NASA Clears Discovery’s Left Wing for Landing, Rest of Shuttle to Follow

A video camera aboard the space shuttle Discovery shows the International Space Station hovering above a blue Earth after the orbiter undocked from the orbital lab on July 15, 2006. CREDIT: NASA TV.

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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