NASA's Discovery shuttle grounded

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Faulty fuel sensor puts launch on hold.

Discovery: Launch delayed... Credit: © NASA

NASA has been forced to postpone the launch of the space shuttle Discovery after a routine test revealed a faulty fuel sensor.

The shuttle's seven crew-members, led by commander Eileen Collins, were already strapped into their seats, with just over 2 hours until blast off, when mission control scrubbed the launch. "I appreciate all we have been through together, but this one is not going to result in a launch attempt today," launch director Mike Leinbach told the crew.

Mission managers say the earliest time Discovery could launch is 1840 GMT on Saturday 16 July, but that may be optimistic. Worryingly, NASA has seen problems with these sensors before, but has been unable to identify the causes. NASA administrator Mike Griffin called it "an unexplained, intermittent fault".

Signal failure

The faulty sensor is one of four that measure the amount of liquid hydrogen inside the external fuel tank. The test involved sending an electrical signal telling the sensors, which read 'wet' when immersed in fuel, to flip to 'dry' instead. But the faulty sensor continued to read 'wet'.

The sensors provide backup protection for the shuttle's engines by shutting them down in the unlikely event of the hydrogen supply running low, due to a leak for example. In a hastily convened press conference a few hours after the launch was scrubbed, mission managers said they had never tested what happened to an engine if the hydrogen dried up, but that it would probably be catastrophic. If the hydrogen runs out, the engines would be fed pure liquid oxygen.

The eyes of the world are watching whether the agency is able to send its astronauts into space two and a half years after Columbia broke up on re-entering the Earth's atmosphere. But the shuttle has never flown with a faulty sensor before, and they are certainly not going to start now.

Bad bodings

A hint of the fuel gauge problem was spotted back in April. To prevent a problem with ice build-up on the tank, Discovery was rolled back into its hanger to have its fuel tank swapped for one with a heater. During tests, engineers also found that two of the four hydrogen sensors were giving intermittent readings.

To try to fix the problem, some of the electronics on Discovery were swapped with those from Atlantis, another of NASA's shuttles. But further testing showed more problems, so the electronics were swapped again, this time with spares from Endeavour. The actual cause of the problem was never identified.

Fresh look

Discovery's external tanks have now been drained of the 2 million litres of liquid oxygen and hydrogen. The investigation into the fault will start with external circuits that connect the shuttle to the fuel tank, but to carry out more tests inside the tank itself, the craft may have to be trekked the 7 kilometres back to its hanger.

The current launch window runs until 31 July. If Discovery misses the deadline, it will have to wait until 9 September, when the International Space Station will again be in the right position for docking. Timing this flight has been more difficult than scheduling previous shuttle missions, because the launch must happen during the day so that ground cameras can watch for falling debris, such as the chunk of foam that caused the Columbia accident.

Earlier in the day, concerns were raised after a window cover fell off the front of Discovery's cockpit and damaged some of the craft's heat resistant tiles. But the repairs were completed in about an hour. The troublesome sensors may not be so easy to fix.

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Peplow, M. NASA's Discovery shuttle grounded. Nature (2005) doi:10.1038/news050711-9

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