Sunita Williams and Barry “Butch” Wilmore, the NASA astronauts whose mission aboard the International Space Station was extended from about eight days to nine months, are finally set to come home.

The return will have to wait a little longer, however. On Wednesday evening, their replacements were set to launch to the orbiting laboratory on a SpaceX rocket and Dragon spacecraft, a key step toward allowing Wilmore and Williams to return to Earth. But less than an hour before the launch, NASA and SpaceX called it off because of an issue with one of the clamps that holds down the rocket before liftoff. SpaceX could try again in the coming days; however, it wasn’t immediately clear when the attempt would be.

The flight, when it occurs, would help bring an end to an extraordinary saga that began when a troubled Boeing spacecraft led NASA to decide to rely on Elon Musk’s SpaceX to get the astronauts home safely — a decision that has since blown up into a political controversy.

SpaceX was planning to launch the replacement crew, known as Crew-10, to the space station at 7:48 p.m. Wednesday from the Kennedy Space Center in Florida. The crew comprises NASA’s Anne McClain and Nichole Ayers, Japan’s Takuya Onishi and Russia’s Kirill Peskov. Once they arrive at the station, Wilmore and Williams would work alongside the new crew for the next couple of days, then hand over operations.

Any number of technical or weather-related issues could force NASA to adjust its schedule, however.

Williams and Wilmore launched to the space station on June 5 in a Boeing Starliner spacecraft. The capsule had never flown people before, and the goal of the mission was to test how it performed over a period of about eight days. From the outset, it did not go well.

Although the vehicle successfully reached the space station, several of the capsule’s thrusters failed to fire properly, and the spacecraft also suffered helium leaks. Boeing spent weeks trying to solve the issues and argued that the spacecraft could safely return the astronauts. Ultimately, however, NASA said it lost confidence in the company’s ability to fly the crew home safely, and decided that SpaceX, the other company under contract to transport crews to the space station, should do it.

That decision was a tremendous blow to Boeing and also required NASA to juggle its rotation of astronauts. To make room for Williams and Wilmore on the return flight, the space agency had to bump two of four astronauts who had been scheduled to fly as part of the next crew rotation mission. Crew-9, NASA’s Nick Hague and Russia’s Aleksandr Gorbunov, arrived at the station in September, joining Wilmore and Williams at the station for a regular six-month stay.

Musk, who has become one of President Donald Trump’s top advisers, has claimed on social media that “SpaceX could have brought them back several months ago. I OFFERED THIS DIRECTLY to the Biden administration and they refused.”

In a recent briefing with reporters, however, Williams and Wilmore said they were unaware of any such offer. “What was offered, what was not offered, who it was offered to, how that process went — that’s information that we simply don’t have,” Wilmore said.

NASA’s leadership at the time, including administrator Bill Nelson and his deputy, Pam Melroy, also have said they were unaware of any offer from Musk. “There was no discussion of that whatsoever,” Nelson told The Washington Post.

The long-awaited return of the stuck astronauts comes as SpaceX has had several setbacks. The past two of its next-generation Starship spacecraft exploded during test flights.

Earlier this month, the Starship lost the first stage of its Falcon 9 rocket, when it landed on a ship at sea. A fuel leak then caused a fire to break out in one of the rocket’s engine compartments, Bill Gerstenmaier, SpaceX’s vice president of build and flight reliability, told reporters during a briefing last week. The fire caused the rocket to fall over and explode.

“The fire was pretty extensive,” he said. “It melted a lot of aluminum components.” But before the booster fire, SpaceX successfully deployed another batch of Starlink internet satellites.

During a Falcon 9 flight in February, Gerstenmaier said, a small liquid oxygen leak had prevented the stage from properly reorienting itself. It was lost, with debris reportedly falling in Poland. No one was injured.

“Flying crews safely is always our top priority,” Gerstenmaier said. “Spaceflight is really hard, and we’re going to learn every day. The more we fly, the more we learn.” He said engineers “have a good understanding” of the problems, and that they “will improve our knowledge and ultimately we will be safer because of what occurred.”

NASA officials expressed confidence in SpaceX and said the company had been working side-by-side with NASA to prepare for this month’s astronaut missions.

“We have a wonderful private-public partnership with SpaceX,” Ken Bowersox, the head of NASA’s human spaceflight division, said during the briefing. “We’re not going to launch before we’re ready. We’re always analyzing the data, and making sure that the rocket is ready to go before we let the SpaceX team hit the button with our crew on board.”