LOS ANGELES, Dec. 3 (Xinhuanet)-- Engineers have found the most likely answer to why the insulating foam outside the fuel tank broke off during the space shuttle launch, according to Michael Griffin, administrator of US space agency NASA.

    Super-cooling of the external fuel tank may have led to cracks in the insulating foam, when it was filled with liquid propellant, making it vulnerable to falling off, Griffin told Saturday's Los Angeles Times.

    Insulating foam is plastered on space shuttle's fuel tank to prevent ice-forming on the surface. NASA engineers have learned that the foam always breaks off during shuttle launch, making an extreme threat to the mission safety.

The shuttle Discovery. (AFP/File)

    A piece of falling foam damaged the wing of shuttle Columbia in 2003, causing the craft's destruction on reentry and the death of all seven astronauts aboard. Another foam piece fell off during the flight of Discovery this summer, resulting in the grounding of the entire US shuttle fleet.

    NASA managers have said that they won't launch the shuttle again before the foam-shedding problem is solved. According to Griffin, engineers are now fairly certain that radical temperature change is "the closest thing to a smoking gun."

    In a series of tests, engineers discovered a series of cracks near an irregularly shaped part of the tank known as the PAL ramp when a fuel tank was filled with propellant and then emptied, Griffin said.

    He said the next step would be figuring out how to prevent the problem from recurring on future flights. A solution could require redesigning the foam application procedure or removing the foam altogether, while it's still not clear that removing it would not cause other problems during the launch.

    Griffin also criticized former space strategy of the United States, saying the country has "frittered away a 40-year lead in space exploration" by investing too much on low-Earth programs such as space shuttle and the International Space Station.

    He said he was not surprised that other countries, particularly China, have made huge strides in catching up with America, but he believed the US should remain at the forefront of space exploration.

    The NASA chief said he will focus on President Bush's space program of sending humans back to the moon by 2020 and then on to Mars. He also promised to stop launching the space shuttle in 2010. Enditem