Cornell space scientists will make an impression with probe's visit to comet

By Alex Kwan

Cornell space researchers are playing a prominent role in scientific planning, image acquisition and data analysis for NASA's forthcoming Deep Impact mission to study deep inside a comet for the first time.

On Jan. 12 a Delta II rocket will lift off from Pad 17-B Cape Canaveral Air Force Station, Fla., carrying a two-part space probe on a six-month, 268 million-mile voyage to comet Tempel 1, where it will send a projectile crashing into the comet's 3.7-mile-wide nucleus.

The impact should create a stadium-sized crater, allowing scientists to study pristine material inside the comet dating back to the formation of the solar system.

"Deep Impact is an active experiment. We make an impact crater by dropping off a several-hundred-kilogram bowling ball of copper," said Peter Thomas, a senior researcher in the Center for Radiophysics and Space Research at Cornell and a Deep Impact science team member.

The two parts of the spacecraft will be the flyby unit, which will move into the orbit of Tempel 1, and the 820-pound smart impactor. Cornell will serve as a depository and processor of data gathered from the two cameras, on the flyby spacecraft and on the impactor, and from the onboard spectrometers.

Twenty-four hours before the July 4, 2005, impact, the flyby spacecraft will point its high-precision tracking telescopes at the comet and release the impactor on a course to hit the comet's sunlit side. The impactor is a battery-powered robot that operates independently of the flyby spacecraft for just one day. It is called a "smart" impactor because, after its release, it takes over its own navigation and maneuvers into the path of the comet. A camera on the impactor will capture and relay images of the comet's nucleus just seconds before the 23,000-miles-per-hour collision.

The entire collision, which could last only a few minutes, will be observed by a combination of cameras onboard the spacecraft and by telescopes on Earth.

The collision will create an impact crater two to 14 stories deep, ranging in size from a house to a football stadium. Ice and dust debris will be ejected from the crater, revealing fresh material beneath. Sunlight reflecting off the ejected material should provide a dramatic brightening, fading slowly as the debris dissipates into space or falls back onto the comet.

"Deep Impact is an experiment where we do things in a classical sense because nobody knows how big the crater is going to be," said Thomas. Despite the force of the impact, scientists are confident that the comet's orbit will not be altered significantly.

Thomas is interested in the surfaces of planets, satellites and asteroids. "I want to analyze the images and map out topography to look at the changes occurring in the comet nucleus after the impact because we really have no idea how dense these comets are," he said. "They can be fluffy as snow or solid as ice." Since comets have elongated and elliptical orbits, they are more difficult to investigate compared with asteroids, Thomas noted.

Joseph Veverka, professor of astronomy and chair of the astronomy department, is a co-investigator on the mission and an expert in photometry. He studies how scattered light can be used to determine the properties of the surface materials.

Comets are time capsules that hold clues about the formation and evolution of the solar system. They are composed of ice, gas and dust, which is the primitive debris from the solar system's earliest and coldest formation period 4.5 billion years ago. As a comet moves across space, it releases trapped gases and dust particles to form a brilliantly visible tail. Astronomers have suggested that comets become asteroids when the trapped gas supply runs out, much like a volcano becoming dormant or extinct. The Deep Impact mission can provide direct evidence.

"Currently our knowledge about comets comes from ground-based observations and several quick flybys," Thomas said. "Deep Impact will put a lot of decades-old information in context."

Comet Tempel 1 was discovered in 1867 by Ernst Tempel. The comet has made many passages through the inner solar system, orbiting the sun every 5.5 years. This makes the comet a good target to study evolutionary change in the mantle, or upper crust.

Alex Kwan is a science writer intern with Cornell News Service.