Astronaut Scott Kelly returned to Earth about two inches taller than when he launched into space nearly one year ago, NASA reports. The sudden growth spurt was likely the result of living for almost 12 months in a microgravity environment.

Kelly has now spent more time in space than any other American. One of the reasons NASA is interested in long-duration human spaceflight is to study how people are able to deal with the rigors of living in space, as the agency prepares to send a human mission to Mars. The 340-day mission ended with the return to Earth of Kelly and Russian cosmonaut Mikhail Kornienko.

Scott Kelly also has an identical twin in the space program, Mark. By comparing the pair of astronauts, medical officials at NASA are able to determine which, if any, genetic changes take place during extended periods off the Earth.

"Kelly and Kornienko specifically participated in a number of studies to inform NASA's Journey to Mars, including research into how the human body adjusts to weightlessness, isolation, radiation and the stress of long-duration spaceflight. Kelly's identical twin brother, former NASA astronaut Mark Kelly, participated in parallel twin studies on Earth to help scientists compare the effects of space on the body and mind down to the cellular level," NASA officials report.

Like Scott, many astronauts on extended missions return home a little taller than they were at liftoff, as disks in the spinal column are no longer compressed as they are on the surface of our home world.

Spending a significant amount of time in the microgravity environment of space can also lead to weak bones and muscles, as space travelers do not need to walk or hold themselves up against gravity. Hearts are frequently weakened during long-term spaceflights, and the organ can even shrink as it does not need to work as hard in space as it does on Earth. Because a greater amount of blood than normal travels to the upper part of the body, astronauts on long flights often experience thinner legs and puffy faces.

While leaving the capsule on return to Earth, many astronauts report feeling disorientated and have trouble with balance. This is due to the effects of a changing gravitational environment on the mechanisms of the inner ear, including the labyrinth, responsible for helping to maintain balance.

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