Barring any unexpected technical problems, a Soyuz rocket will launch Thursday from the Baikonur Cosmodrome in Kazakhstan, carrying just two people: first-time NASA astronaut Jack Fischer and veteran Russian cosmonaut Fyodor Yurchikhin. The launch will occur during the afternoon local time, but it will come early in the morning in the United States, at 3:13am EDT (8:13am UK time).

The rocket launch comes amid some uncertainty after a Soyuz rocket carrying an uncrewed Progress spacecraft exploded about six minutes into flight on December 1, 2016, wiping out 2.6 tons of food, fuel, and supplies. This was due to a problem with the rocket's third-stage engine, the kerosene-fueled RD-0110.

That Progress spacecraft launched on an older Soyuz-U rocket, and crewed launches now take place on a more modern Soyuz-FG rocket, but the two rockets share a common third stage. This is now the second time the third-stage engine has failed in some way in the last five years, as a similar problem occurred during the 2011 launch of Progress 44P.

Fischer and Yurchikhin had been due to launch in March, but officials with the Russian space agency wanted to see the Soyuz rocket fly safely again before putting crew on it. That occurred in late February, when an unpiloted Russian Progress spacecraft launched from Baikonur and safely delivered about three tons of supplies to the space station.

Crew size

The other notable aspect of Thursday's launch is the size of the crew—two rather than three. Russia's space program has decided to reduce the overall number of crew members it flies to the station from six per year to four in order to lower Russia's ongoing station-servicing costs. By reducing the number of its crew members, Russia will need to fly fewer Progress cargo ships, which deliver food and supplies to the Russian side of the station. This frees up two Soyuz seats a year.

NASA will use this opportunity to expand its own crew complement on board the station, and the space agency has already announced that veteran astronauts Joe Acaba and Ricky Arnold will launch to the station in September 2017 and March 2018, respectively. However, there wasn't time to accelerate the training of a crew member to add to this week's launch.

It's rare for the United States or Russia to launch a crew of just two astronauts. The last time the United States did so was in 1981 with the launch of the first space shuttle mission and its crew of John Young and Robert Crippen. Russia last launched a crew of two in 2003, when Expedition 7 Commander Yuri Malenchenko and Flight Engineer Ed Lu launched to the space station from Kazakhstan. The smaller crew size then was due to grounding of the space shuttle following the Columbia accident.