NASA's Mars Opportunity rover was supposed to last 90 days on the red planet. That was 11 years ago, and it's still roving. The Mars Odyssey orbiter had a stated two-year mission, starting in 2002. Today it still collects data and acts as a communications hub for the rovers on the ground—and took the photos that proved near-conclusively that there's seasonal water on Mars.
It's not just that the missions last beyond their expiration dates. It's that they last for decades after, and it's not limited to Mars, either. Both Voyager probes are still kicking at the outer boundary of the solar system. Cassini was supposed to go offline seven years ago, but just keeps sending back stunning pictures of Saturn and its amazing moons. It will be retired into Saturn in 2017.
Of course, there's a public relations advantage to setting reasonable goals and blowing past them, as opposed to setting overly ambitious targets you never reach, especially if you're an organization that has to ask Congress for money. But NASA keeps its crafts going so long through a particular kind of smart building. Erik Conway, a historian at the Jet Propulsion Laboratory, recently wrote a history of Mars exploration called Exploration and Engineering. He has two words for why probes tend to last so long: engineering conservatism.
"What tends to kill electronics is thermal cycling—in other words, expansion and contraction of circuit boards and components due to changes in temperature. JPL's engineers (and their partners at Lockheed Martin-Denver, which builds about half of the Mars spacecraft under contract to JPL) have gotten very good at creating relatively benign thermal environments for the electronics," Conway said in an email to Popular Mechanics.
Conway says that so long as the electronics are thermally protected, the moving parts are the things that are really at risk. These parts are "over-designed" by engineers not wanting their work to die a premature death. Thus, what's most likely to do in a spacecraft on Mars is depleted battery capacity as the batteries become exhausted from constant recharging over a period of years. But giving it a more conservative shelf-life allows for the possibility of exceeding expectations.
"So it isn't so much that the estimates are conservative," Conway says. "The engineering is."
