Astronauts and cosmonauts have lived in space for prolonged periods in the past, but the numbers on any one stint have been few. When the station is completed, crews of seven will serve tours of duty of up to 180 days. And proposed missions to Mars could take two and a half years to complete. Naturally, sexual behavior might occur on such long missions. It is a topic, however, that makes NASA publicists uneasy--as if the issue could somehow make astronauts seem to have less of "the right stuff." (Rumors of unofficial orbital couplings abound, but no one is talking.) Yet sexual tensions could affect crew performance and thus mission success. "It's just one more problem that can potentially cause the whole thing to come apart," says retired astronaut Norman E. Thagard. Of course, sex is only one of the psychological components facing a space crew; isolation, loneliness, bickering, and habits of colleagues can come into play. Besides space living, ...