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The Strange Disappearing Act of the Mars Polar Lander

When sending a multi-million dollar lander to Mars, do your homework.

What's worse than a crashed lander? A silent lander. Landers typically relay telemetry to mission control during their descent towards a target planet. This telemetry is vital. It gives engineers and program managers continual insight into the spacecraft's well being. In cases when landers fail, telemetry is often all engineers have to figure out what happened. But if a lander doesn't call home, its designers are left in the dark. Not the best situation with a multi-million dollar project.

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This was the case of the ill-fated Mars Polar Lander (MPL), sent to the red planet in 1999 not to find life but rather to further scientists' understanding of the Martian climate. It spoke to NASA throughout its interplanetary journey then suddenly disappeared.

The lander traveled to Mars in a conical casing, the base of which was the heat shield that protected it from the heat of atmospheric entry. Just over five miles above the surface, the main parachute deployed, slowing the payload. The heat shield jettisoned and exposed the lander's underside. Less than a mile above the surface, the lander separated from the casing and its retrorockets ignited. Under its own power, the lander slowed its descent further before its legs unfolded from their stowed position. With the legs deployed, the lander slowed to a near standstill and lowered itself toward the surface. The retrorockets shut down the moment sensors in the lander's footpads detected contact with the surface.

NASA had done routine checks on the lander during its interplanetary journey. Prior to atmospheric entry, it had reoriented itself flawlessly and begun the planned loss of communication with mission control – the spacecraft's antenna faced away from Earth throughout its entry, descent, and landing (or EDL). Contact with the lander was expected the resume 45 minutes later when it was safely on the surface with its antenna reoriented towards Earth.

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The expected transmission never came. Contact was lost sometime during the descent on Dec. 3, 1999. NASA tried to call the lander for weeks to no avail. MPL was declared lost on Jan. 17, 2000.

The orbiting Mars Global Surveyor began the hunt for the lost lander soon after loss of communication, transmitting images of the intended landing site to Earth. But even its highest resolution images produced no clear visual evidence of the lander. NASA's Jet Propulsion Laboratory (JPL) put together a team of men and women who had built the lander to investigate its possible fate.

One problem was immediately evident – the lack of telemetry gathered during EDL. Severing communication with the lander during this critical phase had been a cost-saving decision. EDL telemetry hadn't been considered vital to the success of the mission.

JPL was able to compensate for the lack of telemetry thanks to its rigorous pre-launch testing. Engineers had run extensive tests on each piece of MPL's hardware and software. They could reverse engineer the problem – use their intimate knowledge of the lander's systems to work backwards to uncover possible fail points and determine the most likely course of events. There were a host of possible reasons behind MPL's silence. The heat shield could have failed; a faulty design in the lander or its retrorockets could have caused it to impact the surface at an angle and crash; the landing site might have been unsurvivable or the lander could have touched down on a precipice and rolled into a chasm. The casing that held the lander during atmospheric entry could have fallen on top of the lander and crushed it.

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Plausible as these explanations were, they didn't line up with data from the MPL's development and testing. There was one possible fail point, however, that was supported by the test data: the landing sensors.

Each of the lander's legs used a magnetic sensor to determine when the lander arrived on the Martian surface. The sensors were set to react to a hyperextension of the leg's joint as the weight of the lander settled onto it. The signal would in turn shutdown the retrorockets. But the sensors had a fatal design flaw.

When the folded legs deployed during the landers' descent, the joints experienced the same hyperextension as they did at landing. Data from MPL development and deployment tests confirmed that a false touchdown signal occasionally registered with when the legs deployed, prematurely shutting off the retrorockets. The lander's onboard software couldn't distinguish between the landing legs deployment and a touchdown.

In short, the JPL investigation team determined that the lander's legs deployed and the faulty signal triggered a shut down of the retrorockets. The lander was far enough from the surface that its free fall gathered enough speed to destroy the spacecraft on impact. This would explain the lander's good health going into EDL and the total lack of communication after its intended touchdown time.

The faulty signal was a preventable problem. A simple software program could have taught the lander to distinguish between a touchdown signal from a joint quickly snapping into position and a signal from a prolonged joint separation as the weight of the lander settled on its legs; the latter being the rue signal. Unfortunately, no one had included such a program into the Mars Polar Lander.

Experience turned out to be the key to solving the riddle of the lost MPL and the mission wasn't a total failure. Reverse engineering determined the problem and enabled engineers to start working on a solution. The problem was eventually fixed and MPL was resurrected and reborn as the successful Mars Phoenix Lander that arrived on Mars in 2008.

It's a good thing NASA did its homework before sending the spacecraft to Mars. Otherwise losing that lander could have been really embarrassing.

Connections:
Secrets of the First Manned Spaceflight
Saturn V Got Us To the Moon, and Then Disappeared
The Man Who Chose the Moon: President Kennedy and the Bold Decision That Began the Race to the Moon