Mr. Hartley’s Amazing Comet

Today the Deep Impact spacecraft zipped past Comet 103P/Hartley 2 at a distance of about 435 miles (700 km) at 6:59:47 a.m. PDT (13:59:47 Universal Time). After confirming that the spacecraft had survived its 27,000-mile-per-hour brush with this icy interloper, the scientists and engineers who'd gathered at the Jet Propulsion Laboratory in California held their collective breath for about 20 minutes while waiting for the first images to be radioed to the ground.

Comet Hartley 2's nucleus
One of several close-ups of the nucleus of 103P/Comet Hartley 2, taken on November 4, 2010, by NASA's Deep Impact spacecraft. Note the numerous jets of gas emanating from the elongated body, and the smooth "collar" in the lowest area gravitationally.

Watch an extraordinary animation (0.9 MB .avi file) made by morphing the five released flyby images from one into the next. This is the creation of Unmanned Spaceflight forum member Daniel Macháček using Squirlz Morph morphing freeware.

NASA /Univ. of Maryland
Any apprehension quickly turned to joy, as the views revealed the craft's pinpoint targeting and crisp images of an elongated, irregularly shaped body spewing gas and dust from a plethora of jets.

Spacecraft have now photographed five comets at close range, and with a length of just 1¼ miles (2 km), Hartley 2 is the smallest. However, as investigator Jessica Sunshine (University of Maryland) noted today during a press briefing, "It's the most interesting and, for its size, the most active."

Radar images acquired by Arecibo Observatory last week had prepared the mission scientists to expect an elongated body. But no one was anticipating such an unusual visage: the comet's two ends, both roughly textured, are the sources of perhaps dozens of individual jets, while the midsection looks completely smooth, as if covered deeply by a blanket of fine dust. "We have a lot of work to do to try to understand what's going on here," Sunshine admitted.

Project scientist Michael A'Hearn (University of Maryland) noted that images of the comet taken continuously since October 1st had shown periodic surges of carbon dioxide (CO2) emanating from the nucleus. Judging from the close-ups, he says it's now clear that "one area on the comet is incredibly rich in dry ice, and that's what drags out the grains and produces all the phenomena that we see."

Unlike spacecraft that veritably drip with instrumentation, Deep Impact carries a minimalist payload: medium- and high-resolution cameras, along with an infrared spectrometer.

Today's views were not the most detailed frames — those will be radioed back to Earth in the coming days. According to project manager Tim Larson, the spacecraft will continue to photograph the comet's nucleus for three more weeks as it recedes into the distance.

The rendezvous was the second cometary encounter for this spacecraft, which previously looked on as a large copper bullet slammed into Comet 9P/Tempel 1 on January 4, 2005. After that eventful encounter, NASA managers recommissioned the still-viable craft as the EPOXI mission, a combination of its two extended mission components: Extrasolar Planet Observations and Characterization (EPOCh), and the second cometary flyby, called the Deep Impact Extended Investigation (DIXI).

On hand for today's festivities at JPL were Malcolm Hartley and his wife. Hartley discovered this object in March 1986 on glass plates taken with the Siding Spring Observatory's UK Schmidt telescope in New South Wales, Australia. Its high eccentric path carries the comet from a perihelion near Earth's orbit out to beyond Jupiter and back every 6½ years. In fact, right now this periodic visitor is quite close to Earth and putting on a decent showing in the predawn sky.

Finally, here's something for you space-trivia buffs: There's actually been a sixth close flyby of a comet — in fact, it was a fly-through. Do you know which spacecraft it was? Offer your guess in the comments section below; I'll congratulate the first person to post the correct answer in a future story about the Comet Hartley 2 results!

11 thoughts on “Mr. Hartley’s Amazing Comet

  1. Anthony Cook

    I think that ESA’s Giotto was sent to another comet years after its 1986 encounter with comet Halley. Because its imaging system was damaged during the Halley encounter, it did not image the second comet that it encountered.

  2. Frank ReedFrank R

    Hmm. I’ve got a count of seven encounters that would count as flybys (there are also a couple by Ulysses many millions of miles upwind in various comets’ tails but those don’t appear to fit the definition of “flyby”):

    Giacobini-Zinner (ICE) 1985,
    Halley (Giotto, Vega) 1986,
    Grigg-Skjellerup (Giotto) 1990,
    Borrelly (Deep Space 1) 2001,
    Wild 2 (Stardust) 2004,
    Tempel 1 (Deep Impact) 2007,
    Hartley 2 (Deep Impact/EPOXI) 2010.


  3. Jeff Rabb

    I agree with Frank R.

    There have been 7 flybies not 6. You have ICE (now enroute back to Earth) passing within about 8000miles of Giacobini-Zinner and Giotto making a second flyby with Grigg-Skjellerup.

    This also means that Giotto and not Deep Impact was the first spacecraft to investigate two comets.


  4. Dman - Pleasanton CA

    The first Spacecraft to accomplish a “fly through” of a Comet was the International Cometary Explorer (ICE) which it was renamed after its original mission as ISEE-3.

    On September 11, 1985, the spacecraft ICE passed through the plasma tail of the Comet Giacobini-Zinner, but because of its original mission as ISEE-3 and having no cameras, it collected data with on board instruments.

    Also Note; Donald K. Yeomans Scientist and leading asteroid and comet authority, was instrumental in his contributions with the 1985-86 International Cometary Explorer mission which led to the first fly-through of a comet.

  5. Adolf Schaller

    I think perhaps Kelly might be referring to the Ulysses spacecraft which famously passed through the enormously long tail (at least 3.8 AU) of Comet C/1996 B2 Hyakutake in 1996. But if one counts that ‘encounter’, I suppose one would need to include two other comet tails which Ulysses unexpectedly passed through, according to Wikipedia: Comet C/1999 T1 McNaught-Hartley in 1999, and Comet C/2006 P1 McNaught in 2007.

  6. Dave Miller

    There has been some speculation about the strange smooth-looking necked-down section in the middle. It makes me wonder whether the nucleus is actually a binary (two separate objects rotating about a common center of gravity) – the larger one being potato shaped and the smaller more spherical. The smooth section bridging the gap between them might be particles/rubble released by the action of the gas jets. Maybe the particles/rubble would be gravitationally attracted to the space between the two and distributed more toward the larger body than the smaller, with a gravitationally balanced point between, about where the minimum diameter lies. Just theorizing… maybe good for a sci-fi story

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.


This site uses Akismet to reduce spam. Learn how your comment data is processed.