With its own big bang, Deep Impact takes us back 4.6bn years, to solar system's birth

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Taking its name from the 1998 sci-fi blockbuster about a comet heading towards Earth, Nasa's $333m (£190m) Deep Impact mission was being hailed last night as one of the most important landmarks in space exploration ­ the nucleus of a comet has never before been penetrated.

Scientists succeeded in firing the washing machine- sized device, an " impactor", from Deep Impact at a target 3.7 miles wide and from a distance of 537,000 miles, in an operation controlled from a laboratory in Pasadena, California, 133 million miles away.

The battery-powered impactor was fired on Sunday, allowing the spacecraft time to move to a position 310 miles from the comet, where its high-resolution camera and infra-red spectrometer could observe the 23,000mph collision, equivalent to the energy released by 4.8 tons of TNT. Cameras on the impactor were also transmitting until three seconds before collision. When the first pictures came through after the collision yesterday morning, scientists at Nasa's Jet Propulsion Laboratory (JPL) clapped, cheered and hugged each other.

"We hit it just exactly where we wanted to," said a jubilant Dr Don Yeomans, a Nasa scientist. "The impact was bigger than I expected, and bigger than most of us expected. We've got all the data we could possibly ask for."

Dr Charles Elachi, the director of the JPL, added: "We are in the business of opening new frontiers in the exploration of space. When we analyse the data, we will have a whole new insight into the Universe."

Comets are balls of dust and ice that travel around the Sun in long, oval, orbits. A comet has a solid, icy core ­ or nucleus ­ which is surrounded by a cloudy atmosphere, known as a coma.

Scientists know that, although the surfaces of comets have been affected by the Sun's rays, the material inside is "primordial" ­ unchanged since the formation of the solar system 4.6 billion years ago. Little is known about it.

It is also believed that much of the water on Earth arrived from comets that bombarded the planet in the early part of its existence; it may also be that the original organic molecules that gave rise to life arrived on comets.

Professor Iwan Williams, from Queen Mary, University of London, one ofthe many British scientists taking a keen interest in the mission, said: " This is probably a once-in-a lifetime moment. This is likely to be much better than anything we have done to help us in understanding the origins of the solar system. These are clues to our knowledge of the building blocks of life."

Scientists hope analysis of the Deep Impact collision will provide information about the comet's internal structure, the ratio of dust to gas and whether there are larger particles within. "If the comet is made of slushy stuff, then the crater will be long and thin, but if it has a hard and compact surface, then there will be a shallow crater," said Professor Williams.

Full analysis of the results, which are still being transmitted back to Earth is likely to take months. Initial study of the plume of dust and gas created by the explosive impact suggested that the comet was dustier than previously believed, said Professor Williams.

Pictures of the collision were also taken by the Hubble space observatory and by astronomical telescopes around the world. It was also observed by cameras on the European Space Agency's Rosetta spacecraft, which aims to land a probe on another comet, Churyumov-Gerasimenko, in 2014. Professor Williams, who is part of that project, added: "We hope that will be our next once-in-a-lifetime moment."

The mission also seemed to spark the enthusiasm of skywatchers. Officials at JPL said the Deep Impact website had one billion hits, compared to about 400 million hits for the Mars mission. The cosmic collision did not significantly alter the comet's orbit around the Sun and Nasa said that the experiment never posed any danger to Earth.

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