Mars has been a major target of exploration since the beginning of the space age. It also has been an elusive target, as demonstrated once again by Mars Climate Orbiter's failure in September. Overall, about two in three missions to the red planet have failed.

"Mars has probably had more than its fair share of failures," says Ronald Greeley, an Arizona State University geology professor and former NASA scientist, who has worked on several Mars missions.

Among the failed Mars missions were the first six attempts to send probes to the planet, five by the Soviet Union, which occurred in the early 1960s. Another failure was the U.S. spacecraft Mars Observer, which lost contact with controllers in August 1993 as it neared the red planet.

However, Mars also has been the scene of some major successes, such as the Viking project of the mid-1970s. This, says Greeley, "is still the most complicated mission that's ever flown." Viking involved four different craft, two orbiters and two landers.

Comparing success rates of Mars missions to those of other missions may be misleading. Probes to the giant outer planets have had a high degree of success, but there have been far fewer such missions.

Greeley notes that "the track record is pretty good" in exploring Venus, but disputes any notion that therefore Mars is an unusually difficult destination. Venus, he notes, is an extremely harsh environment, especially for landers. The higher failure rate of Mars missions, he thinks, is just the luck of the draw.

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