Washington: After conducting some key research on the International Space Station (ISS) to send humans to Mars in the near future, astronaut Scott Kelly and cosmonauts Mikhail Kornienko and Sergey Volkov safely returned to Earth on Wednesday.

After undocking from the ISS to begin their voyage home, they performed a separation burn to increase the distance from the station before executing a four-minute, 49-second deorbit burn and landed southeast of Dzhezkazgan, Kazakhstan.

All three participated in field tests immediately after landing.

Kelly will conduct functional task tests once he is back at NASAÂ’s Johnson Space Center which will assess how the human body responds to living in microgravity for such a long time.

Understanding how astronauts recover after long-duration spaceflight is a critical piece in planning for missions to deep space missions.

The Expedition 47 crew members, Commander Tim Kopra of NASA, Yuri Malenchenko of Roscosmos and Tim Peake of ESA (European Space Agency), will continue research and maintenance aboard the station.

They will be joined on March 18 by three additional crew members — NASA astronaut Jeff Williams and Russian cosmonauts Oleg Skriprochka and Alexey Ovchinin.

Kelly and Kornienko were launched to the space station on March 27 last year for their one-year mission.

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

After his return, he will have spent a total of 520 days in space across four space missions.

During the 340 days of this mission — which spanned four space station expeditions — Kelly has participated in a variety of research that will help scientists better understand how the human body reacts and adapts to long-duration spaceflight.

That knowledge will play a critical role in future NASA missions deeper into the solar system and on the Journey to Mars, in which a round-trip mission is likely to last 500 days or longer.

“The year-long endeavour has seen Kelly accomplishing several key missions, including growing the first space vegetable and flower,” the US space agency said in a statement.

NASA has partnered with other space agencies to conduct numerous human research investigations to see how the human body changes during a year in space.

Coinciding with the one-year mission is the twins study. Scott’s identical twin brother Mark, himself a former astronaut, is spending the year on Earth, enabling researchers to focus on a near-identical comparison of the brothers’ genomics profiles — something never captured before in human space research.

This new field of study, akin to personalised medicine, produces so much information and data to analyse that it’s hard to imagine.

“The mission will continue even after Scott returns,” said John Charles, chief scientist of the NASA Human Research Programme.

“For the one-year mission research, we will be collecting post-flight medical data three-months and six-months after he is back on Earth. For the twins study research, we will continue to collect data as far out as a year after his return,” he added.

The post-flight data are as important as the inflight data to help scientists learn how to send humans safely to Mars and return them safely to Earth.

To date, NASA has approximately 15 years of data from the space station about how space affects the human body on six-month missions.

Now, the “Human Research Programme” is building on that foundation by proposing more international collaboration on future one-year space station missions.