Comet Hartley 2 looks like a peanut

The first photos were greeted by shouts, applause and handshakes all around. The project's principle investigator - UM College Park astronomer Michael A'Hearn - and the comet's discoverer, Malcolm Hartley, were both on hand.

The icy comet nucleus, about 1.2 miles long, can be seen in the images, tossing off gas and dust from its sunlit end. The outgassing can also be seen at other locations, even the night side. That's been seen before at other comets, but this is "by far the clearest demonstration of that fact," A'Hearn said.

Hartley 2 "probably loses a meter to a meter-and-a-half everywhere on the surface every time it comes near the sun," he said. At that rate, "it probably won't be around very long."

Scientists said the gas jets are primarily carbon dioxide, with less water in the mix than other comets have displayed. Earlier observations also found millions of tons of hydrogen cyanide gas were also being released. And as the gases escape, they drag out dust grains that make the comet easier to see in telescopes.

A'Hearn said that, to a person standing on the comet, the jets would be less forceful than a firehose. "You would feel it, but it's probably not enough to lift you off the surface," despite the comet's weak gravity, he said.

"Every time we go to [a comet] they are full of surprises," said A'Hearn. "The big differences between them have sort of surprised us ... There must be some fundamental differences in the way they work."

"That could mean they formed in different ways, or they came from different parts of the early solar system," he said. "Or, it could be they evolved very differently.'"

Scientists will try to use the differences they see to better understand how the solar system formed, and what materials and physical conditions prevailed in different places.

"Comets are incredibly important," said Edward Weiler, associate NASA administrator for science. "We know that, way back in ancient history just after the Earth formed, the inner solar system was bombarded with comets."

Many people think the water on Earth, and perhaps the organic compounds that formed the building blocks of living organisms may have originated in comets, he said.

The mission to Hartley 2, and the capture of thousands of photos and other data, constituted the "exploration phase" of the mission, Weiler said. "Now we have the fun part. We have to do the science."

The spacecraft is expected to send back a total of 120,000 images. "Sorting it all out ... will take years."

You can read more about the flyby here. The NASA EPOXI web site is here. The University of Maryland site is here.