NASA lost contact with a $158 million spacecraft early Thursday, when the robotic probe was to have left Earth orbit on a journey to explore several comets.

NASA's Comet Nucleus Tour, or Contour, spacecraft was programmed to fire its motor at 1:49 a.m. PDT, boosting it from orbit. At the time, the spacecraft was about 140 miles above the Indian Ocean.

NASA's Deep Space Network of antennas was to have picked up a signal from Contour at 2:35 a.m. By late morning, the antennas in California, Australia and Spain still had not picked up a signal from the spacecraft.

"We really are still in a search mode, trying to communicate with the spacecraft," NASA spokesman Don Savage said.

The mission's operations team was searching along the predicted paths Contour might have taken in an effort to find it, NASA said.

The spacecraft was in Earth orbit since its launch July 3. Thursday's maneuver was to have sent it on its way toward encounters with at least two comets. The mission plan called for it to meet up with comet Encke in 2003, Schwassman-Wachmann 3 in 2006 and perhaps comet d'Arrest in 2008.

The spacecraft was built by Johns Hopkins University, with assistance from Cornell University, for the National Aeronautics and Space Administration.