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.

SpaceX's iconic Falcon 9 rockets have been grounded for the third time in the last three months after part of the rocket uncontrollably crash-landed in the ocean. The rocket was carrying a capsule to be used in the "rescue mission" that will eventually bring home the astronauts who have been stranded on the International Space Station (ISS) since June.

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.