Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more.

A SpaceX mission due to take flight Saturday aims to unite the Boeing Starliner astronauts with the spacecraft that will bring them home. NASA’s Suni Williams and Butch Wilmore have already been on the International Space Station more than 100 days longer than expected.

The mission, called Crew-9, is on track to take off as soon as 1:17 p.m. ET Saturday from Cape Canaveral Space Force Station in Florida. NASA will stream the event live on its website.

The space agency previously delayed the launch attempt from Thursday, rolling the spacecraft back into its hangar as Hurricane Helene threatened Florida and other parts of the southeastern United States. Mission teams reset everything at the launchpad after the danger had passed.

“We rolled out a little late this morning,” said Steve Stich, manager of NASA’s Commercial Crew Program, at a Friday news conference. “We are vertical at the pad.”

A backup launch window is also set for 12:54 p.m. ET Sunday should weather or technical issues force a scrub of Saturday’s attempt.

Unlike other routine trips ferrying astronauts to and from the space station under NASA’s Commercial Crew Program — of which SpaceX has already launched eight — the outbound leg of this mission will carry only two crew members instead of four: NASA astronaut Nick Hague and Roscosmos cosmonaut Aleksandr Gorbunov.

Two other seats will fly empty, reserved for Williams and Wilmore to occupy on the spacecraft’s return flight in 2025.

The configuration is part of an ad hoc plan that NASA chose to implement in late August after the space agency deemed the Starliner capsule too risky to return with Williams and Wilmore. The two rode the Starliner to the International Space Station in early June for what was expected to be about a weeklong test flight.

At liftoff, Hague and Gorbunov will be strapped inside the SpaceX Crew Dragon spacecraft, nicknamed Freedom, as it sits atop a Falcon 9 rocket. The launch vehicle will roar to life, powering on nine massive engines that sit at its base to thrust the 1.2 million-pound (544,300-kilogram) rocket system into the air.

After about 2 ½ minutes, the first stage of the Falcon 9 rocket will quit firing and break away from the rocket’s second stage. The second stage will then ignite its own engine and continue to propel the Crew Dragon capsule to more than 17,000 miles per hour (27,360 kilometers per hour) — or 22 times the speed of sound.

As the crew blazes to faster speeds, the rocket’s first stage will guide itself back and land on a ground pad in Florida so that SpaceX can refurbish and reuse the vehicle.

Once the Crew Dragon capsule reaches orbital speeds, the spacecraft will separate from the Falcon 9’s second stage and begin maneuvering through orbit on its own, using onboard thrusters to adjust its position gradually so it can link up with the International Space Station, expected at about 5:30 p.m. ET Sunday.

A ‘heartbreaking’ crew swap

Williams and Wilmore watched on September 6 as their Boeing-built capsule returned from the station without them.

Engineers had worked for months to understand issues with helium leaks and thruster outages that had plagued the Starliner’s journey to the space station, and NASA ultimately declared too many uncertainties and risks existed to trust the vehicle to transport crew on its return trip. It’s not clear when Boeing’s Starliner might fly again.

NASA remains in the same situation it has been in for four years, with SpaceX as the sole provider for the space agency’s Commercial Crew Program, which was designed to hand over the task of crew rotations at the International Space Station to the private sector. (Boeing and SpaceX were each awarded contracts in 2014, and SpaceX began flying routine trips in 2020, while Boeing has struggled to push Starliner development to the finish line.)

To get Williams and Wilmore home, NASA turned to SpaceX — opting to remove two previously assigned members from its Crew-9 team to make room for the Starliner test pilots.

The space agency announced at the end of August that NASA astronauts Stephanie Wilson and Zena Cardman would be the members booted from the mission. Cardman had been set to make her first trip to space and was expected to be commander of the Crew-9 mission.

Gorbunov, a Russian cosmonaut who got his seat via a ride-sharing agreement inked between NASA and Russia’s space agency, Roscosmos, remained on the crew. And Cardman handed over commander duties — the top position on a spaceflight — to Hague, who had previously been named the pilot for Crew-9.

“Handing the helm to (Hague) is both heartbreaking and an honor. Nick and Alex are truly an excellent team, and they will be ready to step up,” Cardman said in a post on the social platform X, formerly Twitter, after the announcement.

“I only wish (Wilson), Nick, Alex, and I could fly together, but we choose without hesitation to be part of something much larger than ourselves. Ad astra per aspera. Go Crew 9.”

Meanwhile, Williams and Wilmore have folded into daily life on the space station. The duo transitioned from a lighter test mission schedule to taking on roles as full-time crew members, with Williams assuming the role of commander at the orbiting laboratory.

Gorbunov and Hague will join them after docking with the space station, set for Sunday.

When asked if he had trouble adjusting to the prospect of waiting months longer to get home, Wilmore said during a September 13 news briefing from the space station, “I’m not gonna fret over it. I mean, there’s no benefit to it at all. So my transition was — maybe it wasn’t instantaneous — but it was pretty close.”

Williams said that she missed her family and was disappointed to miss some family events this fall and winter, but she added, “This is my happy place. I love being up here in space. It’s just fun. You know, every day you do something that’s work, quote, unquote, you can do it upside down. You can do it sideways, so it adds a little different perspective.”