Eight times during the past four years, SpaceX has provided a regular astronaut transportation service for NASA from Kennedy Space Center in Florida.

Its next flight to the International Space Station, scheduled to launch on Saturday, will not be like the previous eight.

There will be two, not four, astronauts aboard. Two other astronauts who were assigned to the mission will remain on Earth. And the mission, named Crew-9, will launch from a different launchpad.

The shuffling is a consequence of difficulties with a different spacecraft, Boeing’s Starliner, over the summer.

“The word that comes to mind for this flight is ‘agility,’” Steve Stich, the manager for NASA’s commercial crew program, said during a prelaunch news conference on Friday.

Here’s what you need to know about Saturday’s launch, and why it’s unlike other recent NASA astronaut missions.