Friday’s launch of the Soyuz capsule in Kazakhstan marks the beginning of a space mission to test the limits of humans. The crew arrived at the International Space Station Friday just after 9:30 p.m. EDT.

Most space expeditions to the International Space Station last four to six months. So by doubling this length of time, researchers are hoping to learn more about how the human body responds and adapts to this spaceflight, which will set the stage for longer missions like sending humans to Mars.

It’s a mission that will test the human spirit, strength and sense of survival.

American astronaut Scott Kelly suited up to leave planet Earth for an entire year, which will break the current record for the longest mission on the space station. With so much exposure to a zero-gravity environment, Kelly will be the most important experiment.

“To see if you know there’s a cliff out there with regards to bone-loss, muscle loss, our immune system, the effects on our vision,” Kelly said.

His identical twin brother, retired NASA astronaut Mark Kelly, will be used as the comparison in this experiment here on Earth.

But besides the physical aspects, this year-long mission will also examine the astronaut’s psychology. Kelly will be living in isolated and small spaces.

“After awhile you kind of feel like you’re running out of entertainment on board,” Kelly said.

It will be a year away from family and gravity. So to stay grounded, Kelly will conduct research, exercise, keep a journal of his progress and capture the view down below.

Kelly is not the only one embarking on this one year spaceflight journey. Cosmonaut Mikhail Kornienko from Russia is also aboard the Soyuz space capsule. All research from both American and Russian crew members will be shared. This information is key for ensuring the health of astronauts in future space exploration missions.

Five things to know about the duo's extraordinary endeavor:

THE CREW

Both Scott Kelly and Mikhail Kornienko have lived on the space station before. No-nonsense former military men, they were selected as an astronaut and cosmonaut in the 1990s. Kelly, 51, is a retired Navy captain and former space shuttle commander. Kornienko, 54, is a former paratrooper. The pair will blast off with Russian Gennady Padalka, a veteran spaceman who will spend six months at the orbiting lab.

THE MISSION

Kelly and Kornienko will remain on board until next March. During that time, they will undergo extensive medical experiments, and prepare the station for the anticipated 2017 arrival of new U.S. commercial crew capsules. That means a series of spacewalks for Kelly. They also will oversee the comings and goings of numerous cargo ships, as well as other Russian-launched crews. Soprano superstar Sarah Brightman will stop by as a space tourist in September.

THE SCIENCE

Doctors are eager to learn what happens to Kelly and Kornienko once they surpass the usual six-month stay for space station residents. Bones and muscles weaken in weightlessness, as does the immune system. Body fluids also shift into the head when gravity is absent, and that puts pressure on the brain and the eyes, impairing vision for some astronauts in space. Might these afflictions peter out after six months, hold steady or ramp up? That's what researchers want to find out so they can protect Mars-bound crews in the decades ahead.

THE TWINS

NASA's scientists couldn't resist when Kelly's identical twin brother, Mark, a retired astronaut, agreed to take part in many of the same medical experiments as his orbiting sibling. Researchers are eager to see how the space body compares with its genetic double on the ground. They won't follow the same diet or exercise regime, however. Mark, who's married to former congresswoman Gabrielle Giffords, said he has no intentions of consuming bland space-type food or working out and running two hours a day on a treadmill, as his brother will be doing.

THE HISTORY

NASA and the Russian Space Agency announced Kelly and Kornienko as the one-year crew in late 2012. This will be new territory for NASA, which has never flown anyone longer than seven consecutive months. The Russians hold the world record of 14 months, set by a physician-cosmonaut aboard the former Mir station in 1994-1995. Several other Russians spent between eight and 12 months at Mir. All but one of those long-timers are still alive.

Information from the Associated Press was used in this story