Comet ISON has taken the summer off, hiding from the eyes of Earth-based telescopes in the constellation Gemini the Twins. Not only does the comet remain faint, but it’s in the same area of the sky as the sun and overwhelmed by its glare. It won’t return to view until mid-August low in the eastern sky at the start of morning twilight. You might recall this is the fuzzball that some predict will be the “comet of the century.”
The last images the comet I’m aware of were made in early May; they show a faint, teardrop-shaped object with a brighter head and short tail. While ISON appeared relatively bright for its distance around the time of its discovery last September, it brightened little in the ensuing months, plateauing around magnitude 15 from winter through spring of 2013. Only amateur astronomers equipped with larger telescopes caught the barest glimpse of the tiny comet.
Comet ISON is what astronomers call a dynamically new object. In other words, it’s probably making its first-ever trip to the inner solar system after spending untold centuries at the other end of its orbit in the deep freeze of the Oort Cloud.
Its rapid increase in brightness between the time of discovery last September and early January was likely due to a thin coating of fresh, exotic ice (carbon monoxide and carbon dioxide varieties) that quickly vaporize, causing the comet to briefly surge in brilliance.
Once the easily-vaporized ice fizzed away, ISON was left with primarily water ice. That stuff won’t boil until mid-July when the comet is within about 250 million miles of the sun. Although ISON will still be too close to the sun to observe, this “second wind” should mean an increase in its brightness and tail length.
John Bortle, comet expert and writer for Sky & Telescope magazine, is usually right on the money when it comes to predicting comet behavior. Based on its faint appearance for so many months this winter and spring, Bortle expects ISON to brighten more slowly than initially hoped for. When it finally appears in a dark sky before dawn in mid-August, the comet will be a faint puff shining around 13th magnitude, just bright enough to spot in a 10-inch or larger telescope.
Small telescopes owners will spot it at 10th magnitude in mid-October well-placed in the eastern sky before dawn. Watch for a spectacular conjunction of the planet Mars, Regulus and the comet on Oct. 14. On that morning, you’ll see all three lined up in a tidy row.
Comet ISON should first become faintly visible with the naked under dark skies around Nov. 10 a little less than 3 weeks before it reaches perihelion, its closest point to the sun. Bortle expects it to brighten steadily all the way to 2nd magnitude (about as bright as the stars of the Big Dipper) or 3rd magnitude before disappearing in morning twilight.
Don’t expect a bright or long tail before perihelion on Nov. 28 when ISON flies less than one solar diameter (684,000 miles / 1.1 million km) above the surface of the sun. Bortle’s prediction is for the comet to briefly surge to magnitude -6 or some six times brighter than Venus. Under very clear skies, by carefully blocking the sun from view behind a roof, sharp-eyed skywatchers might see the comet as a small spot of light with a stubby tail with the naked eye and in binoculars.
ISON quickly leaves the sun’s vicinity and fades, but the best part of the show begins in early December as the comet approaches Earth. While its head is expected to dim rapidly, the comet’s tail will grow ever longer and become a dramatic sight in the morning sky in the weeks before Christmas. Bortle suspects the comet could disintegrate during its close solar encounter, freeing up even more dust to fuel the tail but causing the head to ultimately fade from sight.
The grand finale of ISON’s warm weather vacation to the inner solar system occurs from about Dec. 10-14 when it will loom like a gigantic feather floating up from the eastern horizon in the dawn sky.
ISON passes closest to Earth the day after Christmas but by then it will be well beyond the sun and fading quickly. Throughout its passage, professional and amateur astronomers will coordinate their observations in a special campaign to study every aspect of the comet’s development. The largest and smallest telescopes will have their eyes glued to this 3-mile wide spinning hunk of dusty ice. Let’s hope for a great show.