According to Elbert Einstein's theory of relativity, if an astronaut travels in the space at a very high velocity, his time will flow slower according to an observer on the earth. And after a long time of such space travel, when this astronaut returns to earth after the trip, the people left on earth will find the astronaut younger than he should be.

However, Frank Cucinotta, the chief scientist for radiation studies at the Johnson Space center, doesn't think this would really happen. He pointed out that the strong radiation in space would accelerate the aging process of the astronaut by damaging the telomeres on the chromosomes of the astronauts faster than on the earth.

Till now, this potential risk has not become noticed, since the astronauts have been traveling inside the protective magnetic field of the earth, which is capable of deflecting most cosmic rays. But by the year 2018, NASA will be sending humans to the moon, to get to Mars eventually. They will be out of the protection of the earth's magnetic filed by then, and NASA will try to figure out the "radiation aging" effect does exist or not, and carry out countermeasures if necessary.