Comet Ison C/2012: between the promise of brilliance and disillusionment

Comet ISON seen by Hubble in 2013 — On October 18, 2013, NASA published a new image of the comet, taken on October 9 by the Hubble Space Telescope. At that date the comet was still intact.
Image source: NASA Hubble Space Telescope (new window)

Scientific summary

The Astronoo article retraces the history of comet C/2012 S1 (ISON), spotted on September 21, 2012 by two Russian and Belarusian amateur astronomers. Originating from the Oort cloud, this dynamic comet was heading toward a very tight perihelion, predicted for November 28, 2013, only 1.16 million kilometers from the solar surface. Models predicted a nucleus of 1 to 2 km and a brightness comparable to that of the full Moon. The scenario turned out quite differently: the nucleus disintegrated under the effect of heat. Observations by SOHO and STEREO nevertheless enriched our knowledge of cometary nuclei.

Did Comet ISON really deserve its nickname of “comet of the century”?

This article is devoted to comet C/2012 S1 (ISON), discovered on September 21, 2012 by two amateur astronomers, Artyom Novichonok and Vitali Nevski, from the International Scientific Optical Network observatory near Kislovodsk. Very quickly, orbital calculations revealed an object from the Oort cloud, making its first passage through the inner Solar System. Its perihelion, set for November 28, 2013, was to bring it only 1.16 million kilometers from the Sun’s surface, an extremely risky approach but also spectacular in power. Astronomers then hoped for a comet visible to the naked eye, even in daylight, as bright as the full Moon. Expectations were immense, fueled by the memory of Hale-Bopp and Ikeya-Seki. Reality was quite different: crushed by solar heat and tidal forces, ISON disintegrated at the time of its perihelion passage. It never delivered the great spectacle announced. Yet continuous observation of its death throes by SOHO, STEREO, and Hubble provided scientists with valuable data on the structure and fragility of cometary nuclei.

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Where do comets come from?

Two reservoirs at the confines of the Solar System

Comets come mainly from two major reservoirs located at the confines of the Solar System: the Kuiper Belt and the Oort cloud. Their origin and evolution are linked to the gravitational dynamics of the primitive Solar System and to the distribution of ices in cold regions.

The Kuiper Belt

The Kuiper Belt is a disk-shaped region located beyond Neptune’s orbit, between 30 and 50 astronomical units (AU) from the Sun. It contains billions of small icy bodies.

The Oort cloud

The Oort cloud is a hypothetical gravitational sphere surrounding the Solar System out to a distance of about 100,000 AU. It is thought to consist of several billion small icy bodies.

Comet ISON: a promising discovery

A discovery made by two amateur astronomers

On September 21, 2012, two amateur astronomers, Artyom Novichonok (Russia) and Vitali Nevski (Belarus), discovered a particularly bright object using the telescope of the International Scientific Optical Network (ISON) located near Kislovodsk. This comet, officially catalogued as C/2012 S1 (ISON), immediately aroused the interest of the scientific community: its highly elliptical trajectory suggested that it was a new dynamic comet from the Oort cloud, heading toward the Sun for the first time.

Orbital and physical characteristics of Comet ISON

A highly eccentric orbit

Comet ISON follows a highly eccentric orbit (e ≈ 1), typical of long-period comets originating in the Oort cloud. Its passage through perihelion was predicted for November 28, 2013, at a distance of about 1.16 million kilometers from the solar surface (i.e., 0.0124 AU). It was then to graze the Sun at very high speed (about 377 km/s), while undergoing extreme heating capable of vaporizing a large part of its volatile ices and releasing a potentially spectacular coma.

A nucleus 1 to 2 kilometers across

From a physical standpoint, early observations estimated that the comet’s nucleus measured between 1 and 2 km in diameter. The volatile compounds it released (water, carbon dioxide, dust) under the effect of early insolation seemed particularly abundant, which reinforced the hypothesis of a spectacular surge in brightness as it approached the Sun.

The promise of an exceptional celestial event

A “comet of the century”

From its discovery, some astronomers called ISON the “comet of the century.” Orbital projections indicated that it could become as bright as, or even brighter than, the full Moon, and even visible in daylight, an extremely rare phenomenon. Enthusiasm spread quickly, fueled by historical precedents such as Comet Ikeya-Seki (1965) or Hale-Bopp (1997).

A unique opportunity for science

Hopes were all the greater because its visibility was to extend over several weeks in autumn 2013, for observers in both the northern and southern hemispheres. For scientists, ISON also offered a unique opportunity to study the primitive composition of an object from the Oort cloud, little altered since the formation of the Solar System.

Disappointment and lessons

Disintegration at perihelion

Alas, as perihelion approached in late November 2013, Comet ISON was disintegrated by the Sun’s intense heat. Images captured by the SOHO and STEREO probes showed a progressive fading of its nucleus, leaving only a diffuse trail. The event certainly drew worldwide attention, but the visual spectacle expected from Earth never took place.

A test bed for cometary models

Despite this disintegration, Comet ISON represented a valuable opportunity to test thermal and dynamic models of cometary nuclei. It also served as a test bed for solar observation instruments and collaborations between professionals and amateurs around the globe.

References

FAQ: Everything you need to know about Comet ISON

What is a long-period comet?

A long-period comet is an icy body whose highly elongated orbit around the Sun exceeds 200 years, and which generally comes from the Oort cloud. Like C/2012 S1 (ISON), many make their first passage near the Sun, which makes them particularly unpredictable.

Why was Comet ISON nicknamed the “comet of the century”?

Because early estimates of its brightness suggested it could rival the full Moon, or even become visible in daylight. This scenario was based on its very close passage to the Sun and the apparent abundance of its volatile ices.

What is perihelion?

Perihelion is the point in a body’s orbit closest to the Sun. For ISON, it was located about 1.16 million kilometers from the solar surface, an infinitesimal distance on the astronomical scale, reached at nearly 377 km/s.

Did Comet ISON survive its passage near the Sun?

No. In November 2013, its nucleus disintegrated under the effect of heat and gravitational stresses. Images from SOHO and STEREO show a progressive fading of the nucleus, replaced by a diffuse dust trail.

Where did comets like ISON come from?

ISON came from the Oort cloud, a spherical reservoir of icy bodies located at the confines of the Solar System, up to 100,000 astronomical units from the Sun. The closer Kuiper Belt, for its part, supplies short-period comets.

Can Comet ISON still be observed today?

No, the comet has no longer existed as an active nucleus since late 2013. However, archives of images (Hubble, SOHO, STEREO) and freely accessible photometric data remain, useful for studying the fragmentation of cometary nuclei.