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Spring 2004

CMPS Scientists Lead First Mission to Explore the Interior of a Comet
Distinguished University Professor Michael A'Hearn, principal investigator for the $312 million Deep Impact mission, and Associate Research Professor of Astronomy Lucy McFadden know exactly where they will be at dawn on December 30, 2004.

With hearts palpitating and stomachs tied in knots, they will be adjacent to Cape Canaveral, Florida, awaiting liftoff of the NASA-sponsored Deep Impact mission to the comet Tempel 1. If all goes according to plan, A'Hearn and McFadden and their team of scientists and engineers will make history.

The Countdown and Launch
The researchers will be on hand to watch as two mated spacecraft - a large flyby craft and a small vehicle known as an impactor - are hurled into outer space by a giant Delta II rocket. Bound together, the spacecraft will race across the inner solar system in order to intersect the comet on its elliptical path around the sun.

Six months later, on July 3, 2005, the flyby and the impactor will approach Tempel 1, a cucumber-shaped comet about the size of northwest Washington, D.C. At a crucial moment, the spacecraft will separate. Both craft will point their telescopes toward the comet. The flyby will fire its thrusters, change course and miss the comet. The impactor, traveling 23,000 miles an hour, will head straight for the nucleus.

The following day, July 4, 2005, the impactor, not much bigger than a garbage can but made of 816 pounds of pure copper, will hit the comet, blasting open a crater the size of a football stadium between seven and 15 stories deep. As the crater's contents, called ejecta, spew into space, the flyby will slow down. The craft will have only 800 seconds (just 13 minutes) to record spectra and other measurements from the crater and the ejecta. After that, the crater will be out of range.

Data from the flyby and the impactor will be transmitted back to earth in near real time to three huge radio antennae at observatories participating in the Deep Space Network. Leading this effort will be the receiving station in Canberra, Australia. Facilities located in California and near Madrid, Spain are also taking part.

In addition, the earth-orbiting telescopes, Hubble and Spitzer, will observe and record the impact. Human observers will also play important roles. Scores of professional astronomers and skilled amateurs all over the world will monitor the comet before, during and after impact through different regions of the spectrum. "These viewers enhance the scientific return of the mission by giving us what amounts to different pairs of glasses, providing information that we will not get from spacecraft," explains McFadden. "Collecting all this disparate data will enable us to understand and interpret what we see," she adds. "This, of course, is the real work of the mission."

Comet Tempel 1

What's a Comet? A comet is a "celestial wanderer" located far from the sun, and that orbits around the sun in an elliptical path. So far as we know, comets are made from ice and dirt. The ice inside preserves a record of conditions when our solar system was formed. Comets have solid nuclei and two streaming tails. One tail contains dust, the other, gas ionized by solar wind. Each time the comet goes around the sun, its surface is heated, dragging dust and gas and eroding the surface, causing major differences between the comet's exterior and interior. Although the nucleus is only the size of Washington, D.C., the tail stretches as far as from the earth to the moon.

The Payoff
So why Deep Impact? What did NASA hope to accomplish when it included this initiative in its solar system Discovery program five years ago? "Deep Impact delivers a lot of bang for its buck," says A'Hearn, an internationally recognized astronomer. "For a relatively small investment, we expect to answer fundamental questions about the physical properties of the comet's nucleus, such as its mass, texture and porosity. We hope to learn how the comet's nuclei differ from its surface. These are questions that astronomers have pondered forever.

"The comet is a kind of laboratory enabling us to study the archeology of the solar system," A'Hearn continues. Comets are like time capsules. Though their surfaces heat when they approach the sun, their deep interiors do not. "The ice inside the comet preserves a record of how things formed at the beginning of the solar system," he explains.

Members of the CMPS Deep Impact Team

Members of the CMPS Deep Impact team include (from left, front row) Administrative Assistant Linda Diamond, Graduate Assistant Donna Pierce, Associate Research Scientist Rosemary Killen, Faculty Research Assistant Anne Raugh, Associate Research Faculty Elizabeth Warner, Faculty Research Assistant Stephanie McLaughlin and Associate Research Professor Lucy McFadden; (from left, back row) Professor Mike A'Hearn, Research Associate Tony Farnham, Undergraduate Student Tim Cline, Senior Research Scientist Casey Lisse, Research Associate Ed Grayzeck and Postdoc Olivier Groussin.

Challenges Ahead
Institutionally, Deep Impact is a fourway collaboration among NASA; CMPS, which is in charge of science, instrumentation, education and outreach; California Institute of Technology's Jet Propulsion Laboratory (JPL), which is managing the space voyage; and Ball Aerospace & Technologies Corp. in Boulder, Colorado, which is building the space vehicles. Hundreds of scientists, engineers and technicians from these institutions are taking part in the design, production, testing and operation of the Deep Impact vehicles and their instruments.

The biggest technical headache faced by all these experts has been designing and testing the computer hardware and software for the space vehicles. These are not the kind of components that can be purchased from a catalog. Each has been custom designed to withstand the force and vibrations of the launch, the heat of exposure to the sun, the cold of deep space and the impact of cosmic rays, solar wind and other conditions. Every component has been tested repeatedly inside a vacuum chamber that simulates the conditions of space flight. After a discouraging year in which far more went wrong than right with the computers, "We turned a corner at the beginning of this year, and we are solving computer problems at a far faster rate than new ones are appearing," A'Hearn reports.

Once the computers have passed muster, the spacecraft will be assembled and subjected to "incompressible" test protocols. Every operation that occurs during the mission will be performed according to a demanding script. Following testing, the spacecraft will be packed and shipped in a heavy vibration-proof truck from Boulder to Cape Canaveral for more tests. A'Hearn expects all of this work to be complete by October.

In November and December, the mating of the spaceships to the rocket will take place and the whole system will be tested again. Toward the end of December, A'Hearn and his team will start worrying about the weather at Cape Canaveral.

In order to successfully carry out the experiment, the space vehicles must intersect the comet at a particular spot in outer space within a very narrow band of time. To achieve the perfect intersection, launching must occur within a five-minute time frame during a clearly defined window that opens on December 30, 2004 and closes 30 days later. If the launch is delayed because of bad weather, the mission could be cancelled.

Short of an unexpected extension of the hurricane season, however, A'Hearn and McFadden optimistically anticipate liftoff in late December or early January. "We are doing everything humanly possible to ensure a safe and successful launch," McFadden says. "No doubt our anxiety will build as we move toward December 30, but I expect this is going to be a very happy new year."

Calling All Kids
NASA wants to share the excitement of space exploration with the public at large, especially young people. To do so, the agency earmarks one percent of Deep Impact's budget for education and outreach. McFadden runs this portion of the Deep Impact program. Her team of scientists and science educators has created hands-on science units that they are distributing to teachers and students via the World Wide Web.

In addition, the team has created the Small Telescope Science Program that elicits the help of amateur space enthusiasts with video cameras rigged to their telescopes who will observe the comet before, during and after Deep Impact. These images will help the team monitor and assess the event. Those who would like to participate can register on the Deep Impact Web site. This site also includes activities for kids, some math-based and challenging, others just plain fun.

The Web site address is: with educational materials at

Please visit the CMPS webpage to read other issues of the Continuum.

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