Comet ISON, the long-traveling iceball that skywatchers hope will turn into the “Comet of the Century,” takes on a fuzzy glow in an image captured two weeks ago by the Hubble Space Telescope and unveiled on Tuesday.
The picture was taken on April 10, using Hubble’s Wide Field Camera 3, when the comet was 386 million miles (621 million kilometers) from the sun and 394 million miles (634 million kilometers) from Earth. That’s just inside the orbit of Jupiter. Right now, the comet’s brightness is roughly magnitude-16, which means it can only be seen with telescopes. But comet-watchers are hoping that ISON will get dramatically brighter as it swings around the sun in late November. Some have said the comet could match the brightness of Venus or even the full moon.
The reason for those hopes — and the reason for all the “coulds” and “mights” — is that ISON appears to be a long-period comet, coming in from the far reaches of the solar system for the first time in living memory. Such comets are unpredictable: Will they shed lots of dust and glowing gas, or will they turn out to be duds? ISON’s orbit is due to bring it within 700,000 miles (1.2 million kilometers) of the sun’s surface. That could cause ISON to crumble like Comet Elenin did in 2011, or it could spark a flare-up of Comet Lovejoy proportions.
Unlike Comet Lovejoy, which lit up the Southern Hemisphere’s skies during 2011’s holiday season, Comet ISON should be visible from the Northern Hemisphere — which means Americans might get an eyeful during this year’s winter holidays. (There’s that pesky “might”!)
The picture from Hubble helps astronomers get a better fix on the current state of Comet ISON: The nucleus appears to be no larger than three or four miles (five to seven kilometers) across. In Tuesday’s image release, the Hubble team says that’s “remarkably small, considering the high level of activity observed in the comet so far.” The comet’s fuzzy head, known as the coma, measures about 3,100 miles (5,000 kilometers) across, or a little less than the distance from New York to Dublin. ISON’s tail extends more than 57,000 miles, far beyond Hubble’s field of view.
Detailed readings from Hubble could unlock the secrets of ISON’s origins, University of Maryland astronomer Michael A’Hearn said in a news release. “We want to look for the ratio of the three dominant ices, water, frozen carbon monoxide, and frozen carbon dioxide, or dry ice,” A’Hearn said. “That can tell us the temperature at which the comet formed, and with that temperature, we can then say where in the solar system it formed.”
Comet ISON was discovered last September and is formally known as C/2012 S1 (ISON). It takes its name from the International Scientific Optical Network, a group of observatories in 10 countries managed by Russia’s Keldysh Institute of Applied Mathematics.