NASA stopped the launch of a rocket holding NASA's second Mars rover with seven seconds left until liftoff Monday night because of a problem with a valve on the rocket.
NASA officials were deciding whether they would try to meet a second launch Opportunity at 11:18 p.m.
Prelaunch preparations were proceeding smoothly for the 10:35 p.m. launch when NASA officials halted the countdown with seven seconds left. NASA Administrator Sean O'Keefe had flown to the Cape Canaveral Air Force Station for the launch.
The launch has been postponed more than a half dozen times because of bad weather, a failed battery cell and a nagging problem with cork insulation failing to stick to the rocket.
NASA has until July 15 to launch Opportunity before Earth and Mars are too far apart. The next chance is in four years.
The failed battery cell was discovered over the weekend and replaced. Fixing the cork on the Delta II Heavy rocket had been more challenging.
The cork is believed to have peeled off from the rocket during fueling, when minus-300-degree liquid oxygen is loaded into the rocket. Workers replaced pieces of the cork and used a stronger adhesive.
Workers also fixed minor last-minute problems Monday night on software and the replacement battery cell which wasn't working as expected with less than two hours until launch.
Opportunity and its sister rover, Spirit, which was launched last month, are scheduled to arrive at Mars in January and land on opposite sides of the planet as part of the $800-million mission.