In a little over a week, the Mars Science Laboratory rover Curiosity will land in Gale Crater on the surface of Mars.

The wonderfully innovative — and frighteningly complicated — “Sky Crane” will deliver the rover to the surface. It’s a complicated entry descent and landing (EDL) profile during which one small failure could have big consequences.

ANALYSIS: When Curiosity Almost Took Men to Mars

For those unfamiliar with the Sky Crane concept, here’s a quick recap: Curiosity is currently on its way to Mars sandwiched between a backshell and heatshield. The heatshield will deflect the frictional heating as the spacecraft enters the Martian atmosphere, slowing its initial descent then falling away when its job is finished. This will uncover the ground-sensing radar on the rover’s underside.

At the same time, the 65-foot parachute will unfurl and inflate, slowing Curiosity’s rate of descent.

A little more than half a mile above the surface, explosive bolts will detonate to release a descent module from the backshell. Its retrorockets will fire, slowing the one-ton rover’s descent then dropping Curiosity on a 65 foot tether 115 feet above the ground.

It will lower the rover until Curiosity’s wheels are on the surface. Then, more explosive bolts will fire, separating the tethers from the rover. The descent module will fly away while Curiosity begins its sojourn on Mars.

There are a lot of moving parts in this EDL sequence, and a lot of places where things could go wrong. Like the bolts that will sever the tether from the rover as Curiosity contacts the surface. Though there are redundancies in place, if just one cord fails to separate the descent module could drag Curiosity to an untimely death across the Martian surface.

The reality of spaceflight is that things can go wrong. Engineers are only human and freak accidents do happen. Mars Polar Lander’s story is a perfect example of how one little thing can make a big difference, turning an expensive mission into debris.

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The Mars Polar Lander (MPL) launched on a Delta II in January 1999. When it arrived at Mars in December, it used a heatshield and parachute in the first stages of its descent. It’s legs were designed to snap out of their folded position then retrorockets, guided by a landing radar, should have enabled a soft touchdown near the edge of Mars’ south polar cap. MPL was designed to collect sampled and analyze them for insights into the planet’s surface materials, frost, weather patterns and interactions between the surface and atmosphere over time.

Unfortunately, the MPL never made it to the science stage of its mission. NASA didn’t monitor the lander during its EDL — cutting out telemetry was a cost-saving measure. Scientists expected to establish communications with the lander once it was operational on the surface, but the expected contact with MPL never came. The lander was declared lost.

Luckily, NASA wasn’t totally in the dark about what killed the MPL. Pre-launch tests had left enough data on the lander’s EDL systems that mission scientists could reverse engineer the problem and figure out what had likely gone wrong.

The MPL investigation was broken into main questions: 1) What were the biggest risks with the mission? 2) How was the system designed to cope with adverse situations? 3) What was the margin for error between how the system worked and the point where it fails?

A number of explanations emerged. MPL could have landed in dangerous terrain, either among rocks that knocked it over or on the edge of some unknown chasm. The heat shield could have failed, either because of a manufacturing defect, an impact with a micrometeoroid, or because it wasn’t up for the stresses of the mission. Uneven thrust between the retrorockets could have brought the lander down at an angle and caused it to fall. The backshell that housed the parachute could have landed on top of the lander and crushed it.

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These were all possible failures but inconsistent with data from the MPL’s development. There was only one weak point that could have really been responsible for the loss of the MPL: the sensors on the lander’s legs.

Each of the lander’s four legs used a magnetic sensor to determine when it was on the surface. The sensors reacted to a hyperextension: overextending the leg broke a connection in the joint, signaling to the lander that its weight was settling on the Martian surface.

But there was a flaw in these sensors’ design. The force of the legs deployment caused the joint to hyperextend. To the sensors, the signal meant the lander was settling on Mars. When the landers’ legs deployed 0.02 miles above the surface, it triggered a shutdown of the retrorockets and the lander fell. It hit the surface traveling 0.01 miles (160 meters) per second — much faster than the planned landing speed of 0.001 miles (1.6 meters) per second. These may sound like small values, but the MPL hit the ground traveling 100 times faster than designed.

This scenario explained the lander’s good health going into its EDL and its sudden silence. What’s worse is that it was a preventable problem. A software program could have taught the lander to distinguish between between a brief signal associated with leg deployment and a prolonged signal of the landers’ weight settling on the surface.

Of course, engineers learned from the MPL missions and sent the Mars Phoenix lander to perform the same mission in 2008. And there’s little doubt that this experience has played some part in ensuring the sky crane will be a success in August.

Image: The Sky Crane lowers Curiosity onto the Martian surface. Credit: NASA/JPL