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On Oct. 31, 2019 an asteroidal object of magnitude 20.5 was discovered near the border of the constellations Orion/Lepus by the Mt.Lemmon project. Due to its nearly parabolic orbit it was suspected to be of cometary nature. On Dec. 6, 2019 H. Sato detected a well-condensed 6" coma of total magnitude 18.5. Comet C/2019 U6 (Lemmon) developed rapidly in the following weeks, already reaching magnitude 12.5 on Mar. 22, 2020, showing a diffuse 2' coma. The comet will pass perihelion at a solar distance of 0.91 AU on June 18, 2020, but will then be located in the constellation Hydra, unobservable from mid-European locations (CBET 4735). Mid-European observers should get a first chance in mid-July with the comet low above the western evening horizon. During the following weeks the altitude will increase, reaching about 45° in October. Until early December the comet will move from the northwestern parts of Virgo, through Coma Berenices, Bootes and the northern parts of Serpens into Hercules.
The comet showed an unusual brightness development, based on 580 observations from 58 observers. At the end of April 2020 the comet had already brightened to 9.0 mag and the high activity parameter indicated a maximum brightness of 4.0 mag! However, as I had expected, the comet experienced a significant change in its brightness development 40 days prior to perihelion (May 9, 2020). Thereafter it experienced an average brightening until perihelion, resulting in a maximum brightness of only 6.2 mag around the days of perihelion. During the 15 days immediately after perihelion the brightness evolution did not show a standard behavior, but instead this period has to be regarded as a transitional one. Thereafter the comet again faded in a standard manner, but rather rapidly. The appropriate formulae are:
t < -40d: m = 5.9 mag + 5×log D + 21.2×log r -40d < t < 0d: m = 6.7 mag + 5×log D + 7.5×log r t > +15d: m = 7.9 mag + 5×log D + 14.8×log r
Pre-perihelion development of the heliocentric magnitude
Post-perihelion the comet faded rather rapidly, reaching 10 mag during the first week of August and 14.5 mag in mid-October.
Total Brightness and Coma DiameterThe coma diameter increased more slowly prior to perihelion than it decreased thereafter. With an apparent coma diameter of almost 2' by mid-March, it increased during the following weeks quite steadily and reached its maximum of 9.5' in mid-June. This value was hold constant for four weeks. Thereafter it decreased rapidly at first, reaching 4' by the end of July. Then the decrease slowed down, with the coma measuring 1.5' in mid-October. Between mid-March and the end of May the absolute coma diameter increased from 160.000 km to the maximum of 400.000 km. Until early July it had decreased to 325.000 km and to 140.000 km by mid-October. The degree of condensation showed a similar development. Between mid-March and mid-May it increased steadily from DC 2-3 to DC 5-6, holding this value for four weeks. Thereafter the coma grew ever more diffuse, with the degree of condensation reaching DC 2-3 in early August and DC 1-2 at the opening of October.
The tail could be observed between mid-May and the end of June 2020, reaching a maximum length of 0.8° (2.5 Mio. km). The tail was oriented constantly towards SE.
Andreas Kammerer
