Astronaut Scott Kelly heading home

NASA astronaut Scott Kelly, who returns next week after nearly a year aboard the International Space Station, has said that the secret to enduring the longest U.S. space flight is marking individual milestones, not ticking days off the calendar.

Since arriving at the space station on March, 27, 2015, Mr. Kelly and Russian cosmonaut Mikhail Korneinko have served with eight different crewmates, unpacked six cargo ships, weathered two botched supply runs and participated in dozens of science experiments.

Mr. Kelly also made three spacewalks outside the $100 billion station, which flies about 400 km above Earth, and Mr. Kornienko made one.

“I’ve tried to do this with a deliberate pace, looking not really at the end, but at the next milestone,” Mr. Kelly told reporters during his last in-flight press conference on Thursday.

“I could go another 100 days; I could go another year if I had to,” he added.

Mr. Kelly and Mr. Kornienko’s stint — about 340 days — is intended to provide medical and engineering information to prepare for three-year missions to Mars.

Four Soviet-era cosmonauts flew longer missions aboard the now-defunct Mir space station. The longest flight was a 437-day mission between January 1994 and March 1995 by cosmonaut Valeri Polyakov.

“Physically, I feel pretty good,” said Mr. Kelly, 52, a veteran of three previous space flights. He said the hardest part of being in space for so long was being separated from his friends and family. Mr. Kelly’s medical tests will continue for months after his return. His identical twin brother Mark Kelly, a former NASA astronaut, is participating in a series of related studies looking for genetic changes caused by the high radiation and weightless environment of space.

The astronauts are due to depart the station on Tuesday. They will land in Kazakhstan.