The first space shuttle mission in more than two years was postponed less than three hours before its scheduled launch Wednesday when NASA encountered a problem with one of the external fuel tank’s low-level sensors.

The shuttle crew, led by commander Eileen Collins, was already strapped into the shuttle when the launch was postponed at about 1:30 p.m. ET. The seven astronauts climbed back out and were driven back to their quarters while the launch-pad team secured the shuttle.

The space shuttle Discovery had been scheduled to lift off at 3:51 p.m. ET on a mission to resupply the international space station and test safety procedures that had been developed in the wake of the shuttle Columbia’s catastrophic breakup in February 2003. All seven of Columbia’s astronauts were killed in the tragedy, which led NASA to ground its shuttle fleet.