Two astronauts are on their way to the International Space Station after a successful SpaceX launch on Saturday.

The Crew-9 mission, helmed by astronauts NASA’s Nick Hague and Aleksandr Gorbunov of Russia’s space agency, launched from Cape Canaveral Space Force Station in Florida at 1:17 p.m. ET.

It’s the first time since SpaceX’s first crewed mission in 2020 that it will carry two astronauts to orbit instead of four. They’re saving room to bring home two NASA astronauts stuck at the ISS.

The stranded pair, Butch Wilmore and Suni Williams, flew to the ISS aboard Boeing’s Starliner capsule on June 5 on what was supposed to be an eight-day test mission. But the two have remained there due to concerns about the safety of Boeing’s capsule. NASA made the “tough decision” to return Starliner to Earth without the crew earlier this month.

During the planned five-month mission at the ISS, the crew will conduct more than 200 science experiments and demonstrations including studies of blood clotting, effects of moisture on plants grown in space and vision changes in astronauts.

Hague and Gorbunov are slated to return to Earth, with Wilmore and Williams aboard, in February.