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Fly-by captures first comet photos
A spacecraft has successfully conducted a close fly-by of the comet Hartley 2, providing the most extensive observations of a comet ever.

Comets are thought to be the left over construction debris from the formation of our solar system, yielding important information about the birth of the Sun and planets, 4.6 billion years ago.

NASA's EPOXI spacecraft passed within 700 kilometres of Hartley 2's surface at 0050 AEDT today, providing the first ever close-up details.

Scientists say initial images are giving new information about the comet's size, shape and chemical composition.

EPOXI uses the already in-flight Deep Impact spacecraft which successfully fired an impactor into the comet Tempel 1 in July 2005.

Images from the EPOXI mission reveal Hartley 2 is about 2.2 kilometres long and has a mass of about 280 million tonnes.
Australia key to mission

NASA's Deep Space Communications Complex at Tidbinbilla near Canberra is playing a major role in the mission, being the primary data receiving station for much of the fly-by.

Facility spokesperson Glen Nagle says the comet was 18 million kilometres away when the first data began downloading about an hour after the closest approach.

"Early images show the peanut-shaped space rock in amazing detail. For the first time, we may be able to connect activity to individual features on the nucleus. You can actually see where some of the comet's jets and gasses are spewing from, and the high resolution images are yet to down-load!"

Nagle says there'll be another three weeks of imaging during the spacecraft's outbound journey.
Cometary poison gas geyser

The Hartley 2 encounter is only the fifth time a spacecraft has come close enough to image a comet's surface. The others were Halley, Tempel 1, Borrelly and Wild 2.

EPOXI Principal Investigator Michael A'Hearn from the University of Maryland says Hartley 2, though smaller, is far more active than the others, spewing five times more gas and dust.

He says earlier readings indicate ice, carbon dioxide and silicate dust particles. "But the biggest surprise was a massive surge of cyanide."

Cyanide is a common ingredient of comet cores. But the size and purity of this outburst was unexpected.

"The abundance of cyanide in the comet's atmosphere jumped by a factor of 5 over an 8 day period - that's huge," says A'Hearn. "Curiously, however, there was no corresponding increase in dust."

"This flies in the face of conventional wisdom. Comet cores are thought to be a mish-mash of volatile ices, rock, and dust particles, generally well mixed.

"When the ice evaporates to produce a jet of gas, dust naturally comes along for the ride. Yet this outburst was pure gas."

"The amount of gas suggests a global event - but how could such an event occur without dust? It's a mystery", says A'Hearn.

Hartley 2 is named after its discoverer Malcolm Hartley, who first saw it in 1986 through the Schmidt Telescope at the Australian Astronomical Observatory.

The mission name EPOXI is a combination of the names for the two extended mission components: the Extrasolar Planet Observations and Characterisation (EPOCh), and the fly-by of comet Hartley 2, called the Deep Impact Extended Investigation (DIXI).

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