On Nov. 6, 2002 the NEAT-team found a new comet near the border of the constellations Taurus/Orion. Comet C/2002 V1 (NEAT) was of magnitude 16 and showed a 0.4' coma and a tail of 0.4' length towards p.a. 250°. According to the current elements (which show that it is rounding the sun on an elliptical orbit with a period of roughly 30.000 years) this comet came as close as 0.1 AU to the sun on Feb. 18, 2003 (IAUC 8010/11). However, it was very uncertain whether this comet would survive this close encounter, because its absolute magnitude of about 11.5m at discovery was well below a "survivance" limit defined by an empiric analysis of John Bortle. Nevertheless, assuming the case of an average brightness evolution it could attain a maximum brightness of 1-2m at perihelion, displaying a 5° tail. However, during those days the elongation would only be 6°. In this scenario it would be brighter than 10m from the end of January to the beginning of March, brighter than 6m from about Feb. 12 to 23. For mid-European observers the comet would get out of view around Feb. 10, if a solar depression of only 12° is assumed.
But against all odds, comet NEAT became a big surprise. Observations starting in mid-November 2002 showed the comet already brighter than expected. Michael Jäger photographed the comet on Nov. 11. He estimated the brightness of the 1.5' coma as 14.0 +/- 0.5m - about one magnitude brighter than expected.
During the following weeks its brightness increased rapidly. Would it have continued at this rate until perihelion, it would have reached -15m! Therefore it was clear that it would either break apart or suffer a drastic change concerning the brightness evolution. The latter took place around Jan. 5 when the comet had already reached 7.5m. During the next weeks (until perihelion date) the activity factor n dropped to only one third of the starting value, with the maximum brightness still predicted to reach -0.5m.
Evolution of the heliocentric magnitude
For the following analysis I could use 54 observations by 8 members of the German comet section and 380 international estimates. While the pre-perihelion brightness increase has to be divided into two distinct phases, the post-perihelion decrease was very smooth, at least until the beginning of April. Thereafter no statements can so far be made about the further evolution, because until now no observations after the second week of April have been published, although the comet was still rather well placed for Southern hemisphere observers.
Total Brightness and Coma diameter
Applying these formulae a brightness of 1.3m results for the evening of Feb. 15 (last observations before perihelion), of -0.3m at perihelion and of 3.0m for Feb. 25 (first observations after perihelion). Not unusual post-perihelion the comet exhibits a higher absolute magnitude and a smaller parameter n than pre-perihelion. All attempts to find the comet during the days of the perihelion passage with binoculars were unsuccessful. Therefore it can be stated that it is impossible to find a 0m comet at elongations of less than 10°. The SOHO probe, however, was able to follow the comet in all its glory.
At the start of the visual apparition (November 2002) the coma diameter measured about 1.5' (80.000 km), but had increased to 12' (440.000 km) at the time the parameter n dropped significantly (Jan. 5, 2003). Thereafter the coma diameter decrased considerably, primarily as a result of the diminished activity, but also due to the solar wind getting stronger and - at least during the last weeks of the pre-perihelion apparition - because of the worsening observing conditions. Thus the coma diameter was estimated as 8' (250.000 km) at the beginning of February, but only 2' (80.000 km) around Feb. 15. First estimates after perihelion (at low altitude and in twilight) showed an almost 2' (90.000 km) coma which increased to 5' (450.000 km) around Apr. 5. Comparing the values of the absolute coma diameter pre- and post-perihelion it has to be taken into account, that the comet was approaching Earth until December (minmum distance 0.80 AU), thereafter receeding rather slowly till perihelion passage, whereas it receeded rapidly from Sun and Earth after perihelion (during the first days of April the geocentric distance had already increased to 2 AU!).
The coma itself was only weakly condensed (DC 3-4) until the change in the brightness evolution occurred. Through a telescope the central condensation was only faintly discernible at this time. During the following weeks the coma became more condensed and began to show a central condensation, which became more and more prominent, displaying a faint false nucleus. At the beginning of February the coma was highly condensed (DC 8-9) and presented itself in binoculars almost as a "defocussed star". In a telescope a dominant star-like false nucleus was observable, which was located at the focus of a parabola-shaped coma that was only marginally brighter than the tail, which extended at full width from the coma. The comet showed the same morphology during the first days after perihelion, but thereafter the degree of condensation quickly dropped. Estimates after Mar. 10 are rather discordant but seem to hint towards a much slower further decline of the DC values.
First visual tail sightings were reported at the turn of the years 2002/03. Not until Jan. 20 became the tail more prominent and longer. Just before the disappearance it was estimated around 3.5° (8 Mio. km). First estimates after perihelion were done in twilight, so they are not applicable. The few reports after the comet entered darker skies indicate that the maximum tail length after perihelion was similar to that before perihelion. The tail decreased rapidly thereafter and was already short of 1° at the end of March. The tail pointed towards N at first, turning to ENE during the following weeks, only to turn back to N around perihelion. Post-perihelion it pointed towards S at first and later towards SE.
Andreas Kammerer
