NASA engineers are running checks to solve the mystery of why the Falcon 9's engine failed to perform a routine maneuver to lower the rocket's altitude.

The Dragon Crew 9 capsule has docked with the International Space Station, carrying a U.S. astronaut and a Russian cosmonaut. The craft will bring home two astronauts stranded on the International Space Station.

The Crew-9 incident was the second Falcon 9 upper-stage issue to crop up in less than three months. On July 11, the rocket experienced a leak of liquid oxygen in its upper stage during the launch of 20 of SpaceX's Starlink internet satellites. As a result, the spacecraft were deployed too low and ended up being lost.