HOUSTON - Shuttle managers cleared the
Discovery orbiter's left wing for landing Saturday, with the rest of the
orbiter expected to follow after a late-night analysis by engineers and flight controllers.
Analysts
found no signs of heat shield damage - from either orbital debris or
micrometeorites - to the vital heat-resistant panels on the leading edge of
Discovery's left wing, based on imagery relayed
to Earth Friday and Saturday by the spacecraft's six-astronaut crew, NASA
officials said.
John
Shannon, NASA's deputy shuttle program manager here at Johnson Space Center,
said analysis is still pending on imagery from Discovery's right wing leading
edge and nose cap received today, but should be completed no earlier than 4:00
a.m. EDT (0800 GMT) Sunday with few surprises anticipated. The spacecraft's
heat shield already received an initial clean
bill of health on July 9.
At about
that same time as the heat shield decision, the six STS-121
astronauts aboard Discovery - slated to make a 9:14 a.m. EDT (1314 GMT)
landing Monday at NASA's Cape Kennedy Space Center (KSC) in Florida - will
begin their flight control systems check to prime the spacecraft for Earth
return after a 13-day
mission to the International Space Station (ISS). They will also check a potential
fuel leak in one of the orbiter's three auxiliary power units (APUs) by
activating the unit.
"You really
wake the vehicle up and get it ready for entry," said Shannon, who chairs
Discovery's STS-121 Mission Management Team, of the systems check. "It's really
a full, top to bottom check of all the instrumentation that you would use for a
normal entry."
Discovery's
STS-121 mission is NASA's second shuttle to fly since the 2003 Columbia accident and the
last of two flight tests before construction resumes on the ISS. The shuttle
crew launched on July 4 and docked at the space station two days later. After
eight days of joint operations, the STS-121 crew cast
off from their ISS berth early Saturday.
STS-121
commander Steven
Lindsey and his crewmates are scheduled to begin that systems check at
about 3:58 a.m. EDT (0758 GMT), and activate APU 1.
The APU is
one of three units powering the hydraulics necessary to for Discovery's
elevons, landing gear, steering and other moving systems required to land the
vehicle safely. Flight controllers are tracking a steady pressure drop in APU 1's
fuel tank, but are unsure if it is harmless gaseous nitrogen or toxic hydrazine
propellant dripping out.
"It's 50-50
that it's nitrogen and not a problem," Shannon said of the apparent leak.
But to be
on the safe side, engineers are treating the glitch as a hydrazine leak - a very
small leak of about six drops an hour. If it is a hydrazine leak and remains
unchanged during the flight control systems check, it should pose a problem for
landing, Shannon said.
If the tank
pressure decay worsens, or the potential leak increases, engineers plan simply
to run the APU 1 until it uses up all its fuel and shut it down.
With two
other functioning APUs, the only system to be affected would be landing gear
deployment since they are linked directly to APU 1, Shannon said, adding that a
series of small pyrotechnic charges could then be used to deploy the landing
gear.
After the
flight control systems check and APU 1 evaluation, the STS-121 crew is also
slated to test fire their primary attitude control jets and run tests on a finicky
flash evaporating system (FES), which is part of Discovery's cooling system.
"It's
really hard to check a FES out on the ground because you're not in the vacuum
environment," Shannon said. "The easiest place to check it out is in space."
Meanwhile,
shuttle officials are preparing NASA's KSC Shuttle Landing Facility for
Discovery's initial landing attempts on Monday.
Current
plans call for the STS-121 crew to land at 9:14 a.m. EDT (1314 GMT) or 9:50
a.m. EDT (1350 GMT), Shannon said. More opportunities arise at KSC and California's Edwards Air Force Base on Tuesday before the shuttle's liquid oxygen supplies -
used for breathing air and power - run low, he added.
"We'd want
to be down on Wednesday," Shannon said.
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