Cold weather might delay Thursday's launch of a U.S. space shuttle mission to repair the Hubble Space Telescope. A technical problem may interfere, too.
The U.S. space agency forecasts a launch temperature of just three degrees Celsius above freezing, the lowest allowed for lift-off under expected wind and humidity conditions.
There is a 40 percent chance the temperature will drop below that, causing a launch delay. But the weather is expected to be several degrees warmer on Friday.
Freezing temperatures were behind the 1986 explosion of the shuttle Challenger after takeoff when flammable gas escaped through shrunken rubber seals in the external fuel tank. Seal heaters prevent that worry now, but ice formation on the tank could pose a hazard.
Weather is not the only concern. Engineers have also discovered new wheel bearings in Columbia's main landing gear were insufficiently tested to determine if they are durable enough to survive the rigors of touchdown.
They are expected to report later today whether the bearings must be replaced, a procedure that would take several weeks.