The date for launching the space shuttle Discovery might be put back as Nasa has failed a set of safety tests.
Extra safety checks were brought in, after another shuttle, the Columbia, exploded in 2003. There have been no shuttle flights since that accident.
An investigation team has now found the shuttles still aren't strong enough to prevent another disaster.
Nasa will make a decision about the launch, set for some time between 13 and 31 July, in the next two days.
Nasa is confident the shuttle is OK. Joseph Cuzzupoli, a spokesman, said: "We feel that it is a safe vehicle to fly."
Columbia disintegrated when returning to earth, killing seven astronauts.
There have been 113 shuttle missions. Columbia was the second fatal accident after Challenger blew up in 1986.