New Scientist reports that cosmic radiation would be too dangerous for NASA to send people to Mars.
NASA's current rules on risk aim to keep each astronaut's lifetime risk of radiation cancer to below 3%. That limit would be reached in under 200 days, but a round trip to Mars would take 750 days.
Read the report here.
It would be relatively easy to shield a spacecraft from the sun's radiation. But galactic cosmic radiation, comprising "protons and heavier atomic nuclei" has higher energy than the sun's, and can cut through DNA in living cells, which damage can lead to cancer. On Earth, we are protected from such bombardment by both atmosphere and the Earth's magnetic field.
It does seem that the particular set of circumstances that fostered evolution and maintains life on Earth is a rare contingency, requiring the right combination of atmosphere, magnetic field, distance from the sun, type of sun and type of planet - even plate techtonics contributes to the ongoing habitability of the planet.
The piling up of such factors could help explain the lack of success in the Search for Extra Terrestrial Intelligence project. It remains inconceivable that those factors could not arise repeatedly elesewhere, but the very delicacy of balance of all these factors is a plausible explanation for why we have fould no near neighbours.
Postscript 20-Oct-09: I have come across two memes that claim to speak to this. First, the Drake Equation, which purports to estimate the number of civilizations in our galaxy (the Milky Way) with which we could establish contact. Seven factors are included, including the rate of star formation and the proportion of life-potential planets that go on to develop life. A current estimate of the solution to that equation is 2.31; however, the equation (and estimates of factors) must be seen as so conjectural that to my mind it's little more than a philosophical exercise (or something akin to economists being asked to estimate something they know very well they don't have enough information for).
There is also a claim that there are "20 factors" necessary for the emergence of life with the complexity that we know. However, I have not found the origin for this meme, and it's debated more in circles religious (both Christian and Muslim) than logic, scientific, or mathematical - invariably to "prove" the small contingency of life. Still, there are necessary factors, and they're worth considering - albeit some of them surely overlap in terms of contingency. For the purposes of debate, some of those mentioned include:
- a liquid iron planetary core (to provide a magnetic field that shields us from some cosmic radiation);
- a moon to pull tides (and circulate oceans) - (how necessary?);
- the sun's composition;
- the planet's distance from the sun;
- distance from the centre of the galaxy... etc.
A scientific enumeration (and discussion) of such a list would be interesting to read (factors in Drake's equation are rather more broad - and conjectural - than these).
Yet I'd have to point out that at least some of these factors only pertain to our version of life. It is hard for us conceive of life emerging in radically different form (and I'm not talking SF bugs or tentacled aliens: more, different formations of cells, etc), but that doesn't mean it can't happen.