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When we talk about judging, about getting real, we're usually talking about limits. In rocket science parlance, we talk about dealing with "constraints."

If Johnny can eat one apple in five minutes, how many apples can he eat in sixty minutes? If you do the math you get twelve apples, but Johnny's mother knows better because she's a realist. She knows he's going to start slowing down on the second apple and his little stomach—which is smaller than his eyes—will probably not accommodate the third apple.

Johnny's stomach has its limits. Johnny might be able to imagine eating twelve apples in one hour, and he might bet his lunch money that he's going to do it—but we know better. (Don't try this bet against Paul Newman, however. In Cool Hand Luke, he bet his prison inmates that he could eat fifty hard-boiled eggs, and won— which just proves that you shouldn't underestimate limits either.)

Knowing your limits is an important aspect of thinking about a problem. When you plan a long trip, you have to allow for a number of limits including the size of your gas tank, the speed limit, your budget, even your physical limitations.

In rocket science, these limits are extraordinarily important. The amount of onboard propellant determines how long a spacecraft can continue to operate in space. In the multi-ton (mobile home sized) communications satellites that make MTV possible, the value of one year's worth of propellant is over \$100 million. That's just for one hundred pounds of propellant. The satellite itself is worth \$1 billion—until it runs out of precious station-keeping propellant, when its value goes down to zero. Then another satellite has to be launched into a 22,000-mile-high orbit. Amazingly, several of these satellites are launched every year.

When Armstrong and Aldrin landed on the moon, they had to deal with some very serious limits. They had to land softly enough to avoid damaging their spacecraft, they had to avoid obstacles, and they had to do it quickly—before their propellant ran out—or they'd fall to their deaths on the lunar surface.

Unfortunately, the Apollo 11 lunar module missed its landing site by four miles (due to a one inch per second velocity error at the beginning of their descent orbit), and Neil and Buzz found themselves hovering over a boulder-strewn field. They had a fuel gauge that was as inaccurate as the one in your car. (We can go to the moon, but why can't we make an accurate gas gauge?) Mission Control back in Houston anxiously called out, "sixty seconds," their estimate of how much longer the fuel would last. Then, "Thirty seconds." Then they waited and listened. Buzz Aldrin read off altitudes and descent speeds, "forty feet, two and a half down. Picking up some dust."

By this time the fuel gauge was on empty. But you know when your car's gas gauge is on empty how it might still have a gallon left—or it might be bone dry? It depends on the car. The lunar module fuel gauge had an error of about 2 percent—and it was registering empty. They could run out of fuel at any second.

Finally Neil Armstrong called out, "Houston, Tranquility Base here. The Eagle has landed."

To which Mission Control replied, "Roger, Tranquility, we copy you on the ground. You've got a bunch of guys about to turn blue. We're breathing again. Thanks a lot."

So the moral of our story is that when you try to solve problems that have all sorts of limits, you are thinking like the rocket scientists and the astronauts who made the first landing on the moon happen. It is easy and fun to say, "Think outside of the box," and there is a time for that kind of thinking. But when you want to get real, you have to stay inside the constraint box—that's where the challenge is.

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