Home=Current Comets: C/2023 P1
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On Aug. 11, 2023 the Japanese amateur Hideo Nishimura discovered a comet in an image he took with a digital camera and 200mm telephoto lens. The comet, positioned in the constellation Gemini, had a diffuse, 5' coma, whose total magnitude he estimated as 10.5-11.0 mag. Follow-up observations of comet C/2023 P1 (Nishimura) revealed a coma of up to 3' diameter with a total magnitude of 9.5 mag and an 11' tail in p.a. 270°. The comet will pass perihelion very close to the Sun (0.22 AU) on Sep. 17 and could then reach 3rd magnitude. Since May (then at approx. 16 mag) its elongations had been less than 40° and will not exceed this value until the end of November (CBET 5285). Thus it will be difficult to observe worldwide. In addition, its absolute brightness is below the Bortle-limit, so that it must be expected that the comet will break up while approaching the Sun. If it should not disintegrate, it should remain brighter than 16 mag until Apr. 2024. During this period it moves through the constellations Gemini, Cancer, Leo, Virgo (perihelion), Hydra, Centaurus and Vela. From Central European locations it first rises above the Northeastern morning horizon, reaches a maximum altitude of just 15° at the end of August and will then quickly sink towards the horizon, finally disappearing in the brighter twilight around Sep. 12. On Nov. 29, the Earth will cross the comet's orbital plane. The comet could be the parent body of the sigma-Hydraids, which are active between the end of November and mid-January. The radiant is at R.A.=117° and Decl.=+5°, with the meteors having an average speed of 60 km/s. The maximum is expected around Nov. 30 (CBET 5290). Further astrometric observations and pre-discovery images back to January 2023 result in an orbital period of 437 years. This means that the comet passed perihelion approximately in the years 302, 723, 1169 (April) and 1588 (July) (CBET 5291).
The comet survived the close proximity to the Sun and became the brightest comet of 2023! On perihelion day it reached 2.7 mag, but could then only be spotted under excellent conditions in the bright twilight. It was also a difficult object in the days before due to its low altitude. Post-perihelion it could not be observed from any place on Earth for five weeks.
Based on 219 observations from 49 observers the comet showed a very steady brightness development, which can be represented very well with the formula
m = 8.5 mag + 5×log D + 8.6×log rTotal Brightness and Coma Diameter
The apparent coma diameter increased from 3' at the beginning of the apparition to the maximum of 6' by the end of August, only to decrease rapidly thereafter, reaching less than 1' while approaching the Sun (although this shrinkage probably should partly be attributed to the low altitude and the twilight situation). At the end of October 3' have been reported. The absolute coma diameter increased rapidly from 200,000 km at the beginning of the apparition to 300,000 km at the end of August, only to shrink in a similar quick manner thereafter, measuring about 70,000 km at perihelion (but see note above). At the end of October it measured 225,000 km. The coma condensed very strongly within a short time, with the degree of condensation rising from DC 3-4 at the start of the apparition to DC 8 at perihelion. At the end of October it was still estimated as DC 5. A visual tail was reported between Aug. 25 and Sep. 12, reaching a maximum length of 3° (7 mio. km) around Sep. 7. After that date unfavorable viewing conditions prevented accurate tail observations. In October the observers reported a length of only 7' (1.5 mio. km). The tail rotated from West to Northwest pre-perihelion; pointing in easterly direction in October.
Andreas Kammerer