Future astronauts sent on long duration flights to Mars or beyond will have to worry about more risks than merely launching into space and reentering planetary atmospheres safely.

Space radiation, prolonged exposure to weightlessness and the psychological impact of extended confinement inside a space ship are just some of the challenges detailed in Naked Science: Spacemen (9 p.m. EDT, National Geographic Channel).

While much of the material covered in Spacemen may seem old hat to dedicated followers of NASA's space exploration efforts, the program provides a basic primer of the fundamental obstacles facing astronauts in Earth orbit today and in the future.

The one-hour program centers around a fictional colonization mission to another world, necessary after our own Earth has grown too uninhabitable to sustain life, to convey its message. Such planets could be found by the Terrestrial Planet Finder - actually two different space telescopes which NASA hopes to launch between 2014 and 2020.

Spacemen touches on everything from antimatter and solar sails to controlled nuclear explosions - illustrated by a nifty video of NASA's Project Orion tests that used conventional explosives to lift a mock payload - as potential propulsion methods for a multi-generational spacecraft required to make the long trip between planets.

Meanwhile, protecting spacefarers during their flight and preparing them for life on an alien planet - including the development of more efficient space suits and shields against space radiation- are vital. Scientists are also studying measures to put humans in hibernation and even expand human life spans, which could prove critical to the centuries-long trip it would take to reach another Earth-like planet. Whether it would be better to sent small families or young couples on such long journeys are still up for debate.

"We don't want to have astronauts...reach their target and then not be able to perform," says Brookhaven National Laboratory's Marcelo Vasquez, who is working to develop better shielding against space radiation.

Among the more novel suggestions in Spacemen is the possibility of tailoring human space colonists to fit their new planetary homes.

Still securely in the realm of science fiction, such genetic manipulation could prove useful to adapt human lungs to differing oxygen levels or other environmental conditions. But researchers say such changes would be unique for each planetary expedition.

"Each group of people that goes to a different planet in a different solar system will become a different species," says geneticist Lee Silver of Princeton University.

Naked Science: Spacemen will air tonight at 9 p.m. EDT on the National Geographic Channel. Check local listings.