CAPE CANAVERAL, FLORIDA, the United States, July 13 (Xinhuanet) --Activities are continuing in NASA about four hours away from the launch of Discovery Wednesday at the Kennedy Space Center, with the fuelling delayed about an hour, said the US space agency in a statement.

    The tanking of supercold liquid oxygen and hydrogen fuel was originally scheduled for 5:30 a.m. EDT. It actually began at 7:11 a.m. in the morning.

    NASA said one of two heaters needed to purge the external tank failed, causing the delay. The problem was quickly resolved. Weather is currently NASA's biggest concern in the launch of Discovery. Heavy clouds on Wednesday morning are expected to be clear around the launch time.

    NASA said the possibility that the weather would block the liftoff is about 40 percent. The liftoff is planned at 3:51 p.m. EDT (1951 GMT) Wednesday.

    This is NASA's first shuttle flight since the 2003 Columbia disaster, in which seven astronauts aboard were killed. Over the past two and a half years, NASA has made many safety improvements for the return of flight.

    NASA chief Michael Griffin said Tuesday the space agency is in good shape and ready to go. Enditem