Predicting whether an incoming comet will be spectacular or not, especially when it's a first-timer, is a risky business. I got my start writing science columns when the editor of our then-local paper called me in early 1986. "I understand Comet Halley is on its way here, and I'd like a story about what it's going to look like from Bellingham."
"It'll look like a faint, fuzzy blob from here, same as from anywhere on Earth," I said.
Pause. "Whatever. Just make it a local story, OK?"
My prediction was astonishingly accurate. Comet Halley, which has been swinging around the sun every 76 years for millennia with clockwork precision, looked exactly like a faint, fuzzy blob from our location in Washington State. Comet ISON, whose path takes it closest to the sun (its perihelion point) on Nov. 28, is a different kettle of ice. Unlike Halley's elliptical trajectory, ISON's path is hyperbolic, meaning this is its first (and probably last) visit to the sun. And because it's fresh from spending the last 4 billion years deep-frozen way out there in the Oort Cloud surrounding our solar system, it has the potential for a great show. The crucial word, of course, is "potential."
Soon after it was discovered last year by a pair of Russian amateur astronomers using a 16-inch reflector telescope from the International Scientific Optical Network (hence ISON), it was hailed as the "Comet of the Century," perhaps bright enough to see in daylight. Now we know that it won't live up to its early promise of grandeur — who ever does? — but will probably be a naked-eye object under dark skies, and (almost!) certainly will be a fine sight in binoculars and wide-field telescopes. The fact that ISON is a "sungrazer," coming really close to the sun's surface at perihelion (just three times the Earth-moon distance) means it will be heated fiercely and fast. Which is good news for comet-watchers, because the closer a comet gets to the sun, the more dust is released and consequently the brighter its tail will be.
Think of a comet as a loose, dirty snowball, before you've squeezed it into a tight missile. As it approaches the sun, solar radiation vaporizes its outer layers of water and carbon dioxide ice, releasing a shower of micron-sized dust grains. Sunlight reflecting off the dust gives comets (the "good" ones) their visible tails. (Comets actually have two types of tails, but the other kind — blue ionized gas tails, which point directly away from the sun — are dimmer than their yellow dust tails.)
However ISON turns out, the best time to see it — assuming it doesn't break up as it rounds the sun — will be during the next two or three weeks, from now until mid-December. That's when it will be outward-bound from the sun, heading roughly in our direction. Get thee to dark skies (Kneeland airport is a favorite venue for local skywatchers) an hour before dawn and scan the eastern sky with binoculars. No promises, but my comet antennae tell me you'll be glad you did. After all, my prediction for Halley was spot-on.
Barry Evans (firstname.lastname@example.org) is hoping for a repeat of comet Hale-Bopp, which exceeded all predictions for brightness following its perihelion in April 1997.