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On July 26, 2021 the PANSTARRS project discovered a comet of magnitude 19.5 in the constellation Pegasus. Follow-up observations of comet C/2021 O3 (PANSTARRS) showed a nearly stellar coma of magnitude 18.5, but no tail. The comet will pass perihelion in the small distance of 0.29 AU on Apr. 21, 2022, expected to reach magnitude 6-7 (n=3) or even 4 (n=4). However, its absolute magnitude is signifanctly below the Bortle limit. Therefore it is probable that the comet will disintegrate before reaching perihelion. It should be brighter than 16 mag between December 2021 and August 2022. During this interval it moves through the constellations Pegasus/Aquarius, Pisces, Cetus (perihelion), Taurus, Perseus, Camelopardalis, Ursa Minor, Draco and Bootes. At the end of May it will approach the North Pole to within 9°. From mid-European locations the comet will disappear above the western evening horizon at the start of February (expected magnitude: 15.0). If it should survive the perihelion passage it would reappear above the northwestern evening horizon during the first week of May (expected magnitude: 5-6), gaining quickly altitude but fading in a similar rapid manner.
The comet survived the close perihelion distance heavily chipped. Observations during the first half of May 2022 showed a very diffuse coma with only a hint of a central condensation. In addition, it appeared several magnitudes fainter than expected. Until early 2022 the comet developed according to the predictions, but reached only magnitude 16. Visual observations in the bright dawn reported a brightness of 7.5 mag on Apr. 19, 2022 and about 9.0 mag on Apr. 22. During the following weeks the comet faded rapidly.
Based on only 21 observations from 12 observers a somewhat different development pre- and post-perihelion can be derived, which can be described by the approximate parameters
pre-perihelion : m0 ≈ 12.0 mag / n ≈ 3 post-perihelion: m0 ≈ 14.5 mag / n ≈ 4The comet peaked at approximately 8 mag at perihelion, brightening and fading rapidly before and after perihelion.
Total Brightness and Coma DiameterThe coma measured about 0.5' (appx. 50.000 km) at the start of the apparition and 2.0-2.5' (about 65.000 km) after perihelion. No estimates of the degree of condensation have been published. Near perihelion a tail of 0.1° (250.000 km) length was observed.
On May 10 Michael Jäger estimated the brightness of the 3' coma to be about 12.5 mag, showing a small, slightly condensed region.
Andreas Kammerer