Dust Up Between the Stars

by Paul Gilster on February 9, 2009

Almost four years ago I wrote a Centauri Dreams entry about Dana Andrews’ views on shielding an interstellar spaceship. The paper is so directly relevant to our recent discussion on the matter that I want to return to it here. Andrews (Andrews Space, Seattle) believes that speeds of 0.2 to 0.3 c are attainable using beamed momentum propulsion. That being the case, he turns in his “Things to Do While Coasting Through Interstellar Space” paper to questions of human survival.

Particles with a Punch

Collision with interstellar dust becomes a major issue when you’re traveling at speeds like these, a fact Andrews is quick to quantify. For a starship moving at 0.3 c, a typical grain of carbonaceous dust about a tenth of a micron in diameter should have a relative kinetic energy of 37,500,000 GeV. Our hypothetical star mission with human crew moving at a substantial fraction of light speed will run into about thirteen of these dust particles every second over every square meter of frontal area.

This gets interesting when put in the context of cosmic rays. Most galactic cosmic rays, which as Andrews notes are completely ionized atoms accelerated to extremely high energy states, have energies between 100 MeV and 10 GeV. You can see the overlap. Travel fast enough and even small grains of dust behave like energetic cosmic rays as our vessel encounters them. Clearly, dust between the stars is something we have to reckon with on any interstellar journey.

Absorbing Particles (and the Cost)

In my post on Friday, I looked at the Project Daedalus dust shield and its role in the journey to Barnard’s Star. A key question: Do we want to absorb cosmic rays and dust particles, or redirect them? The former can be envisioned in terms of a human crew surrounded by layers of supplies and equipment, with an outer shell for the spacecraft composed of 25 cm of multiple layers of polyamides, metal foils and polyethylenes to assist in radiation protection. The mass of structure and shielding obviously becomes a major factor. As Andrews writes:

The striking facts from this mass statement are the large masses associated with structure and shielding, and the small masses associated [with] things like food and water. This is because all waste and CO2 is recycled through growing plants and algae to provide clean air, food, and water.

The author is figuring that three percent of food intake would have to be derived from stored supplies to offer the necessary trace minerals and nutrients. He adds a 33 percent margin to that number and bumps the stored food percentage up to four percent, along with an extra year’s food supply as margin in case of intermittent problems with the life support system along the way.

As to protection from galactic cosmic rays (GCR), we can create a workable but quite bulky shielded environment using these methods:

Since 99% of the GCR is ionized hydrogen or helium, it’s obvious why hydrogen makes the best shielding (because like masses scatter better). Hence, plastics with high hydrogen content were selected for the hull and shielding materials. The total shield thickness for the hull alone is about 20 gm/cm2, which will cut the dose rate to about 25 rem/year. Assuming the sleeping quarters are 3 meters by 4 meters and 2.3 meter high we can shield the walls with approximately 30 gm/cm2 of water storage (top and bottom shielded by dry storage and machinery), which should reduce the annual dose to about 15 rem. That’s three times the recommended yearly dosage for radiation workers in the USA, but within (barely) the overall guidelines for astronauts.

A Magnetic Shielding Option

But there is another way to solve the problem. Magnetic shielding, using large current loops of superconducting wire to create a protective magnetic field, could reflect or deflect charged particles around the habitat. And the advantages of using the magnetic approach are considerable.

Get this: Although the mass of the magnetic shielding support structure is close to that of the hull shielding option we looked at above, the living quarters go from a space seven meters high and ten meters long (in three levels) to a habitat 200 meters in diameter with over 6000 cubic meters of usable volume. Foil bumpers are used to break down incoming dust into atoms and ionize the result, which can be then handled by the magnetic field. We arrive at a design like the one below:


Image: Magnetically shielded interstellar habitat (to scale). Credit: Dana Andrews.

Clearly we have a long way to go before building such a vehicle, considering our own halting attempts at sustainable life support (Andrews points to the problems of Biosphere 2, and to the challenging environment aboard the International Space Station). But there is nothing to prevent us from refining these technologies, while magnetic shielding and dust particle ionization is well within the realm of conventional physics. So there are ways around the dust and radiation problem if we do venture between the stars.

Just how a workable life support system could be maintained is the subject of an intriguing appendix. The paper is Andrews, “Things to Do While Coasting Through Interstellar Space,” AIAA-2004-3706, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, July 11-14, 2004.

ljk February 9, 2009 at 11:56

It would be interesting if we could do something with the energy
from those dust impacts in terms of propulsion for the vessel ala
a Bussard ramjet type design.

Ignoring for a moment the obsoleteness of needing a human crew
for a starship outside of transporting people in the event of a
catastrophe in our Sol system (or a group of baseline humans
who wish to start life anew elsewhere in their current form), I
might think that future human crews would use some form of
virtual reality to escape the potential negative aspects of being
on a very long space journey.

Star Trek was getting there with the idea of the holodeck, but then
again, their ships had FTL drives and they could get to other star
systems in a matter of hours and days.

tacitus February 9, 2009 at 13:41

In the timescales we’re talking about (a couple of hundred of years minimum, probably) before interstellar travel is possible, you also can’t rule out nanotechnology to be in the mix, especially when it comes to fending off radiation damage inside the body.

If you have a swarm of nanobots patrolling your bloodstream and nervous system for cancer cells and other radiation damage, then I suspect you could tolerate a much higher dose of rads over a long period of time.

Given the comparative amount of money being poured into medical and cancer research vs interstellar propulsion systems, I would be willing to bet a considerable amount that we will be about to counter the effects of high doses of radiation long before we set foot in another solar system.

tacitus February 9, 2009 at 14:57

Star Trek was getting there with the idea of the holodeck

I think the Red Dwarf guys are closer to the mark with their “Better Than Life” video game. :)

ljk February 9, 2009 at 15:14

Or go one step further and “just” genetically engineers humans
to survive in all kinds of cosmic environments. At the very least
the survival of the species will be assured.

Administrator February 9, 2009 at 15:44

tacitus, I know zero about video games. What is “Better Than Life”?

tacitus February 9, 2009 at 16:33

Well, it’s a made-up virtual reality game in the Red Dwarf TV series (and novel) that taps into your subconscious mind and feeds you with an experience that is literally “better than life” — all your dreams come true, but you are so fully immersed that you quickly come to think it’s your real life. As a result, it’s the most addictive video game ever invented and people will die of thirst or starve to death because they are unable to quit the game.

The joke in the show is that the hapless Arnold Rimmer is so screwed psychologically that even his greatest fantasies go horribly wrong for him and even begin to infect the virtual lives of his fellow players.

If you haven’t watched Red Dwarf, I would highly recommend it. It’s a British sitcom from the 1990’s set in deep space in the distant future when all but one human being (Lister) has died off and he only has a humanoid descendant of his cat, a hologram, a depressed ship’s computer, and an obsessive cleaning robot for company.

Yeah, it’s a bit silly, and probably an acquired taste, but the scripts were consistently funny and often introduced some quite clever twists on various science fiction ideas and concepts. “Better than Life” was one of those (though the book version is much better realized than the TV show — being somewhat constrained by a budget!).

Administrator February 9, 2009 at 19:04

I love recommendations like this. You have no idea how many good writers I’ve first encountered because of recommendations from readers. I’ll check the Red Dwarf show out for sure. Thanks!

andy February 9, 2009 at 19:18

So long as you stay well clear of the final Red Dwarf series, which was consistently awful…

John Hunt February 10, 2009 at 0:33

The issue of shielding against relativistic dust particles is significant for interstellar missions traveling at significant proportions of the speed of light. I’d like to point out that, for an EGR (frozen Embryo/automated Gestation & Rearing) mission taking about 2,000 years, shielding against relativistic dust particles is much, much less of an issue.

However, the cosmic ray issue remains. When transporting adult humans you need a 25 cm polyamide, etc layers in all directions because of concern for DNA damage. However, the shielding can produce showers of secondaries (especially with the very high eV cosmic rays) which may be more harmful than just taking the hit and letting it pass through.

The issue of cosmic rays is even larger for longer missions (such as EGR) although, working with a cellular form of humanity, you can, in theory replicate those “astronauts” that don’t have DNA damage and eliminate those that do provided there is a means of telling the difference. Using this approach one might be able to live with an only partially effective shield against cosmic rays.

Regarding magnetic shielding, superconductors can produce magnetic fields far greater than the Earth’s (e.g. 27 Tesla) and can do so in “persistent mode” with little energy input for months at a time. Beyond Jupiter, ambient temperatures would negate the need for coolant with current superconductors. Although 27 Tesla is about 540,000 times the magnetic field of Earth, I wonder if the physical size of the magnetic field matters when it comes to diverting the very high magnetic fields. Also, I don’t know that it’s true that superconducting magnets of 27+ Tesla need to be all that large.

djlactin February 10, 2009 at 3:44

two points:
1) Red Dwarf also appears as a series of novels. Dangerously addictive.

2) “…all waste and CO2 is recycled through growing plants and algae to provide clean air, food, and water.” is correct, but simplistic. The loop requires an Energy source. I would think that the kinetic (or other) energy from the colliding dust grains could be harnessed for this purpose.

3) I think the possibility of ‘encountering’ a slightly larger interstellar object (say, a gram?) might be non-negligible.

okay, so that was three points…i’m no mathematician


Adam February 10, 2009 at 6:11

I must’n’ve read that paper too closely as I missed the bit about shielding. Nice that most of my worries about particle showers streaking through the ship like streaks of lightning was an exaggeration. The ionization bumper is a cool idea, though I wonder how much stand-off distance is needed at higher speeds. Pellegrino’s “Valkyrie” has the multi-layer bumper shielding projecting quite a ways forward of the engine and even further away from the cabin. He figured a top speed of 0.92 c was practical, though with an awesome amount of antimatter needed for propellant.

Travelling at 0.3 c seems the minimum for humans as they are presently, else the trip-times are punitive. High current density superconductors would allow better shielding and quicker braking via magnetic-sail. Launching from the ground via magnetic sail would be the ultimate refinement – imagine a ring-shaped vehicle able to propel itself into space from the magnetic poles, then be pushed via a mass-beam or particle-beam to interstellar speeds. Ideally some kind of regenerative braking set-up would cut net energy costs immensely. I read a Ken Macleod short-story that used electromagnetic tubes for two-way trade – then as instruments of gun-ship diplomacy, launching off those nasty relativistic missiles as the need arose. Made for a fun story – and a chilling reminder that regardless of how clever we get, we’re still human.

James M. Essig February 10, 2009 at 10:34

Hi Folks;

Perhaps a layered or tiered combination of water shields, in conjunction with shields made of very high heat capacity materials such as isotopically optimized artificial diamond; and also vacancies between such layers where very strong permanent magnets could be judiciously arranged inorder to deflect the plasma that would punch through the water and carbon shields, could permit space craft to travel at velocities of atleast 0.2 to 0.3 C, and perhaps velocities with a gamma factor in the 10 EXP 3 to 10 EXP 4 range, which is within the range of the Tevatron at Fermilab at the lower end and the LHC at the higher end.

The larger the space craft, the larger the radius the magnetic field permeated vacancies between the layers can be. Any collected mass during the deflection and filtering process could perhaps be broken down into pure energy by some yet to be discovered means or atleast used as fusion fuel and/or reaction mass. A mechanism to convert the kinetic energy of the incoming plasma to ship based kinetic energy would be great, also, inorder to reduce the net drag on the spacecraft by the interstellar medium.

The idea of using a liquid water shield appeals to me since the water within the shield would immeadiately flow back into position after tha plasma dust particle left its track through the water.



dad2059 February 10, 2009 at 13:13

Many science fiction stories I have read either many years ago, or recently use ice as shielding to protect interstellar vehicles at .1 to .3 C.

Even if one goes by the premise of using downloaded quantum DNA into a small Kuiper Belt type comet/planetoid ice matrix, one would need to bury the DNA qubits pretty deep, since the trips would still take a century or more and interstellar space is pretty dusty.

As for baseline, or near baseline humans, leisurely trips from Oort Cloud to Oort Cloud taking hundreds, or thousands of years would just require icy bodies for resources and shielding.

andy February 10, 2009 at 19:44

downloaded quantum DNA

DNA qubits

Sorry, do those things actually mean anything or are they just bunches of transhumanist buzzwords strung together to make it sound all techy? Seriously, WTF?

Darnell Clayton February 11, 2009 at 0:33

Wouldn’t it be simply easier if our species found a way to prolong life by a factor of ten? (or at least slowing the process of aging during the trip)

We then would not need to worry about traveling FTL and have to redesign ships to deal with particles that we could barely see with our eyes.

Adam February 11, 2009 at 2:29

Oh andy, don’t get nasty. He’s talking in Orion’s Arm speak which does at least try to be “hard” SF. But you’re quite right, it is nonsense to talk of DNA qubits AFAIK. There’s a really interesting set of interviews with Craig Venter and other luminaries of biological theorising worth downloading from the Edge organisation…

Life: What A Concept!

…which raised my eyebrows on the difficulties of kick-starting life from chemicals and made me question just how easy it will be to really synthesise life. John Hunt’s frozen embryoNauts might be the option for a while yet. But eventually I think coding life in more durable media will be needed for long-duration transfer of the relevant code with high-fidelity e.g. intergalactic travel.

kurt9 February 12, 2009 at 12:43

There is a more basic reason why 0.3c is the minimum velocity for an interstellar voyage. At that velocity, the travel time is short enough (10-20 years) that you can reach your destination before the boys back home develop a relativistic space drive or an FTL.

Rick S. February 12, 2009 at 14:28

I’ve brought this up before on a few sites that printed articles on intra/inter-stellar particles and cosmic rays affecting space travelers, so I’ll go at it again.

If one takes a toroid and extends it lengthwise (a tube), wraps conductive coils around the outside and inside you get a bar magnet. The magnetic field wraps comes out of one end and into the other end and travels through the middle of the toroid correct? Correct. Now if the the walls of the tube are thickened greatly one can build-out the inhabited areas within those walls.

The magnetic field would serve three purposes. The first is to shield the inhabitants from cosmic rays and charged particles. The second is to funnel these particles into the fusion reactor for fuel, thereby reducing the amount of stored fuel mass required for the journey. The third is to provide electricity (assuming there are more particles of one charge vs. the other in interstellar gas and cosmic rays) to the vessel for life support.

I would love to provide a drawing of such a vessel but cannot as it isn’t possible to submit one here.

Adam February 13, 2009 at 2:14

Hi Rick S.

A Solenoidal spaceship is an old idea as that’s basically what a Bussard ramjet would look much like. And, as any veteran around here knows, Bussard ramjets are very hard to get net thrust out of because of energy losses in compressing the incoming hydrogen for fusion. Bob Bussard himself believed that Daniel Whitmire’s suggestion of CNO catalytic fusion would have sufficient power levels for a practical fusion reactor size. But the drag issue is still unsolved and complex. Just how the hydrogen is captured and its kinetic energy is stored, then restored to it, is an open question. Room temperature superconductors with high current densities would be helpful, as would very high energy density capacitors.

Tobias Holbrook May 3, 2009 at 9:45

But what of it’s use as radiation protection? Would a Solenoid Ship protect it’s occupants from dangerous radiation, or attract it?

Positive Ions seem to be the most dangerous, so we need the shield to be positive.

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