Analysis of Comet Apparitions

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C/2021 A1 (Leonard)

On Jan. 3, 2021 G.J. Leonard discovered a comet, using the 1.5m-telescope at Mt. Lemmon Observatory, near the border of the constellations Canes Venatici / Bootes. Comet C/2021 A1 (Leonard) showed a significantly condensed 10" coma of total magnitude 19.2 and a 5" tail in p.a. 250-270°. It will pass perihelion on Jan. 3, 2022 in the solar distance of 0.62 AU, expected to be as bright as magnitude 8.0. On Dec. 12, 2021 it will approach Earth to within 0.23 AU, expected to peak at magnitude 6.0 (CBET 4907). However, the coma diameter during those days should reach about 20', requiring a dark sky background to be well discernible. It should be brighter than 16 mag between August 2021 and July 2022. During this interval it moves through the constellations Ursa Major, Canes Venatici/Coma Berenice, Bootes, Serpens, Ophiuchus (minimum Earth distance), Serpens, Sagittarius, Microscopium (perihelion, than retrograde), Sagittarius, Corona Australis and Scorpius. At mid-European locations it will sink towards the evening NE-horizon until the start of September 2021, being visible in the morning sky until Dec. 10 (reaching maximum altitude of 50° at the end of November in the morning sky).

On the Comet's Mailing List Michael Mattiazzo pointed out that the orbital geometry will favor a significant forward scattering in mid-December, resulting in a short-lived brightness increase of up to 1-2 mag around Dec. 16 (unfortunately the comet will then be very low during brighter twilight). Earth will cross the comet's orbital plane on Dec. 8, so that the tail will display an higher surface brightness and even an anti-tail may be observable.

Around the days of perigee the comet could be observed with the unaided eye – if the weather was cooperative, what was not often the case. After Dec. 12 the comet actually disappeared for mid-European locations and could only be followed further at more southerly latitudes. There was a miniscule chance of detection very low in the evening twilight sky until the Christmas days for mid-European observers, but I did not receive any observation from members of the German Comet Section.

Based on 442 observations from 81 observers the brightness development can be represented quite well with the formulae

Thus the comet peaked at magnitude 3.8 on Dec. 14, 2021. However, actually the brightness development was not uniform. Between mid-November and Dec. 12 the comet was considerably fainter than the formula would predict, causing some rumor about a possible early disintegration. However, during the following two weeks the brightness increased significantly, perhaps partly caused by the proposed forward scattering. In consequence the above mentioned brightness was hold for 10 days. However, not constantly, because the comet showed several short-time outburst during the second half of December. In consequence the comet reached a short-time maximum of 3.0 mag on Dec. 20/21, 2021. After the turn of the year the comet began to fade rapidly, reaching 8.5 mag at the end of January 2022.

The surprisingly small number of observations in spring 2022 is caused by two facts: first, the comet could not be seen worldwide between end of January and end of February. Second, when it reappeared the comet displayed only a very diffuse, large coma without a significant central condensation. Thus it seems that the comet fragmented eventually near perihelion. This scenario could neatly explain the numerous small outbursts of comet Leonard during the weeks around perihelion.

The apparent coma diameter increased from 0.5' at the start of the apparition to its maximum value of 16' on Dec. 8, 2021. Thereafter it decreased comparatively fast, reaching 2' at the end of January 2022. That the maximum apparent coma diameter was observed almost one week before closest approach to Earth could have been caused by the period of low activity between mid-November and Dec. 12 mentioned above. The absolute coma diameter increased from 80.000 km at the start of the apparition to the maximum of 360.000 km around Nov. 7, 2021. Thereafter it decreased rapidly, reaching only 100.000 km on Dec. 15, only to increase again, measuring 250.000 km on Dec. 25 and 325.000 km on Jan. 5, 2022. Eventually it decreased steadily, reaching 140.000 km at the end of January 2022, paralleling the variations in activity. The common effect of seeing the faint outer coma only partly with a comet very near Earth due to the then low contrast of it should have had only a minor impact for this comet. The coma was mostly well condensed with a degree of condensation of DC 3 at the start of the apparition. It increased to DC 6-7 until the Christmas days. During January 2022 it decreased from DC 6-7 to DC 4.

Observers equipped with CCD cameras began reporting a tail in early October 2021, visual observers in November. The tail reached a length of 2.5° (2.5 Mio km) around Dec. 8. At the turn of 2021/22 visual observers saw a tail up to 10° (16 Mio km) long, while it even stretched over 40° on photographs. Photographs recorded a very dynamic tail with lots of structure that showed rapid changes. Until mid-January the visual tail length had decreased to 1° (4 Mio km). At first the tail pointed towards North, slowly turning towards Northwest during the following weeks. It abruptly turned towards East during the days of perigee and onwards.

Andreas Kammerer

FGK observations