Washington  - An old-fashioned magnetic stress test convinced NASA managers that the ageing Discovery shuttle could launch next Wednesday, saving about a month's time in the programme's hectic final two years.

With the pressure of ten launches by September 2010 weighing on them, NASA hit a glitch last month over worries about cracked fuel valves breaking off and damaging the spacecraft during launch.

It seemed Discovery could not launch before mid-April, two months after originally planned.

But on Friday, NASA managers gave the green light for Discovery to launch next week to the International Space Station, where its crew is to install the final set of solar-electricity panels that will double station capacity to six astronauts.

The launch is scheduled for 9:20 pm Wednesday (0120 GMT Thursday).

At issue in the delays were three valves - really, small tubes with a lip at the top, shaped something like a large hollow nail. They pop up and retreat to maintain proper air pressure as hydrogen fuel is fed into the launch engine.

In November, a piece broke off of one during liftoff. Although there was no damage to the spacecraft, the incident raised concerns about whether such falling material could cause problems in future launches, recreating another disaster like the one that destroyed the Columbia shuttle in 2003.

John Shannon, the space shuttle programme manager, said it took an old-fashioned commercial technique normally used to inspect boltheads to determine whether the valves were cracked.

"It worked so well we found cracks we had not seen with our electron microscope," Shannon told reporters in pod-cast remarks.

The technique involved putting a magnetic field around the circumference of the bolt to look for eddies in the current to reveal faults - a technique used to inspect bridges and high rise steel construction.

Shannon said that through the tests, scientists found three valves that were not damaged and installed them in the fuel systems.

Bill Gerstenmaier, associate administrator for space operations, said the clearance to launch next week meant other missions would stay on track: The mission to repair the orbiting Hubble telescope is set for May, followed by another two shuttle missions to the space station in June and late August or early September.

Including the Discovery mission, another 10 shuttle flights are planned by September 2010.

Some of the shuttles have been in service nearly 30 years. Shannon said the valve problem showed that NASA was "pushing the state of the art" and "approaching fatigue in the metal."

After 2010, only Russian Soyuz spacecraft will be able to service the space station. NASA meanwhile will be designing a new spacecraft with a focus on returning to the moon and using it as a launch pad for further space exploration. (dpa)