Updated 1 a.m. Eastern March 4 with comments from post-launch briefing.

WASHINGTON — A Crew Dragon spacecraft is on its way to the International Space Station with four Americans and Russians on board after a launch March 3.

A Falcon 9 lifted off from Launch Complex 39A at 10:53 p.m. Eastern on NASA’s Crew-8 mission. The Crew Dragon spacecraft Endeavour separated from the upper stage 12 minutes after liftoff.

Endeavour is scheduled to dock with the Harmony module of the ISS at about 3 a.m. March 5. It will deliver to the station NASA astronauts Matthew Dominick, Michael Barratt and Jeanette Epps and Roscosmos cosmonaut Alexander Grebenkin for a six-month stay.

The launch had been scheduled for early March 1 but delayed because of forecasts of poor weather along the launch corridor in the event of an abort. NASA and SpaceX started preparations for a launch on the evening of March 2 but called off the launch more than three hours before liftoff because of weather.

Sarah Walker, director of Dragon mission management at SpaceX, said at a post-launch briefing it was an “extremely quiet countdown” with the one issue being a minor imperfection or crack spotted in the top layer of a seal in the spacecraft’s side hatch. That issue, spotted during reviews of photos taken during the closeout of the hatch, was cleared only about 10 minutes before liftoff.

She said the size of the crack in the room-temperature vulcanized (RTV) seal, about 0.13 square centimeters, was within the qualified limit of the deal’s design based on past tests. “That was ultimately the flight rationale that cleared us for launch,” she said, with additional margin coming from the limited heating that part of the spacecraft gets during reentry and because the heating would “self-heal” the crack.

Steve Stich, NASA commercial crew program manager, said the agency agreed with SpaceX’s analysis. “I thought they had exactly the right rationale for this size of little imperfection in the seal.”

The mission is the first for Dominick, the Crew-8 commander, and mission specialists Epps and Grebenkin. Barratt, the pilot, is making his third flight, having flown to the ISS on a Soyuz for a long-duration mission in 2009 and who went to the station again in 2011 on the STS-133 shuttle mission.

The flight is a long time coming for Epps in particular, who is the last member of the 2009 astronaut class to go to space. Epps was assigned to a Soyuz mission to the station in 2018 but abruptly removed six months in advance for reasons that NASA did not disclose and which Epps said in a late 2018 event were not explained to her. She was later assigned to the first operational flight of Boeing’s CST-100 Starliner mission before being reassigned to Crew-8 after continued Starliner delays.

“I was confident that I would fly,” Epps said a January press conference about Crew-8. “The way I kept my spirits up is that we continue to train weekly, daily. We train vigorously for any mission that we’re assigned to. So, I’ve been pretty busy over the last few years still training, still working towards the goal of going to the space station.”

The flight is the fifth for Endeavour, the Crew Dragon spacecraft used for the first flight with astronauts on board, Demo-2 in 2020. Endeavour is the first Crew Dragon to reach the five-flight milestone, which is the current limit that NASA has certified the spacecraft for. However, at a Feb. 25 briefing, NASA and SpaceX said they are studying extending that certification to as many as 15 flights.

Endeavour is one of four active Crew Dragon spacecraft. SpaceX is building a fifth that it will have ready late this year for the Crew-10 mission in 2025. “Whenever NASA wants to go ahead and use that vehicle, it will be available sometime in the fourth quarter of this year,” Bill Gerstenmaier, SpaceX vice president of build and flight reliability, said at the Feb. 25 briefing.

This is the second of as many as five crewed SpaceX flights this year after the Ax-3 private astronaut mission that launched in January. After Crew-8, SpaceX plans to launch NASA’s Crew-9 mission to the ISS as soon as August, along with the Ax-4 private astronaut mission to the ISS in the fall of the year and the Polaris Dawn mission, which will not go to the ISS, as soon as the summer.

Once Crew-8 docks with the ISS, that crew will begin a five-day handover with the Crew-7 mission which has been on the ISS since August. They will return to Earth no earlier than March 11 on the Crew Dragon spacecraft Endurance.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...