IT WAS a celestial fireworks display that left even NASA speechless. After travelling for 172 days and 431 million kilometres, the agency鈥檚 Deep Impact spacecraft smashed into comet Tempel 1 on 4 July at a speed of 36,800 kilometres per hour. And for a few spectacular moments, the debris shone six times brighter than the comet鈥檚 nucleus.
鈥淛eez! And we thought it was going to be subtle,鈥� exulted Don Yeomans of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淭hat was considerably brighter, and more material came out, than I had expected.鈥� Team member Peter Schultz of Brown University in Providence, Rhode Island, says the results exceeded his 鈥渨ildest dreams鈥�.
Launched in January, the spacecraft was a combination of an impactor and a fly-by probe. The 370-kilogram impactor was released on 3 July into the comet鈥檚 path, and NASA guided it towards Tempel 1 before it switched over to autonomous flight for the final 2 hours before impact.
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The collision was observed by cameras on board the impactor and the fly-by probe, and by observatories on Earth plus four orbiting telescopes, including Hubble. The plan is to study the debris and the crater formed from the impact in a bid to understand more about the interior of comets. These bodies of ice and dust a few kilometres across are believed to be well-preserved relics of the primordial material that formed the solar system 4.5 billion years ago.
Within hours of the impact, the dramatic plume of debris that erupted had disproved some theories about the structure of comets. Some scientists thought the comet鈥檚 nucleus might be so porous that the impactor might sink with hardly a trace. But it didn鈥檛. 鈥淐learly it rules out extremely porous material,鈥� says the mission鈥檚 chief scientist, Mike A鈥橦earn of the University of Maryland in Baltimore.
According to Schultz, the images seem to show that the plume went through four distinct stages. First, there was an almost horizontal movement of dust outwards, followed within 150 milliseconds by a very brief but bright flash. The flash was apparently caused by the explosion of the impactor after it had tunnelled into the comet鈥檚 nucleus, producing gas so hot that it glowed. 鈥淎 lot of it is incandescent gas, producing its own light,鈥� said Schultz.
Then a narrow vertical plume spewed out, followed by a much broader fan of material spreading out from the impact point. This sequence suggests the impactor encountered different layers of material 鈥� perhaps a loose, fluffy surface, with denser ices beneath, Schultz says.
Before the impactor was vaporised by the collision, it snapped the best high-resolution pictures of the comet ever taken. It kept taking pictures until 3 seconds before it hit the comet; the best planners had hoped for was images up until 30 seconds before impact. These pictures, and those taken by the fly-by probe, show numerous perfectly circular features, which could be either impact craters or sinkholes, as well as long linear features and varied topography with some rough areas and one large very smooth feature. The images show 鈥渆verything that a geologist would love鈥�, Yeomans says.
Over the coming weeks, the data from spectrographs on the Keck telescopes in Hawaii, as well as NASA鈥檚 orbiting Chandra and Spitzer telescopes, should provide new insights into the comet鈥檚 internal composition. The incredible volume of images and spectral measurements returned by this mission in less than a day represents 鈥渁 wealth of data that will take me into retirement鈥�, A鈥橦earn says.
Meanwhile, instead of being irreparably damaged by the debris field of the comet鈥檚 inner coma, the fly-by spacecraft survived intact 鈥� perhaps to study yet another comet at close range.
