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Retired astronaut Mark Kelly and his twin brother, U.S. astronaut Scott Kelly, will be participating in a NASA study, focused on the way extended periods of weightlessness affect the human body in space. NASA TV

NASA will use identical twin astronauts as subjects for a series of tests of how living in space affects the human body.

Retired astronaut Mark Kelly -- better known as the husband of former Rep. Gabrielle Giffords -- and his astronaut brother Scott Kelly crafted the idea on their own, pitching it to NASA, reports Florida Today. Scott will launch into space in March 2015 for a mission on the International Space Station that will last a year. Mark will remain on Earth, allowing scientists to compare a host of medical data and samples taken from the two brothers.

“As currently conceived, this project will center on established plans for blood sampling on the flying twin at regular intervals before, during and after the one-year ISS mission, and will obtain corresponding samples from the nonflying twin, who will otherwise maintain a normal lifestyle," said NASA in a solicitation for experiment proposals.

The tests will take into account other samples from the astronauts such as blood, cheek swabs and saliva in addition to psychological and endurance tests. According to the solicitation, the tests will continue only if they “illuminate one or more aspects of transient or long-term effects of spaceflight on humans” and don't affect the main research of the study.

The twins' experiment is a part of a larger initiative to better understand the effects lengthy stays in weightlessness have on the human body. NASA and 15 international partners are using the space station for a flurry of studies on the topic, reports USA Today. This information will be used to improve measures to protect astronauts on longe missions to places such as Mars or the moon in the future.

The 49-year-old twins are the only siblings to have ever flown in space.

Scott Kelly has participated in two space shuttle flights and Mark Kelly has participated in four.

NASA is currently calling for proposals for experiments that can be conducted on the brothers. The link to the project -- titled "Differential Effects on Homozygous Twin Astronauts Associated with Differences in Exposure to Spaceflight Factors" -- was shared by NASA Watch.

Proposals can be submitted through Sept. 17.

NASA