In less than 48 hours, Scott Kelly and Russian cosmonaut Mikhail Kornienko will come blazing into Earth's atmosphere in the Soyuz spacecraft at more than 17,000 miles per hour before landing on Kazakhstan soil, a familiar trip for the veteran American astronaut who has compared it to going over Niagara Falls in a barrel on fire.

Kelly's safe return home - he is expected at Ellington Field by Wednesday - will be one for the record books. After 340 days aboard the International Space Station this go-round, Kelly will become the first American astronaut to have spent almost a year in space. And with four trips to the space station soon to be under his belt, Kelly now has the most cumulative hours in space among American astronauts, beating the old record held by Mike Fincke.

More importantly, Kelly's mission could represent a major step toward NASA's ambition to send humans to Mars by the 2030s, as it will provide new evidence about how the body holds up to the stresses of long-duration space travel. Until now, U.S. scientists have had data from only six-month trips; while useful, it doesn't show them much beyond the immediate effects of microgravity and radiation.

In the days, weeks and months ahead, Kelly will continue to be poked, bled and probed at Johnson Space Center before traveling to Russia for more of the same. Likewise, Kornienko will visit Johnson, where scientists will learn more about the mission's impact on the astronauts' cardiovascular system, bones and vision.

"This is a significant step for us," said Dan Huot, a NASA spokesman. "It's what a lot of people have hoped for."

There are skeptics who question whether NASA will ever make it to Mars, given their current funding levels and wavering political support for such a mission.

Life without gravity

Over the past year, however, Kelly has won over new fans for NASA, which was already riding a wave of good publicity from the discovery of water on Mars and the release of the hit film "The Martian."

From his perch in the space station, Kelly, a social media machine, has taken otherworldly photos of volcanos, deserts, storms and city lights, giving his more than 900,000 Twitter followers an idea of what it is like to see Earth from space. He's shared fascinating tidbits of space travel - who knew zero gravity could make your feet baby soft? - while taking part in earthly pleasures such as watching the Super Bowl (he's an avowed Texans fan) and seeing "Star Wars: The Force Awakens."

He told reporters last week that while he's not overly eager to leave space, he looks forward to seeing his loved ones and jumping in the pool at his home, even if it's three in the morning.

"The space station is an amazing place and an incredible science facility," Kelly said. "It's been a privilege to fly here. ... I hope more people have the opportunity to do that in the future."

Kelly's charisma and accessibility may be the key to earning him legions of admirers - his fans include President Barack Obama, who encouraged the astronaut to send more photos via Instagram - but it's his body fluids that most interest Dr. John B. Charles, a physiologist who is chief scientist at NASA's Human Research Program, based at Johnson Space Center.

Scientists already know that fluid tends to accumulate in the upper part of the body during zero gravity. They believe this additional cranial pressure causes some astronauts to have irreparable damage to their vision, but aren't sure what causes the variability in harm.

Over the past year, Kelly and Kornienko have been testing a Russian-designed pressure suit that is supposed to keep body fluids more evenly distributed. Simply testing the suit has proved a herculean task, since it is on the Russian side of the station and cannot be moved because it is attached to real-time monitoring equipment that must not experience any interruptions.

So Kelly - and all the American testing equipment - had to go to the Russian Zvevda module.

"Who likes to have their bedroom taken over in the name of science? But they did it, they did it three times, and they did it safely," Charles said of the "Chibis" suit testing.

A not-so-secret weapon

The one-year mission will provide data for about 10 different studies that seek to examine the physical and cognitive effects of long-duration space travel. In addition to the fluids test, another will try to identify biological clues that might suggest whether an astronaut will develop atherosclerosis, or hardening of the arteries, while another will examine how the immune system wards off viruses in space.

Richard Simpson, an associate professor at the University of Houston, is one of the key scientists who will be probing the molecular depths of the astronauts' blood to better understand the risks that latent viruses might pose to future space travelers.

Astronauts' immune systems become weaker during spaceflight but recover fairly quickly once back on Earth, Simpson said. If longer flights - such as a three-year journey to Mars - are being contemplated, scientists need to know more about potential viral dangers, he said.

"If a reaction occurs of a latent virus, such as the one that causes shingles, during an astronaut's mission, that would be very concerning," Simpson said. "We want to identify an immunity mechanism to keep something like that from happening."

As scientists sort through the trove of data the one-year mission will provide, one not-so-secret weapon they'll have at their disposal is Mark Kelly, Scott's identical twin, a retired astronaut and husband of former U.S. Rep. Gabrielle Giffords.

During the mission, scientists have collected samples from the earthbound Kelly to help them better understand the biological changes going on with his twin in space.

It will likely be at least a year before any scientific conclusions from the mission can be published, officials with the Human Research Program say. Some of Kelly's and Kornienko's blood, urine and saliva samples aren't set to arrive back from the space station until April, and then all the work must be checked, rechecked and rechecked some more.

"This mission isn't really a one-year mission. It isn't over once they're on the ground," Charles said. "It will continue for us for quite some time."

Easier this time

During his last teleconference with reporters before returning home, Kelly said he "feels pretty good," but acknowledged the data from the tests might tell a different story.

"Yeah, I could go another 100 days. I could go another year if I had to," Kelly said.

The hard part about living on the space station, he said, is the isolation and being away from the people he loves. He has often spoken about missing his two children and his girlfriend, Amiko Kauderer, a NASA public affairs officer.

In many ways, Kelly said, his most recent trip to the space station five years ago was much more difficult, even though he's been gone almost twice as long this go-round.

That's because two months before Kelly was scheduled to return to Earth, Giffords was shot in the head and given grim odds of surviving. Earlier this month, Giffords and Mark Kelly accompanied former President George H.W. Bush and wife Barbara as they visited mission control, where they spoke to Scott Kelly and the other astronauts aboard the space station.

'The price of the ride'

Fincke, the astronaut whose record Kelly broke in October, said he always found the return home from the space station bittersweet.

"The bitter part is leaving your friends aboard the space station, giving up the views you see of the Earth that sort of make you feel like Superman, you know?" said Fincke, now the branch chief of NASA's commercial crew program. "The sweet part is seeing your family again, sleeping in your own bed, enjoying earthly foods and earthly beverages."

No doubt, Kelly also will soon have to manage throngs of well-wishers, the media and even colleagues who want nothing more than to shake his hand and welcome him home.

And that, Fincke said, is one of the responsibilities of the job.

"It's very expensive to fly us on these kinds of missions, and one of the ways we give back is to share our story, and Scott's done a tremendous job already of that," he said. "Look, it's a grand adventure, and this is part of the price of the ride."