It’s launch day for four astronauts who will soon be on their way to the International Space Station.

The otherwise routine flight will be closely watched because it kicks off a series of events culminating in NASA astronauts Suni Williams and Butch Wilmore’s long-awaited trip back to Earth.

Williams and Wilmore were expected to stay on the ISS for just a week as part of the first crewed test flight of Boeing’s Starliner spacecraft, but problems with the vehicle forced them to remain in orbit, where they’ve now been for nine months.

The astronauts launching Wednesday on a mission known as Crew-10 will take over from Williams, Wilmore and two fellow crew members onboard the ISS.

The four set to depart are NASA astronauts Anne McClain and Nichole Ayers, Japanese astronaut Takuya Onishi and Russian cosmonaut Kirill Peskov. They’re expected to lift off at 7:48 p.m. ET from NASA’s Kennedy Space Center in Florida, aboard a SpaceX Falcon 9 rocket.

The foursome is expected to arrive at the space station Thursday at around 6 a.m. ET. Once there, preparations will be underway for Williams and Wilmore to finally return home.

The Boeing capsule on which the pair launched encountered several issues during its docking process at the ISS, including malfunctioning thrusters. Because of those problems, NASA decided to bring the Starliner craft back to Earth with no one onboard, leaving Wilmore and Williams to wait for a ride back to Earth.

The two will travel home in a SpaceX Dragon capsule that arrived at the space station in September, carrying NASA astronaut Nick Hague, Russian cosmonaut Aleksandr Gorbunov and two empty seats for their “stuck” colleagues.

The unusual situation became a political talking point for SpaceX CEO Elon Musk and President Donald Trump, who have made unsubstantiated claims that the Biden administration held up the flight back to Earth for political reasons. Trump said recently that he had tasked Musk with bringing the astronauts home, but NASA’s plan to use a SpaceX vehicle for their return journey has remained the same since it was announced in August.

Hague and Gorbunov will return to Earth with Wilmore and Williams, capping their monthslong mission at the ISS.

NASA has not yet announced a date for the crew’s undocking and landing. There is typically a short handover period after new crew members arrive at the space station, during which all the astronauts can exchange information about ongoing science experiments, maintenance projects and other protocols.