Flaw Found in Mars Lander's Design

Updated 3:41 AM ET February 18, 2000

By MATTHEW FORDAHL, AP Science Writer

PASADENA, Calif. (AP) - Engines controlling the final descent of the Mars Polar Lander might have shut off prematurely, sending the $165 million probe crashing to the planet's surface, according to a new scenario being investigated by scientists.

Engineers say they have found a design flaw that could have caused the engines to switch off while the probe was still more than 100 feet above the surface. A fall from that altitude would have destroyed the NASA probe.

The flaw is one of several possibilities under investigation in the loss of Polar Lander, engineers said Thursday.

"The problem has been identified with the design," said Sam Thurman, the lander's flight operations manager at NASA's Jet Propulsion Laboratory. "That's one of several possible causes of loss of mission that have been identified during the investigation."

The last signal from the probe was received Dec. 3, just before its descent to the surface for a 90-day mission to study the climate history of Mars.

Two review boards are examining the various failure possibilities and implications for future Mars missions. However, because no information was returned during the lander's descent, the reason for the failure may never be conclusively known.

Other scenarios include the possibility that the spacecraft landed on the steep side of a canyon and toppled over, that its complicated descent system thrusted unevenly or that its radios broke down. It was not known if any scenario has been ruled out.

The latest theory to emerge suggests jarring during descent, possibly from release of the craft's back shell, might have triggered an early shutoff of the engines that were supposed to control and slow the lander's descent.

The dozen thrusters were designed to turn off after one of the probe's three legs made contact with the surface. Engineers say a movement of the legs similar to landing could have led the probe to think that it had touched down when it had not.

Polar Lander's failure followed the embarrassing loss of its companion orbiting spacecraft, the Mars Climate Orbiter. Engineers discovered that a failure to convert English measurements into metric units caused the orbiter to burn up in the atmosphere as it was beginning to circle the planet on Sept. 23.

In 1993, the $1 billion Mars Observer suddenly disappeared just three days before it was to begin circling the Red Planet. In that case, a four-month investigation determined that ruptured fuel lines were the most likely cause.