91av

A CONFUSED sensor may have sealed the fate of NASA’S Mars Polar Lander,
condemning the craft to crash into the Red Planet. Tests have shown that
deploying the spacecraft’s landing gear could fool the sensor into thinking the
craft had landed, triggering a command to shut down engines that should have
slowed the lander’s descent.

Ground control lost contact with the Mars Polar Lander when it was due to
land on Mars on 3 December 1999, though no one was quite sure why. The lander’s
three legs were supposed to swing into position after a parachute had opened and
the lander had dropped its heat shield. Then thrusters should have fired at
about 1.4 kilometres above the ground, slowing the spacecraft’s descent and
turning it to land vertically.

Analysis of pre-launch tests has now shown that opening the landing legs
could have triggered the landing sensor, sending a command to shut down the
descent thrusters. No one realised that such a scenario could happen, probably
because different teams tested different elements of the landing system.

“It is one of the scenarios the review board is looking at,” a spokeswoman
for NASA’s Jet Propulsion Laboratory in Pasadena told 91av. The
panel NASA commissioned to study the problem is also working on other
possibilities, and its final report is not due until the middle of this month.
The next lander, based on the same design and originally planned for launch in
April 2001, is “not going to fly as is”, says one scientist who worked on the
project. The launch may not go ahead, and if it does, software will have to be
altered to make sure the sensor resets after the landing legs open.