Altogether, Mars Observer had seven instruments. Replacements for five of these were carried by Mars Global Surveyor, which was launched in 1996 and is now orbiting the red planet. The seventh and final replacement piece for Mars Observer's science payload, a gamma ray spectrometer, will be carried on Mars Surveyor 2001, a spacecraft scheduled for launch on March 30, 2001.

Mars Observer 's failure helped usher in the "new paradigm" of smaller, cheaper missions, says David R. Williams, a planetary scientist at NASA's National Space Science Data Center. This paradigm was reflected in the lost Mars Climate Orbiter's mission, a craft that was launched separately from Mars Polar Lander and carried only two instruments. A key advantage of this approach, notes Williams, is that "if you lose, you don't lose a billion-dollar mission."

As for what exactly happened to Mars Observer, "no one quite knows," says Williams. A review board set up to investigate the failure concluded that a fuel line rupture was the "most likely" cause. Such a rupture could have emitted volatile gases, damaging or diverting the craft. Whether Mars Observer ended up in orbit around Mars, or in a broad orbit around the sun, is unknown.