On Sep. 21, 2012 Vitali Nevski (Belarus) and Artyom Novichonok (Russia) discovered an asteroidal object of magnitude 18.5 in images taken with the 0.4m telescope of the International Scientific Optical Network (ISON). Although they suspected a comet, due to its motion, they did not mention anything in their discovery report. Thus, after the object appeared on the NEOCP page, other observers first noted and reported the cometary nature. In consequence the comet was given the project's name. Comet C/2012 S1 (ISON) displayed a 9" coma and a faint tail in p.a. 280°. The central condensation seemed elongated. Shortly thereafter the comet was identified on images taken on Dec. 28, 2011 and Jan. 28, 2012, which allowed the quick determination of accurate orbital parameters. Accordingly this comet will come extremely close to the Sun on Nov. 28, 2013 (0.012 AU). Since the discovery brightness indicates an intrinsically bright object, the comet could become very spectacular (CBET 3238). Assuming an average brightness evolution (n=4) the comet should reach magnitude -10 at perihelion, with the visual tail stretching about 40-50° during the following days. After perihelion the comet will move strictly northward, thereby approaching Earth. It will come within 0.43 AU of our planet on Dec. 27, when it could still be as bright as 4 mag, with a coma the size of the Moon. On Jan. 7/8, 2014 it will pass the northern celestial pole at a distance of only 3°. In case it should be a new comet (n=3), the predicted values are -5 mag / 8° / 6 mag / 7.5 mag.

Brightness Prediction

Prior to perihelion comet ISON will be observable in a reasonably dark sky until Nov. 20, 2013 for mid-European observers. After perihelion it will again become observable in a reasonably dark sky after Dec. 5. On these days the comet will be visible in the morning sky at a magnitude of about 1 mag (n=4). It should be brighter than 10 mag from the beginning of October 2013 until the end of February 2014.

Visibility

Relative positions comet - sun

During this period it moves through the constellations Leo, Virgo, Libra and Scorpius (pre-perihelion) and through the constellations Serpens, Corona Borealis, Hercules, Draco, Ursa Minor, Cepheus and Camelopardalis (post-perihelion). Incorporated in the diagram is the direction and expected length of the gas tail. The much more prominent dust tail will deviate significantly from this direction only during the days around perihelion. Between the start of September and the end of October the comet will be situated less than 5° from Mars (minimum distance will be 0.9° on Oct. 18), which can be used as a comfortable guidance.

Path on the Sky

Orbit in 3D

First visual sightings and a couple of images of the comet were reported at the start of 2013. These showed the comet to be about 0.5 mag brighter than initially expected. At the beginning of April the brightness of the strongly condensed 0.5' coma was estimated to be 14.0 mag. Surprisingly a short tail was already discernible, both visually and photographically. Due to the miniscule data base not even a rough prediction could be made at that time.

During the weeks following its reappearance (in the morning sky) at the end of August 2013 it showed a disappointing evolution. Until Nov. 12 its brightness increased rather slowly from 12.5 mag to 8.0 mag. A period of much more rapid brightening followed, which was superimposed by 2 outbursts. The first (on Nov. 13/14) peaked at 5.0 mag, the second (on Nov. 18/19) at 4.5 mag. This behavior, accompanied with a decrease in the heliocentric brightness – and a significant decrease in the production rates – during the very last days of terrestrial observations (when it reached 4.0 mag) strengthened the fear that the comet may be disintegrating (heralded by the outburst on Nov. 13/14). STEREO observations showed the comet intact, but indicated a brightness standstill.

Just before the comet appeared in the SOHO field, first non-gravitational parameters were determined, one further indicator of a disintegration of the nucleus. The comet looked very similar to comet C/2011 W3 (Lovejoy) when it appeared in the SOHO field on Nov. 27 around 2hUT. During the following hours it brightened, reaching a maximum of –2 mag about 14 hours prior to perihelion (Nov. 28.2 UT). During the next hours it faded gradually and, shortly before entering the obscuration disc in the C3-coronagraph (around 15:30 UT), got more diffuse. With the C2-coronograph comet ISON could be followed until 17:30 UT. With time it became ever more diffuse, showing a broadening brightness distribution. When it reappeared behind the C2-occultion disk (briefly after 19:30 UT) it was very faint with a very flat brightness maximum, indicating that the comet survived perihelion only as a cloud of debris. Surprisingly it got more condensed during the following hours, accompanied with a significant increase in surface brightness. In addition, it showed two tails: one pointing towards the Sun and one at a right angle to the first. This sparked new hopes, which however vaporized when the comet began to fade and got more diffuse again. It is likely that the brief increase in brightness was due to forward scattering. The remnant faded continuously and became ever more diffuse until it left the C3-field around 0hUT on Dec. 1. The pre-perihelion tail could be discerned until 15hUT on Nov. 29. in the C3-field. The remnant was last seen on Dec. 7, when it left the STEREO-field. All trials to observe it from Earth (beginning on Dec. 8) failed – neglecting two very uncertain visual observations. Negative results did not only concern visual observations, but also deep and wide angle photography (CBET 3767).

Böhnhardt et.al. (Max Planck Institute for Solar System Research) concluded that the tail pointing in the direction of the Sun was the result of the dust production about 1 to 2 days prior to perihelion. The perpendicular oriented (to the east) part of the tail should have been the result of the dust production during perihelion passage (CBET 3731). Observations and simulations at the Max Planck Institute for Solar System Research showed, that the nucleus ceased to produce dust 2 hours past perihelion. The tail pointing in the direction of the Sun consisted of particles generated before perihelion, the second tail of particles generated in the two hours after perihelion.

Daylight observations of ISON were not successful as well. On Nov. 27/28 this failure was tried to explain by pointing on comet Seki-Lines (1962 III). Before plunging in the bright twilight as well as after its reappearance in similar conditions it was estimated as -2 mag. Based on this observations a maximum brightness of -7 mag was deduced. However, numerous attempts to find this comet in the daylight remained unsuccessful.

Evolution of the heliocentric magnitude

Until the end of the terrestrial apparition (Nov. 23) 34 observations from 8 members of the German Comet Section and 215 international observations augmented for the analysis. Furthermore I used 13 observations by SOHO between Nov. 27 and Nov. 30. The comet showed an erratic brightness evolution. Nevertheless, by neglecting the two short-term outburst on Nov. 13/14 and Nov. 18./19, it can be described reasonably well by the following formulae:

t < -16d: m = 8.7 mag + 5×log D + 4.7×log r

Total Brightness and Coma Diameter

The coma diameter swallowed very rapidly. In early September measuring only 0.7' (100.000 km) it increased to 8' (375.000 km) on Nov. 14, only to decrease immediately afterwards. The rather small coma was characteristic for this comet. It was only moderately condensed (DC 4), thus looked rather dull in a telescope until the end of October. Furthermore the false nucleus was unusually faint. Thereafter it condensed rapidly, with the degree of condensation reaching DC 8 at the end of the terrestrial apparition.

Degree of condensation (Moving weighted 3-days means)

The only positive fact was the tail, which could be quite well seen, starting in early October. Until Nov. 16 its length increased to 3° (7 Mio. km), thereby pointing constantly WNW.

Tail Length

In retrospective the high hopes for a spectacular show vaporized in the heat of the Sun. Nevertheless the comet showed numerous interesting features, and the development near perihelion should keep the scientists busy for many months or even years. Comet ISON confirmed one experience: you cannot trust on a "new" comet, approaching the Sun for the very first time. Their evolution is very uncertain, because their nuclei have not yet had the chance to prove their power of endurance.

Andreas Kammerer

FGK observations