Before we address the different ways a human being would grow old on an alien planet, we need to look at the simple consequences of leaving this planet in the first place. It turns out Father Time (at least as we know him) has a home address, and it's right here on Earth.

Outside the friendly confines of our home planet and our daily routines, time goes a little wonky. Every astronaut who comes back down to Earth is, for each day he or she has spent in orbit, actually 0.000023 seconds younger than a person born at the exact same moment who has never left Earth. Changing a person's relativistic reference frame in this way can have a cumulative effect. After spending a year on the International Space Station, a space traveler will gain back 0.0085 seconds of life that would have elapsed for someone living on the surface of the planet [source: NASA]. But don't start thinking that a life of interstellar adventure is your path to eternal youth -- in many ways, leaving Earth may have something of the opposite effect.

The passage of time affects the human body differently in space. Within the first decades of space travel, scientists learned that astronauts could lose as much as 1 percent of the density of load-bearing bones each month while in space. The lack of gravity keeps the body from strengthening its muscles naturally through resistance -- on Earth, even standing up is a kind of exercise. This means that space flight has some parallels with the natural aging process, which includes bone weakening and muscle atrophy [source: Bearden]. Space flight can also alter sleep patterns -- in addition to the general tension and excitement of being in space, astronauts orbiting the Earth see a sunrise every 90 minutes. This fitfulness mirrors the decreasing need for sleep and generally lighter sleep cycles that seniors experience [source: NASA].

Environmental conditions during travel are also a concern, because the nearest possibly habitable planet is Mars. Visiting Mars would involve astronauts being in interplanetary transit for a year or more -- which could include exposure to cancer-causing radiation. "We know how much radiation is out there, waiting for us between Earth and Mars, but we're not sure how the human body is going to react to it," says Frank Cucinotta of NASA's Space Radiation Health Project at the Johnson Space Center. "We can't yet estimate, reliably, what cosmic rays will do to us when we're exposed for so long" [source: NASA Science].

Even if astronauts were to reach Mars without any negative health effects, living conditions would be impossible without a triumph of engineering. Not only is Martian gravity little more than a third of what we experience here on Earth, Mars is also extremely cold and wrapped in a thin, low-pressure atmosphere, composed predominantly of carbon dioxide. "I think what people are not keeping in mind is what a truly horrible place Mars is," says astronomer Steve Squyres of Cornell University [source: Tyson]. There's no question that the raw Martian atmosphere is strictly uninhabitable for humans, and even with technological help, living there would probably be extremely hard on the body. Earthlings on Mars would also continue to suffer the deleterious effects of the small planet's weak gravity.

Traveling to Mars is logistically difficult, and returning home is even tougher, given the challenges of lifting off from the red planet and hauling all that fuel for the ride home (or manufacturing it on the Martian surface). Thus some scientists have proposed making our initial piloted journey there a one-way trip. "The amount of infrastructure, the amount of support that you have to put in place on the surface to sustain people indefinitely, I think may be prohibitively expensive," Squyres says, concluding that a one-way mission is "totally unrealistic" [source: Tyson]. It's fairly certain that more generations will reach maturity on Earth before we learn definitively how humans age on other planets.