Shuttle Astronauts Prepare to Leave Space Station

The International Space Station transfered the Leonardo module to the Discovery shuttle's cargo bay today, July 14, 2006. CREDIT: NASA TV

HOUSTON - Six astronauts are spending their final day at the International Space Station (ISS) as NASA's shuttle Discovery is primed for a Saturday undocking.

Discovery's STS-121 astronauts retrieved their Italian-built Leonardo cargo module from its berth at the space station's Unity node and tucked it away in the shuttle's cargo bay for the return flight to Earth.

"They're still working hard and still talking to each other," said Tony Ceccaci, lead shuttle flight director for Discovery's STS-121 mission, who spoke one-on-one with the flight's commander, Steven Lindsey, earlier today. "He said everybody's doing great and they're ready to do late inspection."

Later today, the crew will check for micrometeorite impacts to Discovery's port - or left - wing leading edge using the shuttle's sensor-laden inspection boom. The crew will undock from the ISS at 6:08 a.m. EDT (1008 GMT) on Saturday and inspect the orbiter's starboard wing edge and nose cap.

The shuttle's primary reaction control thrusters, rather than its smaller vernier engines used for fine maneuvers, will be used to back away from the ISS Saturday, NASA officials said. The change will mean that Discovery's motions will be larger, and rather crude according to some astronauts, but should not be a challenge for Lindsey and STS-121 pilot Mark Kelly.

"The crew is trained to do that, they understand what they're looking for, so there's no concerns on that," Ceccacci said. "Basically, you don't have as fine control as you would on verniers."

Discovery is set to land at NASA's Kennedy Space Center in Florida at 9:07 a.m. EDT (13067 GMT) after a 13-day mission that began with a July 4 launch.

The spaceflight is NASA's second orbiter test mission since the 2003 Columbia accident. Discovery's crew delivered about 7,400 pounds (3,356 kilograms) of supplies, hardware, tools and other equipment to the ISS along with a new crewmember - European Space Agency (ESA) astronaut Thomas Reiter - and completed several ISS repairs and several tests of shuttle heat shield repair tools and methods.

Engineers study APU issues

While Discovery's shuttle crew prepares for undocking, a team of engineers continues to pore over issues with two of the shuttle's three Auxiliary Power Units (APUs), Ceccacci said.

Thermostat failures have lead two heaters on APU 3 to fail in the "On" mode, which - though unexpected - is not thought to be a major problem.

The other problem concerns a possible leak in APU 1, either with its nitrogen supply or toxic hydrazine fuel, Ceccacci said. A nitrogen leak is rather benign, but a hydrazine leak could lead to a fire akin to that seen during STS-9 in 1983, though engineers are studying the issue and will update Discovery's Mission Management Team (MMT) later today, he added.

According to a mission summary uplinked to Discovery's crew this morning, the threat of a hydrazine fire appears to be low.

"In fact, at this leak rate, the hydrazine vapor concentrations will remain well below the flammability limits in the aft compartment for reentry," the crew's Flight Day 11 execute package reported, adding that engineers are still unsure if it is hydrazine or nitrogen leaking from APU 1. "The teams continue to work around the clock on the ground to quantify the effects of the small N2 of fuel leak and the forward plan for reentry."

ISS set for new construction

With Discovery's Leonardo cargo pod back inside its payload bay berth, the ISS is now set for future construction flights to expand its living area and crew size.

"After 121's successful completion here we can continue with assembly ops," said Rick LaBrode, NASA's lead ISS flight director for STS-121. "And if we weren't successful, we weren't going to be able to do that."

During Discovery's flight, STS-121 spacewalkers Piers Sellers and Michael Fossum restored the station's railcar-like Mobile Transporter - vital for ISS construction - to full operation by repairing a redundant power and data cable system.

Among the notable ISS deliveries for Discovery's crew, Reiter's arrival stands out, followed by a new Minus Eighty Laboratory Freezer for ISS (MELFI) to store biological samples, a European Modular Cultivation System (EMCS) for plant growth studies, and a rebuilt exercise cycle - dubbed Cycle Ergonometer with Vibration Isolation and Stabilization (CEVIS).

"On this particular flight, we've transferred more payload cargo than any other shuttle flight," LaBrode said.

The station now has 15 water bags, redundant air conditioners, a U.S. Oxygen Generating System, six new laptop computers, a fresh printer and new lights scavenged from inside Leonardo, he added.

Discovery is also hauling about 4,600 pounds (2,860 kilograms) - 300 pounds (136 kilograms) more than expected - of unneeded cargo back to Earth aboard Leonardo, with an additional 1,800 pounds (816 kilograms) worth to be tucked in its middeck lockers.

"It looks very close, we're right in the ballpark," LaBrode said.

NASA will broadcast Discovery's Saturday undocking live on NASA TV beginning with a crew farewell ceremony at 3:38 a.m. EDT (0738 GMT). You are invited to watch the activity via SPACE.com's NASA TV feed, which is available by clicking here.  

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