CAPE CANAVERAL, Fla. What's one thing astronaut Scott Kelly can't do without when he moves into space this week for a year? A belt.

Kelly went beltless during his five-month mission at the International Space Station a few years back, and he hated how his shirttails kept floating out of his pants. So this time, the 51-year-old retired Navy captain packed "a military, tactical-style thing" that can hold a tool pouch.

Actually, scratch "pouch." He prefers "superhero utility belt."

Kelly's partner on the yearlong stay at the space station Russian cosmonaut Mikhail Kornienko can't do without his vitamins. When their Soyuz rocket blasts off from Kazakhstan on Saturday (Friday afternoon in the U.S.), three bottles of over-age-50 vitamins will be on board.

After more than two years of training, Kelly and Kornienko are eager to get going. It will be the longest space mission ever for NASA, and the longest in almost two decades for the Russian Space Agency, which holds the record at 14 months.

Medicine and technology have made huge leaps since then, and the world's space agencies need to know how the body adapts to an entire year of weightlessness before committing to even longer Mars expeditions. More yearlong missions are planned, with an ultimate goal of 12 test subjects. The typical station stint is six months.

"We know a lot about six months. But we know almost nothing about what happens between six and 12 months in space," said NASA's space station program scientist, Julie Robinson.

Among the more common space afflictions: weakened bones and muscles, and impaired vision and immune system. Then there is the psychological toll.

Russian cosmonaut Gennady Padalka, a frequent flier who will accompany Kelly and Kornienko into orbit, predicts it will be the psychological not physical effects that will be toughest on the one-year crew.

"Being far away from Earth, being sort of crammed, having few people to interact with," Padalka said. He'll break the record for most time spent in space during his six-month stay, closing in on a grand total of 900 days by the time he returns to Earth in September.

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US astronaut flying 'superhero utility belt' on 1-year mission; Russian packing megavitamins