipated.

  • Eniac September 2, 2010, 17:23

    Mark: I don’t think friction would be a problem. If you compress the ISM column between here and a nearby star to normal density, you get a layer only a micrometer or so thick, clouds notwithstanding. There is no way this can take significant momentum from a macroscopic ship. It will take away a lot of momentum, yes, but the amount will be insignificant compared to the humongous momentum of the ship and do very little to slow it down.

  • Stan September 2, 2010, 19:00

    What about moving through dark matter at high speeds? Any problems, considering there is presumably so much of it out there?

  • Mark Wakely September 2, 2010, 20:45

    Eniac: I sincerely hope you’re right about the density (or lack thereof) of dust in interstellar clouds, although we won’t really know how serious a problem it is until we have the ability to directly measure that density. My fear is that we might be shocked at how much sub-micron (and larger) dust is actually floating out there among the stars. “Empty” space might not be so empty after all.

  • Eniac September 2, 2010, 21:57


    Eniac: I sincerely hope you’re right about the density (or lack thereof) of dust in interstellar clouds, although we won’t really know how serious a problem it is until we have the ability to directly measure that density.

    I think we do have good ways of measuring the density of the ISM. That’s how we know about all those clouds and bubbles in the first place. I am not an expert, but I think we still have a lot to learn about the particle nature and size distribution, but not so much about the overall density. If I am wrong, someone please set me straight, here.

    My fear is that we might be shocked at how much sub-micron (and larger) dust is actually floating out there among the stars. “Empty” space might not be so empty after all.

    There is of course also a limit to that density given by cosmology. I don’t know how we know that Dark Matter is not just interstellar dust, but in any event there could never be more dust than the mass we attribute to Dark Matter. Surely we know Dark Matter is not dust, so the limit should be much stricter than that. Plus, we have the measured extinction coefficients. All these ought to exclude the most shocking surprises, but perhaps someone more in the know should comment.

  • Ron S September 3, 2010, 15:21

    Stan, the cross-section of interaction between dark and ordinary matter is so incredibly low that for our purposes here it is non-existent. This is the reason why it is so incredibly difficult to detect and understand. While there is a gravitational interaction, for most interstellar flight paths it will be present at a constant density and therefore cancels out entirely.

  • Adam September 4, 2010, 2:48

    Hi guys

    There’s good material online about the ISM, especially it’s dust component. One striking comparison I’ve read is that if the ISM was compacted to air density then it’d be like being in a room filled with thick smoke, since 1% particulate dust is quite thick in terrestrial terms. Fortunately it’s spread out so thin that we can see through thousands of light-years without too much reddening – incidentally that’s how we know the stuff is out there to start with, by its optical effects. And because of how it affects different parts of the spectrum differently quite a lot is known about its size range – nanometres to microns in size. Sand grain and pebble size ‘dust’ aren’t seen, so large particles must be spread pretty thin.

  • bigdan201 September 4, 2010, 4:30

    Often space is described in nautical terms, as if it were a giant ocean.. especially in fiction. And like a seafaring vessel, a spaceship must make long and difficult voyages through a hazardous medium.

    Issues like this further highlight the difficulty in making interstellar voyages. It will certainly be possible, but there will be many challenges to be met by innovative ideas.

    Perhaps an ionization beam could be sent out ahead of the craft to prepare a path to the star.

  • Adam September 4, 2010, 6:40

    Hi again

    Good link that summarises what we know of the ISM…

    Astro553 Lecture 9

    …with quite a lot of useful detail. The smokiness of the ISM is quite a surprise.

  • James M Essig September 4, 2010, 15:09

    Hi Folks;

    One way to potentially deal with dusts particles traveling relativistically relative to a ship and her shield using ordinary periodic table atomic and molecular matter based materials is to have a layered shield that is thick enough to cause all incoming dust particles to be completely reduced to a divergent atomistic state. The massive jets of decay products would then be diverted by electric and/or magnetic fields set up in vacancies between the shield’s layers. Neutral particles that were produced but which somehow passed through an initial layer would be subject to transformation into decay jets, which could include charged species, that could then be diverted by other layers, and the process could repeat itself until all of the energy was absorbed before it could irradiate the crew members or sensitive ship electronic equipment. Nanotech self-assembly repair mechanisms or other microbots could continually refashion the shielding components including field generation components to compensate for radiation damage.

    A good bulk shield material might be pure diamond, perhaps improved forms of diamond that have higher heat conductivity and better refractive properties then the best natural diamonds. Such improved diamond is theoretically possible (and has been produced in limited quantities) by altering the ratio of carbon isotopic composition of the diamond, which accordingly manifests itself in artificial diamond with improved characteristics.

    Thermal energy phonon quasi-particle conductors have been proposed as theoretically possible to greatly enhance heat conduction over traditional thermally conductive materials. Imagine if the energy impinging on the front of the space craft could be conducted very rapidly away from the front of the shield and collected and exhausted out the back of the space craft such as by a photon rocket, electron rocket, ion rocket, or even a neutrino rocket. The frontal shield might then have a temperature maintained below the impinging radiation temperature, perhaps even below the ambient CMBR temperature, and the craft should in theory, be pulled forward by the radiation imbalance in an effectively perpetual manner so long as the process remains operative. Phonon conductors may in theory conduct heat from cold to hot material portions.

    We should not give up hope in the potential joys of extreme gamma factor interstellar manned travel with all the mystique and potential mysteries that such might offer as may have been overlooked in traditional Special Relativity.

  • Kennth Harmon September 6, 2010, 12:37

    Andy and All,

    A couple of times you and a few others have mentioned Epsilon Eridani as a potentially superior destination compared to Alpha Centaruri for an early Interstellar probe or ship,unless a planetary system is found around one of the Alpha Centauri Stars. This said, Epsilon Eridani is a very young Stellar System, and while it may one day be a destination for humanity we are talking about millions of years in the future before it settles down enough to be of direct interest for anything other then a Scientific probe to explore Stellar Planetary formation. There is however another Epsilon, Epsilon Indi, near our Sol system which may have a planetary system and seems to be given recent redating research much older then previously thought. Given its recalculated age Epsilon Indi seems to be old enough to harbor some form of complex life if it turns out to contain a viable planetary system.

    It would be interesting if other members of this forum including James Essig would comment on the top five Stellar Systems within ~20 Light Years of Sol/Terra for a potential probe or even manned exploration in the next 200 years or so. It is still very early in the process, and we have very little hard data, but perhaps a survey should be taken here of the best candidates, especially given the latest information on things like the LIC and the G Cloud. Personally, I believe that Alpha Centauri should still rank number one out to 20 Light Years given its many interesting attributes. However, I would certainly place Epsilon Indi as a strong second pending future discoveries.

    Jill Tarter’s survey for TPF is still “the Gold Standard” for indentifying the best candidates for habitable Star Systems near Sol/Terra and as candidates for a future TPF to focus on, but she went much further out then 20 Light Years from Sol/Terra, and there were many more interesting candidates with this expanded data sample starting with 55 Cancri, which seems to be high on everybody’s list. However, over the next 200 years or so Humanity’s radius of physical reach is unlikely to exceed 20 Light Years unless there is a revolutionary physics breakthrough so while we should have a destailed survey out to about 60 Light Years, and a general survey out as far as we can go, we are probably stuck with the Stellar Systems within about 20 Light Years of Sol/Terra in terms of where we can successfully send either a probe or manned ship. The question is who if any are interesting candidates within a 20 Light Years radius of Sol/Terra

  • James M Essig September 6, 2010, 15:46

    Hi Kennth Harmon;

    Thanks for the offer.

    There is a total of 83 star systems containing 109 stars and 8 brown dwarfs known to be located within 20 light years of Earth. This is a great deal of territory to explore.

    If we can obtain velocities of 0.4 C such as by some high end M0/M1 ratio fusion rockets, of course assuming an effective Isp of between 0.1 and 0.119 for fusion fuel, then we can reach the furthest of these stars in only about 50 years ship time even under 0.2 G to 1/3 G acceleration.

    A 10 micron wide sized dust particle would have a mass of about 1 red blood cell or about 1 nanogram. Now 0.4 C corresponds to a gamma factor of 1.091 or a relativistic kinetic energy of about 9,000 joules for the 10 micron dustspeck. The Barret brand 50 Calibre Sniper Rifle has a muzzle energy equal to about 2 1/2 times this value, yet the round fired by such a gun will not phase an M1-A2 American main battle tank.

    If we had a very large space craft with a very thick shield, perhaps one consisting of water ice followed by a thick carbonacious very strong material such as diamond, carbon nanotubes, boron nitride nanotubes, Osmimium, which has a bulk modulous that 7 percent greater than pure diamond, or even traditional graphite fiber, followed by another deposition of water and so on, my guess is that we could completely absorb 10 micron dust impacts with no radiation breach of the craft’s forward shield. The track left by the particle as it passed through the water would simply be refilled by the surrounding water. The frontal shield might even take the form of a highly elongated solid cone inorder to provide for an enhanced grazing incidence of the dust, which although still having ionizing characteristicss with respect to shield, would produce a plasma that might be partially diverted by an electro-dynamic-hydrodynamic-plasma drive mechanism.

    The ship might have an elongated aspect ratio simmilar to a passenger rail train for reduced frontal cross-section and much thicker forward shields.

    The good news is that even though such a highly relativistic ship traveling at 0.4C to 0.5 C would be massive and thus require a lot of fusion fuel, fusion fuel is very, very cheap and in fact is the most common natural potential real energy supply known to exist within the observable universe.

    I remember Werner Van Braun’s crude liquid fueled rockets that fizzled out at a few kilometers in hieght and the huge Space Shuttle systems or the Russian Proton Booster rockets in comparison. Likewise I feel confident that if our first fusion rockets were limited to 1 GigaWatt, (The NERVA Fission Rocket was 4 GigaWatts), I beleive we could scale up to 10 EXP 18 Watts or more in relatively short order with a Global Manhattan style R&D effort and thus enable the launch of truely huge starships. Heck, we already know how to produce powers of about 10 EXP 25 watts such as in the 10 nanosecond peak energy release of a 25 megaton hydrogen bomb or about (10 EXP 17)Joules/(10 nanoseconds). A fusion pulse mechanism be it of a project Icarus scale or a huge Project Orion starship should enable space craft to achieve 0.4 C when the most efficient fusion reactions are used, and electrodynamic shielding is used to safely back-reflect the bomb or fuel pellet plasma.

    I do not go for this “No Nukes in space nonesence!”. Nukes might be our best short time ally for reaching the stars this century. Pure fusion devices would be nice in this regard.

    Upon reaching 0.4 C, essentially half of our currrent light cone of 13.7 Billion LY is fair game for space travel over the next 10 billion years. Nanotech controlled hyberbation, suspended animation, or indefinate human life span extension would also help for journeys to distant galaxies. However, I think such journies can and will be accomplished one day.

  • Ronald September 7, 2010, 5:57

    @Kenneth Harmon, top five Stellar Systems within ~20 Light Years:

    Of couse this will all depend on near future (spectroscopic) analysis of existing planetary systems, but tentatively guessing, in order of distance:

    – Alpha Centauri A/B, G2/K0 at 4.3 ly, particularly B (close binary, so I have my doubts, but you have to keep good relationships with the neighbors);
    – 40 Eridani (= Omicron 2 Eridani), K1, 16.5 ly, rather low metallicity, but probably not too low;
    – Eta Cassiopeia A, G0 (or G3?), 19.4 ly, rather high luminosity and rather low metallicity, plus medium close/wide binary (closest approach 36 AU), so not very ideal;
    – 82 Eridani, G5 (or G8?), 19.8 ly, rather (but not too) old, rather (but not too?) low metallicity;
    – Delta Pavonis, G5 (or G8?) IV-V, 19.9 ly, rather old and remarkably high luminosity for its spectral type, gradually getting brighter and hotter, high metallicity!

    I could also have mentiond as runners-up within 20 ly:

    – Tau Ceti, G8, 11.9 ly, but quite low metallicity, so probably failed planetary system (lots of dust, asteroids);
    – Epsilon Eridani, K2, 10.8 ly, has a real planetary system, but on the dim side (about 0.28 solar lum) for a solar type star and quite young, terrestrial planets, if present, maybe suitable for terraforming;
    – Sigma Draconis, K0, 18.8 ly, but probably a variable star, and rather low metallicity (?).

    So few ideal candidates within 20 ly.

    Between 20 and 30 ly, life gets better, I would mention as my personal most promising candidates (and also of Porto de Mello et al.):
    – Beta Canum Venaticorum (= Chara), G0, 27.3 ly, a TPF-Top25 target star of Turnbull & Tarter;
    – 61 Virginis, G5, 27.8 ly, one of my personal darlings within 50 ly; though it has a few subgiants (Neptune class) and superearths in its inner system, it may still appear to have real terrestrials in its habitable zonde as well;
    – Zeta Tucanae, F9, 28 ly, rather on the bright side for a solar type (1.31 solar lum).

    If you don’t mind flying on a few more lightyears, another darling of mine:
    – Alpha Mensae, G5, 33.1 ly, has everything going for it: luminosity, metallicity, age, quite promising and similar to 61 Virginis. And if you have a terrestrial planet in the habitable zone, you’re set for many stable gigayears.

    Finally, for the really seasoned star travellers, I can’t help mentioning a few very interesting candidates between 30 and 50 ly:
    – Zeta 1 and 2 Reticuli, G1/G2, 39.4 ly, extremely wide binary, rather low metallicity (about 0.6 solar), but possibly not too low, formerly thought to be very old population II, now considered about 3 gy old. Big advantage: two solar type stars within 0.1 ly together.
    – 18 Scorpii, G1/2, 45.7 ly, solar analog, near-solar twin, TPF-Top25.

  • JoeP September 10, 2010, 11:09

    I recall reading about smoke particle shield, although I do not recall the source. Once cruising speed is reached, a probe moves out significantly ahead of the main spacecraft and generates a very large cloud of smoke particles that leads the spacecraft for the entire journey. The idea is that the smoke cloud will intercept and atomize the majority of the dust particles before the main spacecraft reaches that area of space.

  • Tobias Holbrook September 13, 2010, 8:27

    “I recall reading about smoke particle shield, although I do not recall the source. Once cruising speed is reached, a probe moves out significantly ahead of the main spacecraft and generates a very large cloud of smoke particles that leads the spacecraft for the entire journey. The idea is that the smoke cloud will intercept and atomize the majority of the dust particles before the main spacecraft reaches that area of space.”
    Hmmm. Surely that would take quite a bit of dust, since the smoke would be disappated eventually and need replacing? I suppose you could hold a plasma shield, and idea I’ve been considering – the heat to keep it ionized might come from the energy of vaporization?

  • JoeP September 13, 2010, 22:36

    Tobias — now I recall the origin of the dust/smoke particle shield. It was from Project Daedalus. I think Paul Gilster mentioned it in his Centauri Dreams book.

    I guess the dust shield was for relatively short duration, like when the spacecraft was about to enter a system and dust levels are significantly higher.