In article , R F L Henley wrote:
According to BBC Online:-

President George W Bush will announce proposals next week to send Americans
to Mars

but . . .

unless and until we have robotically established conclusively that there is
or is not life on Mars, we can't put humans on the planet because they will
inevitably bio-contaminate it.

Problem: It is, in theory, easy to prove there is life on Mars - you
find some (although doing this is difficult). It's next thing to
impossible to prove there isn't; even if you manage to examine a
statistically significant amount of the surface (and 'a few square
yards' don't really count...) you have to consider the prospects for
life in deep rifts, caverns, that sort of thing. As someone has pointed
out, since we started sending probes to Mars we've discovered two entire
sets of life we didn't think existed on *this* planet...

[I'm idly reminded of the /Mars/ trilogy; the protagonists find some
very scabby lichen at the bottom of a *probably* isolated deep
drillshaft, and can't tell if it's indigenous or introduced by them...
almost certainly the latter, but they just Can't Prove It. Oops.]

--
-Andrew Gray