Nasa will try to launch Discoveryon the first space shuttle mission in more than two years next Tuesday.
The agency may go ahead with lift-off even if there's a repeat of the fuel gauge problem that halted last week's countdown.
Mission managers decided last night to bypass another fuelling test of Discoveryand go straight for the real thing in an effort to understand and either fix or work around the fuel gauge failure.
The most probable cause is an electrical grounding problem lurking inside the spacecraft.
Multiple safety nets are in place "to ensure we don't proceed unless we feel we're safe to go fly," shuttle programme manager Bill Parsons said.
But in what would be an almost certainly controversial move in the wake of the 2003 Columbiatragedy, Nasa might also proceed with the lift-off if the fuel gauge problem recurs but is considered well understood.
That would mean revoking a launch rule requiring all four hydrogen fuel gauges at the bottom of Discovery's external tank to be working properly, and instead relying on just three out of four.
That looser three-out-of-four rule was thrown out after the 1986 Challengerlaunch explosion. The fuel gauges are intended to keep a shuttle's main engines from shutting down too early or too late after lift-off, both potentially disastrous situations.
Only two of the four are needed to ensure safety, but ever since the Challengeraccident, Nasa has required all four to be operating.
AP