The launch of an unmanned Delta 2 rocket from Vandenberg Air Force Base in California was called off less than a minute before liftoff Tuesday when the launchpad's water system failed, a live NASA Television broadcast showed.

The rocket, built and flown by United Launch Alliance, a partnership of Lockheed Martin Corp. and Boeing Co., was due to lift off at 2:56 a.m. from a launchpad that had not been used in nearly three years.

The pad's water deluge system helps reduce the high temperatures and potentially damaging acoustic vibrations from launch.

The rocket carries NASA's $465 million Orbiting Carbon Observatory. Built by Orbital Sciences Corp., the rocket is designed to measure where carbon dioxide, a greenhouse gas tied to climate change, is moving into and out of the atmosphere.

United Launch Alliance had just 30 seconds to get the rocket off the launchpad to properly position the satellite at the front of a train of polar-orbiting spacecraft that passes over Earth's equator at the same time every afternoon.

"As we only have a 30-second launch window, launch will not be occurring this morning," NASA launch commentator George Diller said.

Once the rocket was drained of propellants, technicians were able to find the source of the problem — a stuck valve, NASA said in a status report.

A second launch attempt was planned for 2:56 a.m. Wednesday.