Within days after a new comet is first discovered, astronomers can tell you exactly what its path through the solar system will look like. They can calculate when it will make its closest approach to the Sun, how near it will be to Earth at any given moment and even when — or whether — it’s likely to make a return visit.
What they can’t say for sure is how brilliant a show it will put on for us. Back in the 1970’s, Comet Kohoutek was billed as the “Comet of the Century,” but it turned out to be so disappointing that it ended up as a laugh line for Johnny Carson on the Tonight Show. Even a well-known comet like Halley’s, whose 1986 appearance was ballyhooed years in advance, can turn out to be a dud.
With that caveat noted, however, there’s a reasonable chance that Earth is in for a celestial display just about a year from now. Comet ISON, discovered by two amateurs — one from Belarus, one from Russia — in September, shows early signs of being truly spectacular. At its brightest, in fact, ISON could put out as much light as the full Moon but concentrated into a smaller area — and if that turns out to be true, the term “dream comet,” now floating around the internet, would be an understatement.
There’s a legitimate possibility this might indeed be the real deal: for one thing, ISON (named for the International Scientific Optical Network, of which discoverers Artyom Novichonok and Vitali Nevski members) was first seen when it was nearly 600 million miles (965 million km) from the Sun, well beyond the orbit of Jupiter. That’s unusually distant for a comet to be spotted: these interplanetary chunks of debris usually live in the frigid realms out beyond Neptune and are more or less invisible until solar heat begins boiling ice and dust from their surfaces, forming a light-reflecting halo (known technically as its coma), that makes them seem bigger than they really are.
The fact that ISON can already be seen means it may be reasonably large — perhaps a couple of miles across — which suggests that when it dips to less than a million miles (1.6 million km) above the Sun’s fires next November 28, it may be robust enough to avoid the breakup that often happens to smaller comets. And if it does survive, ISON could go on to light up the night sky in the Northern Hemisphere for much of December, 2013 and on into January.
Comet experts are also making much of the fact that ISON’s path is very similar to that of Kirch’s Comet, a.k.a. Newton’s Comet, a.k.a. the Great Comet of 1680, which was bright enough to be seen in daylight and had a magnificently long tail (it was also the first comet ever discovered with a telescope). It’s not the same object, but it’s quite possible that both are chunks of a much larger body that broke apart long ago, maybe during its own passage through the inner solar system. The fact that such larger bodies exist isn’t in doubt: Pluto, for example, is essentially a gigantic chunk of dirty ice.
If ISON is truly destined to become one of the greatest comets in history, we won’t have to wait until November to find out. By August, it will still be more than 200 million miles (320 million km) from the Sun, but that’s close enough for it to start forming its halo. How bright it is then should be a good indication of how much brighter it could get.
Even then, comet-lovers would be wise to stay calm. “Comets are like cats,” the great amateur astronomer and comet hunter David Levy, who has found 22 comets himself, has said. “They have tails, and they do precisely what they want.”