When arguing for the human exploration and settlement of space, two standard counter arguments are:

1) Human physiology degrades under weightlessness.
2) The radiation dose experienced outside of the ionsphere due to a huge solar event can sometimes be fatal.

Both of these arguments are essentially bogus, e.g. you can provide artificial gravity, radiation shielding or minimize trip time. However it does not stop people who are opposed to Space Exploration from trotting out these same old arguments again-and-again-and-again because they're really just looking for any sort of technical argument to justify their position. It's a bit like saying that a manned Moon landing is impossible because the lander would sink in the lunar dust or that human beings would become so disorientated by weightlessness that they couldn't function. Any excuse will do if you're against Space Exploration.

Gary