Discovery is already in position for its launch from NASA's Kennedy Space Center.© NASA

The space shuttle Discovery could be in orbit this time next month, according to NASA engineers who have given the craft's troublesome fuel tanks a clean bill of health.

"Based on what we know today, there is no reason not to launch on July 1," says N. Wayne Hale Jr, manager of the shuttle programme. The assessment comes after engineers removed about 16 kilograms of insulating foam from the shuttle's main fuel tank (less than 1% of the total), to reduce the risk of falling debris.

NASA have been trying to stop chunks of foam popping off the tank since the Columbia shuttle disintegrated when it re-entered the Earth's atmosphere on 1 February 2003. Investigators found that a 700-gram piece of foam debris had gouged a hole in Columbia's wing during take-off, allowing hot gasses to pour through the fuselage when the shuttle returned from its trip to the International Space Station (ISS). All seven astronauts on board died.

The foam helps to keep the liquid hydrogen and oxygen fuel cold during launch, and also stops dangerous ice build-up on the outside of the tank. Before the next shuttle - Discovery - went up, NASA changed the way the foam was applied to the tank so that it stuck better. The shuttle's wings were also strengthened and sensors installed to detect similar impacts (see 'The long road to a safe launch').

The improved shuttle flew on 26 July 2005, but engineers were sent back to the drawing board after a 400-gram piece of foam fell off during launch, narrowly missing the craft.

Photographs showed that the chunk came from the tank's protuberance air-load (PAL) ramp, which shields pipes and cables from air turbulence during lift-off. The latest modifications have removed that ramp altogether.

On Schedule

The redesign doesn't mean that there will be no debris shed during launch. "There will continue to be foam coming off the external tank," cautions Hale. "What we have done... is eliminate the largest hazards." Engineers are confident that the shuttle should be strong enough to withstand the inevitable shower of smaller bits of foam debris, no more than 50 grams each, nicknamed 'popcorn'.

The review was no mere rubber-stamp for the shuttle's tanks. There was much debate within the team of around 100 engineers about whether more foam should be removed from 34 brackets that support pressurized fuel pipes. Hale says that this foam does pose a hazard, but that the effects of removing the PAL ramp should be flight-tested before making a second major change. Getting rid of the extra foam on the brackets will be "the number one thing we have to work on next", he says.

All aboard

The next shuttle mission, designated STS-121, will bring an extra crew member, equipment and supplies to the station to do tests and repairs.

NASA estimates that after the Discovery flight, pegged for 1-19 July, and an Atlantis flight in August, 16 more flights will be needed to finish building the ISS before the shuttle programme's anticipated retirement in 2010. The problem with the shuttle's fuel tank has pushed back so many flights that it could prove tough to squeeze the number needed into the available time. Sections of the station that have been built on Earth are now queuing up, waiting for a launch slot.

NASA has a flight-readiness review meeting planned for 16-17 June to see if the entire shuttle is ready for a July launch. But the tank, which has now been cleared, was expected to be the most problematic part of the craft.

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