C/2019 Y4 ATLAS
Hopes for an Unaided Eye comet were dashed when it disintegrated on its way to perihelion.

Closest to Earth on 2020 May 23 at 0.78AU
Closest to Sun on 2020 May 30 at 0.25AU
Maximum brightness : 8 in late March 2020 prior to disintegrating
Orbital period +5,000 yrs

2020 March 29

Discovered on 2019 December 28 by the ATLAS survey, Y4 had a very interesting orbit, which gave it the potential for reaching naked eye visibility in May 2020.
It is related to the great comet of 1844 and has shown to be the smaller fragment of a comet that broke apart 5000+ years ago.

In late February 2020, the comet was seen to develop much faster than anticipated.
I imaged the comet remotely from New Mexico using Itelescope on 2020 Feb 18 at 09:20UT and found the comet to have a large 4’coma
and large aperture photometric V magnitude of 12.5, as in below image.

By the start of March 2020, it had brightened to magnitude 11 with a 15’ coma.

By March 15th, observers were estimating a magnitude of 8.5.

C/2019 Y4 ATLAS. 2020 March 16, courtesy of Terry Lovejoy. 2 minutes exposure C11 RASA T68 in New Mexico.

By March 25th, observers were estimating a magnitude of 8.0.
In late March however, the comet was seen to decline in brightness. A break in the lightcurve is a very bad indicator for disintegration.
The nucleus still appeared to be intact as shown in the below images taken 2020 April 4 to 6

As April progressed, the comet took on a more elongated appearance. The nucleus was slowly eroding as it approached perihelion.
Hubble space Telescope images of the comet nucleus on April 20 showed about 30 fragments.

Below images are a sequence from April 24 to 27 using a remote telescope in New Mexico. The fragments are too small to resolve
but you can see the trail of debris left by the fragmenting comet.

Attempts to recover the comet post-perihelion had failed. Its close passage to the Sun resulted in complete disintegration.

COBS lightcurve