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Comet C/2019 Y4 (ATLAS), 17 March - 20 May 2020

Comet C/2019 Y4 (ATLAS) was discovered by the ATLAS (Asteroid Terrestrial-impact Last Alert System) telescope system in Hawaii on 28 December 2019. It was the last comet discovery of the year. The comet had a magnitude of approximately 7 on discovery, making it the brightest such object of the year. The dates of closest approach to the Earth and of perihelion were calculated from observations as 23 May and 31 May respectively. In early April 2020, astronomers reported that the nucleus had fragmented.

Although there were initial hopes that the comet might become visible to the naked eye around the dates of perihelion and closest approach to the Earth (see e.g. a tweet by the BAA on 08 March), once fragmentation had occurred, it was clear that this would not be the case.

17 March 2020

The image below shows two views of the comet as follows: the LH panel tracks the stars and accordingly, the comet appears as a streak, and the RH panel tracks the comet itself, so the stars appear as streaks sliding past it. There is also a video of the comet taken on this date.



24 March 2020

The following image shows two views of the comet (explanation as above).


25 March 2020

The following image shows two views of the comet (explanation as above).


26-27 March 2020

On the night of 26-27 March, there were no clouds, but high altitude haze, the sort that turns the daytime sky white. The comet transited the northern meridian at 74° in the evening sky. For me, this is an awkward part of the sky where the observatory dome needs to turn through 180° very quickly. The composite image and video below show the motion of the comet across the sky. (The video exhibits a few glitches in guiding towards the end.)



31 March 2020

The first image below was taken with a 200 mm Celestron Edge HD with SBIG8300 monochrome cooled camera; the second with a smaller 90 mm Megrez scope and a Sony A7S colour camera, providing a much wider field of view.



11 April 2020

The image below was taken under a very hazy sky and was terminated when clouds rolled in just before 23:00 UT. It shows that the coma has fragmented, with several of the fragments travelling forward of the main (brightest) part of the coma. A better image is here, taken with a 0.9 m Cassegrain reflector in Mirasteilas Observatory, Switzerland, under much superior skies!


14 April 2020

There are now many reports of the comet breaking up, so I set out to image what was left of it. The sky was hazy.

Unfortunately, images of the coma taken on 14 April were not well defined. Aligning images with reference to the stars showed the comet as a smudge moving against them, but were not worth showing. Aligning images on the coma (a technique used successfully on the earlier images) proved very tricky and required some educated guesswork. The result, below, lacks detail. However, all is not lost! The second image below shows the intensity profile along the axis of the comet. Normally the profile would show a relatively bright coma and an intensity in the tail steadily decreasing with distance from the coma. The profile below reveals a peak in intensity in the tail, possibly caused by debris resulting from fragmentation of the nucleus. (Note that the scale of the intensity profile is five times that of the image of the comet in the background.)



15 April 2020

The comet has become much fainter, such that in single exposures there is no discernible head. The image below is a stack of exposures which brings out the head and also the bright area in the tail, although this is not as prominent as yesterday. To create the stacked image I had to find how much to offset each subframe. The JPL Horizons ephemeris (https://ssd.jpl.nasa.gov/horizons.cgi ) provided the motion of the comet as 90 arcseconds per hour and https://nova.astrometry.net/ provided the location of the static frame and, more importantly, the orientation of the baseline star field.


19 April 2020

The first image shows nothing remaining resembling a head.

The second image clearly illustrates the demise of the comet. It contains eight 60 second exposures, each processed in the usual way (i.e. darks and flats), then set to the same absolute brightness scale. All were taken at about 21:00 UT, except for the image on 14/04, taken at 22:00 UT. The altitude decreased from 73° for the first image to 50° for the last. The differing background brightnesses reflect variation in sky conditions. The sequence shows clearly how the head of the comet faded significantly following fragmentation of the nucleus, first reported on 06 April.



20 May 2020

The comet appears as a faint smudge in an image also showing C/2020F8 (SWAN).

Nigel Evans