People have targeted Mars as a primary destination for solar system exploration since space travel began. Most missions to the Red Planet, though, have ended in failure.

But failure is not a reason to shy away from an aggressive space-exploration program, many NASA officials argue. Failures are a necessary part of making advances, and all missions entail some risks, they say.

Perhaps the greatest catastrophe, both in scientific and economic terms, was the 1993 loss of the Mars Observer just days before it was to reach the planet to begin a detailed mapping and scientific mission.

The observer, which cost nearly $1 billion, carried seven scientific instruments. Although replacements for several of those experiments were re-flown aboard the Mars Global Surveyor, which reached Mars in 1997, NASA has still not made up for the loss of the observer.

One Mars Observer instrument -- the Pressure Modulator Infrared Radiometer (PMIRR) -- was lost a second time when the Mars Climate Orbiter crashed into Mars' atmosphere in September. A copy of the PMIRR was one of the two instruments aboard the climate orbiter.