How unique is Comet ISON?

A journalist asked me recently “When is the last time there was a dynamically new Oort Cloud comet that was also a sungrazer? In other words, how long has it been since the last Comet ISON?” It is an interesting question that other people will likely ask, so I thought it was worth discussing here.

The quick answer? At least 200 years!

The full answer, complete with caveats, is longer and a bit more technical, so thanks in advance for sticking with me.

The term “sungrazer” is rather ill defined. Historically, just about all known comets that came extremely close to the Sun were members of the Kreutz group, which includes Ikeya-Seki (seen in 1965), C/2011 W3 Lovejoy (seen in late 2011 and early 2012), and more than 2000 small comets discovered by space-based observatories like SOHO in the last few decades. The Kreutz comets all have similar orbits that pass just above the Sun’s surface (typical close approach distances to the center of the Sun, known as “perihelion distance” in astronomer jargon, are about 1-2 solar radii, or about 0.005-0.010 AU) and are thus known as “sungrazers.” The Kreutz comets are believed to have been produced by a series of splits from a single progenitor comet sometime in the last few thousand years. Kreutz comets have periods of 500-1000 years and are therefore NOT dynamically new since they (or their parents) have been through the inner solar system before.

Comet ISON is not a member of the Kreutz group, and has a perihelion distance of 0.0124 AU, which is a bit larger than is typical for the Kreutz comets. To the best of my knowledge there has never been a dynamically new Oort Cloud comet that had a smaller perihelion distance than ISON. Note, however, that reliable orbits go back only about 200 years, and a comet must be well observed to conclusively determine that it is newly arrived from the Oort Cloud as opposed to just having a long period. The only dynamically new comet of which I am aware that came close to ISON’s perihelion distance is C/1962 C1 (Seki-Lines) at 0.0314 AU. C/1865 B1 (Great Southern Comet) also came close (0.0258 AU) but its orbit was not measured well enough to determine if it was new or not.

As I mentioned above, there is no formal definition of “sungrazing.” However, a logical distinction (and one that I am advocating for in a paper I am currently writing) is comets that come close enough to the Sun to be subject to tidal disruption. What is tidal disruption? It’s when an object (comet, asteroid, moon, etc.) is close enough to a larger body that the gravitational tug on the near side of the object is so much stronger than the gravitational tug on the far side of the object that the object becomes elongated and eventually pulled apart. Astronomers call the distance at which this occurs the Roche limit. The exact distance depends on a comet’s density, but for typical comet densities is ~3.7 solar radii (0.017 AU).

Since both C/1962 C1 and C/1865 B1 are comfortably beyond that distance, I'm inclined to consider them to be “sunskirters” rather than “sungrazers.” Thus, there have been no known dynamically new sungrazing comets in at least 200 years. Comet ISON really is unique!

A couple of final caveats:
1. The Great Comet of 1680 (C/1680 V1) has an orbit rather similar to ISON’s and actually came closer to the Sun (perihelion distance 0.0062 AU). Despite arriving so long ago, it was observed well enough that a reasonably reliable orbit was calculated by the Minor Planet Center that indicates it was not new.
2. There are a handful of SOHO-discovered comets that are not Kreutz but are sungrazing by the above definition. However, none were ever observed beyond the SOHO field of view and therefore have too short an orbital arc to determine if they are new or not.