CALL it the great comet drought. For two decades, astronomy enthusiasts have longed to see a spectacular comet, one that outshines most of the stars in the sky and sports a long tail stretching through several constellations. The tail of the most famous, Halley鈥檚 Comet, in 1910, was so long that even the Earth passed through it. On average, one spectacular comet appears every decade; two appeared in 1910. But nearly twenty years have elapsed since the last great comet graced our sky.
The drought may finally be over. On the night of 22 July this year, Alan Hale and Thomas Bopp, two observers working independently in the south west of the US, spotted a new comet that some commentators are beginning to call the comet of the century 鈥� one that may become incredibly bright when it passes close to the Sun and the Earth in 1997. Hale and Bopp found the comet amid the many stars of the constellation Sagittarius. At first, it seemed no different from those occasional ones that approach the Sun each year. Like most comets, it was too faint for the unaided eye to see.
But this time there was a twist, for astronomers soon discovered that Comet Hale-Bopp was far away, between the orbits of Jupiter and Saturn. If it is visible when it is so far from the Sun, it should brighten considerably as it nears the Sun and its ice begins to vaporise.
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Hale-Bopp may turn out to be a great comet that will give astronomers a wonderful chance to introduce millions of people to the beauty of the sky. But some astronomers may be dreading its arrival, for new comets are notoriously unpredictable. Although astronomers know the comet鈥檚 future path, they cannot say how bright it will become, and Hale-Bopp may be unable to live up to the grand claims now being made about its future performance.
Comet Hale-Bopp underscores one of the ironies of astronomy. The science deals both with extremely well-determined quantities, such as the clockwork timing of a pulsar鈥檚 spin, and with extremely uncertain ones, such as the density and destiny of the Universe. The behaviour of a new comet definitely falls at the wild end of astronomy鈥檚 predictive spectrum.
Today, hovering over any alleged great comet is the spectre of Comet Kohoutek, which was supposed to amaze us all in 1973. But what astronomers had extravagantly hailed as the 鈥渃omet of the century鈥� turned out to be the disappointment of the decade. As it approached the Sun, it failed to brighten as predicted and didn鈥檛 even match the faintest star of the Plough.
Unfortunately, because of the Kohoutek disaster, many folks missed out in 1976, when a great comet really did appear. Bruised by Kohoutek, astronomers initially dismissed Comet West, which did not promise to be spectacular. But as it approached the Sun, the comet began to exceed predictions, and after emerging from behind the Sun it put on a tremendous display in the predawn sky. Alas, because astronomers had warned people that West might be another Kohoutek, many did not bother to look for it. As Astronomy magazine reported (May 1976), 鈥淐omet West was played down so successfully that astronomers were faced with a barrage of 鈥榳hy didn鈥檛 you tell us it was coming?'鈥�
Ten years later, longtime favourite Halley鈥檚 Comet returned, but it was faint, as astronomers expected, as this time around it passed far from the Earth. In 1990, a comet named Austin was briefly advertised as the next great event. But it, too, turned out to be a damp squib, and its discoverer, Rodney Austin, wrote in the magazine Sky and Telescope (November 1990): 鈥淣ow I know how Lubos Kohoutek must have felt.鈥�
So what to do with Comet Hale-Bopp? On the one hand, astronomers could hype it and hope that it performs well. That will attract many people, at least temporarily, to astronomy. But the risk of another Kohoutek fiasco is high, so astronomers might want to take the opposite strategy and underplay the comet, as they did Comet West. Then no one will be disappointed 鈥� except, of course, the millions of people who will miss out if Hale-Bopp really is the comet of the century.
For now, though, all we can say for sure is that the comet鈥檚 course through the sky is ideal. When Hale-Bopp is at its brightest, it will be in the northern celestial hemisphere, so most of the world鈥檚 people will have excellent seats for the performance. The most optimistic scenarios have Hale-Bopp appearing brighter than any star, the most pessimistic say it will be no brighter than the stars of the Plough. So the comet鈥檚 future brightness is quite uncertain.
And one more thing. Hale-Bopp will pass closest to the Sun around 1 April 1997. That鈥檚 right, April Fools鈥� Day. So if the comet fizzles, we can鈥檛 say it didn鈥檛 warn us.