(CBS SF) — NASA is set to carry out its longest mission ever in an effort to study the effects of space travel on the human body.

Twin brothers and NASA astronauts Scott and Mark Kelly are the subjects of the study.  Scott will take off in a Russian rocket Friday afternoon for the International Space Station, while Mark undergoes tests to help compare the aging effects on the two 51-year-olds.

NASA Ames Research Center in Mountain View trains astronauts for missions like this one, and will be studying the results carefully.  The study will help NASA answer the questions of whether the human body can withstand the effects of space travel long enough for missions to Mars or Europa.

The study will last one year, making it one of the longest stints in space for any astronaut.