BEIJING, July 15 -- Six astronauts will end a nine-day visit to the International Space Station (ISS) Saturday morning while NASA engineers on Friday focused on a slow leak aboard shuttle Discovery.

US space shuttle Discovery Mission Specialistswork inthe shuttle's aft cargo bay 12 July, 2006. (NASAvideo)Discovery's pilot Mark Kelly is scheduled to undock the shuttle at 6:08 a.m. and slowly back to a distance of 450 feet before performing the first of two engine firings that will separate the spacecraft. The shuttle will linger within 45 miles of the station until a check of Discovery's heat shield is completed Saturday.

If all goes as planned, Discovery will be on track for a 9:07 a.m. Monday landing at the Kennedy Space Center. The long-range weather forecast predicts favorable conditions at Cape Canaveral for the shuttle's return.

While the astronauts are spending their final day at ISS, engineers continue to monitor a small leak in one of the ship's power generators.

John Shannon, the shuttle program's deputy manager, said the problem was unlikely to affect the shuttle's return to the Earth, but engineers were closely monitoring the leak in the pipeline of an auxiliary power unit that controls hydraulic steering and braking maneuvers.

It is leaking at a rate of "about six drops per hour," and could be leaking harmless nitrogen or flammable hydrazine fuel, Shannon said.

Engineers believe the leak is inconsequential but because there is not enough proof, they are proceeding cautiously.

Just in case, NASA will turn on the power unit with the leak early Sunday as part of its normal testing and then see if the leak rate changes. If it does, NASA may burn off the hydrazine and shut down the power unit before the shuttle returns to the Earth to eliminate any fire hazard, Shannon said.

If that happens, the shuttle would land with just its two other power units for the first time in the spacecraft's history. Enditem

(Agencies)