The Speculist

Phil,

I was a naysayer about push prizes until the X-prize was won two years ago. The second big blow was the DARPA grand challenge for automatically navigated vehicles in the desert.

I've said it before, and I'll say it again: we need a huge push prize for either or both of the Moon and Mars exploration. If I were Bill Gates, or at least had his money, I'd offer $10 billion of my personal money for such a purpose.

Any comments on the recent Scientific American article on the difficulties of protecting astronauts from cosmic ray damage?

Larry --

Cosmic radiation is the elephant in the room for any discussions about long-term space travel. We may not be able to get much of anywhere in space until nanotechnology pushes us up a few more rungs on the materials science ladder.

Ok, I don't understand the cosmic ray problem. Apparently, a good chunk of cosmic rays are charged and can be diverted by magnetic fields which is partly why it's less of a problem for LEO astronauts. Why isn't this considered practical in a deep space environment? Is the problem that you'd need powerful (or at least large volume) magnetic fields (and hence substantial extra mass) to divert charged ions enough? Or perhaps that even with the reduction, you get too much of a radiation dose?

Incidentally, according to this 2003 Sky and Telescope story, astronauts would receive roughly 120 millirem of radiation per day from cosmic rays. Apparently, three years of that is within NASA's "career limit" for astronauts.

Here is the overview from the article:
A spherical shell of water or plastic could protect space travelers, but it would take a total mass of at least 400 tons—beyond the capacity of heavy-lift rockets.

A superconducting magnet would repel cosmic particles and weigh an estimated nine tons, but that is still too much, and the magnetic field itself would pose health risks. No other proposed scheme is even vaguely realistic.

Biomedical researchers need to determine more precisely how much long-term exposure to cosmic rays a person can tolerate and whether medicines could stimulate the body’s natural repair mechanisms.

On dosages, they write:

The implications were recently studied by Wallace Friedberg of the Federal Aviation Administration’s Civil Aerospace Medical Institute in Oklahoma City and his colleagues. In a report published last August, they estimated that Mars astronauts would receive a dose of more than 80 rems a year. By comparison, the legal dose limit for nuclear power plant workers in the U.S. is five rems a year. One in 10 male astronauts would eventually die from cancer, and one in six women (because of their greater vulnerability to breast cancer). What is more, the heavy nuclei could cause cataracts and brain damage. (To be sure, these numbers are highly uncertain.)

The constant hailstorm of cosmic rays is not the only radiation threat, of course. The sun, too, can unleash tremendous bursts of protons and heavier nuclei traveling at nearly the speed of light. Such bursts occasionally deliver in excess of a couple of hundred rem over an hour or so—a lethal dose to an unshielded astronaut. The great fl are of February 23, 1956, is a notorious example. Whatever measures are taken to ward
off cosmic rays should also protect against these solar tempests. Even so, it might be wise to schedule a trip to Mars during the years of minimum solar magnetic activity.

Larry:

A space elevator would completely change the equation.

It would make getting that much water into space doable. You could shield the entire living quarters in ice, and also use it as a water source, oxygen source, and fuel source.