“Interstellar travel may still be in its infancy,” write Gregory Matloff and Eugene Mallove in The Starflight Handbook (Wiley, 1989), “but adulthood is fast approaching, and our descendants will someday see childhood’s end.” The echo of Arthur C. Clarke is surely deliberate, a sign that one or both authors are familiar with Clarke’s 1953 novel about the end of human ‘childhood’ as we learn about the true destiny of our species in the universe. But becoming a mature species isn’t easy, nor is figuring out interstellar flight.
Awash in Hard Radiation
Consider just one layer of complexity. Suppose we somehow discover a propulsion system that gets us to relativistic speeds in the range of 0.3 c. That seems a minimum for regular manned starflight given the times and distances involved, but suddenly attaining it doesn’t end our problems. Interstellar space isn’t empty, and when we accelerate to cruising speed at a substantial percentage of the speed of light, our encounter with interstellar gas becomes a nightmare. Indeed, this haze of gas between the stars acts as a flow of nucleonic radiation bombarding the starship as we push ever higher into relativistic realms.
Image: As a playful example of science fiction mixing with science, this photo shows the luminosity from hot gas used in a hypersonic “super-orbital expansion tube X2” test rig at the University of Queensland, Australia. A toy model of the fictional starship Enterprise is subjected to a Mach 5 flows. (Credit: Tim McIntry, Queensland Physics Department/Marc Millis).
And let’s not forget high-energy cosmic rays and dust, all of which demand protection. Because sensitive electronics are susceptible to damage as well as humans, we have to work out the hazards whether our mission is manned or not. A non-relativistic capsule moving in interstellar space would, according to Oleg Semyonov (State University of New York at Stony Brook), experience a radiation dose of 70 rems per year, while the safety level for people is considered to be between 5 and 10 rems in the same period. Going relativistic drives the dosage level to far greater extremes.
Did I say ‘greater’? Try this much greater: Thousands or hundreds of thousands of rems per second, comparable to conditions in the core of a nuclear reactor. All this from a ship moving at high relativistic speeds through interstellar gas. But even slower velocities are a problem as we move through this medium.
Puzzling Out Shielding Options
Semyonov plots the radiation involved from encountering interstellar gas versus velocity and finds that at speeds much above a comparatively sedate 0.1 c, an astronaut could not be outside the hull without layers of shielding. Shielding the entire ship is problematic. A radiation-absorbing windscreen installed in front of the vehicle is possible, a titanium shield of 1-2 cm workable up to 0.3 c but becoming ‘dramatically thicker with acceleration.’
Water? It’s not a bad idea because the crew needs water anyway:
Placing a water tank (or an ice bulge) in front of a ship is advantageous in comparison with a shield made of metal or another solid material because it eliminates the damaging embrittlement of solids under intense nucleonic radiation; for a given cruising speed, the penetration depth of monoenergetic nucleons will be the same and a layer located near the penetration depth inside a solid shield will be largely damaged because all the nucleons deposit the bulk of their kinetic energy at the end of their penetration depth dislocating atoms from the lattice, weakening the material, and causing peeling or flaking.
On the other hand, our water shield adds significant mass to the vessel, and at speeds close to that of light, it would need to be tens of meters thick to form an effective barrier. So we can contemplate titanium or aluminum hull shielding up to about 0.3 c, but 0.8 c demands several meters of titanium or the water barrier.
The Cosmic Ray Hazard
Cosmic rays are a hazard to any interstellar mission, relativistic or not. Water is again an option, but Semyonov notes that a ship would require a ’round shell of water of 5 m in thickness,’ a huge increase in mass, and still insufficient for absorbing the highly penetrating secondary gamma and muonic radiation that will bathe the ship, demanding an additional shield of its own. Usefully, cosmic rays become increasingly beamed as we increase velocity, so that a frontal shield for interstellar gas can also absorb them.
Isotropic cosmic rays are subjected to relativistic beaming when a starship is moving with a relativistic speed. For the ship’s velocities closer to the speed of light, most of cosmic rays form into a beam directed toward the front of the spaceship. While they do present a hazard, they can be easily absorbed or deflected by a frontal shielding system that is required anyway protecting the crew and electronics against the hard radiation of the oncoming flow of interstellar gas. Cosmic dust will also contribute to the radiation hazard, because the dust particles are actually lumps of high-energy nucleons at relativistic velocities. A serious problem will be the sputtering of a ship’s bow or a radiation shield by the relativistic dust particles. Nevertheless, the shielding of relativistic starships from hard ionizing radiation produced by interstellar gas and cosmic rays does not seem to be far beyond existing technology.
In other words, if we can figure out the key question of propulsion, we should be able to overcome the shielding issue. Semyonov considers a range of options including combinations of material and magnetic shielding in arriving at this conclusion. His discussion is a wide-ranging and sobering reminder of how many barriers interstellar flight presents beyond tuning up the right kind of engine. Childhood may end, but we biological life-forms remain fragile creatures indeed when flung into the interstellar deep.
The paper is Semyonov, “Radiation hazard of relativistic interstellar flight,” Acta Astronautica 64 (2009), pp. 644-653.
Comments on this entry are closed.
According to this paper, even a starship in a warp bubble
would be full of Hawking radiation:
How about encasing a starship inside a comet? Lots of
frozen water in the outer Sol system for just such a mission.
The paper brought up by ljk also mentioned how the warp bubble would become rapidly unstable once superluminal speeds are reached. Seems like we need both a quantum certainty stabilizer and some really efficient shielding targeted to heat and Hawking radiation to send biological beings through the light barrier.
In the paper we read “we find that in the center of the bubble there is a thermal flux of particles at the Hawking temperature corresponding to the
surface gravity of the black horizon.”
That is a lot of energy. Why not use it for work?
So to address the radiation, how about an efficient way to convert this radiation into energy redirected to shielding the ship? Give the system some pretty good overhead capacity so that it provides its own feedback loop.
Alternatively, use the H radiation to assist in powering the system itself. Design the ship magnetically so that all planck particles are redirected into raw materials for thrust instead of colliding with our delicate cells. That way, the energy used to create the bubble is mostly needed at the front-end, and the resulting radiation contributes to fuel it’s continuation. Pull the plug at the destination, and the bubble collapses.
Just me speculating. I don’t have math to back any of this up but it’s fun to talk about.
Jeff, Hawking radiation takes energy out of the horizon, so what the quantum “leakage” means is that the warp would rapidly decay into a spray of “Planckons” – particles at Planck energy. You couldn’t recycle it especially since every particle packs about 2 gigajoules of energy.
Interestingly there’s no Hawking radiation if the warp remains below lightspeed, so potentially a warp-drive would make a very high acceleration sub-light drive – if we can make the negative energy needed. With the original warp-drive the Casimir effect was used and that necessitated confining the negative energy to a very thin layer – which is what causes the Hawking radiation to have a ridiculously high temperature. If the negative energy could be spread over a metre or so, then the temperature drops to very, very low levels. But how do you make negative energy?
BTW the sublight warp-drive is also a good protection against radiation and debris according to a paper from a couple of years ago.
Just a thought. With protection against the interstellar medium of whatever sort on the front of the vessel what happens when it turns round to decelerate? The deceleration effector (whatever it is) would presumably have to work without a shield in front of it so what protection could then be provided to the vessel?
A frozen embryo interstellar mission (EGR) overcomes nearly all of these problems.
Traveling at sub-relativistic speeds, the following issues become much smaller:
– impacts from dust,
– nucleonic radiation flow,
– the amount of energy needed to accelerate a much reduced mass,
– the cost of launching a large mass craft to LEO,
– convincing the government to pay for launching a massive interstellar craft to LEO,
– needing significant space-based infrastructure to power beams,
– life-support systems in transit, and
– getting a safety/ethics panel to approve the transit portion of the mission,
My point is simply this. Get away from relativistic-speed missions and the difficulty of star flight becomes much smaller.
If such an interstellar mission were to last 2,000 years instead of 43 years then an EGR mission would be exposed to about 46 times the cosmic radiation. But the 5-10 rem/yr safety level is probably based mostly on the risk of developing cancer. Given that there are about 50 trillion cells in the human body and only a few cells in an EGR mission, then a 70 rem/yr exposure means that the odds of a frozen embryo being damaged may be small.
But, if protection is needed then a 27 Tesla (540,000 times the Earth’s magnetic field), low-powered, persistent superconducting magnetic field could reduce the rem exposure towards acceptable levels. Similarly, since an EGR mission is dealing with single cells which can replicate, the problem could be addressed on the other side; namely using spermatogonia tissue culture, super anti-oxidant, or a deinococcus radiodurans approach to preserving the DNA sequence.
Also, I understand that the secondaries produced by shielding may pose a greater problem to cells than just suffering a direct hit from cosmic rays and then letting the bulk of the secondary energy pass out to the other side of the ship.
My brother John once brought to my attention the concept of using an atmosphere that would be contained within a cone or cylindar wherein accelerations of one or more Gs would keep the atmosphere within the cone or cylinder.
The cone would be located in front of the ship and would act as a radiation shield and as a means to burn up small dust particles at relativistic velocities.
The shield could also be used to collect interstellar mass which might somehow be processed in exothermic nuclear or sub-nuclear reactions for the purposes of propelling a highly relativistic starship closer to C. If the reactions can be made to power a very efficient photon rocket which has an exhaust stream of C, then perhaps the jet engine analogue of a perfectly efficient matter antimatter rocket can be achieved providing we find a way to break matter down into pure electromagnetic energy.
Given the very numerous branching ratios for decays of the known unstable leptons, fermions,hadrons, mesons, baryons etc, perhaps there is some decay sequence yet to be discovered and/or hidding in the midst of the complexity of known decay sequences which can be used to efficiently convert matter to antimatter and then use such reactions to power a highly directional photon rocket.
But before we can do any of this we need a way to shield craft traveling at high gamma factors and a way to efficiently extract mass from the interstellar or intergalactic medium. Once we can do that, perhaps the sky is the limit for high gamma factor craft.
I do not claim to be the originator of the above shield concept, but it seems like a cool idea.
It’s obvious to anyone who watches Star Trek (The Original Series) that your warp-capable ship needs a functioning deflector shield before it can go to warp. The deflector is capable of deflecting asteroids as well as smaller particles, including particles of highly energetic radiation. We need to get to work on that before we build the warp engine.
Where’s Scotty when we need him?
Chris asked about protection during decceleration. One thought is that the plasma exhaust of an interstellar reaction drive would make for quite a formidable shield against material and ions. During decceleration it’s firing along the direction in question.
Alternatively there’s Charles Pellegrino & James Powell’s “Valkyrie” design which employs a large multi-layer series of Whipple shields and a liquid droplet debris cloud defense system. Check out the graphics on Charles’s web-site…
…there’s a clearer view on Winchell Chung’s page here…
…though I’m not sure why the droplet cloud is operating during decceleration in the reverse direction.
You could fire a laser ahead of you or a particle beam (possibly charged would be better) to encourage the material ahead of you to move away before the main craft passes through