Work on Your Average Performance

Rocket scientists have a term they use to describe the average expectation for a space mission. They say everything is "nominal" when things are going as planned, when the spacecraft is following the average trajectory out of thousands they simulated in their computers before flying the real mission.

All those simulations are like the practicing that athletes, musicians, and actors do before the actual performance. There is a big difference between knowing (the theory of) how to do something and the actual performing. Parents say that their toddler understands a lot, even though she doesn't talk yet. This is because understanding speech is easier than speaking. Listening is understanding language, but performing is putting language into practice.

A coach was asked how his world-class figure skater would do in a competition for world champion. He said, "If she manages to do her average performance, she will win, because no one can beat her average performance." This is why people say you should prepare for the worst and hope for the best.

Spaceflight is a performance. Years of planning, simulation, and practice are behind it. Rocket scientists spend a great deal of time simulating what could go wrong and how to fix it when it does. They worry about Murphy's law. They know something will go wrong—that's why all good designs have plenty of backup systems. But in the end, in the real mission, usually something in between the extremes happens. The worst-case scenario doesn't occur; neither does the best.

The spacecraft does its nominal performance. The rocket scientists are very happy to hit their average—it is a very high standard, which is why spacecraft sometimes last a lot longer than the original planned mission. Often, rocket scientists are just a little lucky and the spacecraft does better than the nominal mission.

The Voyager 1 and 2 spacecraft were planned to encounter Jupiter and Saturn with a 95 percent probability of success. If all went well with Voyager 1, then Voyager 2 would be targeted to Uranus with a 60 percent chance of success and to Neptune with only a 40 percent chance of success. So the scientists didn't really expect that Voyager would make it all the way to Neptune. It was a bit of a long shot.

In fact, both spacecraft had problems shortly after launch. The rocket scientists had tried to make the twin Voyagers smart (by adding fault-protection software), but then the spacecraft started acting funny (refusing to obey commands because the spacecraft got too twitchy about taking risks) and so their programming had to be ripped out and replaced (all this during flight) to make the spacecraft obey direct orders from Mission Control. A few times the babysitters, who constantly monitored the health of the spacecraft, lost contact, and the engineers had to do some detective work to figure out what went wrong and how to reestablish the communication link.

One time, one of the Voyagers locked on the wrong star (a case of mistaken identity) and pointed its antenna not toward Earth but in an entirely different direction—where it thought Earth was.

Fortunately Voyager 2 had a great mission operations team, and the spacecraft eventually made the whole trip from Jupiter to Saturn to Uranus and to Neptune—traveling a distance of nearly 4 billion miles from the Earth in twelve years. It was one of the most successful missions in the history of space exploration.

Voyager 2 performed just a little better than its average.

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