Scott Kelly and ISS crew head home after record-setting mission in US spaceflight
Scott Kelly inside the Cupola, a special module which provides a 360-degree viewing of the Earth and the space station, July 12, 2015. REUTERS/NASA

NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko headed back toward Earth on Tuesday after nearly a year aboard the International Space Station, ending a record-long U.S. spaceflight intended to pave the way for human travel to Mars.

The men, accompanied by Russian cosmonaut Sergey Volkov, strapped themselves inside a Russian Soyuz capsule and departed the station at 8:02 p.m. EST (0102 GMT on Wednesday).

They are expected to make a parachute landing in Kazakhstan at 11:25 p.m. EST (0425 GMT on Wednesday).

Kelly and Kornienko have been aboard the space station for 340 days, about twice as long as previous crews. Their flight sets a record for the space station and for the longest U.S. space mission.

The men hugged their crewmates goodbye and floated inside their Soyuz capsule at 4:43 p.m. EST (2143 GMT), along with Volkov, who has been in space for 5-1/2 months.

"The journey isn't over," Kelly posted on Twitter before leaving the station. "See you down below!"

Throughout their nearly year-long stay in space, Kelly, 52, and Kornienko, 55, have been the subjects for dozens of medical experiments and science studies designed to learn more about how weightlessness and the high radiation environment of space impact the human body.

The research is intended to help the U.S. space agency and its partners develop plans for eventual human missions to Mars that will last at least two years.

Kelly, along with his identical twin brother Mark Kelly, a former NASA astronaut, also are participating in genetic studies, the first to assess if genetic changes take place during long spaceflights.

Kelly’s 340-day mission eclipses the previous U.S. record-long spaceflight of 215 days set by former astronaut Michael Lopez-Alegria aboard the space station in 2007.

The world’s longest missions were made by four Soviet-era cosmonauts aboard the now-defunct Mir space station, including a January 1994 to March 1995 flight spanning nearly 438 days by record holder Valeri Polyakov, a physician.

The International Space Station, a joint project of the United States, Russia, Europe, Japan and Canada, followed Mir and has been permanently staffed by rotating crews since 2000.

About the size of a five-bedroom house, the $100 billion station flies about 250 miles (400 km) above Earth.

(Reporting by Irene Klotz; Editing by Grant McCool and Lisa Shumaker)