SpaceX prepared a Falcon 9 rocket and Crew Dragon capsule for launch early Wednesday to ferry four professional astronauts to the lab complex for a planned four-and-a-half-month stay, just one day after bringing four commercial fliers down from the International Space Station.

Launch is scheduled for 3:52:55 a.m. EDT from pad 39A at the Kennedy Space Center, roughly the time when Earth’s rotation carries the rocket into the plane of the space station’s orbit — a requirement for rendezvous missions. There was a 90% chance of good weather, according to forecasters.

NASA’s fourth operational commercial crew launch to the space station follows a fully commercial flight to the outpost, which ended Monday with a successful Atlantic Ocean splashdown after a 17-day stay. Engineers had to review data from the Axiom-1 mission to make sure no technical issues had arisen that could affect subsequent flights before moving forward with NASA’s Crew-4 mission. According to Steve Stich, manager of NASA’s Commercial Crew Program, “it was a very clean flight overall, really no major issues.”

“A lot of the data has been examined by the teams,” he said. “They examined the thermal protection system as well as a large amount of the flight’s (guidance, navigation, and control) data. This flight’s parachutes were obviously spotless…. We had our launch readiness review this morning, and everyone said ‘go.'”

Kjell Lindgren, pilot Bob Hines, geologist-astronaut Jessica Watkins, and European Space Agency astronaut Samantha Cristoforetti will board the Falcon 9 shortly after 1 a.m. Wednesday to await launch.

It will be Hines and Watkins’ first rocket ride. Lindgren and Cristoforetti have both been to the International Space Station before, aboard Russian Soyuz spacecraft.

“Flying to space is a once-in-a-lifetime experience,” Lindgren said. “It’s even more surreal to be able to do it again. I have a general idea of what to expect during the launch. But it means a lot to me to be able to launch from here, from the Kennedy Space Center, from a historic launch pad with an incredible history of exploration.”

Assuming a successful launch, the Crew Dragon will reach the space station late Wednesday night, docking at the Harmony module’s space-facing port around 8:15 p.m.

The 16-hour 22-minute rendezvous was planned to get the Crew Dragon to the station ahead of a Russian spacewalk scheduled for Thursday. Currently, the SpaceX capsule will dock while the three cosmonauts on board the station are sleeping.

A traditional welcome-aboard ceremony for the Crew-4 astronauts will be delayed several hours until after the cosmonauts — Oleg Artemyev, Denis Matveev, and Sergey Korsakov — get up to begin their busy day, rather than waking them up early on the eve of a complex spacewalk.

Lindgren’s team is taking over. Raja Chari, pilot Thomas Marshburn, submariner-turned-astronaut Kayla Barron, and European Space Agency astronaut Matthias Maurer are the members of Crew-3, which launched to the space station in November.

The Crew-3 astronauts will familiarize their replacements with the ins and outs of station operations, as well as safety procedures and ongoing research, before returning home. If everything goes according to plan, Chari and his crew will undock next Wednesday and return to Earth, bringing the 174-day mission to a close.

The Crew-4 astronauts plan to stay on the station until mid-September, when they will be replaced by another Crew Dragon.