NASA probe beams in close encounter with comet

A NASA probe returned close-up views of a comet Thursday, in a flawless 435-mile-high flyby.

Jets of melting ice streamed from both ends of the comet Hartley 2 as the $333 million Deep Impact probe passed overhead at 27,000 mph. The flyby was humanity's fifth close look at a comet.

"It's just amazing," said mission scientist Jessica Sunshine of the University of Maryland, at a NASA briefing. "For the first time, we are seeing the jets going off all over the surface of a comet."

Comets are essentially frozen leftovers from the dawn of the solar system about 4.6 billion years ago. They may have delivered water and life's early ingredients — organic chemicals — to Earth's oceans in a period of frequent impacts more than 3.9 billion years ago. Planetary scientists hope to learn more about the chemistry of those eras from these long-tailed snowballs.

Hartley 2's jets sprouted as the comet approached within 98 million miles of the sun late last month, the warmest leg of its orbit. The famous tails of comets such as Halley's arise from them melting as they reach the warmer inner solar system, shedding a cloud of ice in their wake.

The real surprise of the flyby was the discovery that melting dry ice, carbon dioxide, powered the jets emerging from the comet rather than water ice, said project chief investigator Michael A'Hearn, also of the University of Maryland. About 1.25-miles long, the peanut-shaped Hartley 2 circles the sun once every 6½ years. Earlier this week, it delivered a small meteor shower to Earth as its icy wake splashed across the nighttime sky.

"The atmospheres of Venus, the early Earth and Mars all contain a lot of carbon dioxide. We may very well be looking at where the atmosphere of early planets came from," said mission scientist Jay Melosh of Purdue University in West Lafayette, Ind.

As comets go, Hartley 2 is quite small, and its jets are quite active. Nearly 5 feet of its surface depth melts away each time its orbit takes it into the warmer inner solar system, A'Hearn says. "It won't be around for long."

"We could not have asked for a better performance from our spacecraft," said mission manager Tim Larson of NASA's Jet Propulsion Laboratory in Pasadena, Calif. The encounter about 11 million miles away from Earth passed within 2 miles of its target altitude and was the second such comet visit for Deep Impact. The same spacecraft released a crater-blasting probe into the comet Tempel-1 in 2005, flying overhead to film the impact.