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.
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.
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.