Mark and Scott Kelly are the subjects of the first twin study about space exploration.
Mark and Scott Kelly are the subjects of the first twin study about space exploration. Photograph by Marco Grob

After a record-breaking stint in space, astronaut Scott Kelly is finally coming home tomorrow.

Kelly and his Russian counterpart, Mikhail Kornienko, have spent nearly a year living in the International Space Station. Scott now holds the U.S. record for longest time in space (he’s logged over 500 days in total), and NASA’s hoping his long-term stay could shed light on what will happen when they send astronauts to Mars–a journey that could take 2 or 3 years, round-trip. Specifically, they’re hoping to learn how space affects a person’s eyesight, immune system, stomach bacteria, bone and muscle loss, mental health, and more.

Although other astronauts and cosmonauts have spent a full year in space before now, Kelly’s case is unique in that he has an identical twin brother remaining on the ground, making it easier for scientists to look for changes induced by the space environment.

The experiment isn’t over yet–doctors and scientists plan to do a lot more testing on the twins–but Scott Kelly’s trip is. On Tuesday evening he’ll return to Earth from the International Space Station, and you can watch it here:

Here’s the tentative schedule of livestream events, according to NASA:

  • 4:15 PM Eastern: Kelly and Kornienko say goodbye to the space station and settle into the Soyuz capsule that will carry them home. The hatch closes at 4:40PM.

  • 7:45 PM: The astronauts prepare to undock from the space station.

  • 8:05 PM: The Soyuz capsule undocks.

  • 10:15 PM: The spacecraft begins to leave orbit, entering the Earth’s atmosphere and landing at 11:27 PM.