The difficulties.

One difficulty is psychological. Astronauts would have to live together, in a tiny space, for over a year. The living conditions would be cramped, everyone would be in everyone else’s way, and there is a real chance that that the crew couldn’t live with each other.

The other hard part of going to Mars is surviving the radiation that you’ll pick up over the course of eighteen months in space. When the sun flares, which it does periodically, a lightly protected human could pick up a lethal dose of radiation.

“Lightly protected” in this context means a few millimeters of metal and plastic. That’s about what the walls of a spacecraft are made of, and you can’t afford to make them any thicker, or else the craft becomes too heavy and expensive to boost. Remember that the part of the rocket that moves fast enough to get out to Mars is just a tiny fraction of the whole. Thus, for every pound of shielding you add, you need to add hundreds of pounds of booster rocket and fuel.

So, weight is at a premium, but radiation cannot be ignored. The conventional solution is to have a small, heavily-shielded closet on the spacecraft that people can retreat to, during a solar flare. But, even with the heaviest closet that one could imagine launching, the radiation dose during a trip to Mars will be large.

There is a solution:

• Find a rock that goes on a suitable orbit between Earth and Mars. We want a smallish rock, just a few meters across.
• Intercept it and break it to fragments.
• Then, your astronauts rendezvous with it, and (carefully) set up living quarters amidst the rubble. The rock provides shielding against radiation.

The advantage of this proposal is that you can have thick shielding over a large area, because you are not paying to lift the shielding from Earth: it is in orbit already.

Lots of shielding mean that radiation isn’t a problem, and you may be able to make the crew quarters big enough so that the astronauts could tolerate each other.

Is it possible?

Does such a rock exist?

Probably. There are thousands of asteriods that are bigger than a kilometer that cross the Earth’s orbit, and small rocks are vastly more common. Almost certainly, there are many rocks going from Earth’s vicinity to Mars’s vicinity. As yet, we don’t have a suitable telescope to find the right rock, but we know how to build it. The Large Synoptic Survey Telescope is almost the right telescope.

How do you break the rock?

A good question. Probably explosives and a tough net. This would be an interesting challenge in space engineering.

What do you put inside?

It the right approach may be to inflate a tough balloon inside the rubble. (This would be something like the TRANSHAB design studied in L.C. Simonsen, J. W. Wilson, M. H. Kim and F. A. Cucinotta Health Physics 79(5) (2000) 515.) Inside, you wouldn’t have to worry about radiation or meteroids, and you might well be able to shield a comfortably large volume.

This could help a lot with the psychological aspects of long-term space flight. Astronauts might even be able to have something like a room of their own.

How thick a layer of rubble do you need?

Interestingly enough, thin shields can actually add to the radiation damage experienced by the crew. Many cosmic rays have energies over a gigavolt, and they go right through the astronauts and out the other side. On the way through, they cause damage, of course, but not all their energy gets deposited in the human. However, if one puts a thin shield in the way, especially if it is made of elements with high atomic numbers, like lead, a cosmic ray can hit the nucleus of a lead atom and blow it into fragments. The astronaut is then hit by a number of fragments and more total damage is done.

So, the crew’s radiation exposure is a complex balance between stopping some of the cosmic rays and generating more, lower energy radiation by fragmenting atomic nuclei in the shield. The appropriate literature to look at is then mission plans for permanent lunar bases, which can be assumed to be shielded by many centimeters of lunar soil. Lunar soil is similar enough to asteroid rubble, and the radiation environment on the lunar surface is not too different from what a spacecraft would experience on route to Mars (except that the lunar habitat doesn’t get radiation from below). Comparison with published designs of lunar habitats (Lisa C. Simonsen, Chapter 4, pages 43-77 in Shielding Strategies for Human Space Exploration, NASA conference publication 3360, edited by J. W. Wilson, J. Miller, A. Konradi, and F. A. Cucinotta) suggests that about 2 meters of rubble would provide sufficient shielding.

Assuming a 2 meter thick shield where the rubble is half-rock and half empty space, a 5 meter diameter rock would produce enough rubble to shield a spherical crew compartment that is about 4 meters in diameter. A 10 meter diameter rock could shield a 12 meter diameter crew compartment.

Note: Stephen Ashworth seems to have come up with essentially the same idea. See “Transport for Areopolis”, presented at the British Interplanetary Society symposium, 19 November 2008. Also see Maurizio Morabito’s blog for a discussion. Thanks to Mark Hempsell for pointing out an error in my rubble thickness computation.

Riding a Rock to Mars by Greg Kochanski is licensed under a Creative Commons Attribution 2.0 UK: England and Wales License Based on a work at kochanski.org . Copyright 2008 Greg Kochanski. 29 November 2008. (Originally posted 29 November 2008, details added 4 December.)