When President Bush announced plans to send humans to Mars last week, he talked of weightlessness and radiation as being the key dangers. But there is increasing evidence to show that one of the greatest hazards lies in the crew itself. The hostile space environment and the hardware will, of course, be crucial factors in a Mars mission. But so will the software of our brains. The proposed expedition will require humans to spend longer in close proximity with each other in a confined capsule than ever before. Some psychologists believe an all-female crew would be best suited to such a mission.