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Apache Point Observatory will watch comet collision

When NASA's Deep Impact space probe collides with comet Tempel 1 just before midnight July 3, New Mexico State University astronomer Nancy Chanover will catch the action with Apache Point Observatory's 3.5-meter telescope.

knows quite what to expect from this first-ever impact of comet and man-made spacecraft, Chanover said. Astronomers hope to learn what the nucleus of the comet is made of, and that could provide new insight into the formation of the solar system.

"The support of ground-based telescopes is critical to understanding what happens after the impact," she said. "We are helping out in that regard."

Using the APO telescope's infrared instrument, Chanover will look for the signatures of molecules or isotopes created by the impact. Because every element has unique spectral lines, studying the material thrown out by the collision could tell a lot about the nucleus of the comet.

"Normally if you take a spectrum of a comet, you're seeing what's on the surface of the nucleus," Chanover said. "This should tell us something about the pristine layers below the surface, as well as how tightly or loosely bound the comet nucleus material is."

Comet Tempel 1, located low in the southern sky now and visible only with the aid of a good telescope, is hurtling through space at about 6.3 miles per second. The comet measures about 8.7 miles by 2.5 miles. The Deep Impact probe, made largely of copper, is about the size of a coffee table.

The collision is not expected to have any appreciable effect on the comet's trajectory around the sun. But it is expected to create a large crater, ranging from the size of a house to the size of a football stadium, throwing out ice and debris and revealing what lies beneath the surface.

The collision is expected to occur at about 11:52 p.m. Mountain Daylight Time; in Eastern time zones it will already be Independence Day. But don't look for a major fireworks display.

The debris cloud thrown up by the impact will cause Tempel 1 to brighten, but "estimates of how much the comet is going to brighten vary quite a bit," Chanover said. "It may brighten very briefly to the point where it could be visible to the naked eye. We also don't know how long it will stay bright."

Whatever the outcome, a worldwide network of telescopes, plus several orbiting observatories, will be watching.

The Deep Impact spacecraft itself will get the closest look. Early on July 3, the spacecraft will deploy its small impactor probe. While the probe uses autonavigation to move into the path of the comet, the flyby spacecraft will maneuver away from the comet's path enough to observe the impact. The spacecraft is expected to have about 13 minutes to take images and spectra before it is overtaken by a blizzard of particles from the collision.

Orbiting and ground-based telescopes will have different vantage points for making observations in a variety of wavelengths during and after the impact.

"The last 24 hours of the impactor's life should provide the most spectacular data in the history of cometary science," Deep Impact principal investigator Michael A'Hearn of the University of Maryland has said. "With the information we receive after the impact, it will be a whole new ballgame. We know so little about the structure of cometary nuclei that almost every moment we expect to learn something new."

Chanover is a planetary scientist, but she's not without experience in observing comet impacts. In 1994 she and other NMSU astronomers used the 3.5-meter telescope at Apache Point to observe the collision of comet Shoemaker-Levy 9 with Jupiter. Chanover was a doctoral student at that time and the 3.5-meter telescope had just recently been commissioned.

Apache Point Observatory, operated by New Mexico State University for the Astrophysical Research Consortium, is located high in the Sacramento Mountains near Cloudcroft. It is home to the 3.5-meter telescope shared by the consortium members, a 1-meter telescope owned by NMSU, and the Sloan Digital Sky Survey 2.5-meter telescope, a unique instrument used by an international collaboration that includes NMSU.