Will Comet ISON Thrill Us This Fall? A Fresh Look At Prospects

Comet C/2012 S1 ISON is currently in Gemini and too near the sun in the sky to view. The sun’s position is shown for July 1. Credit: NASA

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.”

One of the last photos of Comet ISON before it was lost in the solar glare was taken on May 2 by Nick Howes and Ernesto Guido of the Remanzacco Observatory taken with a 79-inch (2-meter) telescope. The comet had changed little in brightness since January.

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.

Comet ISON photographed with the Hubble Space Telescope on April 10 this year when it was 223 million miles (359 million km)  from Earth. Credit: NASA/ESA

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.

The comet will be in stellar company on and around October 14 when it joins Mars and Regulus for a trio conjunction. This map shows the view at the start of dawn Oct. 14. Created with Stellarium

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.

Comet ISON will be visible – especially for northern mid-latitude observers – low in the western sky during early evening twilight during the first half of December. Best views  – with the comet higher up – will happen at dawn during the same time. Created with Chris Marriott’s SkyMap software

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.

Simulated views of Comet ISON in the early dawn sky during the first part of December 2013. The comet’s head will likely fade but the tail should be a spectacular sight from dark, moonless sky for both northern and southern hemisphere skywatchers. This map shows the sky facing southeast about 45 minutes before sunrise. Created with Stellarium

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.

The comet’s trajectory around the sun around the time of closest approach or perihelion. Heat and gravitational stress during the encounter could cause it to break to pieces and disintegrate. Credit: NASA

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.


9 Responses

  1. Lynn

    Hi Bob
    As you know that NASA has found their 10,000 neo, but I thought they had 600,000 catalogued, so is the 10,000 the ones that are over the 1,000 km size or have I got that wrong, I remember Don Yeomans saying that and I think you said it too on one of your blogs. Thanks 🙂

    1. astrobob

      The 10,000 are near-Earth asteroids both large and small that have been cataloged. There are something like 300,000 asteroids overall – near-Earth, main belt and Kuiper Belt – that have been cataloged.

  2. Lynn

    Do you know why then that Don Yeomans said to me that they have 600,000 known asteroid’s and they have them all catalogued, you mentioned this not long ago in one of your blogs too.

    1. astrobob

      Sorry, I just quickly looked up the number and grabbed what I thought was a reputable source. It may not be. Stick with the number in my earlier blog. Yes, 600,000 known and cataloged asteroids that include all varieties as I wrote before. There are millions of others that have not been observed (yet) or cataloged.

  3. Lynn

    That’s ok Bob, does that mean that there is 600,000 known asteroid’s but NASA have discovered the 10,000 near earth asteroid’s, as I read 10,000 also but I took that as near earth asteroid’s, not all known ones, hope that’s right 🙂

    1. astrobob

      Yes, the 600,000 includes EVERY known asteroid. The 10,000 Earth-approachers are a small subset of that number.

  4. Edward M. Boll

    I am surprised at how bright Lemmon is. It may be yet around magnitude 10 in mid August. As far as ISON goes anything is yet possible. While I have downsized my earlier prediction there still is a possibility that it could loom brighter than the Full Moon on Thanksgiving Day. I expect peak brightness around 4:15 Central Standard Time.

    1. astrobob

      Hi Edward,
      I think full moon is way on the high end but ya never know. I looked at Lemmon last night. Still a very nice comet at mag. 9 and beautifully paired up with the rich open cluster NGC 7789 in Cassiopeia.

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