Mar 2013 Update!
Two months after the article below, our view of Comet ISON is clearing, and I've written an update on the comet's activity, meteoroid stream, and orbit.

Discovery and Orbit

Comet ISON discovery image

Comet ISON discovery image by Nevski and Novichonok. Read their story at

The comet was discovered by Vitali Nevski and Artyom Novichonok on September 21, 2012 using a 40-cm (16-inch) telescope in Russia [1]. The telescope is part of the International Scientific Optical Network (ISON), a group of observatories for observing asteroids and space debris [2]. Comets are usually named after their discoverers, but because the initial report from Nevski and Novichonok did not indicate a cometary appearance for this object, and because other observers discovered it as cometary, the comet was named ISON after the telescope used to discover it [3].

Accurately determining a comet's orbit can take several days, or even weeks of new observations, but for Comet ISON it came much more quickly. Pre-discovery observations from the Mount Lemmon Survey (observed Dec. 28, 2011) and Pan-STARRS (observed Jan. 28, 2012) were identified in the Minor Planet Center's database [1] and it was immediately realized that we had a sungrazing comet on our hands. The orbit's perihelion distance, or the closest distance to the Sun, is 0.012 AU. 1 AU, or Astronomical Unit, is the average distance between the Earth and Sun. Comet ISON's perihelion is well inside the orbit of Mercury (0.4 AU), and closer than 3 solar radii!

Over the past decade, hundreds of sungrazing comets have been discovered, but they are usually small, short-lived, and only seen by spacecraft designed to observe the Sun. However, Comet ISON was discovered at the great distance of 6 AU (just beyond Jupiter's orbit), and the pre-discovery observations go back to more than 8 AU from the Sun! Because it was discovered so far away, the comet must be much larger, and therefore brighter, than the typical sungrazer. Currently, Comet ISON is faint, 16th magnitude in astronomy-speak, or 10,000 times fainter than the human eye can see. But it is brightening as it approaches the Sun. When it is near perihelion in late November, it may be possible to carefully(!) see in the daytime with the naked eye. It is this comet's intrinsic brightness and small perihelion distance that combine to make it so exciting.

Observing comets close to the Sun is a tricky business. Do not look directly at the Sun, and especially do not look at the Sun through any lens, telescope, or binoculars. If this comet is bright enough to see in the daytime, look for observing tips from reputable sources, like Sky and Telescope.

The Science

So, why are astronomers so excited about sungrazing comets? Or, even any comet? Comets are frozen left-overs from the formation of our Solar System. While comets have been in a deep freeze for the past 4 billion years, planets and asteroids have changed a lot from their original compositions. Better understanding of their ices, dust, and organic matter, and how they have changed over the past billions of years, tell us about the origins of our Sun, the planets, and, possibly, life on Earth. To astronomers, every bright comet is an opportunity to learn more about our Solar System.

Comet ISON and the Great Comet of 1680

Staartster (komeet) boven Rotterdam, Lieve Verschuier

A painting of the Great Comet of 1680: Staartster (komeet) boven Rotterdam; Lieve Verschuier (c.1680).

The orbit of Comet ISON has an interesting similarity with another bright sungrazing comet, the Great Comet of 1680. The two comets don't have the same orbits, but the similarity between these two bright sungrazers is remarkable. One thing, however, is for sure: they are not the same comet. Comet 1680 is currently 250 AU from the Sun, and isn't due back for 9000 years. But the similarity suggests they have a common origin. Perhaps they are two fragments of a much larger comet?

Some orbital elements for Comet C/2012 S1 (ISON) and three Great Comets: C/1680 V1, Comet C/1965 S1 (Ikeya-Seki), and C/2006 P1 (McNaught). Note the similarities between ISON and C/1680, but the large differences between Ikeya-Seki and McNaught.
Orbit element ISON C/1680 Ikeya-Seki McNaught
perihelion distance (AU) 0.01250.0060.0080.17
inclination (deg) 62 61 142 78
longitude of the ascending node (deg) 296 277 347 267
argument of perihelion (deg) 345 351 69 156

What might we expect?

Comet ISON's absolute magnitude, that is, its intrinsic brightness, seems to be between those of two spectacular comets: Comet C/1965 S1 (Ikeya-Seki) and Comet C/2006 P1 (McNaught). Both of these comets made very close passes by the Sun, and were possibly the two brightest since the 1930s. Specifically, their absolute magnitudes are 3.9 (Ikeya-Seki), 6.0 (ISON), and 9.5 (McNaught). The magnitude system is ordered such that fainter objects have higher values. It is also on a logarithmic scale, where a difference of 2.5 magnitudes corresponds to a factor of 10 in apparent brightness; a 5 magnitude difference is a factor of 100. Given the same distance from the Sun and distance from the Earth, Comet ISON would be roughly 25 times brighter than Comet McNaught, and one-seventh as bright as Comet Ikeya-Seki. Thus, Comet ISON is in good company, and we may expect a show similar to these two comets, which makes for a very exciting prediction.

Earth and Mars Close Approaches

Comet ISON makes two interesting close approaches. The first, in October 2013, we will watch the comet pass by Mars at the small distance of only 0.07 AU. This distance is small on the grand scale of the Solar System, but it is still 10 million kilometers. The close encounter may make comet ISON observable to NASA and ESA's spacecraft orbiting Mars. Maybe we can even hope to see a picture of ISON from NASA's Curiosity rover?

Although there is no possibility of such a close approach between the Earth and comet, the second close approach I want to bring up will be between the comet's orbit and the Earth. In January 2014, the Earth will swing past a part of space that Comet ISON already traveled through, at a small distance of only 0.03 AU. This encounter brings up the possibility of a meteor shower on Earth. Meteor showers from Oort Cloud comets are rare events indeed. Stay tuned while astronomers consider this possibility.

Comet ISON and Mars

Comet ISON and a close approach to Mars (green and red circles, respectively).

Comet ISON and Earth

A close approach between Earth and Comet ISON's orbit. Will this encounter produce a meteor shower? The green orbit is Comet ISON's orbit and has been split into two parts: the solid line is the path before grazing the Sun, and the dotted line is after.

Will Comet ISON survive perihelion?

Most sungrazing comets do not survive perihelion; they are disrupted by the Sun's intense radiation and gravity. Yet most sungrazing comets are quite small, only tens of meters across [4]. Comet ISON, being so bright, is more like Comet Ikeya-Seki in size, which did survive its encounter with the Sun. Their larger sizes insulate their interiors from the Sun's energy. If Comet ISON does survive, it will zoom back out to the outer-solar system, and on it's way give people in the Northern Hemisphere one last opportunity to see it, only this time at night.

Anything can happen

Comet C/2011 W3 (Lovejoy)

Comet C/2011 W3 (Lovejoy), post-disintegration.

(credit: Dan Burbank/NASA)

Comets are notoriously unpredictable, and anything can happen. Well, maybe not anything, but they can certainly be surprising. Some possibilities include:

  • The comet could fizzle. Comets are made of ices and dust, and the relative proportions of each material, as well as the structure of the nucleus, determine its activity levels. Currently, these details on Comet ISON are unknown, which throws a small wrench into our prediction machines. We won't really know if this comet is going to be a firecracker or a dud until it happens.
  • The nucleus may fragment or undergo a spectacular outburst. These can occur at anytime with any comet for many reasons.
  • Complete disintegration might occur well before perihelion. Although they were not sungrazing comets, comets C/1999 S4 (LINEAR) and C/2010 X1 (Elenin) are good examples of disintegration.
  • It may fragment near perihelion. The great stresses caused by passing so close to the Sun can cause the comet to eject short- or long-lived fragments, which will be comets in their own right. Indeed, if comets ISON and C/1680 are related, then this scenario has already happened.
  • Disintegration near perihelion. The nucleus may completely crumble into many sub-kilometer-sized pieces, all short-lived. Even if this occurs, we may still have a spectacular show. Recently, sungrazing Comet C/2011 W3 (Lovejoy) did just that, yet a remnant tail persisted for a few weeks after the event.

While astronomers will do their best to predict what Comet ISON will become, we may be disappointed. The classic let-down in the business of comet predictions was Comet Kohoutek in 1973, but based on what we know about Comet ISON so far, the Kohoutek scenario seems unlikely. Our brightness estimates will improve as the year progresses. Whatever happens, astronomers will be watching, and waiting to learn more about these fantastic objects.


  1. Novski et al. 2012, Central Bureau Electronic Telegrams, Comet C/2012 S1 (ISON), 3238.
  2. Molotov et al. 2008, Advances in Space Research, International scientific optical network for space debris research, 41, 1022-1028;

    Molotov et al. 2010, 38th COSPAR Scientific Assembly, ISON Near-Earth asteroids project, B04-0088-10.
  3. Cometary Science Archive 2012, Comet C/2012 S1.
  4. Knight et al. 2010, Astronomical Journal, Photometric Study of the Kreutz Comets Observed by SOHO from 1996 to 2005, 139, 926-949.


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