“Kryptonite” Comet Making Closest Approach to Earth

Imagecometha A pale green interloper among the stars of Cassiopeia, Comet Hartley 2 glows like an escapee from Krypton at the center of this exposure taken on the night of Sept. 28, 2010, by NASA astronomer Bill Cooke. Still too faint to be seen with the unaided eye, the comet was 18 million miles away from Earth at the time. Cooke took this image using a telescope located near Mayhill, N.M., which he controlled via the Internet from his home computer.

In 2007 the comet became the primary target for a NASA spacecraft called Deep Impact. That probe's first mission had been to launch a projectile into comet Temple 1 in 2005, kicking up a plume of ice and dust so that astronomers could study the comet's composition.  After that mission ended, the Deep Impact "mothership" still had enough fuel for followup experiments, so NASA redirected the probe for an encounter with comet Hartley 2.

Now called EPOXI, for Extrasolar Planet Observation and Deep Impact Extended Investigation, the spacecraft is closing in on Hartley 2 and is due to make a flyby on November 4. The craft will dive within 600 miles (965 kilometers) of the comet's surface, getting close-up images of its craters and sources of dust and gas plumes.

Based on the large differences seen between previous comets visited by robotic probes—including comets Halley, Wild 2, Borrelly, and Temple 1. "The mission may answer whether comet Hartley 2 has a family resemblance to one of these, or if it is also unique," said Anthony Cook, an astronomer at the Griffith Observatory in California . "Its composition is also of interest, as it is suspected that at least some comets may have formed outside of the solar system."

Comet 103P/Hartley 2, a small periodic comet, was discovered in 1986 by Malcolm Hartley, an Australian astronomer. It orbits the sun about every 6.5 years, and on Oct. 20, the comet will make its closest approach to Earth since its discovery.

In this case, "close" means 11 million miles, or 17.7 million kilometers. A moonless sky will make for promising viewing conditions in the northeastern skies, especially just before dawn.

A comet discovered in 1986 will soon make a close pass by Earth, giving sky-watchers the best view of this cosmic body in decades. The visit comes a few weeks before a NASA probe is due to fly by the icy space rock.

Comet 103P/Hartley 2 was discovered by Australian astronomer Malcolm Hartley, who calculated that the object orbits the sun about every 6.5 years. Until now, however, gravitational interactions with Jupiter kept shifting the comet's path, sending it closer to the sun and thus farther from Earth during each subsequent return.

This year comet Hartley 2 is on course to make its closest pass by Earth at a mere 11 million miles (17.7 million kilometers) on October 20—and a dark, moonless sky in mid-October will help create ideal viewing conditions, astronomers say. Comet Hartley 2 was discovered only recently because, before 1986, the comet had not been on an orbital path that brings it near Earth. Three close encounters with Jupiter in 1947, 1971, and 1982 shifted the comet's orbit, making it finally visible.

"Before mid-October, Northern Hemisphere observers will be able to see the comet nearly all night long in the northeast," Cook told National Geographic. "After mid-October it can be seen as early as 11:30 p.m. [local time] but is best just before dawn."  Starting in late November observers in the Southern Hemisphere will have a good view of the comet as it heads away from Earth, Cook added.

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Casey Kazan via National Geographic and http://nssphoenix.wordpress.com/2010/07/01/comet-hartley-2/

Image below: Comet Hartley 2, seen in a 1997 NASA picture.

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