SpaceX Scrubs Launch of Crew-10 Astronauts for NASA to the I.S.S.

Four astronauts will have to wait at least until Friday to head the International Space Station.

This otherwise routine rotation of crew on the space station is garnering extra attention because it will at last allow the return to Earth of Suni Williams and Butch Wilmore, two NASA astronauts whose brief scheduled visit to the space station last June was unexpectedly stretched to more than nine months.

The scrub of the flight attempt on Wednesday night, which occurred less than 45 minutes before the scheduled liftoff, means Mr. Wilmore and Ms. Williams’ stay in orbit will also be extended at least one more day. During the countdown, SpaceX encountered a hydraulic issue with a clamp arm that holds onto the Falcon 9 rocket before it launches.

NASA announced Wednesday night that the next launch attempt would be on Friday at 7:03 p.m. Eastern time. Mission managers decided not to attempt a launch on Thursday because of high winds and rain predicted along the launch path.

Launch teams are working to fix the problem with the clamp arm.

Here’s other information about the mission, which is named Crew-10 because it is the 10th such mission by SpaceX ferrying crew to and from the space station.

Who is going to space on Crew-10?

Anne McClain of NASA is the commander of Crew-10, and Nichole Ayers of NASA is the pilot. The other two crew members are Takuya Onishi of JAXA, the Japanese space agency, and Kirill Peskov of Roscosmos, the Russian space agency.

This will be the first spaceflight for Ms. Ayers and Mr. Peskov and the second spaceflight for Ms. McClain and Mr. Onishi.

Will Suni Williams and Butch Wilmore be rescued after getting stranded in orbit?

Sort of.

Not really.

The spacecraft that will bring back Suni Williams and Butch Wilmore has been docked at the space station since late September and could have returned to Earth at any time.

Ms. Williams and Mr. Wilmore launched to the space station in June last year for a test flight of Starliner, an astronaut capsule built under a NASA contract by Boeing. Because of propulsion problems, NASA officials decided that Ms. Williams and Mr. Wilmore would not return to Earth in Starliner. In early September, the spacecraft undocked from the space station, re-entered the atmosphere and landed in New Mexico without any problems.

Just as when an airline scrambles to rebook passengers after a flight is canceled, NASA had to find seats on a ride home for Ms. Williams and Mr. Wilmore.

The next spaceflight was Crew-9, which lifted off a couple of weeks after Starliner left the space station without anyone aboard. Two astronauts assigned to the flight were bumped off, leaving two seats in the SpaceX Crew Dragon capsule for Ms. Williams and Mr. Wilmore on the return trip.

Thus, the Crew-9 capsule could have brought back the two astronauts any time after that, but that would have left the space station understaffed, affecting scientific experiments, operations and maintenance.

NASA and SpaceX could have rushed the Crew-10 mission to launch earlier, but NASA officials decided it was best for Ms. Williams and Mr. Wilmore to join the space station crew and keep the planned schedule for Crew-10.

When will Williams and Wilmore return to Earth?

Once Crew-10 gets to the space station, preparations for the departure of Crew-9 will begin.

The astronauts of Crew-9 and Crew-10 will overlap for a few days at the space station. About four days after Crew-10 launches, Ms. Williams and Mr. Wilmore — along with Nick Hague of NASA and Alexander Gorbunov, the two astronauts who arrived with Crew-9 — will climb aboard their spacecraft and head home.

Their stay could be extended again if there is bad weather at the possible splashdown sites off the coast of Florida.

What have the astronauts said about their extended stay?

In an interview last month, Michael Barbaro, the host of “The Daily,” asked the astronauts, “So, if not stuck, exactly how do you describe this scenario you find yourselves in?”

“That’s a great question,” Mr. Wilmore said. “I would say it’s work. It’s wonderful enjoyment. It’s been fun. It’s been trying at times, no doubt. But stranded? No. Stuck? No. Abandoned? No.”

This was the third trip to space for both Ms. Williams, 59, and Mr. Wilmore, 62, and they realize it might be their last one. “We’re heading home,” Ms. Williams said. “And it makes you really want to enjoy every bit of your time that you have up here.”

Michael Barbaro contributed reporting.