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Comet 103P/Hartley (P=6.48a) will pass perihelion on Oct. 12, 2023 at the solar distance of 1.06 AU. Thereby will it move roughly parallel to Earth for many weeks, reaching its minimum distance to Earth (0.38 AU) on Sep. 26, 2023. The comet became brighter than 16 mag in July and should continue to brighten rapidly during the following weeks. During the days around perihelion, when it will be close to Earth too, the comet is expected to peak at magnitude 7.5. Thereafter the distance to the Sun and Earth will increase, with the activity decreasing much slower than it increased pre-perihelion. In consequence the comet should not become fainter than 16 mag in April/May 2024. During the period brighter than 16 mag it will move through the constellations Pegasus, Andromeda, Perseus, Auriga (perigee), Gemini (perihelion) and Cancer, reaching Hydra in early November, where it will draw an opposition loop. From mid-European locations it is an object in the morning sky at first, being situated almost at the zenith in September. Thereafter the altitude decreases and the comet moves towards the night sky. Starting in March 2024 it will be visible in the evening sky. During the first week of November the Earth will be close to the comet's orbital plane.
The comet remained noticeably fainter than predicted during this apparition. Based on 158 observations from 42 observers a brightness development according to the parametersm0 = 10.5 mag / n = 7.5
results which, however, reflects the development only moderately well. The comet reached a maximum brightness of 9.0 mag in the first week of October 2023 - 1.5 mag fainter than predicted.Total Brightness and Coma Diameter
The apparent coma diameter increased from 1' (30,000 km) at the beginning of August to a maximum value of 9' (160,000 km) at the opening of October, starting to decrease thereafter. The coma has been quite diffuse throughout the apparition with a constant degree of condensation of DC 3. Sightings of the westward pointing tail by CCD observers have been very rare so far and do not exceed 10' (150,000 km).