Engineers fueled a workhorse Soyuz booster for launch Friday to ferry NASA astronaut Scott Kelly and cosmonaut Mikhail Kornienko to the International Space Station for a marathon 342-day mission, the longest flight ever attempted by an American.

Kelly, Kornienko and Soyuz TMA-16M commander Gennady Padalka were scheduled for launch at 3:42:57 p.m. EDT (GMT-4; 1:43 a.m. Saturday local time) from the Baikonur Cosmodrome in Kazakhstan. The launching was timed to roughly coincide with the moment Earth's rotation carried the pad into the plane of the station's orbit.

With Padalka strapped into the Soyuz command module's center seat, flanked on the left by flight engineer Kornienko and on the right by Kelly, the spacecraft was expected to slip into its preliminary orbit eight minutes and 45 seconds after launch.

Following a fast-track four-orbit trajectory, Padalka, one of Russia's most experienced cosmonauts, plans to monitor an autonomous rendezvous and docking at the station's upper Poisk module around 9:36 p.m. Standing by to welcome them aboard will be Expedition 43 commander Terry Virts, cosmonaut Anton 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 with 878 days in space over five missions. Kelly and Kornienko, both space station veterans, will remain aloft until March 3, 2016, logging 342 days in space.

Four Russian cosmonauts -- Valery Polyakov, Sergei Avdeyev, Vladimir Titov and Musa Manarov -- participated in flights aboard the Russian Mir space station lasting between 366 to 438 days, but the last such flight ended in the 1990s. Kelly and Kornienko will be the first ISS crew members to spend nearly a year in space and Kelly will set a new endurance record for American astronauts.

The Soyuz TMA-16M spacecraft on the pad at the Baikonur Cosmodrome in Kazakhstan.

NASA

"This is not Russia's first venture having people stay in space for a year or longer," Kelly said Thursday. "But ... this is the first time we're doing it as an international partnership, which is what I think is one of the greatest success stories of the International Space Station.

Continue reading here:

NASA has high hopes for one-year station flight