A periodic comet normally far too faint to see without a large telescope has brightened dramatically since June, after undergoing two unexplained outbursts that have made it visible in binoculars and long-exposure cameras. NASA's Astronomy Picture of the Day featured the comet, formally designated 220P/McNaught, on Monday, showcasing its green coma and short dust tail in an image captured from South Africa roughly ten days earlier.
The first outburst, in June, brightened the comet by roughly seven magnitudes — a jump of more than 600-fold in apparent brightness — according to observations compiled by Universe Today. A second flare-up in August added still more brightness, leaving the comet roughly 20,000 times brighter than its typical appearance, according to NASA's account of the event.
Comet 220P orbits the sun roughly once every five years, traveling in the region between Mars and Jupiter, and was first identified by astronomer Robert McNaught, for whom it is named. Astronomers say the cause of its outbursts is unknown, but leading theories point to a sudden release of gas trapped beneath the comet's surface, or a structural fracture — sometimes called a "comet quake" — that exposes fresh ice to sunlight and triggers a burst of sublimation. Because the comet is small and only weakly active in normal times, even modest changes to its surface can produce outsized swings in brightness.
A Narrow Window
The comet is expected to keep approaching Earth through October, when it will close to about one astronomical unit — roughly the Earth-sun distance — before fading as it recedes into the outer portion of its orbit. Skywatchers hoping to catch it are advised to look toward dark skies with binoculars or a small telescope in the coming weeks, before the window closes.
Sudden brightening events are not unique to 220P/McNaught. Comets are unpredictable bodies, more akin to dirty, ancient snowballs than solid rock, prone to venting gas unevenly as they warm on approach to the sun. The phenomenon has drawn renewed attention from amateur astronomers, who have posted a steady stream of images tracking the comet's evolution over the past two months, and has offered casual observers a rare chance to see with modest equipment an object that would otherwise require a research-grade telescope.
