This story was updated at 1:05 p.m. EDT.
A glitch on the space shuttle Atlantis' scanning sensor system has slowed down the inspection of the orbiter's sensitive heat shield.
The inspection is a normally scheduled check to hunt for any signs of damage on the shuttle's insulation tiling that could pose a risk when it re-enters Earth's atmosphere. But a balky system called the pan-and-tilt unit that controls the sensor's motion has stalled the six-hour scan. After hours of troubleshooting, the shuttle astronauts determined that a cable leading to the unit was being pinched by another piece of hardware on Atlantis, restricting how far the sensor can tilt.
"It is crystal clear exactly where the pinch is," Atlantis commander Ken Ham radioed to Mission Control. "No matter of panning is going to help change that situation."
Eventually, NASA instructed the astronauts to switch to a secondary sensor that uses a laser camera system and a digital camera to take photos of the heat shield. This process is normally used toward the end of a shuttle mission for the so-called late inspection. The secondary sensor system requires lighting, so the crew was forced to work only during times when the space shuttle was passing over the daylight side of Earth, further slowing down the process.
The heat shield scan will help the shuttle's six-man crew make sure Atlantis is in good health for its mission to the space station, where the astronauts plan to deliver a new Russian research module and loads of spare parts and supplies, including a set of solar array batteries and a space-to-ground antenna. Docking is set for Sunday morning.
Even when it runs as usual, the inspection is a laborious process.
"It's a little bit of a long day, a long inspection," Atlantis pilot Dominic "Tony" Antonelli said before launch in a NASA interview. "We're going ... to inspect the leading edge of both wings and the nose. It's a long series of automated robotic procedures."
Antonelli is one of three Atlantis astronauts who will trade off using the shuttle's 100-foot (30-meter) inspection pole to scan sensitive heat shield panels along the vehicle's wing edges and nose caps for dings. Cameras and laser instruments will record the health of those panels and the data will be beamed to Earth for analysis.
"We have to do the top, the front, and the bottom so it'll be several of these in a row, and it'll take most of the day," Antonelli said.
Atlantis soared into orbit from Florida Friday afternoon and is flying its last scheduled mission before retiring to museums. After this trip, there are only two more shuttle flights planned. [Atlantis' launch photos.]
The shuttle is slated to dock with the International Space Station Sunday at 10:27 a.m. EDT (1427 GMT). They will spend about a week linked with the orbiting lab to deliver a new Russian research module and a load of spare supplies, including a set of replacement batteries and a new space-to-ground antenna.
Scanning the heat shield is a normal precaution to ensure that the vehicle was not hurt by any foam debris falling from the shuttle's external tank during liftoff.
The inspections have been a standard part of space shuttle missions after the loss of the shuttle Columbia and its seven-astronaut crew in 2003. A piece of foam debris damaged that orbiter's heat shield during launch, leading to its destruction during re-entry.
But a preliminary look at video from Atlantis' launch has not indicated any cause for concern, mission managers said.
"We had a couple little foam losses, pretty typical to what we've seen before ? nothing real significant," said Bill Gerstenmaier, NASA's associate administrator for space operations, said after the launch.
Today's heat shield inspection is one of several planned during Atlantis' space station-bound flight.
Before arriving at the orbiting lab Sunday, Atlantis commander Ken Ham will fly the spacecraft through an orbital flip so astronauts on the station can snap photos of the thousands of black heat shield tiles lining the shuttle's belly.
Data from those photos, as well as a final inspection identical to today's survey, will also be examined before Atlantis is given a clean bill of health for its planned May 26 landing.
Before Atlantis arrives at the space station, it is offering a rare treat for skywatchers on Earth. The shuttle and space station are flying over much of the United States and should be clearly visible, weather permitting, to observers on the ground. Venus and the moon can also be seen in the night sky along with both spaceships.
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SPACE.com is providing complete coverage of Atlantis' STS-132 mission to the International Space Station with Senior Writer Clara Moskowitz in Cape Canaveral, Fla., and Managing Editor Tariq Malik based in New York. Click here for shuttle mission updates and a link to NASA TV.