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As part of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) project, a 19th magnitude asteroidal object was discovered in the northern part of the constellation Virgo on Mar. 1, 2023. Additional observations detected cometary morphology. Comet C/2023 E1 (ATLAS) exhibited a high-density, 6" coma of total magnitude 18.5, but no tail. The comet will pass perihelion on July 1, 2023 at the solar distance of 1.03 AU (well north of the ecliptic) and should approach Earth to within 0.38 AU on Aug. 19, expected to reach maximum brightness of 14.0 mag (CBET 5233). Between early May and mid-September 2023 it should be brighter than 16.0 mag. During this period it will move through the constellations Ursa Major, Draco, Ursa Minor (perihelion, approaching Polaris to within 9° on July 9), Draco, Cepheus, Cygnus (perigee), Pegasus and Aquarius. From central European locations the comet is visible in the evening sky throughout the period, but can be observed equally well in the morning sky before perigee. Earth crosses the comet's orbital plane on Sep. 9, 2023.
Although it was visually not an eye-catcher with its rather diffuse coma, this comet was observed intensively. Based on 323 observations from 55 observers, the comet showed a rapid increase in activity pre-perihelion and a much slower decline in activity post-perihelion. The brightness development can be represented quite well with the following parameters:pre-perihelion: m0 = 10.5 mag / n = 7
The comet peaked at 9.0 mag around Aug. 10, 2023. Thereafter it faded, reaching 13.0 mag by mid-October.Total Brightness and Coma Diameter
Due to the rather diffuse coma, the observations of the coma diameter scatter noticeably. The apparent coma diameter rose steadily from 2.5' at the beginning of the apparition, reaching the maximum of 9' around Aug. 20, 2023 (when the comet reached minimum distance to Earth). By mid-October it had decreased to 3.5'. The absolute coma diameter of the comet initially rose rapidly from 90,000 km to 200,000 km (around Jun. 10), thereafter decreasing more slowly. It measured 110,000 km in mid-October. Thus, the development of the apparent coma diameter was primarily the result of the varying distance to Earth.
The degree of condensation rose steadily from DC 1-2 at the beginning of the apparition to DC 3-4 around July 20. Thereafter it decreased steadily, reaching DC 2-3 by the end of September. Very few sightings of a southeast-pointing tail have been reported by CCD observers between mid-June and mid-July, with a maximum length of about 20' (800,000 km).