US: Barring summer thunderstorms and a few last-minute technical problems, Nasa officials said the space shuttle Discovery was on track to launch today.
But even if the weather co-operates, it is possible that a few late technical problems could hold up the launch, Nasa managers said.
Some officials have raised concerns about corrosion in fittings on the umbilical hoses that supply fuel and electricity to the shuttle. On Monday it was revealed tests had shown that the skin of the shuttle might gain too much heat as it accelerates through the upper atmosphere to its final speed of 17,500 mph.
Managers seemed confident that the problems could be cleared by launch day.
Nasa has said it has become much more safety-conscious since the loss of Columbia, which broke up during re-entry on February 1st, 2003. It turned out that a piece of lightweight insulating foam had fallen off the shuttle's external tank and torn a hole in the left wing on launch.
Discovery's 13-day mission will take it to the International Space Station, where it will dock and install a replacement gyroscope. It will also carry supplies for the station and return with accumulated junk. Three space walks are planned, one of which will be used to test new techniques to repair damage to the shuttle's insulating tiles.
Shuttle commander Eileen Collins said it was time for the spacecraft to launch again. She said the crew would carry with them a photograph of the Columbia crew and would pray for them each day. "We will always remember our friends,'" Ms Collins said. "But it's time to move on."