Before sending humans to Mars, the frontiers of long-term space travel must be explored. Estimated flying time is at least a year in each direction, and the Soviet Union has the most first-hand knowledge of the demands involved. During the past two decades, the Soviets have tested the endurance of their cosmonauts on small space stations. Vladimir Tito and Moussa Manarov hold the record with a 366-day flight that ended in 1988. The longest manned American flight was three months.

``The Soviets have settled into a pattern of five-to-six-month flights,'' say Kelly Beatty, senior editor of Sky & Telescope. ``There seems to be a diminishing return in keeping people up much longer than that. It puts needless stress on the cosmonauts.''

According to Mr. Beatty, the Soviets have found ways to counteract the body deterioration supposed to occur in a weightless environment and now are studying the effects of long-term radiation.

Valentin Lebedev, who spent 211 days aboard Salyut 7, wrote of the mental challenges of life in space in his book, ``Diary of a Cosmonaut.'' While visiting Boston for the ``Soviet Space'' exhibit at the Museum of Science, he shared two points about crew selection for a Mars mission.

``If you have people with different interests - astrophysics, geology, biology - that would lead to a complementary situation. It would allow these people to develop and feel good about themselves throughout the whole flight.

``The members of the crew should also have good families because if they have problems at home that will disrupt relationships on the spacecraft. In their minds and their memories the crew members still live to some extent through their families. On their way back from Mars, they'll have a desire to get back together with them.''

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