NASA has delayed launching the Shuttle Atlantis before Friday because of problems with one of the craft's fuel cells.
It was initially put off until Thursday, but engineers said they need more time to work out how serious the problem is.
When the mission managers switched the fuel cells on, one malfunction caused a voltage spike in the other two. For a launch all three must be working, NASA says, but reports vary as to what this means for Atlantis.
"The launch rules say you need to have three good, operating fuel cells," NASA spokesman Bruce Buckingham told the BBC. "We're going to see if it's something that's a real problem or whether it's something we can rectify."
However, in the New York Times, shuttle program director N Wayne Hale Jr said that under the rules the Shuttle could launch without the problem being fixed, but that engineers wanted to understand it properly.
Although it is working well enough to launch, if it fails in flight, the mission would have to be ended early, the paper reports.
If the cell has to be replaced, NASA will miss the current launch window. The procedure normally takes about a week.
Adding to an already complicated situation is the fact that NASA doesn't know exactly how the fuel cell works, ABC reports.
"The vendor sold us the thing [in 1976] and didn't exactly tell us how it works, amazing as that might be," Hale said*.
Friday is the last possible launch date before scheduling conflicts (with the planned Russian visit to the International Space Station) will force NASA to wait until late October before trying again. ®
*Bootnote: Our thanks to reader Roger Costello for suggesting that NASA might want to take a look at this explanatory document. Published by...er...NASA.