National Geographic News
Comet Hartley 2 in 1997.

Comet Hartley 2, seen in a 1997 NASA picture.

Image courtesy NASA

Andrew Fazekas

for National Geographic News

Published September 16, 2010

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.

(Find out about two asteroids that recently swung between Earth and the moon's orbit.)

"Before mid-October, Northern Hemisphere observers will be able to see the comet nearly all night long in the northeast," said Anthony Cook, an astronomer at the Griffith Observatory in California.

"After mid-October it can be seen as early as 11:30 p.m. [local time] but is best just before dawn." (See comet pictures.)

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.

As the comet nears the sun, it may become bright enough to see with the naked eye from dark, rural areas, where it will appear as a glowing smudge in the sky. And even the smallest optical aid will help tease out details, Cook said.

"I would recommend binoculars as the best way for the beginner to observe comet Hartley 2," he said. "But through a telescope, the comet may fill the field of view, with more structure in the bright center and a faint tail that could become observable."

NASA Probe Due for Comet Flyby

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.

(Related blog: "Ancient Greeks Made First Halley's Comet Sighting?")

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. (See pictures of the Deep Impact collision.)

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.

(Also see "NASA to Visit Asteroid Predicted to Hit Earth?")

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—expectations are high that Hartley 2 won't disappoint.

"The mission may answer whether comet Hartley 2 has a family resemblance to one of these, or if it is also unique," Cook said. "Its composition is also of interest, as it is suspected that at least some comets may have formed outside of the solar system."

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