What's that in the sky? UNT astronomy lab program director says it's an unusual comet

Photograph of the comet taken at UNT's Rafes Urban Astronomy Center.
Comet 17P star chart
Comet 17P star chart
Ron DiIulio
Director of the University of North Texas Astronomy Laboratory Program and planetarium.
Thursday, November 1, 2007

Maybe you've seen it, and didn't realize what it was. That bright object, visible in the northeast sky even from a well-lit area over the past few days, is a rare kind of comet. Ron DiIulio, director of the astronomy lab program at the University of North Texas, says the object is known as Comet 17P/Holmes.

The comet has the imagination of astronomers and space enthusiasts since Oct. 24, when it erupted and brightened by a factor of nearly 1 million. Such comet eruptions typically result from the sudden release of particles, or outgassing.

Comet 17P/Holmes was discovered by British astronomer Edwin Holmes in 1892 - literally by accident. Holmes noticed that an object in the sky appeared to be rapidly brightening, and determined that it must be a comet. To the naked eye, the comet looks like an out-of-place star in the constellation Perseus. Through a moderately powered telescope, however, it resembles a planetary nebula.

DiIulio, who is a NASA/Jet Propulsion Laboratory Solar System Ambassador, says two factors make Comet 17P/Holmes unusual.

Because of the Earth's position relative to the comet, stargazers cannot see a typical comet tail on 17P/Holmes, he says.

"The tail of a comet points away from the Sun, and we are seeing the comet from the front. It's like looking into the headlight of a car," DiIulio says.

In addition, he says, 17P/Holmes does not appear to go around the Sun, "as most comets are thought to do." Instead, 17P/Holmes orbits between the planets of Mars and Jupiter, completing an orbit around those planets in less than seven years.

DiIulio adds that astronomers think that the comet is bigger than Jupiter-the largest planet in the solar system. He thinks it could be visible to the naked eye, in the northeast part of the sky, for another two weeks.

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