Two astronauts.

Astronaut Scott Kelly and his identical twin brother Mark.

Credit: NASA

On 2 March this year, two long-term International Space Station residents returned to Earth. Scott Kelly and Mikhail Kornienko both spent 342 days in space. Their year-long mission included space walks and Earth photography. But the most important experiment was on themselves. How does a person react to a year in space?

Throughout their expedition, scientists conducted many experiments on the duo, checking reaction times and making sure they were still healthy. But while Scott and Mikhail were floating in space, there was also a third participant.

Scott’s identical twin brother Mark stayed at home on Earth throughout the mission. This means there was one brother in space and one on the ground. For the first time, scientists had a regular Earth-dweller acting as a control for their experiments.

One change was very noticeable to both Mark and Scott. The two brothers used to be about the same height. But upon his return, Scott was about five centimetres taller than his twin. Floating weightless, the joints in Scott’s spine relaxed and stretched out. Meanwhile, Mark’s spine was compressed every day by the weight of his own body. However, it took Scott just two days to return to his usual Earth height.

There is still a lot to learn about long space expeditions. Even a fast trip to Mars takes a year to get there and back. We need more research before sending people on such a long and dangerous mission.

More information

NASA’s One Year Crew homepage
Try this experiment to see if your spine gets longer and shorter

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