Comet Hartley 2 Makes Closest Pass by Earth in 24 Years

A comet passed silently by Earth today (Oct. 20), coming closer to our planet than it has since 1986, when astronomers first discovered the icy wanderer.

Comet Hartley 2 passed within 11 million miles (17.7 million km) of Earth at about 3 p.m. EDT (1900 GMT) today — much closer than the Earth is to the sun (about 93 million miles, or 150 million km). [Photo of Comet Hartley 2.]

The close Earth pass comes just two weeks ahead of another flyby for the comet. NASA's Deep Impact spacecraft is chasing Hartley 2, gearing up for a Nov. 4 comet flyby, during which the probe will come within 435 miles (700 km) of the cosmic ball ice and rock.

"It's unusual for a comet to approach this close," said Don Yeomans, head of NASA's Near-Earth Object Office at the Jet Propulsion Laboratory in Pasadena, Calif., and a member of Deep Impact's mission science team, in a statement. "It is nice of Mother Nature to give us a preview before we see Hartley 2 in all its cometary glory with some great close-up images less than two weeks later."

Backyard stargazers with binoculars or telescopes — and dark, clear skies — should be able to see Comet Hartley 2 this week.

To view the comet in the middle of the week, look northwest before dawn, about 75 degrees above the horizon -- almost directly overhead. Hartley 2 will be near the bright star Capella, in the constellation Auriga (the charioteer), according to StarDate magazine.

The nearly full moon will set about two hours before the sun rises, allowing about 90 minutes of prime comet-viewing time.

This sky map shows where to look to see the comet. More information is available in this Comet Hartley 2 skywatching guide.

By Oct. 22, the comet will have passed through Auriga, continuing its journey across the night sky in the direction of the constellation Gemini, scientists said.

Deep Impact's mission

Comet Hartley 2 is relatively small, with a diameter of about 0.9 miles (1.5 km). It orbits the sun once every 6.5 years or so.

The aim of Deep Impact's mission is to gather details about what comprises Hartley 2's nucleus and compare it with other comets, scientists have said. Since comets spend much of their time far from the sun, the cold temperatures preserve their composition — which could reveal details about the solar system's early days.

The Deep Impact spacecraft was originally built as the mothership for NASA's Deep Impact mission, which intentionally crashed a probe into Comet Tempel 1 in 2005 to study the object's composition.

Now, the Deep Impact probe is being put to other uses — it is tracking and studying various celestial objects under the umbrella of NASA's broad EPOXI mission. The name is derived from the mission's dual science investigations — the Extrasolar Planet Observation and Characterization (EPOCh) and Deep Impact Extended Investigations (DIXI).

• Video Sky Map: How to Spot Comet Hartley 2
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• Comet-Hunting Spacecraft Nears Icy Target