CNN  — 

The dangers of space exploration are all too clear. Last month, we observed the 50th anniversary of the Apollo 1 disaster, when a cabin fire during launch rehearsals claimed the lives of all three crew.

It was a dreadful start in NASA’s quest to reach the moon. January 28 marked 21 years since space shuttle Challenger broke up 73 seconds into its flight, killing all seven aboard. Last week, NASA commemorated the lives of seven crew members who perished 14 years ago when space shuttle Columbia disintegrated above Earth during its re-entry.

Even on successful missions, crews face a multitude of issues in space. Astronauts aboard the International Space Station are exposed to radiation levels 10 times that of Earth, leading to increased cancer risks. Seventy-nine percent of astronauts are affected by space motion sickness. The heart deconditions; the immune system weakens; bones lose minerals and density at a rate of 1% per month. Fluids redistribute themselves around the body and away from the legs, and for older astronauts, near vision deteriorates.

It goes without saying that astronauts are made of stern stuff. Now retired, Capt. Scott Kelly and Shuttle Commander Mark Kelly are two of a select bunch to leave our planet and put everything on the line in doing so, not for the betterment of themselves but for mankind. For that reason, above all, CNN Chief Medical Correspondent Dr. Sanjay Gupta has chosen the identical twins for the CNN series “My Hero.”

“It’s easy to look at it in retrospect and say ‘that was a really cool thing that you did,’ but the reality is that they had to take a lot of chances,” Gupta said. “They had to take a lot of risks. They had to say goodbye to their family and friends and with not the clear understanding that they’d ever see them again.

“That’s a really hard thing to do in the pursuit of trying to learn something for all of humanity.”

Mark Kelly was selected as an astronaut in 1996, flying in four missions from 2001 to 2011 aboard Endeavor and Discovery. He visited the ISS four times and clocked over 50 days in space. Kelly’s final mission, the last flight of the space shuttle Endeavor, took place as his wife, then-Congresswoman Gabrielle Giffords, recovered from an assassination attempt in January 2011.

Also selected for the space program in 1996, Scott Kelly took part in four missions from 1999 to 2016, the first and second aboard NASA shuttles, the third and fourth on Russian Soyuz spacecraft. He totaled 520 days off Earth, the second highest of any astronaut.

Scott Kelly and Russian cosmonaut Mikhail Kornienko participated in the One-Year Mission.

It was Scott’s last mission that would unite the twins, who never went into space at the same time.

In November 2012, Scott was chosen to participate in the One-Year Mission, a year-long stay aboard the ISS with cosmonaut Mikhail Korniyenko. The purpose of the mission, to discover more about the long-term effects of space on the human body, required extensive testing of Scott aboard the ISS. But to strengthen the results, the test also required a control subject. Enter Mark.

Part of NASA’s Human Research Program, the Twins Study was formed of the Kelly twins along with ten researchers, who analyzed samples from Mark and Scott taken before, during and after the mission. Their initial findings, first released January 26, have begun shedding light on the transformations the body undergoes when in space.

At a biological level, the team found a decline in bone formation during the second half of Scott’s mission, as well as altered gene expression and increased levels of a metabolite in his gut. But the body’s response to a flu jab remained the same in orbit, while a “space gene” – a hypothetical change to Scott’s genome sequence, triggered by space – continues to elude scientists (they’re still looking).

Scott Kelly takes a selfie during the One-Year Mission.

Part of the Twins Study involved tests measuring body response, posture and dexterity. The thrust behind the move is to see how the human body will fare on a six month journey to Mars – and how fit astronauts will be when they reach the planet. The Functional Task Test found no substantial difference between the results of astronauts in space for six months and a year. However reaction time and accuracy decreased in microgravity.

Ahead of a full report due this year, John Charles, chief scientist at the Human Research Program, said in a statement that the results “have not identified any show-stoppers for longer human spaceflight missions.” The race for Mars is well and truly on.

“Probably the biggest impediment to just going (to Mars) is the cost,” Scott said in September, adding that if that could be overcome, the technology would follow. He also said he’d put himself forward for the three-year mission. With a bevy of data on Scott’s body in space, it would be hard to discount the retired astronaut.

Dr. Sanjay Gupta interviews Scott Kelly aboard the ISS.

As well as an exploratory mission, the venture could lay the groundwork for something more profound.

“NASA believes in redundant systems, and Earth is a system, so if at some point it became feasible to maybe make Mars somewhat like Earth, then there would be a lot of value to that,” Scott said.

“We don’t know how climate’s going to affect this planet that we live on,” Gupta argued. “We need to start thinking about other places in the universe that we may have to live one day. In order to understand how to do that, we’ve got to understand what it’s going to do to us first.”

That’s why, in the long term, the Twins Study – and the sacrifices of Mark and Scott Kelly – is so important. When the full report comes out this year, we’ll be one step closer to the Martian surface – and perhaps a new home for us all.