## Look Behind

Gramps (my Dad's father) never looked through his rearview mirror when he drove. When asked about this he'd say, "What do I care about where I've been?"

But rocket scientists have to look backward and forward when they are conducting a space mission. During spaceflight, they have to determine precisely where the spacecraft is. This is a very difficult task because they cannot see the spacecraft and there is no Global Positioning System (like we have around Earth for pinpoint terrestrial navigation) in the solar system. Rocket scientists need the spacecraft's precise location and velocity so they can predict where it will be in the future—how close it will come to its destination (a planet, an asteroid, or a comet). A tiny error in this determination can mushroom into a very large error down the road—causing the spacecraft to miss its target by millions of miles. (This problem is closely related to that of navigating a ship on the open seas, where a small error in the ship's chronometer could result in missing an island by hundreds of miles or crashing on a reef; for a brilliant account, see Dava Sobel's national bestseller, Longitude.)

To reduce the error, rocket scientists must know where the spacecraft has been in the past through a process called trajectory reconstruction. It comes down to the simple concept that, "If you don't know where you were or when you were there, it's going to be difficult to go where you want to go."

Which reminds me of a stupid joke I heard at JPL about engineers and mathematicians. (We had lots of them—engineers and mathematicians, and stupid jokes.) Managers at the lab would complain that, "The mathematicians will tell you something that is absolutely true (and they can prove it) but it's absolutely useless; the engineers will give you precise numbers (which they obtained from accurate measurements) that mean precisely nothing." The crux of the joke turns on the cultural difference between the two disciplines. Ready? Two engineers were stranded in a lifeboat on the ocean. A balloon with two mathematicians drifted overhead. The engineers shouted up at the mathematicians, "Can you tell us for certain where we are?" The mathematicians yelled down, "Yes, absolutely—but first tell us our precise location." Then the engineers, after a while, shouted up at the mathematicians, "You are in a balloon precisely ten meters above us. It is a number you can rely on." Then the mathematicians yelled down at the engineers, "You are in the Pacific Ocean. There can be no doubt of it."

See—I told you it was stupid.

But jokes like these remind rocket scientists about their dilemma in space navigation—it is difficult to get precise information about where you are because everything is relative. The spacecraft may be close to a planet, but where is the planet? We don't have precise knowledge of where the planets are located.

A lesson in life can be drawn. Sometimes it helps to reflect on where we've been when we make decisions about our future. Who are we? What do our lives stand for? Where are we going? We get meaning and guidance from the background of our lives. A wise person said, "Every day I become more the person I was meant to become." Socrates said, "The unexamined life is not worth living."

It bothered a lot of people that Gramps never looked back (especially the drivers behind him). He focused only on getting to where he was going, not on where he had been. Fortunately (somehow), he never had a serious accident and lived to the ripe old age of 91. But his driving habit left a lot to be desired.

Not looking behind you can be as dangerous as not looking around you or ahead of you.

Ask any insurance agent.

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