Getting from Here to There: A Probe’s Tale

Scientists continue to analyze debris from last week’s collision between a space rpobe and the Tempel 1 comet. Scientists hope, by getting a look at the inside of the comet, they can better understand the formation of the universe. Getting to the comet, though, required some thought – in fact, a lot of thought.

Here’s how they did it:

Traveling from one place to another on Earth involves heading in the proper direction. A trip from Columbus to Cincinnati means heading south on I-71. Sending a probe to a comet would be like trying to drive to Cincinnati if Cincinnati kept moving. I-71 would be useless because by the time you reached where Cincinnati was supposed to be, it would have moved on. The trick is to know where Cincinnati will be at the time you want to arrive.

In the case of the Tempel 1 comet, NASA scientists wanted the Deep Impact probe to arrive when the comet was at perihelion, its point closest to the sun as it swung through the solar system. With a destination and time in mind, one now has to back up.

Imagine the following picture:

A discus thrower is standing on a merry-go-round with the intention of hitting the cotton-candy vendor standing outside the carrousel. If the discus thrower releases his discus while on the merry-go-round the discus will swing out from the ride in an arc traveling further from the carrousel, but still curving in the direction of the carrousel.

The Deep Impact probe acts much the same way when launched from Earth towards the impact point.

The discus thrower has to backtrack in his mind from the moment of impact to the moment he releases his discus. He has to keep in mind that he will be spinning on the merry-go-round and the merry-go-round is also moving in a circle. If he times things correctly the discus will leave his hand and move out from the carrousel in a wide spiral and smash into the unfortunate cotton-candy man. He might have to release the discus before he even sees the cotton-candy man, so timing is everything.

Professor Richard Pogge of the OSU Department of Astronomy says that like the scene on the merry-go-round, the probe and comet are governed by Newton’s Laws and the force of gravity. While the interaction between objects is complex, Pogge says that computers can use Newton’s laws to compute the positions of the objects with surprising accuracy. The only piece not easily determined is the exact position of the comet because tiny jets of water coming from the comet’s surface cause it to jiggle around within the orbit determined by Newton’s Laws.

“What’s remarkable,” Pogge says, “about Newton’s Laws of motion is that they are so very close to being deterministic. I can almost throw a rock and tell you exactly where it will land – except for the forces I can’t predict, like the jets in the comet and things like that.”

For the unknown forces, the piece of the probe that hit the comet was equipped with small jets that steered it to its target.

Pogge says that the only luck that came into the journey of half of a million miles came from the equipment itself.

“I think the luck aspect is does the computer work when it wakes up after six months in orbit, does the rocket not blow up on the pad or get aborted half way up because it goes crazy, I mean, that happens every now and then,” Pogge says.

Despite the consistency of Newton’s Laws, Pogge wants to stress that this was still no easy feat for NASA’s Jet Propulsion Laboratory.

Pogge says, “And you can see, of course, if you saw the videos of the JPL crowd, they were going nuts. And they deserve to, after a six month trip hitting it, just right down the line, it was just unbelievable. It was also really cool to watch.”

Whether a discus from a carrousel or a probe in space, the gap between moving from Newton’s theories to practical action takes skill and practice. Though NASA’s future missions involve continued probes to Mars, a much larger target than Tempel 1, that gap between theory and action still exists, meaning there are plenty more chances to hit that cotton-candy man.

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