American astronaut Scott Kelly and cosmonaut Mikhail Kornienko of Russia are concluding a yearlong mission in space, orbiting aboard the International Space Station (ISS). Although this stay, lasting 340 days, was the longest since humans began permanently occupying the orbiting outpost, this was just the fifth-longest space mission yet undertaken.

Vladimir Titov and Musa Manarov became the first humans to spend more than a year in orbit after launching into space in December 1987. Their flight lasted a total of 366 days before the Soviet cosmonauts were brought back to Earth.

In January 1994, Valeri Polyakov arrived at the Mir space station, where he would spend the next 437 days. This marathon space mission remains the longest stay in orbit yet carried out by any human traveler. Polyakov finally returned to Earth in March 1995.

Sergei Avdeyev lifted off to spend six months aboard Mir, but after another crew member's schedule was shifted, the cosmonaut was required to spend 379 days aboard the orbiting outpost. Space travelers move ever-so-slightly into the future, due to the effects of relativity first deduced by famed physicist Albert Einstein.

Avdeyev took three trips aboard Mir, accumulating 748 days aboard the space station. During these two years, he traveled at a velocity of 17,500 MPH while riding aboard Mir, moving 0.02 seconds into the future. This is the greatest amount of time ever recorded for a human experiencing the weird effects of time dilation. However, when he returned from his final mission, the body of the cosmonaut had weakened so much in the microgravity environment that he was taken out of his capsule aboard a stretcher.

The 53-year-old Scott Kelly is a veteran of three space missions before lifting off on his latest flight.

"The landing of Expedition 46 marks the end of a year in space for Kelly and Kornienko. This is the first time that extensive research using exciting new techniques like genetic studies has been conducted on very long-duration crew members," NASA officials reported in a blog detailing activities conducted aboard the ISS.

As NASA gears up to send humans to Mars in as little as 20 years, further additional long-term spaceflights will be needed to test how equipment — and humans — respond to months or years in space. Muscles, including those in the heart, are especially prone to atrophying when outside the realm of normal gravity.

"I'd like for the legacy of this flight to be that we can decide to do hard things that will take us farther away from the Earth, and this is one of them. I am hopeful, and I think we will learn a lot about longer-duration spaceflight and how that will take us to Mars someday," Kelly stated during a press conference held aboard the space station.

Kornienko and Kelly are scheduled to return to Earth on March 2, with landing of their Soyuz spacecraft due to take place in Kazakhstan. Kelly now holds the record for the longest space mission ever undertaken by an American, although no such distinction awaits Kornienko on his return home.

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