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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