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AS Comet Hale-Bopp makes its closest flyby in 4000 years, scientists have discovered that it is home to many molecules never before seen in a comet. And the comet is throwing off thousands of tonnes of gas every second.
The last time Hale-Bopp streaked across the skies was during the Bronze Age, around 2000 BC. Observations from the Hubble Space Telescope have shown that the comet has an unusually large nucleus—about 40 kilometres in diameter, seven times that of Halley's comet. "It's a monster," says David Jewitt of the University of Hawaii. Because it is so bright, Hale-Bopp was visible as long ago as 1995, when it was still outside the orbit of Jupiter.
Jewitt and his colleague Henry Matthews of the Joint Astronomy Center in Hilo, Hawaii, have used a submillimetre telescope to watch Hale-Bopp as it approaches. In February they found that the comet was throwing off 40 times as much dust as Halley, says Matthews. And in early March, it was already releasing 10 times as much water vapour as Halley did at its peak, when it was closer to the Sun. Hale-Bopp was throwing out about 10 000 tonnes of gas every second, says Jewitt.
Matthews and Jewitt also measured the abundances of various isotopes of carbon, nitrogen and sulphur on the comet. These turned out to be similar to those on Earth, which was just what astronomers expected as the comet probably formed in the disc of dust and gas around the Sun that also formed the planets. But Hale-Bopp also contains molecules that have never been seen on a comet before, including sulphur oxide and the carbon-chain molecule HCCCN.
Curiously, the relative abundances of some compounds streaming from the comet seem to be changing. For instance, Matthews and Jewitt found that the ratio of hydrogen isocyanide (HNC) molecules to hydrogen cyanide (HCN) molecules was about 7 per cent early this year. But in early March, measurements from the California Institute of Technology submillimetre telescope showed that the ratio had climbed to 25 per cent, says Mike A'Hearn of the University of Maryland. No one knows what has caused the variation. One possibility is that different parts of the comet formed from different parts of the disc around the young Sun.
Another puzzle is the comet's rotation period. Early variations in brightness suggested that dark and bright regions were rotating into view with a period of a couple of weeks, but more recent data suggest that the period is only 11.5 hours. A'Hearn says that more observations should resolve the problem. "Nobody suggests the rotation period has changed appreciably," he says.
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