Analysis of past comet apparitions

C/2006 P1 (McNaught)

On Aug. 7, 2006 R.H. McNaught discovered yet another comet in the course of the Siding Spring Survey. Comet C/2006 P1 (McNaught) was of magnitude 17.0, positioned in Ophiuchus, near the border of Scorpius. Despite bright moonlight it displayed a 20" coma. First orbit calculations indicated perihelion during summer 2007 in a solar distance of about 1.5 AU, with the comet reaching magnitude 10-11 (IAUC 8737). Additional astrometric observations then showed that this comet will pass the sun at a very small distance (0.17 AU) in mid-January 2007, perhaps reaching 2 mag. However, mid-European observers can follow it only until mid-October very low above the western evening horizon, when it is expected to have reached only 14 mag. After perihelion it will move quickly southward (MPEC 2006-R53).

Surprisingly, Michael Jäger found the comet on a CCD image taken on Sep. 13/14 to be of magnitude 13.5 - 2.5 magnitudes brighter than expected! The coma diameter was 2.5'.

During the following seven weeks comet C/2006 P1 (McNaught) continued to evolve rapidly, reaching 9.5 mag and a significantly condensed 3' coma by Nov. 7 (r = 1.66 AU), according to images taken by Michael Jäger. If it would continue this pace it would become a bright comet! However, nearly all comet experts (me too...) expected that the comet would experience a sudden drop in its brightness evolution (similar to comet C/2002 V1 (NEAT), which experienced this drop at a solar distance of r = 1.3 AU), resulting in a very short-termed maximum of less than 0 mag or that it even would disintegrate completely before reaching perihelion.

Yet, the comet surprised everyone and in January 2007 became the brightest and most spectacular comet since Ikeya-Seki in 1965, so that it will be remembered as the Great Comet of 2007. It may serve as an excuse for the experts, that comet C/2006 P1 (McNaught) was difficult to observe during its approach of the sun. From mid-October 2006 onwards it could not be observed in a dark sky, hence only few observers succeeded in seeing or imaging it. One of them was indefatigable Michael Jäger who hunted down the comet again and again. He was also one of the very few experts who relied in the comet's great potential.

Despite the positive evolution, documented by Michael Jäger, the scepticism about the comet remained until the end of December 2006. In the last days of December reports were published which stated that the comet was already of magnitude 4, which excited the comet community. During the next days the events developped rapidly: on Jan. 3 the observers estimated the brightness at 2 mag, on Jan. 5 already at 1 mag. However, the observing conditions remained problematic with the comet only a few degrees above the rather bright evening or morning horizon. The best conditions could be found at high northern latitudes, from where the first pictures were published which showed the comet with an astonishingly bright tail against the bright twilight sky. During the night Jan. 6/7 the comet reached 0 mag. Despite the great brightness one has to regard, that under the very unfavourable conditions of those days it could already be regarded a success to even glimpse the comet. During the following days the brightness continued to increase rapidly, so that the comet - now at magnitude -2 mag could even be glimpsed at an altitude of only 1° during civilian twilight with binoculars, showing a bright inner coma and a short tail. It was now better placed in the evening sky and could be seen with the unaided eyes - if the rather awful weather permitted observations. Eventually, on Jan. 11 the first really good image was taken as one of the very first operations of the satellite STEREO-B, showing a tail at least 7 long with numerous striae.

Despite the great brightness during those days it has to be stressed, that the comet was never a conspicious object with the unaided eye, although a few observers could even glimpse a 1-3° tail against the bright twilight sky. The mid-European dusk visibility ended on the evening of Jan. 13 with the comet at magnitude -5. After that the comet could be found - only 6° from the Sun - with binoculars and telescopes in the daylight sky! Telescopic observations showed - depending on the sky transparency - the bright false nucleus with a faint coma and even the brightest few arcminutes of the tail.This was made possible by a favourable Comet-Earth-Sun geometry, allowing strong forward scattering, resulting in a brightness increase of 2.3 mag, as was predicted by J.N. Marcus (IAUC 8793).

After these breathtaking events all mid-European observers expected, that now the chapter of comet McNaught was closed for them. But the comet still surprised with a short-lived post-perihelion show. Due to the strong curvature of the very long and broad dust tail the most northerly striae could be seen above the western horizon from the Northern hemisphere until Jan. 21. They reached up to the southern edge of the Pegasus square! These might have been observable for a few more days, but now the Moon began to interfere.

For the observers in the southern hemisphere the really spectacle began on Jan. 18, when the comet could be observed in a reasonably dark sky for the first time. Until the Moon's interference began around Jan. 23 all observers had a gorgeous view. From the extremely bright head an impressively bright and structured dust tail, which was strongly curved, extended so far that its complete extension could only be fixed by combining photographs taken from both hemispheres. The evaluations are not finished yet, but it seems that the tail extended for about 60°! In addition, the width of the tail of about 20° was enormous. Visually about 35° tail length could be seen. The structures within the tail could be discerned visually, but showed in full glory on photographs, and reminded of descriptions/drawings of the Great Comet of 1744.

The coma showed interesting features too: From the nucleus emanated the tail under an angle of 60°. Furthermore the coma showed a bright sunward segment, which was at a very oblique angle with respect to the solar direction. Near-nucleus observations on Jan. 29 with large telescopes showed a broad fan of material extending from the nucleus in the solar direction for over 13.000 km. In addition, spiral-like jets were found, extending up to 32.000 km at position angles 70, 120, 210 and 330. Spectral observations showed a bright sodium emission within the anti-solar area of the coma, most likely indicating a sodium tail, which was first observed in comet Hale-Bopp (CBET 832).

When the Moon no longer interfered in February, all the luster of the comet was gone. The comet had faded to 4 mag and showed a tail of low surface brightness. Faint tail structures could only be discerned on images, which showed the tail to be still about 20° long. At the end of April the comet had faded to about 9.5 mag and by the beginning of July to 12 mag.

With the exception of the brightness evolution the following analysis has still to be regarded as preliminary, assuming that the number of reports will increase further during the next weeks. It is based on 9 visual observations by 5 members of the German Comet Section and 344 international observations. These show that the brightness, pre- and post-perihelion, evolved surprisingly continuous, as is shown by the formulae below. The maximum brightness of -5.5 mag was reached on the morning of Jan. 14 - 2 days past perihelion passage - due to the optimal conditions for the mentioned forward scattering, which resulted in a short-term brightening in the order of 1-1.5 magnitudes.

t < -2d: m = 6.2m + 5×log D + 12.0×log r

t > +2d: m = 4.1m + 5×log D + 11.2×log r

Regarding the coma diameter, it is obvious that the observers did not gave this parameter high priority. In addition, due to the bad observing conditions prior to about Jan. 18 all coma estimates have to be regarded with caution. At the end of September 2006 the apparent coma diameter was estimated at 1.5', and during the first days after its spectacular appearance in the southern skies at about 3'. Thereafter it increased, reaching 7' around March 20. During the following weeks it decreased, measuring 4' at the end of April and 1' at the beginning of July. The absolute coma diameter was about 250.000 km during fall 2006. Shortly after perihelion it measured about 150.000 km. Then it swallowed to a maximum of 525.000 km around March 20. Thereafter it decreased, reaching 325.000 km at the end of April and 125.000 km at the start of July.

Total Brightness and Coma Diameter

The coma was quite diffuse (DC 1) at the beginning of the apparition, but condensed to DC 2-3 until mid-October. During the last days of December 2006 the coma was highly condensed (DC 8), still increasing to nearly DC 9 around Jan. 20. Thereafter it decreased almost constantly, reaching DC 4 around Mar. 20 and DC 1-2 at the end of April.

The evolution of the tail can still only be described crudely. First visual tail sightings happened briefly before the turn of the year. Until the occurrence of the comet in the dark southern skies the estimates (always done in twilight) did not exceed 4-5. Current results show a maximum short of 30° around Jan. 25. Taking into account the photographs, the publication of even larger estimates of the visual tail length is likely. Regarding the very strongly curved tail it has to be discussed, if it is adequate to measure the length of a tail in respect to the angular distance between head and tail's end. Perhaps for such a curved tail it is more adequate to measure the arc's length, which would stretch McNaught's tail length by additional 5-10° roundabout. At the beginning of March the majority of the observers estimated the tail's length at about 2°, but values of 10° were also reported. A more accurate description will have to wait until additional estimates are published. Of course, the same holds for the absolute tail length. With the current data basis the absolute tail length may have reached 80 Mio. km. The tail was oriented northward until briefly before perihelion, turning to Southeast and further to South until Feb. 10. Thereafter it reversed its movement, pointing to Southeast around Mar. 20.

Apparent Tail Length

According to the current astrometric data the comet seems to be a dynamically "new" comet, i.e. it made its first perihelion passage (IAUC 8801).

Andreas Kammerer

FGK observations