NASA feels it has a good shot at sending shuttle Discovery to the international space station on Sunday following repairs out at the launch pad.
But "the proof of the pudding" won't be until Discovery's external fuel tank is filled once again, launch director Mike Leinbach said Friday. That's when NASA will know, with certainty, whether the repairs to the hydrogen vent line worked.
Leaking hydrogen gas — a dangerous situation because it could ignite and cause an explosion — forced NASA to call off Wednesday's launch attempt.
A half-dozen or so workers were out at the launch pad Friday, putting in a new hydrogen vent line hookup for Discovery's external fuel tank, along with two fresh seals. The removed pieces were being scrutinized for any signs of damage that might have led to Wednesday's leak.
One seal was nicked, but that may have nothing to do with the leak, Leinbach said. Testing at normal outside temperatures with helium — nowhere close to the frigid conditions of the super-cold hydrogen fuel — came up empty.
"We don't have any smoking guns yet," Leinbach told reporters.
The leak was just the latest delay for Discovery's space station construction mission. Earlier postponements were caused by hydrogen gas valves inside Discovery's engine compartment, and pushed the flight back by a month. The leak occurred out at the fuel tank.
Because of all the delays, NASA is up against a tight deadline for launching Discovery. Russia is sending up a fresh space station crew on March 26, and the shuttle needs to have undocked by then.
The desire to avoid a traffic jam means that Discovery and its seven astronauts must be flying by Tuesday. That gives NASA just three days to get the space shuttle up. It also means that the shuttle flight — originally a 14-day mission — will be shorter with fewer spacewalks.
If Discovery lifts off Sunday, the shuttle will be in orbit 13 days and its crew will carry out three spacewalks instead of four. A Monday or Tuesday launch will mean an even shorter flight with even fewer spacewalks.
To make way for Discovery, the Air Force indefinitely put off its launching of a military communication satellite aboard an unmanned rocket, which had been scheduled for Saturday.
The top priority for the shuttle mission is to deliver and install the final set of solar wings for the space station.
NASA also wants to carry out a crewmember exchange — replacing space station resident Sandra Magnus with Japanese shuttle astronaut Koichi Wakata — and drop off and set up a machine for turning astronauts' urine into drinking water. The urine processor that's up there now is not working properly.
Good weather is forecast for Sunday's 7:43 p.m. ET (0043 GMT) launch attempt. Forecasters put the odds of acceptable conditions at 80%.