This photo shows the elongation of the comet's nucleus. Photo by Nick Haigh in the UK.

Find out when and where to see Comet ATLAS (C/2019 Y4) — and stay tuned to see how bright (or not) this comet gets.

UPDATE APRIL 7, 2020 — POSSIBLE BREAKUP
Photos taken on April 2nd (left) and April 5th with the 0.6-m Ningbo Education Xinjiang Telescope show how the pseudo-nucleus of Comet ATLAS has elongated, indicating a possible fragmentation of its core.
Quanzhi Ye and Qicheng Zhang
No, no, no. Say it ain't so. In a recent Astronomical Telegram, astronomers Quanzhi Ye (University of Maryland) and Qicheng Zhang (Caltech) report that photographs taken on April 2nd and April 5th of the comet revealed a marked change in the appearance of its core or pseudo-nucleus from starlike and compact to elongated and fuzzy. A second team of astronomers led by I. A. Steele (Liverpool John Moores University) confirmed the discovery. This change in appearance is "consistent with a sudden decline or cessation of dust production, as would be expected from a major disruption of the nucleus," wrote Zhang and Ye. An elongated nucleus is often a bad sign and could mean the comet's headed for disintegration much like what happened to Comet Elenin (C/2010 X1) prior to its September 2011 perihelion passage when its core crumbled and the object rapidly dissipated. Addition evidence of ATLAS's breakup comes from an unexpected shift in the direction of its orbital motion caused by "non-gravitational" forces. Fragmentation exposes fresh ice to sunlight which quickly vaporizes. The expanding gases act like a natural rocket engine and gently push the comet from its appointed path.

No cause has been assigned to the fragmentation but recent studies indicate that accelerating spin may play a significant role in nuclear breakups. Jets of gas blasting away from the comet can spin it up, stressing the icy body until it develops cracks that lead to a breakup. In late March Comet ATLAS began to plateau in brightness; now it appears to be slowly fading. The latest estimates place the comet around magnitude 8.5–9. I strongly encourage you to observe it at every opportunity and share your observations in the comments area. Observers with larger instruments working at high magnification may be able to discern larger nuclear fragments. I spotted the comet with my 15-inch scope in bright moonlight on April 8.14 UT and observed that the brightness and density of the nuclear region had declined from an observation made 10 days ago.

While it's possible ATLAS has lost only a single small fragment and will soldier on to perihelion and naked-eye visibility, the optics aren't looking good at the moment. Full Moon occurs on April 7th but a dark-sky window returns as soon as the 9th. Seize the night to enjoy Comet ATLAS at every opportunity before it (possibly) is no more.Nick Haigh of Southampton, UK used a 12-inch Skywatcher 300p with an ASI1600MM camera and 682 10-second exposures on April 6th to make this amazing photo showing what appear to be three separate fragments in Comet ATLAS's formerly single nucleus.
Nick Haigh