A new comet discovered at the end of 2019 is likely to reach naked-eye brightness during May. The comet was discovered using the 0.5 m telescope on Mauna Loa, Hawaii, on 28 December via the Asteroid Terrestrial-Impact Last Alert System (ATLAS) survey. At that time it was magnitude 19, but it has brightened more rapidly than expected since then, and is currently at magnitude 8.5 in Ursa Major. It is not yet easily visible, however, and probably can’t be seen using binoculars from the average UK location. The comet’s designation is C/2019 Y4 (ATLAS) and is not to be confused with comet C/2019 Y1, which is also in the sky but is less easily observed.
Recent observations listed in IAU CBET 4734 include a visual magnitude estimate of 8.2 on 14 March by J J Gonzalez of Leon, Spain, using 10 × 50 binoculars and 9.3 from Alan Hale in Cloudcroft, New Mexico, also with 10 × 50 binoculars. The same circular also says that ‘Numerous long-period comets have experienced steep brightness increases well before perihelion, only to see the brightness curve flatten when nearing perihelion, which may be due to volatile ices being sublimated more rapidly, initially, by comets that have rarely if ever approached the Sun closely before.’ Their visual magnitude predictions are as follows:
27 March 8.1
1 April 7.8
6 April 7.5
11 April 7.1
16 April 6.7
21 April 6.3
26 April 5.8
1 May 5.3
6 May 4.6
11 May 3.8
16 May 2.8
21 May 1.6
SPA Forum member Nigel Joslin observed the comet from Galloway on 15 March with a 355 mm Dobsonian reflector. He says that he spotted it quite easily, although it was quite small, using magnifications of 66 and 110. There was no visible tail but with a magnification of 330 he could see the central condensation.
If the comet continues to brighten at its present rate it could become as bright as the crescent Moon, although comet experts are very wary about this possibility. On more conservative estimates it will reach magnitude 2 in the twilight evening sky towards the end of May as it passes through Perseus. Although this is naked-eye visibility, the comet may not be as easily visible as a second-magnitude star, given that it will be diffuse. However, during April and early May it should be visible in a dark sky some distance from the Sun, and while it’s best not to expect too much, there are grounds for cautious optimism.
The comet reaches perihelion on 31 May. It will not be particularly close to the Earth at that time – about 0.78 astronomical units – although it will then be closer to the Sun than Mercury.
At this stage it is not possible to say how much of a tail the comet will display. Comets are notoriously unpredictable, as we found with comet ISON in 2013, which fizzled out on its closest approach to the Sun.
Experts have noted that the orbit of the comet is similar to that of a bright comet that appeared in December 1844, which had a 10º tail and reached magnitude 2.5 as seen from the southern hemisphere in January 1845. It may be a fragment of the same larger body as the 1844 comet.
Use the finder charts shown here to find the comet. The one below shows the predicted track on the sky as seen looking north-west in mid March at 23:30 UT. Note that the positions given are for 0h UT on the date shown, so if you are observing in the evening the comet will be closer to the next day’s position.
During May the twilight will interfere as the comet is getting close to the Sun. Also shown is a detailed map of the track during March and April showing stars to magnitude 9.