Astronaut Scott Kelly sets record for most days in orbit

As of today, International Space Station Commander Scott Kelly has spent more consecutive days in space than any other American.

With 216 days in orbit, he surpasses Michael Lopez-Alegria, a Spanish-American astronaut who spent 215 days on the International Space Station from September to April 2006-07.

.@StationCDRKelly breaks the record today for the longest single U.S. spaceflight: https://t.co/vBFxauELaX pic.twitter.com/HEGn67SVFF

— NASA (@NASA) October 29, 2015

Kelly and his ISS companion, cosmonaut Mikhail Kornienko, are nearly two-thirds of the way through a year-long stay at the station. Although most astronauts only journey to the space station for four to six months, Kelly and Kornienko are part of a project which aims to study how the human body reacts to longer spaceflights. The project looks at the astronauts’ sleep patterns, exercise routines, metabolic rates and even measures pressure changes inside their skulls.

As part of the study, scientists are also comparing Kelly’s data to that of his earth-bound identical twin brother Mark Kelly — a retired astronaut. The comparison tests measure both physical and psychological differences in the twins over the course of a year. NASA will use the data in to determine whether and how to undertake longer spaceflights, including possible trips to Mars.

On Wednesday, Kelly celebrated his first spacewalk. He tweeted a picture of himself outside the station with the hashtag #SpaceWalkSelfie.

#SpaceWalkSelfie Back on the grid! Great first spacewalk yesterday. Now on to the next one next week. #YearInSpace pic.twitter.com/7qXLiKzaKA

— Scott Kelly (@StationCDRKelly) October 29, 2015

A few hours later, Kelly tweeted about his record-shattering day.

“It’s an honor to be here,” the tweet read. “I look forward to our progress in space as we continue to break these records.”