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Watch live: NASA astronaut Scott Kelly launches into space for one year

On Friday, Scott Kelly was sent to the International Space Station on a Russian Soyuz rocket — the start of a yearlong mission that will see him become the first NASA astronaut to spend a whole year in space.

Here's a replay of the launch, from the Baikonur Cosmodrome in Kazakhstan:

Kelly was joined by Russia's Mikhail Kornienko — who's also spending a year on the ISS — and Gennady Padalka, who will return in September.

The extended stay is part of a project aimed at better understanding  how the human body holds up during a full year in microgravity— in order to better prepare for a potential mission to Mars in the 2030s. NASA astronauts typically spend six months on the space station, though three Russian cosmonauts did spend more than a year on the now-defunct Mir in 1980s and '90s.

Among other things, scientists will track Kelly and Kornienko's cellular metabolisms, the millions of bacteria that live inside their digestive tracts, and their cognitive health (with both MRI imaging and mental testing). As a control, NASA will compare Kelly with his identical twin Mark Kelly, a former astronaut who'll stay back on Earth.

Read more: No NASA astronaut has ever spent a year in space. Scott Kelly's about to change that.