Analysis of Comet Apparitions

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C/2020 F3 (NEOWISE)

On images taken by the satellite NEOWISE in the infrared on Mar. 27, 2020 a comet was discovered in the southern part of the constellation Puppis. Follow-up observations of comet C/2020 F3 (NEOWISE) showed a quite diffuse coma of diameter 2' and of total magnitude 16.0. The comet will reach its perihelion at the small solar distance of 0.30 AU on July 3, 2020. It may reach 8th magnitude, but will then be located close to the Sun (CBET 4740/42). In addition, the derived absolute magnitude is clearly below the Bortle-limit, thus making a disintegration of this comet on its way towards perihelion likely. Earth will cross the comet's orbital plane on May 22.

The comet clearly exceeded the original expectations and developed to a nice "Comet with a tail" for the unaided eyes. Only the notoriously low altitude and at times the twilight or bright noctilucent clouds interfered a bit with the display. If those factors would have been optimal, the dust tail would have been more impressive and the gas tail not just a difficult detail visually.

Following a change in the initial rapid brightness development 69 days before perihelion (on Apr. 24), the brightness increased significantly more slowly but very steadily, so that a peak brightness of 0 mag seemed plausible by mid-June. Last doubts were dispelled by images taken by SOHO with the "healthy"-looking, 2 mag comet passing the LASCO-C3 field-of-view between June 22 and 27. Just one day after perihelion the comet was recovered by observers in the bright dawn. Brightness estimates were difficult due to the bright sky background, the few adequate comparison stars, extinction corrections and the different background brightness at the comet's and the comparison stars' position. The analysis of 980 observations from 106 observers yields a maximum brightness of 0.8 mag at perihelion. During the following days the comet became ever more prominent even though the dawn still disturbed. Not before mid-July could it be observed under a dark sky. Post-perihelion the comet showed a very steady decrease in brightness, with the absolute brightness higher and the activity parameter lower than pre-perihelion a typical behavior for a long-periodic comet. The following formulae describe the development very well:

t < -69d: m = 0.5 mag + 5×log D + 45×log r
-69d < t < 0d: m = 7.1 mag + 5×log D + 13.1×log r
t > 0d: m = 6.3 mag + 5×log D + 10.0×log r

Estimates of the coma diameter were not a priority for this comet, instead the observers concentrated on the tail. In consequence the number of coma estimates is lower than average. According to these the apparent coma diameter increased only slightly from 4.5' to 6' prior to perihelion. Just after perihelion the estimates state a diameter of 1.5', but it is unclear if the observers could discern the full extent of the coma in the bright sky. The apparent coma increased until mid-July, when its diameter reached the maximum of 12'. Thereafter the decreasing activity, combined with the increase in distance to Earth resulted in a decrease of the coma diameter, which measured 3.5' at the end of September.

In absolute dimensions the coma diameter increased from 310.000 km to 380.000 km prior to perihelion. Immediately after perihelion, the coma diameter measured 90.000 km (but see the remarks above), but increased rapidly, reaching a maximum of 475.000 km in mid-August. Thereafter it shrunk slowly, measuring still 400.000 km at the end of September.

Total Brightness and Coma Diameter

Pre-perihelion the degree of condensation was constant at DC 2-3 until mid-May. Thereafter it increased, reaching DC 5 when it disappeared in early June. Post-perihelion the degree of condensation remained constant at DC 7-8 until mid-July. Thereafter it decreased steadily, reaching DC 2 at the end of September. The coma showed a conspicuous central condensation until about July 25, which was even brilliant during the first days.

Length of Dust Tail

An impressive dust tail, clearly bent in clockwise direction, was displayed by the comet after perihelion, showing the parabolic shape around the central condensation usual for bright comets. The comet showed a tail of length about 1.5° (15 Mio. km) in the LASCO images. Even just after perihelion in the bright dawn the brightest parts of the tail could be seen. But the observers had to wait until the comet gained altitude and entered darker sky regions to see it in its full extent. Between July 15 and 20 the maximum length of 15° (30 Mio. km) was estimated visually. Thereafter the visually discernible length of the tail decreased rapidly, reaching 2° (4 Mio. km) on Aug. 1. Last tail reports date around Aug. 20 with the tail length reported as 0.5°. Beside the curvature the tail showed a well- defined eastern and an ill-defined western edge. At times a distinct contrast effect could be observed in telescopes at the eastern edge: the sky background appeared unusually dark. In binoculars it could be discerned that the dust tail got broader with distance from the coma, with the surface brightness decreasing rapidly, making the end of the tail difficult to define. Photographs showed a number of striae (dust streaks) which appear due to local activity centers on the rotating comet's nucleus. During the first ten days a slightly darker, narrow "shadow of the nucleus" could be seen in telescopes, stretching tailwards of the central condensation. Beside the prominent dust tail, a narrow and linear gas tail was observable in binoculars between about July 15 and 20 July. While it was a difficult feature visually, it is prominent on photographs, showing a lot of internal structure. On the LASCO images the dust tail was oriented in p.a. 200°. Post-perihelion it pointed towards p.a. 300° at first, turning via North to East until early August.

CCD images taken by the Virtual Telescope Project on July 15 and applying a rotational filter showed several dust shells similar to those of comets C/1995 O1 (Hale-Bopp) und C/2001 Q4 (NEAT). The dust shells expanded with a velocity of 1.300 km/h. The distance between the innermost and the next outer shell measured short of 10.000 km. From the observations a rotation period of 7.5 hours can be derived. The current orbital elements yield the year 2.375 BC for the preceding perihelion passage and the year 8.655 AC for the next one. Thus the orbital periods can be derived as about 4.400 and 6.650 years (CBET 4816).

FGK observations