The mechanisms on a spacecraft have the same function as the joints in our bodies; they are used to rotate, deploy, eject, open and close things. Designers always attempt to keep the number of mechanisms in a spacecraft as low as possible, because they have a relatively high chance of failure and usually a breakdown is critical to the mission.

For instance, if a mechanism that has to release a folded solar array does not work, the spacecraft will not be able to generate enough electricity or even, perhaps, no power at all. Galileo's stuck umbrella antenna mentioned before is another example of how a problem with one relatively simple mechanism can severely upset mission plans.

Developing mechanisms for space is pretty hard, mainly because of the high temperature changes they have to endure and the need to maximize their reliability. Lubrications used in mechanisms on Earth, such as oil or grease, evaporate or freeze in space. Moreover, if a space probe mechanism gets stuck in space there is no one around to give it a kick and get it working again.

Mechanisms for the deployment of solar arrays, antennas etc. are usually activated soon after launch, to give the space environment as little chance as possible to have adverse effects. Deployment mechanisms are normally needed only once; after they have done their work it is no longer important whether their lubrication dries up or their wheels freeze and stick.

For these one-shot mechanisms spacecraft engineers often employ pyrotechnical devices - small explosives set off by a small spike of electric current. These are small, simple, cheap and relatively reliable, but they can be used only once. As the explosives can be dangerous to people, much care for safety has to be taken during testing and integration with the spacecraft on the ground.

Pyro devices are used for various tasks: to separate a spacecraft from its launch vehicle, to permanently deploy antenna booms; to release instrument covers, to permanently open or close valves in propellant lines; to jettison heatshields; and to deploy parachutes on lander systems.

NASA's Mars Pathfinder mission with its Sojourner rover depended on the operation of 42 pyro device activations during entry into the Martian atmosphere, the descent and finally the landing. First the 11-meter (36-foot) parachute had to be shot out of the lander. Then the front heatshield was pyro technically ejected, followed by the separation of the lander from the protective backshell.

Just before hitting the ground, the airbags were released by pyrotechnically cutting the ties on their wrapping during the flight to Mars. These airbags were subsequently inflated with gas generated by burning small amounts of solid propellant in three separate gas generators.

Solid propellant rocket engines were fired to slow the lander down in the final seconds before landing, and the lander was finally cut free from the parachute with a pyro device. Like a giant beach ball Pathfinder then bounced over the surface of Mars 15 times for almost 1 kilometer (0.6 mile), rising as high as 15 meters (50 feet).

More recently, each of the larger Mars Exploration Rovers, Spirit and Opportunity, fired no less than 126 pyro devices during the same mission phases.

Other mechanisms need to work properly and continuously during the entire mission, such as antenna-pointing mechanisms and the Solar Array Drive Mechanisms that keep the solar arrays aimed at the Sun. These mechanisms require special attention and are therefore rather expensive, in the order of half a million dollars each.

Special mechanisms are those on planetary rovers, such as the electric motors driving the wheels. Their lifetime is not only measured in absolute time, but also in revolutions — that is, the number of times the wheels have gone round. There is a lot of wear and tear by Moon or Mars dust that gets into delicate mechanisms and grinds carefully polished surfaces each time a wheel makes a turn. Given enough time, the wheel mechanisms will be so badly damaged that the internal friction becomes higher than the engines can overcome, at which moment the rover will be stuck. The lifetimes of planetary orbiters are measured in years, but the birthdays of a rover on the surface of Mars are celebrated a month at a time.

On the other hand, knowing what may happen to spacecraft equipment used in harsh environments may inspire engineers to make their mechanisms so sturdy that they actually perform much better and longer than strictly required. The NASA rovers Spirit and Opportunity were, for instance, designed to last three months on the surface of Mars, but at the time of writing they are still going strong over two years after landing.

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