Shuttle veteran Scott Kelly first heard about NASA's plans to send an astronaut to the International Space Station for nearly a full year shortly after he completed his third space flight in 2011, a 159-day stay aboard the orbital lab complex.
The idea wasn't particularly attractive.
"At first, I'll be honest with you, I wasn't all that interested," he said. "I hadn't given it a whole lot of thought, and it was soon after I had gotten back from my last flight. So the difficulty of living and working in space for a long period of time was still kind of fresh in my mind."
But he thought about it. Then he thought some more.
Finally, after "mulling it over and talking about it with my family, friends, girlfriend, I decided the challenges that staying in space for a whole year presented were appealing to me, even considering the sacrifices you and your family are in for to do that kind of thing."
In November 2012, Kelly and Russian cosmonaut Mikhail Kornienko, veteran of a 176-day stay aboard the station in 2010, were assigned to what NASA bills as the "One-Year Mission." Now, after more than two years of training in the United States, Russia, Europe and Japan, they're finally ready to go.
Joined by Soyuz TMA-16M commander Gennady Padalka, one of Russia's most experienced cosmonauts, Kelly and Kornienko are scheduled for blastoff from the Baikonur Cosmodrome in Kazakhstan at 3:42:57 p.m. EDT Friday (GMT-4; 1:43 a.m. Saturday local time), departing from the same launch pad used by Yuri Gagarin at the dawn of the space age more than 50 years ago.
If all goes well, the trio will dock at the space station's upper Poisk module around 9:36 p.m. after a four-orbit, six-hour rendezvous. Standing by to welcome them aboard will be Expedition 43 commander Terry Virts, cosmonaut Alexander Shkaplerov and European Space Agency astronaut Samantha Cristoforetti.
Padalka will return to Earth in September, becoming the world's most experienced spaceman in the process. He will have logged some 878 days off planet over five missions. Kelly and Kornienko will remain aloft until March 3, 2016, logging 342 days in space.
Asked what they will miss the most during their extended stay aboard the station, Kelly and Kornienko both mentioned family and friends, along with Earth's changing weather.
"As our song says, we dream of the grass by our home," Kornienko said. "It's true, that's what we dream of, the water that doesn't fly around in bubbles but water you can actually swim in, the forest the fields, that kind of thing."
Joked Padalka: "I don't miss anything on Earth, and that's why I'm flying for a fifth time."
Medical Research a Top Priority
During the course of the One-Year Mission, Kelly and Kornienko will serve as test subjects for dozens of experiments to measure the long-term effects of weightlessness and to evaluate mitigation measures that will be needed for eventual multi-year missions to Mars.
"If we're ever going to go beyond low-Earth orbit for longer periods of time, spaceflight presents a lot of challenges to the human body with regard to bone loss, muscle loss, vision issues that we've recently realized people are having, the effect on your immune system, the effect of radiation on our bodies," Kelly said.
"If a mission to Mars is going to take a three-year round trip, we need to know better how our body and our physiology performs over durations longer than what we've previously on the space station investigated, which is six months. Perhaps there's a cliff out there with regards to some of these issues that we experience and perhaps there aren't. But we won't know unless we investigate it."
Four Russian cosmonauts logged flights lasting between 366 to 438 days aboard the Mir space station -- Valery Polyakov, Sergei Avdeyev, Vladimir Titov and Musa Manarov -- but the last such flight ended in 1999. Medical technology has advanced since then and an international team of researchers has been assembled to study the results.
"It's an opportunity to push deeper into space, it's a stepping stone for that," Kornienko said. "The last long-term space mission was on the Mir station, and it brought (back) major data for investigations and research about how humans will feel during long term flights into space. I hope that our mission will be an opportunity for others who will follow in our footsteps and take space exploration further."
Taking advantage of the unique opportunity, Kelly's twin brother Mark, also a veteran shuttle commander, is participating in control experiments on the ground that will help researchers better understand changes his brother experiences in orbit.
"Because our DNA is so similar, they can look at a lot of this before flight, in flight and after flight and see if there are any like shocking differences between my brother and I that you wouldn't expect if we were both living here on Earth," Kelly said. "In other words, they were the result of spaceflight, whether it's the microgravity environment or radiation, or maybe how our immune systems respond. So it really is a unique opportunity."
Julie Robinson, NASA's lead space station program scientist, said the twin studies represent a "unique opportunity."
"Mark will participate as sort of a ground control to really help us understand this nature-versus-nurture question," she said. "A group of 10 really premier scientists looking at the genetic basis of disease and the genetic basis of many of the different processes that affect astronauts has partnered in this twin study to really make it a state-of-the-art investigation."