Astronauts Scott Kelly, Mikhail Kornienko leave on yearlong space mission

An American astronaut and Russian cosmonaut blasted off into space today, on their way tothe International Space Station, where they will live for an entire year.

The trip is NASA's first attempt at a one-year spaceflight, anticipating Mars expeditions that would last two to three years.

Their Soyuz space capsule set off from Russia's manned space launch facility on the steppes of Kazakhstan at 3:42 p.m. ET Friday (early Saturday local time) and is scheduled to dock with the International Space Station about six hours later after making four orbits of the planet.

Cosmonaut Gennady Padalka of Russia was also aboard their Soyuz capsule. He is scheduled for the standard six-month tour of duty aboard the space station.

• Astronaut twin study could provide valuable data for Mars mission

At a news conference Thursday at the Russian manned-space facility in Baikonur, Kazakhstan, Kornienko said "we will be missing nature, we will be missing landscapes, woods." He admitted that on his previous trip into space in 2010 "I even asked our psychological support folks to send me a calendar with photographs of nature, of rivers, of woods, of lakes."

Kelly was asked if he'd miss his twin brother Mark, who also was an astronaut.

"We're used to this kind of thing," he said. "I've gone longer without seeing him and it was great."

The mission won't be the longest time that a human has spent in space four Russians spent a year or more aboard the Soviet-built Mir space station in the 1990s.

"The last time we had such a long duration flight was almost 20 years and of course all ... scientific techniques are more advanced than 20 years ago and right now we need to test the capability of a human being to perform such long-duration flights. So this is the main objective of our flight, to test ourselves," said Kornienko."

"One of the differences here is that we're doing it as an international partnership, and if we're going to go beyond low-Earth orbit again, perhaps to Mars, because of the cost and the complexity it will most likely be an international mission so we see this as a stepping stone to that," Kelly said.

"If you're ever going to go to Mars, going from a place like this would be, you know, a step in the right direction," Kelly said of Baikonur, set amid the vast and barren steppes of central Asia.

It will be NASA's first stab at a one-year spaceflight, a predecessor for Mars expeditions that would last two to three times as long.

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

1. 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 blastedoff with Russian Gennady Padalka, a veteran spaceman who will spend six months at the orbiting lab.

2. 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.

3. 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.

4. 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 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.

5. 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.