(CNN) -- The two test pilots for the inaugural crewed flight of Boeing's Starliner spacecraft -- NASA's Suni Williams and Butch Wilmore -- left Earth for the International Space Station under the impression their trip would last only about one week.
Months later, after determining that the technical issues the Starliner experienced on the first leg of their journey posed too much of a risk, NASA decided to return Williams and Wilmore home aboard a SpaceX Crew Dragon capsule.
But that did not mean the astronauts would get the first flight back to Earth.
A SpaceX Crew Dragon, on a mission dubbed Crew-8, left the International Space Station on Wednesday, but Williams and Wilmore will once again be left behind for the simple reason that they are not assigned to that specific spacecraft.
Notably, the four astronauts that are assigned to Crew-8's mission -- NASA's Matthew Dominick, Michael Barratt and Jeanette Epps as well as Alexander Grebenkin of the Russian space agency, Roscosmos -- have been on the ISS since March 5, roughly three months longer than Williams and Wilmore.
That's because the Crew-8 team is part of a regular crew rotation at the orbiting laboratory, and routine missions typically last about six months.
The team disembarked from the ISS Wednesday afternoon and is heading for a splashdown off the coast of Florida. That's slated to happen on Friday at 3:30 a.m. ET, according to NASA.
Williams and Wilmore, meanwhile, are assigned to return home aboard SpaceX's Crew-9 mission.
NASA and SpaceX even had to reconfigure the Crew-9 capsule and astronaut assignments before the mission launched September 28 to ensure there would be enough room for Williams and Wilmore to return home aboard the vehicle when it completes its ISS mission no earlier than February 2025.
That's just one reason why the duo can't just hop on the next ride home. Spaceflight tends to be more complex -- and requires more prior planning -- than that.
Williams and Wilmore did not go to the ISS as part of a normal crew rotation.
Instead, the pair ventured to orbit as part of a historic effort: Starliner's first crewed flight. They departed on June 4, mostly without their own toiletries and other personal comforts, expecting a prompt return to Earth.
Williams and Wilmore were then left in limbo by several crucial issues, including gas leaks and thruster problems, that plagued their Starliner capsule during the first leg of its trip. NASA then chose to leave the capsule -- along with Williams and Wilmore -- at the ISS while engineers worked to hash out what went wrong.
After weeks of uncertainty, NASA made the call in late August that it would be too risky to return Williams and Wilmore home aboard Starliner.
And the space agency had scrambled to find an alternative route home. That's how NASA arrived at the decision to put Williams and Wilmore on Crew-9, a mission that at the time had not yet left Earth.
Two astronauts who had been slated to fly on Crew-9 before the Boeing Starliner debacle -- NASA's Zena Cardman and Stephanie Wilson -- had to be booted off in order to make room for Williams and Wilmore on the return trip.
The Crew-9 mission then launched with just two astronauts, two empty seats, and some ballst -- or deadweight -- to balance out the physics of flying without a full crew of four. The Crew-9 spacecraft arrived at the ISS on September 29.
Williams and Wilmore, for the record, have now been on the space station more than 18 weeks.
So what exactly would Williams and Wilmore do for another four or five months in space?
When the pair arrived at the ISS, they were guests. But now, they're at work, having joined Expedition 72, or the international crew of astronauts currently serving as the space station's official staff.
Williams even took over as the commander of the entire space station on September 22.
NASA has said they've seamlessly integrated with the group, picking up everyday tasks aboard the orbiting laboratory.
As part of Crew-9 and the formal expedition, Williams and Wilmore will take on typical crew work, including conducting spacewalks outside the space station, maintaining the orbiting laboratory and carrying out a tight schedule of science experiments.
And NASA previously confirmed the Starliner astronauts are prepared to make such a shift.
"A couple years ago, we made the decision -- knowing that this was a test flight -- to make sure that we had the right resources, supplies and training for the crew, just in case they needed to be on ISS, for whatever reason, for a longer period of time," said Dana Weigel, NASA's manager of the International Space Station Program, during an August 7 briefing.
"Butch and Suni are fully trained," Weigel added. "They're capable and current with EVA (spacewalks), with robotics, with all the things we need them to do."
During recent updates about ISS activities from NASA, the space agency said Williams, Wilmore and their crewmates, NASA astronaut Nick Hague and Russia's Aleksandr Gorbunov, will help move their Crew-9 capsule from its current docking port to the port that opened when Crew-8 began its journey home from space. They recently spent some time training for that task, according to updates from NASA.
Williams and Wilmore have also had some downtime lately as they waited days to support Crew-8's departure, which had been repeatedly delayed by bad weather surrounding Florida.
But the pair have also kept busy with experiments and other daily duties. One October 18 update from NASA, for example, states that during a half day of work, Williams took a cognition test and "checked connections on radio frequency identification hardware," while "Wilmore activated a fluorescence microscope to observe how particles of different sizes gel and coarsen."
Williams and Wilmore had already logged a combined total of 500 days in space before launching on the Starliner test flight this year. Williams even said that she cried after she left the space station following her last mission in 2012, unsure if she would ever return.
"This flight is a dream for her," one NASA commentator said during a June 5 livestream of the Starliner launch.
It's not uncommon for astronauts to unexpectedly extend their stay aboard the space station -- for days, weeks or even months.
NASA astronaut Frank Rubio, for example, was slated to spend about six months aboard the International Space Station for his inaugural trip to low-Earth orbit that kicked off in September 2022. He instead logged 371 days in space following the discovery of a coolant leak coming from his original ride -- a Russian Soyuz capsule -- while docked to the orbiting outpost.
Rubio's yearlong stay ended up setting a US record for the most continuous days spent in orbit.
Astronauts also routinely extend their stays on the station for days or weeks at a time for a variety of factors, including poor weather on Earth or other schedule adjustments.
When asked in a September news conference if he had trouble adjusting to the prospect of waiting months longer to get home, Wilmore said, "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 at the same news briefing that she missed her family and pets and that she 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."
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