CAMBRIDGE -- Back when Joseph E. Palaia IV and the former Melissa Blom were college sweethearts in New Jersey, at a time when most new couples in love think of being together, not apart, he told his future wife that if she wanted to be with him she'd have to let him leave for several years to live on Mars. ''In fact, it was a criteria. She needed to accept the fact that I'm going to Mars one day," Palaia recalls. ''This is who I am." Palaia is a 26-year-old MIT graduate student who recently cofounded 4Frontiers, a company dedicated...