Mars or bust, maybe bust

Scott Kelly had spent 520 continuous days in earth orbit, the longest of any NASA astronaut. He's a guinea pig in an experiment to test human ability to live in space in preparation for a trip to Mars. It's not looking good.

I've spent 340 days alongside Russian astronaut Mikhail "Misha" Kornienko on the International Space Station (ISS). As part of NASA's planned journey to Mars, we're members of a program designed to discover what effect such long-term time in space has on human beings. This was my fourth trip to space, and by the end of the mission I'd spent 520 days up there, more than any other NASA astronaut. Amiko has gone through the whole process with me as my main support once before, when I spent 159 days on the ISS in 2010-11. I had a reaction to coming back from space that time, but it was nothing like this.

I struggle to get up. Find the edge of the bed. Feet down. Sit up. Stand up. At every stage I feel like I'm fighting through quicksand. When I'm finally vertical, the pain in my legs is awful, and on top of that pain I feel a sensation that's even more alarming: it feels as though all the blood in my body is rushing to my legs, like the sensation of the blood rushing to your head when you do a handstand, but in reverse.

I can feel the tissue in my legs swelling. I shuffle my way to the bath room, moving my weight from one foot to the other with deliberate effort. Left. Right. Left. Right. I make it to the bathroom, flip on the light, and look down at my legs. They are swollen and alien stumps, not legs at all. "Oh shit," I say. "Amiko, come look at this." She kneels down and squeezes one ankle, and it squishes like a water balloon. She looks up at me with worried eyes. "I can't even feel your ankle bones," she says.

"My skin is burning, too," I tell her. Amiko frantically examines me. I have a strange rash all over my back, the backs of my legs, the back of my head and neck – everywhere I was in contact with the bed. I can feel her cool hands moving over my inflamed skin. "It looks like an allergic rash," she says. "Like hives."

I use the bathroom and shuffle back to bed, wondering what I should do. Normally if I woke up feeling like this, I would go to the emergency room. But no one at the hospital will have seen symptoms of having been in space for a year. I crawl back into bed, trying to find a way to lie down without touching my rash.

I can hear Amiko rummaging in the medicine cabinet. She returns with two ibuprofen and a glass of water. As she settles down, I can tell from her every movement, every breath, that she is worried about me. We both knew the risks of the mission I signed on for. After six years together, I can understand her perfectly, even in the wordless dark.

Walking in space:

I didn't get to spend time outside the station until my first of two planned space walks, which was almost seven months in. This was one of the things that some people found difficult to imagine about living on the space station, the fact that I couldn't step outside when I felt like it. Putting on a spacesuit and leaving the station for a space walk was an hours-long process that required the full attention of at least three people on station and dozens more on the ground.

Space walks were the most dangerous thing we did in orbit. Even if the station was on fire, even if it was filling up with poison gas, even if a meteoroid had crashed through a module and outer space was rushing in, the only way to escape the station was in a Soyuz capsule, which also needed preparation and planning to depart safely. We practised dealing with emergency scenarios regularly, and in many of these drills we raced to prepare the Soyuz as quickly as we could. No one had ever had to use the Soyuz as a lifeboat, and no one hoped to.

Back on earth:

I tell my flight surgeon, Steve, that I feel well enough to get right to work immediately upon returning from space, and I do, but within a few days I feel much worse. This is what it means to have allowed my body to be used for science. I will be a test subject for the rest of my life. A few months after arriving back on Earth, though, I feel distinctly better. I've been travelling the country and the world talking about my experiences in space. It's gratifying to see how curious people are about my mission, how much children instinctively feel the excitement and wonder of space flight, and how many people think, as I do, that Mars is the next step.

I also know that if we want to go to Mars, it will be very, very difficult, it will cost a great deal of money and it may likely cost human lives. But I know now that if we decide to do it, we can.