SpaceX also uses a Dragon capsule for its own privately run missions. The next one of those is due to fly next week on a trip chartered by Axiom Space, a Houston company.
Cargo versions of the Dragon capsule are also used to ferry food and other supplies to the orbiting lab.
Russia's Soyuz capsules are the only other means of getting crews to the space station right now.
The Soyuz capsules hold three people at a time. For now, each Soyuz launch carries two Russians and one NASA astronaut, and each SpaceX launch has one Russian on board under a barter system. That way, in an emergency requiring a capsule to return, there is always someone from the U.S. and Russian on board.