Deep Impact Closes in on Comet Collision

Story by Lee Tune

As a kid, did you ever hit a rock with a hammer to see what was inside? Maybe it was a chunk of sandstone that exploded in your face, or perhaps a weathered brown pebble that broke open to reveal an interior of gleaming quartz. With one well-aimed blow you learned more about a rock's strength and the nature of its inner "stuff" than you ever would have from looking at its surface.

Deep Impact, the much-anticipated NASA mission led by University of Maryland astronomer Michael A'Hearn, takes this basic but powerful childhood experiment out into the solar system. The object of the Deep Impact experiment is not a rock, but the frozen core of comet Tempel 1, a potato-shaped chunk of dust and ice some six miles long and a third as wide. Tempel 1 now is nearing its perihelion, the point at which it is closest to the Sun. It is there, more than 80 million miles from Earth, that the Deep Impact "hammer" will strike, giving humanity its first look at the inside of a comet.

This hammer is actually a self-guiding, camera-carrying "impactor" spacecraft that is about the size and shape of a wide garbage can. Joined to a larger "flyby" spacecraft, the impactor has nearly completed a six-month arc that will end with a bang on July 4th when the impactor spacecraft is run over by the speeding comet— Tempel 1.

This smashing culmination to some six years' work and more than $300 million will actually begin its final stages early on July 3rd when the flyby spacecraft releases the impactor into the path of the onrushing comet. After releasing the impactor, the flyby craft will maneuver itself out of harm's way and slow its speed so that at impact it will have a prime but relatively safe viewing position to the side and front of the comet.

The 820-pound impactor and the comet will collide at an impact speed of some 23,000 miles per hour, creating a football-field sized crater that A'Hearn and his fellow scientists expect will tell them much about the structure and composition of interior of the comet. The impact will have no perceptible effect on the orbit of Tempel 1, whose path poses no threat to Earth. It is expected that the expanding cloud of dust will increase the comet's visual brightness and make it detectable in backyard telescopes.

"What makes Deep Impact such a big deal is the fact we currently know so little about the structure and composition of a comet's nucleus, the frozen core of a comet that is surrounded by the coma cloud of dust and gas that we see when we look at comet," says A'Hearn.

"Through this mission we will learn a great deal about a comet's strength and density from the size, shape and depth of crater that is formed, and from data on the amount of material ejected and the angle at which it is ejected," he says. "Even if the unexpected occurs, such as the comet breaking apart or the impactor plowing all the way through the comet, it will still tell us a lot about the comet and its interior."

According to A'Hearn, such information not only has tremendous scientific value, it might even prove critical should humanity ever need to deflect a comet that threatens Earth.

"Data from Deep Impact also promises to improve our understanding of how the solar system formed," says University of Maryland research scientist Lucy McFadden. "And it may even shed a little light on how life on Earth might have formed," says McFadden, a member of the science team and director of education and public outreach for the mission.

But, no matter how high tech its equipment or how rigorous its science, Deep Impact can trace its linage to back to something far more simple. "The desire to explore and understand the things that make up the world around us is the essence of science, whether you are using a hammer, a telescope or a space probe," says A'Hearn.

And as any child knows, if you get to smash something in the exploration process, what a bonus! —TERP

Comets 101

Comets are chunks of ice, gas and dust that orbit the Sun. The cloud of dust and gas surrounding a comet is created by the surface heating that occurs each time a comet’s orbit takes it near the Sun. Scientists believe that the permanently frozen cores of comets contain primitive debris from the coldest and most distant regions of the disk of material out of which our solar system formed some 4.5 billion years ago.

Comets may have played life-and-death roles in our planet’s evolution. Some researchers believe comet impacts brought the water and organic molecules that allowed life to evolve on our planet. An asteroid or a comet hitting Earth also may have resulted in the extinction of the dinosaurs.

Tempel 1 poses no threat to Earth, nor does any other currently known comet. Though no comet has hit our planet in millions of years, the possibility of a future strike remains.

Flyby Spacecraft

Spectrometers—one on the flyby spacecraft and others on space and Earthbound telescopes—allow scientists to analyze the inner contents of the comet expelled or exposed by the creation of the crater.

The flyby ship carries two imaging instruments—essentially digital cameras with powerful telephoto lens—that serve as “eyes” that guide the flyby spacecraft and record images and data before, during and after the impact. The largest of these is known as the high-resolution imager (HRI). It feeds both a visible-light camera and spacecraft’s infrared spectrometer. In-flight testing has shown that the resolution of the HRI is not as good as expected. However, the problem is not a critical one and the Deep Impact team continues to expect a highly successful impact and mission.

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