In most sci-fi movies, the problem with the long time periods required for interstellar travel is solved by putting those traversing the stars into some kind of "stasis," often in a comfy pod. Upon waking, they rub their necks, stretch their arms, and within about 30 seconds are good to go fight aliens, despite having been lying still in a pod for the last couple of months or years.
Unfortunately, it looks like that's not really how things are going to work:
The first cellular analysis of muscles from astronauts who have spent 180 days at the International Space Station shows that their muscles lost more than 40 percent of their capacity for physical work, despite in-flight exercise.
No matter how good their shape was before the astronauts left, they returned with muscles tone that resembled that of the average 80-year-old. In fact, the astronauts who were in the best shape before they launched were the most likely to come back with withered, or atrophied, muscles.
NASA currently estimates it would take a crew 10 months to reach Mars, with a one year stay, and 10 months to get back, for a total mission time of about three years. These studies suggest they would barely be able to crawl by the time they got back to Earth with the current exercise regime.
Well that's a bummer. The folks at NASA are optimistic that with the right exercise machines, they can overcome this problem. But it looks like when the day comes that we need to flee our dying planet to get to the newly terraformed Mars, we're going to be a bunch of flabby puddles of meat when we get there. Time to get working on that quantum teleportation device.
-- Paul Waldman