“Right now, it still feels a little bit surreal,” she said in an interview three weeks ago. She said she thought it would continue to feel that way until T+1 in the countdown — one second after the rocket had left the launchpad.

Last summer, Ms. Cardman’s plans were scrambled by the troubled test flight of Boeing’s Starliner. NASA decided that Suni Williams and Butch Wilmore, the two astronauts who traveled to the I.S.S. in the Starliner, would return to Earth another way, and that set off a domino effect for the crews of later missions.

The next available spacecraft headed to the space station was Crew-9. But if two of the seats were to be filled by Ms. Williams and Mr. Wilmore on the return trip, those two seats had to be empty on the way up.

It then became a question of which of the four astronauts assigned to Crew-9 would be bumped.

“As we knew we were launching without half of our crew, there’s no easy option,” Ms. Cardman recalled.