US: The United States celebrated its Independence Day yesterday with a space spectacular, involving a high-speed collision with a comet that evoked the glory days of the early space programme.
An impactor the size of a washing machine (although more than twice as heavy) collided with Comet Tempel 1 at a speed of 10 kilometres a second - unleashing the energy equivalent of four and a half tonnes of T.N.T. and sending tonnes of ice and dust spewing into space.
Time-lapse photography from the impactor showed the comet loom ever larger on television screens at Mission Control in Pasadena, California, and at the European Southern Observatory near Munich, which is co-ordinating European observations. The last image was transmitted just three seconds before the probe was vaporized in a huge flash that was itself photographed by the Deep Impact mothercraft from a safe distance as it flew past.
"The image clearly shows a spectacular impact," said Deep Impact principal investigator Dr Michael A'Hearn of the University of Maryland, College Park.
The celestial collision was the climax of a series of highly complex deep space manoeuvres that required the impactor to navigate automatically using quaternions, a mathematical technique that was first developed by the Irish scientist William Rowan Hamilton more than 150 years ago.
In its last 90 minutes of life the probe repeatedly recalculated and adjusted its trajectory to ensure that it hit the comet in a way that maximized the chances of excavating large amounts of material from beneath the comet's surface.
"We thought it was going to be subtle!" exclaimed mission scientist Donald Yeomans as images of the impact began streaming in. "We've had a far bigger explosion than we anticipated . . . I can't imagine how this could go any better."
The large flash was bright enough to be seen from Earth, and the first ground-based images of the explosion were released onto the internet by a team of school students from Britain and Northern Ireland who were using the pioneering Faulkes Telescope in Hawaii, a dedicated research facility for schools that is controlled from European classrooms over the internet. Students from the Royal Belfast Academical Institution and from Glenlola Collegiate School in Bangor hope to present detailed results at next year's Young Scientist Exhibition.
"I'm at a loss to explain just how on earth our little washing machine-sized impactor caused such a disturbance some 83 million miles (133 million kilometres) away," said Dr Yeomans. "This is going to take some work to explain, but it's sure taken me by surprise."
Since comets contain icy material, they must originate somewhere much colder than the relatively warm inner solar system. Scientists believe that Comet Tempel 1 was formed in the Kuiper/Edgeworth Belt, a broad flattened disk of icy debris extending far beyond Pluto, which was first proposed by astronomer Kenneth Edgeworth from Streete in Co Westmeath. But the structure and composition of these primordial objects have been a profound mystery.
Scientists said last night they might be closer to unravelling some of those mysteries through computer enhancement of the moment of impact. It is thought the probe penetrated a soft dusty layer of the comet, punching deep into the sub-surface; hit a layer of hard ice and exploded; sent a huge pillar-like jet of dust and ice into space; triggered an even more massive eruption deep underground, which created a hole in the comet perhaps the size of Croke Park.
Last night, the Deep Impact mothercraft could still see a vast cloud of dust and ice billowing away from the comet across thousands of kilometres.