Scott Kelly being helped by ground personnel in Kazakhstan

NASA's website, quoting administrator Charles Bolden, says, "Scott Kelly's one-year mission aboard the International Space Station has helped to advance deep space exploration and America's Journey to Mars".

Before Kelly and his crew took off from the ISS in the landing capsule that would bring them the three-and-a-half hour journey back to Earth, he snapped a picture and posted it on Twitter.

The lengthy sojourn was part of NASA's One-Year Mission, an experiment on the effects of long stays in space on the human body.

Before landing, Cmdr Kelly told reporters during a live link-up: "Physically I feel pretty good... but the hardest part is being isolated in the physical sense from people on the ground who are important to you".

The research aims to help the USA space agency and its partners develop plans for eventual human missions to Mars lasting at least two years.

Kelly surpassed the previous record for time spent in space by a USA astronaut on October 16, 2015.

During his highly publicized mission, Kelly also did an incredible amount of public outreach.

NASA astronaut Scott Kelly is closing the door on a space mission that has spanned a U.S.-record 340 days.

The record-breaking pair, however, had launched into space on 23 March last year to embark on what would be called the year-long mission, despite the fact that they spent just a total of 340 days in space.

Kelly returned to earth via the Russian Soyuz spacecraft.

Kelly and two Russian cosmonauts are now inside the Soyuz and the hatch has been closed.

During his final news conference from space last week, Kelly said NASA will put him through a battery of tests as soon as he returns to measure any changes in his health.

Kelly, who handed command of the International Space Station to NASA astronaut Tim Kopra Monday, admitted that he had mixed feelings leaving his orbital home'.

The ISS has been inhabited continuously since November 2000, welcoming more than 200 visitors to enjoy its outer-worldly view.

The space explorer landed in Kazakhstan at 11:26 p.m. ET on March 1 after spending almost a full year on the International Space Station in an effort to understand how the human body responds to long periods of weightlessness.

Though Kelly is the only American astronaut to have stayed in a spaceship for this long, this was not the longest space sojourn in history.