Amid heavy white smoke, glaring red flames and huge roars, the shuttle rocketed into the sky at exactly 10:39 AM EDT (1439 GMT) as planned, leaving a straight column of smoke behind.

The shuttle separated from the twin rocket boosters at 2 minutes 5 seconds into flight, and shut down the main engines after reaching the space 8 minutes and 23 seconds into flight, all as planned.

NASA Administrator Michael Griffin described the liftoff as with "power and majesty".

However, he and other NASA officials at the news conference about one and a half hour after the shuttle lift off restrained from calling the mission now a successful flight. The flight would not be successful until after Discovery's 12-day mission and landing, said officials.

"We had a very, very clean countdown," but only after the 12-day mission and landing could we be comfortable, said shuttle launch director Mike Leinbach.

"We've still got a lot of work to do," said Bill Parsons, shuttle program manager.

Almost 2,500 guests of NASA, including first Lady Laura Bush and Florida Gov. Jeb Bush, witnessed the shuttle lift off about 5 km away from the launch pad, NASA said.

It added that the first Lady went to the launch command center where she hugged and congratulated workers there. "Thank you so much for your hard work. You're inspiring everybody," she said.

NASA workers at the command center later gathered for traditional beans and cornbread to celebrate the launch success.     

Tuesday's launch came after the first try on July 13 was called off because one of the four fuel sensors malfunctioned during the pre-launch testing.

After exhaustive efforts, NASA last weekend decided that it had eliminated all the possible causes of the sensor problem and would have a second try. It also hoped tests during the fueling of the shuttle's external tank would help find out what exactly caused the launch glitch on July 13.

All the sensors turned out to be good during pre-launch testing on Tuesday, licensing a green light to the shuttle liftoff. However, the cause of the sensor problem remains unexplainable and requires further research by engineers.

During the liftoff, cameras installed by NASA to closely monitor the flight safety observed small debris falling off the external fuel tank.

The US space agency said it needs time to have a professional and careful review on any information from the many cameras. A decision on a second post-Columbia flight within the September launch window, by Atlantis, has not yet been made at the moment, said NASA.

Discovery's mission is to test the safety improvements NASA had made over the past two and a half years, and to provide supplies and equipment for the International Space Station.

If any problem would threaten the flight safety, Discovery's seven-member crew would stay in the space station and wait for the rescue by Atlantis.

NASA has made major modifications in the design of the external fuel tank and added heater to the tank in order to minimize the debris risk from foam insulation or ice, which can form when the tank is filled with super cold liquid oxygen and liquid hydrogen fuel.

The Columbia disintegration during re-entry in Feb. 2003 was blamed on a suitcase-sized foam insulation that fell off the external tank and harmed the shuttle wing. NASA engineers last April found that ice breaking off the tank could pose an even bigger threat than foam.

As the fuel sensor problem is only post-Columbia, it is a possibility that the new safety measures might cause electronic magnetic interference with the sensor and affect its work. The fuel sensor is designed to shut down the main engines when the level of hydrogen fuel in the tank would run unexpectedly low.

Misleading readings from faulty sensor could cut off the main engines too early or too late during liftoff, leading to a possible emergency landing or wrong orbital position of the space shuttle, or even threatening the shuttle safety.

Discovery, 37.2 meter long and with a wingspan of 23.8 meters, first flew a space flight in 1984. After the 1986 loss of challenger and the 2003 loss of Columbia, NASA's shuttle fleet is currently consisting of Discovery, Atlantis and Endeavor.

(Xinhua News Agency July 27, 2005)