Aerospace manufacturer Martin Marietta says it won't claim a $21.3 million "on-orbit performance fee" for the Mars Observer spacecraft and will return the $17 million already paid.

The Mars Observer was lost last Aug. 21, only three days short of entering orbit around Mars after a 11-month journey of 442 million miles.Martin Marietta, which built the spacecraft, said Monday it felt that not collecting the incentive award was "appropriate in light of the unsuccessful mission."

By plan, the spacecraft's radio communication with ground controllers was cut off while its components were being pressurized for an engine firing to slow the probe so it could be captured by Martian gravity. When ground controllers tried to re-establish contact they met with only silence, which has continued to this day.

An investigating panel concluded that a broken fuel line was the most likely cause. It said that a small amount of liquid nitrogen tetroxide - about two tablespoons - probably leaked during the long, cold journey and condensed upstream. The oxidizer then mixed rapidly with the spacecraft's propellant, monomethyl hydrazine, and ignited spontaneously.

Timothy Coffey, director of research at the Naval Research Laboratory and head of the panel, told a news conference on Jan. 5 that the most probable cause was "a massive rupture of the pressurization system."