Analysis of Comet Apparitions

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After more than 70 years comet 12P/Pons-Brooks (P=71.32a) will reach its 4th observed perihelion. On Apr. 21, 2024 it will pass the Sun at a distance of 0.78 AU. Since it will not get closer than 1.55 AU to Earth during this apparition and will move on the opposite side of the Sun as seen from Earth it is unlikely to become brighter than 4-5 mag. Around perihelion it should also reach its maximum coma diameter (about 10') and tail length (visual: about 2-3°), as indicated by my empiric formulae. The comet should be brighter than 16 mag between July 2023 and December 2024. During this period it moves through the constellations Draco, Hercules, Lyra, Cygnus, Lacerta, Andromeda, Pisces, Aries (perihelion), Taurus, Eridanus, Lepus, Canis Major, Puppis, Vela, Centaur, Lupus and Norma. As seen from mid-European locations it can be spotted quite low above the Northeastern morning horizon between December 2023 and February 2024. However, this comet is primarily visible in the evening sky at this apparition. While it is almost at the zenith at the end of twilight at the beginning of the apparition, it steadily loses altitude during the following months and will disappear above the WNW horizon in the first week of April 2024. The Earth will cross the comet's orbital plane on Dec. 8, 2023 and on June 6, 2024.

The comet has shown a very interesting development since the end of July with several outbursts, each of which led to a temporarily very interesting coma morphology. Although the comet is still rather faint the analysis can already be based upon 330 observations from 56 observers (end of October 2023). Most striking is the large outburst on July 20/21 with an amplitude of 5 mag (from 16.5 mag to 11.5 mag)! Interestingly, the comet subsequently maintained this much greater absolute magnitude. Additional outbursts (until end of October) did brighten only the central regions of the coma for several days: Sep. 3 (amplitude: ≈0.4 mag), Sep. 23/24 (0.9 mag), Oct. 4/5 (4.0 mag), Oct. 22/23 (≈0.4m), Oct. 30/31 (≈0.4 mag). During each outburst, the comet showed a compact coma of high surface brightness with a tailward appendage that showed a "shadow" behind the nucleus and thus had the shape of two "horns". Due to the erratic behavior and the fact that the observations after July 21 so far show an activity parameter near zero, no prediction can be made for the next weeks. Instead it has to be referred to the behavior of this comet during the previous perihelion passages. According to these the comet should reach a peak brightness of 4-5 mag.

Total Brightness and Coma Diameter

The coma diameter and degree of condensation also show the erratic behavior. After the outbursts on July 20/21 and Oct. 4/5, the coma diameter increased from about 0.5' (50,000 km) to about 5' (650,000 km) within 2-3 weeks, thereafter slowly decreasing. Outside the outbursts the degree of condensation was in the order of DC 3, but reached DC 8-9 during the outbursts. So far tail sightings are rather rare, with lengths up to 6' (3.5 mio. km). The tail was oriented to the WSW until mid-July, thereafter to the Southeast.

Andreas Kammerer

FGK observations