Of 25 U.S. and Russian missions to Mars since 1962, 11 have failed and four did not have complete missions. Some recent failures:

_ Mars Climate Orbiter: The $125 million spacecraft was lost Sept. 23 as it was about to go into orbit to study weather and look for signs of water. Investigators blamed the loss on a failure to convert English-style units of measurement _ feet and inches _ to the metric system.

_ Mars Observer: The $1 billion NASA spacecraft disappeared on Aug. 21, 1993, just before going into Mars orbit. It is believed to have exploded during fuel line pressurization.

_ Mars 96: Russia's $300 million probe suffered rocket engine failure during the launch and crashed into the Pacific Ocean on Nov. 16, 1996.

_ Phobos 1: The first of a pair of Soviet probes received an inadvertent suicide command at launch in July 1988. Phobos 2 stopped transmitting in 1989 after collecting data about Mars and one of its moons.