SpaceX may be working its Falcon 9 rocket a little too hard. The normally reliable launch vehicle suffered another malfunction and has been grounded for the second time in a month. Falcon 9 launched the Crew-9 mission on Saturday,

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 upper stage malfunction apparently occurred after the Falcon 9 successfully deployed SpaceX's Crew Dragon spacecraft carrying NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov on SpaceX's Crew-9 mission. Hague and Gorbunov safely arrived at the space station Sunday to begin a five-month stay at the orbiting research complex.