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.
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!