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 this morning.
Using the shuttle's robotic arm, astronauts will scrutinise 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's called a "focused inspection".
Mr Cain said the two-hour manoeuvre was being done out of an abundance of caution and will not 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 on Wednesday. Initially, the photos showed seven sites with dings or gouges, but six of them were further analysed 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 location and size gives engineers a bit of confidence that the damage is not the type that caused the disintegration of the space shuttle Columbia in 2003.
They also note that similar damage on Endeavour in 2007 - coincidentally commanded by Scott Kelly, brother of current commander Mark Kelly - turned out not to be a problem.
Mr Cain told reporters that it is so unlikely that the gouge will be problematic that Nasa has not even considered making contingency plans for fixing the tile in flight.
Nasa can repair damaged tiles using a souped-up version of a caulking gun during a spacewalk.
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 re-entry 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.