NASA has accepted that it made a mistake in its calculations about a tear in space shuttle Atlantis' heat protection.
But this should not prevent the ship's safe return to earth, the agency added.
Deputy shuttle program manager John Shannon said an area beneath a torn heat-resistant blanket near the shuttle's tail had gotten hotter than the scientists first thought during Atlantis' launch into space on June 8.
While the difference in temperatures was relatively small -- 410 degrees Fahrenheit (210 degrees Celsius) instead of 350 degrees Fahrenheit (177 degrees Celsius) -- and would not have damaged the shuttle, errors involving shuttle heat shields raise eyebrows since the Columbia disaster in 2003.
Despite the error, officials said the shuttle was on course for landing on Thursday at Kennedy Space Center in Florida if weather permits.
Shannon blamed the error on a faulty assumption about what materials were beneath the inches-long (centimetres) tear. The error was spotted when astronaut Danny Olivas went out to repair the blanket, which protects part of the shuttle from heat when it is in the earth's atmosphere.
Instead of a cushioning layer NASA had expected, there was a graphite material.
Shannon, in a briefing at Johnson Space Center, blamed bad paperwork for the mistake, but said the error posed no threat to Atlantis.
Later, shuttle managers officially deemed the shuttle fit for landing.
Columbia broke apart in 2003 as it came back to Earth because of an undetected break in its heat shield caused by a strike of loose insulation from the fuel tank at launch. Seven astronauts on board were killed.
Landing flight director Norm Knight said NASA hopes to bring Atlantis back to Kennedy Space Center in Florida on Thursday, but the weather forecast is for low clouds and possible nearby thunderstorms, both of which would prevent landing.
''I know the forecast now is no-go, but weather changes,'' Knight said. ''If weather doesn't cooperate we will keep the crew safe and the vehicle safe and go around for another day.'' If Atlantis cannot land today, NASA will consider directing it to Edwards Air Force Base in California on Friday, he said. The shuttle has enough supplies to stay in space until Sunday.
Atlantis was docked to the International Space Station for nine days, during which the crew installed a new pair solar power panels to generate additional electricity.
The crew conducted four spacewalks to help prepare the $100 billion orbital outpost for the arrival of new laboratories built by Europe and Japan.
A key set of computers on the station crashed for several days but were restored by an improvised rewiring job.
The US space agency plans 12 more construction missions to the space station, which is slightly more than halfway built, before the shuttle fleet is retired in 2010.
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