Atlantis pilot Gregory "Ray J." Johnson fired the shuttle's twin orbital maneuvering engines for more than two minutes to drop Atlantis into space less cluttered by debris for half of its orbit. The move, completed at 11:02 a.m. EDT, came about 90 minutes after a final thruster burn pushed Atlantis away from the Hubble Space Telescope, which the crew released just before 9 a.m.
The fifth and final Hubble servicing mission faced an above average risk of a catastrophic hit from micrometeoroids and orbiting debris, or MMOD.
The odds were 1 in 229, compared to about 1 in 300 for a mission to the International Space Station.
Hubble's orbit of about 350 miles above Earth has more debris floating in it than the International Space Station's orbit more than 100 miles below.
Atlantis' engine burn dropped the low point of its orbit from 350 miles to 184 miles above the planet.
The move was one of the steps managers devised to keep the mission's odds better than a 1-in-200 threshold that would have required approval from top NASA officials.
A little before 1 p.m., Atlantis will begin a final inspection of its heat shields to make sure debris hasn't damaged them during the flight's first nine days.
As on the mission's second day, the astronauts will attach a 50-foot boom to the shuttle's robotic arm. Equipped with cameras and laser sensors, the boom will scan the orbiter's wing leading edges, nose cap and belly.
By Wednesday, analysts should know whether there's any damage needing repair or whether Atlantis is cleared for a planned 10:03 a.m. Friday landing at Kennedy Space Center.
A clean inspection would also release Endeavour from its role as a rescue shuttle.
Endeavour been standing by on launch pad 39B at Kennedy Space Center in case Atlantis sustained irreparable damage, because the Atlantis crew cannot reach the space station for shelter.
If cleared, Endeavour will roll to pad 39A later this month to prepare for a targeted June 13 launch to the station.
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