Comet ISON: A Legend in the Making?


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Infrared Images of Comet ISON from the Spitzer Space Telescope from June 13, 2013. The image on the left shows the fine dusty tail. The image on the right shows an atmosphere of neutral carbon dioxide gas evaporating from the comet’s nucleus. Image courtesy of NASA

In September 2012, two Russian astronomers discovered Comet ISON with the International Scientific Optical Network. Comets are traditionally named after their discoverers. Comet ISON, however, takes its name from the acronym for the discovery telescope network rather than the individual astronomers.

The earliest predictions of Comet ISON’s brightness suggested that it might be as bright as the full Moon. Hence some media outlets have billed ISON as the comet of the century. Will Comet ISON live up to the early predictions?

Will Comet ISON Make History?

After the initial predictions, Comet ISON has consistently been about 2 magnitudes fainter than the original predictions. On August 12, 2013 amateur astronomer, Bruce Gary, captured the first image of Comet ISON since it became lost in the Sun’s glare near the end of May. It was still about 2 magnitudes fainter than the initial predictions. Two magnitudes is a little over 6 times as faint as predicted.

Comet ISON’s parabolic orbit provides a clue as to why it is fainter than predicted. Most comets have elongated elliptical orbits, meaning they return at regular, even if long, intervals. Some comets have either hyperbolic or parabolic orbits. Hyperbolas and parabolas are both open rather than closed geometric figures, so these comets fly past the Sun once and never return. The parabolic orbit tells astronomers that Comet ISON is making its first pass near the Sun.

Comets passing close to the Sun for the first time often do not brighten as much when they approach the Sun. Hence they do not become as bright as predictions based on their brightness when they are first discovered far from the Sun.

Comet Kohoutek is the most famous example of this effect. Based on predictions it was billed as the comet of the century. As it approached the Sun in late 1973 and early 1974, the comet was much fainter than predicted because it was making its first pass around the Sun. It is possible that Comet ISON will follow this pattern and not live up to its early predictions.

Comets, especially those encountering the Sun for the first time, are however notoriously unpredictable.

Comet ISON Observations and Predictions


Comet ISON: Less dramatic than astronomers expected? Image courtesy of NASA

According to a September 24, 2013 NASA press release, many amateur astronomers are observing Comet ISON. These amateur astronomers continue to report brightness estimates about 2 magnitudes fainter than the initial predictions. These observers are, however, able to see Comet ISON and its tail with relatively small telescopes.

These reported observations lead comet expert, John Bortle, to predict that Comet ISON will not be as bright as initial predictions suggested. Bortle does, however, think that as Comet ISON approaches its closest distance to the Sun on November 28, 2013, it will become bright for a short time. It may even be as bright as the planet, Venus, for a few hours.

Sungrazing Comet: Will ISON Survive The Encounter?

On November 28, Comet ISON will pass within about 700,000 miles of the Sun’s surface during its closest approach. Hence, astronomers classify ISON as a sungrazing comet.

During this near encounter, is possible that the Sun’s tidal forces will tear apart Comet ISON. If ISON survives its close solar encounter, however, it will most likely be very impressive. ISON should be easily visible to northern hemisphere observers throughout December.

During a close solar encounter, solar heat can evaporate enough of a sungrazing comet’s icy nucleus to produce a long impressive tail.

Comet Lovejoy was also a sungrazing comet in 2011. After its close solar encounter, Lovejoy’s tail extended across half of the night sky, for southern hemisphere observers. With luck ISON might provide a similar treat for northern hemisphere observers during December, 2013.

Comet ISON: Potential to Impress or Fizzle

Even if Comet ISON is not spectacularly bright enough to justify its billing as comet of the century, it has the potential to be quite impressive. It may however also fizzle. We won’t know for sure until it approaches the Sun. Whether Comet ISON is spectacularly bright, disappointingly faint, or somewhere in between, the century is still young. There is plenty of time for one or more great comets to appear during this century.


Phillips, Tony. Amateur Astronomers See Comet ISON. (2013). NASA Science News. Accessed September 27, 2013.

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