Video of Comet C/2012 S1 ISON compiled from images taken by NASA’s Deep Impact spacecraft on Jan. 17-18, 2013.
Comet ISON looks like a tadpole in the video, but we all know that tiny tadpoles grow into big green frogs. So will the comet when it brightens to naked eye visibility later this year. Well, not a frog exactly. A great comet is more like a colossal tadpole with a long, bright tail that can stretch many degrees across the sky.
NASA’s Deep Impact probe, the one that gave us amazing closeup pictures of comets Tempel 1 and Hartley 2, is back in the business of comet watching. NASA directed the spacecraft to track and image Comet ISON as part of a campaign to learn all we can about this frozen ball of ice and dust during its first trip to the inner solar system.
Like Comet C/2011 L4 PANSTARRS, ISON is believed to come from the Oort Cloud, a gigantic, spherical reservoir of comets in the distant outer solar system that reaches a third of the way to the nearest star system, Alpha Centauri. As the sun travels around the center of the Milky Way galaxy, gravitational nudges from neighboring stars can give an Oort Cloud comet a little push and send it falling toward the inner solar system.
The ensuing sun-bound voyage is THE definition of a slow boat to China. It takes millions of years for a comet like ISON to inch toward the inner sanctum, but when these first-timers finally get a taste of the sun’s heat, pristine, dust-laden ices are quick to vaporize.
When the photos were taken, ISON was 493 million miles from the spacecraft or about as far as Jupiter is from the sun. Despite this vast distance, we can already see a haze of gases called the coma from vaporizing ice and a short tail more than 40,000 miles long pointing southeast. Amateur astronomers have recorded a similar appearance with telescopes and cameras here on Earth.
Comet ISON is a hot topic whether you’re a professional astronomer, amateur or brand new to skywatching because it belongs to a special group of comets called sungrazers. As the name implies, these comets passed exceptionally close to the sun before swinging back out into the depths of space. ISON will arc only 130,000 miles from the sun or a little more than half the Earth-moon distance on November 28, 2013. Assuming the comet doesn’t break to pieces under the intense heat and gravitational stress, we should see a long-tailed spectacle with the naked eye both before and after closest approach.
Although I don’t have the details on the comet’s size, many astronomers believe ISON is big enough to survive intact and make for a great show this fall and winter in both northern and southern hemispheres.
Have no fear about any effects on Earth except for jaws dropping in wonder (assuming predictions hold true); Comet ISON will only come as close as 40 million miles on Dec. 26, 2013.