HOUSTON - NASA ordered Endeavour's crew to take an unusual close-up look at a damaged tile in the space shuttle's delicate heat shield early Saturday morning.

Using the shuttle's robotic arm, astronauts will scrutinize the gouge on the shuttle's underbelly with a high-resolution camera and a laser attached to a boom.

"There's nothing alarming here, and we're not really concerned," said LeRoy Cain, chairman of the shuttle mission management team that decided Friday to order what is called a "focused inspection."

Cain said the two-hour maneuver was being done out of an abundance of caution and won't cause any disruption to the crew or its 16-day mission to the International Space Station.

The damaged tile was spotted in photos snapped by the station crew just before the shuttle linked up Wednesday. Initially, the photos showed seven sites with dings or gouges, but six of them were further analyzed and turned out not to be a problem.

The one site that remains a concern is the size of a deck of cards, just below the rear landing gear.

The delicate tiles are part of an intricate heat-protection system that keeps the shuttle, especially its bottom and edges, from burning up during its fiery reentry into Earth's atmosphere. In 2003, damage to the edges and tiles allowed too much heat in, destroying Columbia and killing seven astronauts.

Since then, shuttles have been checked in flight for any ice or foam debris damage from liftoff, to make sure the shuttle is safe to fly home. This is only the fifth time an extra inspection has been needed in 21 flights.

Friday turned out to be a day of small concerns for NASA, after an early-morning space walk had to be cut a tad short because of a sensor problem on a spacesuit.

Nearly five hours into the 61/2-hour space walk, mission controllers noticed that Gregory Chamitoff's carbon dioxide sensor wasn't working. NASA needs to know if levels of carbon dioxide - expelled when you breathe - get too high.

It's likely that moisture caused the infrared sensor to fail, lead space-walk officer Allison Bolinger said.

The levels were probably not too high, but controllers told Chamitoff and spacewalking partner Drew Feustel not to finish installing an antenna on the space station because it would take too much time.