C/2019 Y4 (ATLAS) is a comet with a near-parabolic orbit and an orbital period of about 6,000 years. It is also known as comet ATLAS and was discovered on December 28, 2019 by a reflecting telescope atop Mauna Loa in Hawaii as part of the Asteroid Terrestrial-impact Last Alert System (ATLAS). It was about 3 AU from the Sun and shone at a magnitude of 19.6 in the constellation of Ursa Major when it was discovered for the first time.
In April 2020, it moved into Camelopardalis, and next month it will move into the constellation of Perseus. On April 6, University of Maryland astronomer Quanzhi Ye and Caltech astronomer Qicheng Zhang reported on the possible disintegration of comet ATLAS. “Images taken on April 5 from the 0.6-m Ningbo Education Xinjiang Telescope (NEXT) showed an elongated pseudo-nucleus measuring about 3 arcsec in length and aligned with the axis of the tail, a morphology consistent with a sudden decline or cessation of dust production, as would be expected from a major disruption of the nucleus,” they wrote in the Astronomer’s Telegram. “A disruption event could also potentially explain the large non-gravitational forces acting on the comet.”
Images captured on April 12 by the 1-m telescope at Lulin Observatory, Taiwan, showed the presence of at least two fragments of the comet. On April 18, a large team of astronomers confirmed that the comet fragmented into several pieces, designated A to D. “Images taken during the past week display clear evidence of the fragmentation of the comet,” they said. “Greater than expected astrometric residuals, with respect to the computed orbit of the comet, also suggest unusual activity in the nucleus and significant non-gravitational forces acting over the past several weeks.” University of Maryland’s Dr. Ye and University of Hawaii’s Dr. Man-To Hui then used Hubble to observe fragments of comet ATLAS. “Single-epoch observation with Hubble on April 20 showed only two remaining major fragments, likely A and B, each consisting of 1 to 2 brighter components and a couple more fainter components, situated 4 arcsec from each other along their common orbit,” they said. “Fragments C and D seem to have been reduced into a handful of fainter fragments. At the time of the observation, the comet was at a heliocentric distance of 1.10 AU and a geocentric distance of 0.98 AU.”