"The mission team and scientists have worked hard for this day," said Tim Larson, of NASA's Jet Propulsion Laboratory, Pasadena, California, which is monitoring the mission that saw the EPOXI probe fly billions of miles to capture images of the comet.

"It's good to see Hartley 2 up close," Larson said.

The EPOXI -- its full name is Extrasolar Planet Observation and Deep Impact Extended Investigation -- reached the Hartley 2 after a 2.5-year journey across the solar system, a distance of some 4.6 billion kilometers (2.9 billion miles).

The EPOXI mission flew within about 435 miles (700 kilometers) of the comet at about 10 am (1400 GMT) Thursday and about an hour later began sending back its never-before-seen computer images.

"We are all holding our breath to see what discoveries await us in the observations near closest approach," said EPOXI principal investigator Michael A'Hearn of the University of Maryland, College Park, which is also tracking the course of the comet.

The flyby of Hartley 2 is only the fifth time ever that a spacecraft has come close enough to take pictures of a comet.

The peanut-shaped Hartley 2, about 2.2 kilometers (1.4 miles) long and weighing about 280 million metric tonnes, is composed of ice, carbon dioxide and silicate dust particles.

Scientists said studying the Hartley 2 comet, one of billions of frozen balls of ice, rock and dust left over from the time that the Earth was formed, can yield important information about the forming of the sun and planets more than four billion years ago.

"Comets are incredibly important. There are billions of comets, most of them sitting way beyond the orbit of Neptune... that surround the solar system," said Ed Weiler, director of scientific programs for NASA, speaking on the US space agency's television station.

"We know that way back into ancient history, four billion years ago just after the Earth formed, that the inner solar system was bombarded by comets," he said.

The EPOXI began its mission in 2005 with a scientific first by dropping a projectile on comet Tempel-1 to study the plume it lifted off its surface.

After the Deep Impact study and further astronomic observations, NASA in 2007 decided to send the spacecraft to an encounter with Hartley 2.

Since then, the EPOXI has traveled across our solar system a total of 18 astronomical units (one unit equals the Sun-Earth distance), or some 2.7 billion kilometers (1.7 billion miles).

It swung three times by the Earth for added gravitational boosts to its speed.

Hartley 2, is named after its discoverer Malcolm Hartley, who first saw it in 1986 through the Schmidt Telescope in Australia's Siding Spring observatory.

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