NASA officials are confident that a potential fuel leak in one of the three auxiliary power units (APU) aboard Discovery will not affect its landing if no changes in the leak occur. The leak is estimated to be about six drops an hour, or about 100,000 times below the fire hazard limit. John Shannon, NASA’s deputy shuttle program manager, said the worst case scenario is that fuel is leaking and not harmless nitrogen. This is yet to be determined.  “We’re okay where we are right now,” Shannon said, adding that extra checks of the APU are planned to Sunday. “If it’s hydrazine, at the current leak rate we really don’t have any concerns with using it right now.”

The Shuttle APUs power the hydraulic systems that move an orbiter’s elevons, vertical stabilizer flaps, landing gear and other systems required during landing. The APUs use hydrazine for fuel and gaseous nitrogen to generate pressure. Either one of those materials may be the source of the leak in APU 1. The leak, if it's there, could be responsible for the small but steady drop in tank pressure. “The question is what’s leaking,” said Shannon. “There is some anecdotal evidence, actual even better than anecdotal evidence, that it’s nitrogen and if it is than it’s no issue at all."

NASA managers and engineers are assuming the worst – a hydrazine leak – and will test to determine whether the drip is stable or could degrade further. If a Sunday checkout of the system shows pressure drops more than the current rate NASA plans to run the APU until its fuel is exhausted and take it offline during reentry.

The shuttle can land on one APU but all three are preferred for redundancy. APU 1 is the only unit powering Discovery’s landing gear. If it is taken offline the shuttle crew will have to fire a set of pyrotechnic charges to deploy its main and nose landing gears, Shannon said.

But NASA said there is no risk of a repeat fire such as that seen during NASA’s STS-9 shuttle flight in 1983 when a large hydrazine leak sprayed the toxic fuel on a hot surface and ignited two APUs. “Right now I don’t see any change for the landing plan,” Shannon said. The Discovery scheduled to land at NASA’s Kennedy Space Center on July 17 at 9:14 a.m. EDT (1314 GMT).

NASA sighed with relief as data from Discovery show no concerns over the health of the vehicle. NASA’s deputy shuttle program manager John Shannon, said early image analysis of the space shuttles external tank, nose cap and heat shield-lined wing edges revealed items of interest but nothing to worry flight controllers or the crew. “Right now it’s zero,” said Shannon, "We’ll get the truth data tomorrow.”

After arriving at the International Space Station (ISS) on July 6, two Discovery astronauts will conduct a high-resolution photographic survey of the orbiter’s tile-lined belly during the spacecraft's back flip maneuver. The images from that survey will complete the health check of Discovery’s heat shield.

Earlier today, STS-121 mission specialists Lisa Nowak and Stephanie Wilson, put Discovery’s sensor laden orbital boom to work, scanning the heat-resistant reinforced carbon carbon (RCC) panels along the orbiter’s wing leading edges. These scanning revealed nothing that could impact heat shield performance. The boom survey did find a protruding gapfiller jutting out a half-inch from the underside of Discovery’s port wing - an area where gapfillers presented no concern.

As NASA's ground and Discovery crew anxiously await the shuttle's new launch date on the 4th of July, today's aborted lift-off has cost the agency about $1 million. The process of standing down from the cancelled launch attempt, topping of Discovery's fuel cells and revving back up to a space shot on Tuesday has been called by NASA as a challenging task. Pad workers drained Discovery's external tank and will replenish the super-cold liquid hydrogen in the orbiter's fuel cells, and change out some payloads in preparation for the next launch date.

But NASA officials are not concerned about the cost of the delays. "After a year of preparation, and after a very careful countdown, you don't want to do something that's not smart from a weather standpoint," said John Shannon, NASA's deputy shuttle program manager, during the briefing. "Nobody is going to remember that we scrubbed a day or two days a year from now. But if we go launch and get struck by lightning or have some other problem that will be very memorable."