Comet Hartley 2: Up Close and Personal

Comet Hartley 2 during the November 4, 2010 NASA/EPOXI
flyby missionThursday, November 4, 2010: the morning I got up, drove to work, poured my coffee, and whizzed past a mountain-sized chunk of space ice at a hair over 27,000 miles per hour. My job just never gets old….

If you were with us early that morning, you were one of a lucky and intrepid few to experience live the flight of NASA's EPOXI mission (aka the Deep Impact spacecraft) past the nucleus of tiny Comet Hartley 2—a comet that we had been watching through our telescopes for a few weeks. Live space exploration as a public experience is a rare and extraordinary opportunity, and we strive to share it with you whenever we can. Kudos to those who got up before 6:00 AM to join us.

EPOXI is the name for the extended mission of the re-purposed Deep Impact spacecraft, which you may remember lobbed a projectile at Comet Tempel 1 back in 2005. Since that encounter, the spacecraft has been orbiting the Sun, keeping itself busy with other observational activities, including looking for extra-solar planets. But the time had come, the orbital trajectories matched, for an encounter with Hartley 2, a smallish comet circling the Sun between the orbits of Earth and Jupiter. The EPOXI flyby was the fifth time in history that we have captured close-up images of a comet nucleus, and the first time a single spacecraft has bagged two.

What are scientists looking for by sending spacecraft to these distant space-bergs that they can't get with big ground-based telescopes, or the Hubble? In a word, details. From afar, we have observed comets for a long time, and can point telescopes at their nuclei, their comas (the shroud of gas they develop when the comets' ices are warmed in a pass by the Sun), and their long tails—yes, plural: comets typically develop two tails, one a trail of dust and the other a plume of gas, blown by the ever-present breeze of plasma from the Sun called the solar wind.

But comets are tiny objects, generally--even famous comet Halley is less than ten miles across—and usually relatively far away from Earth (a fact that we can give thanks for). Comet Hartley 2 is only about a mile across. This makes seeing details of comet structure and surface difficult even with the biggest ground-based telescopes. And though EPOXI's sweep is considered a close encounter, even that situation would be like using a modest-sized telescope to examine Emeryville, California from Los Angeles.

When EPOXI flew past the comet nucleus that morning, coming to within 435 miles, the comet was quite active, spewing out jets of carbon dioxide gas and traces of cyanide (!) in several directions. With its cameras trained on the nucleus during the flyby, EPOXI captured stunning detailed images that revealed which parts of the comet's surface were the sources of the gas plumes—two dots that had never before been connected in our observations of comets. Adding to the excitement of this encounter, an unexpected discovery was made after the flyby, when the data collected by EPOXI was analyzed: unlike the other comet nuclei that we have probed up close (Halley, Borrelly, Wilde 2, and Tempel 1), Hartley 2 was surrounded by a cocoon of snow!

Seeing the structure of the comet (which turns out to be quite peanut-shaped), the surface details across different parts of it, and the nature of the regions spewing out the gases, scientists can gain insight into the overall nature of this comet. Stay tuned for more as EPOXI data is further analyzed....

Back to our own flyby experience: sitting in our Megadome Theater watching 30-foot images from NASA/JPL Mission Control and waiting for our first up-close glimpse of the comet nucleus would have had me sitting on the edge of my seat—if I could have sat down. It was that exciting….

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