See Comet ATLAS in the evening sky

It’s proving to be a special year for comets, with the astronomical gods looking down favourably on us in these strange times. With the memory still strong of the spectacular Comet C/2020 F3 (NEOWISE), which totally dominated last summer’s observing scene, Comet C/2020 M3 (ATLAS) is now in the late-evening sky among the bright stars of Orion and is bright enough to be viewed through a pair of binoculars from a dark-sky site.

Comet C/2020 M3 (ATLAS) passes a few degrees west of the Orion Nebula (Messier 42) on 8 November. Image: Michael Jaeger.

Comet C/2020 M3 (ATLAS) was discovered on 27 June 2020 by the Asteroid Terrestrial-Impact Last Alert System (ATLAS), based on the Hawaiian islands. Another comet discovery by the programme, Comet C/2019 Y4 (ATLAS), briefly offered the exciting prospect of a naked-eye comet last spring before it was destroyed by the Sun’s massive and unyielding gravitational forces.

This time around though, today’s ATLAS has already passed through perihelion (closest point to the Sun), on 25 October, and survived intact and in fine fettle to offer potential binocular visibility in a dark sky. Over the summer months the comet was too far south to view from UK shores, as it tracked through Sculptor and Formax. Then, during October, it took a northward turn into Lepus and passed into mighty Orion at the beginning of November. On 14 November it was at its closest to Earth, passing us at a distance of 53.6 million kilometres (33.3 million miles).

Comet C/2020 M3 (ATLAS) tracks through northern Orion and Taurus before entering Auriga for a close conjunction with brilliant Capella (alpha [α] Aurigae) as the beginning of the new year. AN Graphic by Greg Smye-Rumsby.

Presently, Comet C/2020 M3 (ATLAS) is shining at magnitude +7.8 to +8.0, around its predicted peak brightness, and shows a large, diffuse coma of around 10 to 15 arcminutes, equating to a physical diameter of over 300,000 kilometres. A short tail has been reported immersed within its greenish coma.

Visually, it appears very diffuse and is tough to spot from light-polluted locations. Away from major sources of light pollution, a small telescope should show the comet without too much trouble. The Moon phase (new on 15 November) is favourable until around the start of the last week of the month, when it exhibits a swelling gibbous phase on the way to full Moon on 30 November.

Comet C/2020 M3 (ATLAS) had just entered the southern constellation of Eridanus by 13 September 2020. NGC 1365, a great barred spiral galaxy and a resident of neighbouring Fornax, is pictured to the lower right. Image: José J. Chambó

The comet is continuing to track northwards through northern Orion, passing a couple of degrees west of magnitude +3.4 Meissa (lambda [λ] Orionis) on its way out of the constellation. Northernmost parts of Orion are well up in the south-eastern sky by 10pm GMT, and transit the southern meridian (culminate) at about 1.30am from London, at which time the comet achieves a healthy altitude of around 45 degrees.

By the night of 23/24 November, Comet C/2020 M3 (ATLAS) enters Taurus, the Bull, and its continued northward motion takes it just half a degree west of magnitude +1.6 Elnath (beta [β] Tauri), the star that marks the tip of the Bull’s northern horn, on 3/4 December.

Comet C/2020 M3 (ATLAS) imaged on 27 September 2020 as it buzzed NGC 1531 in Eridanus, a barred spiral, popularly known as Haley’s Coronet Galaxy, which is interacting with the dwarf galaxy NGC 1531. Image: José J. Chambó

The comet’s light-curve as of 17 November shows visual and electronic scatter of around two magnitudes centred around magnitude +8. If Comet C/2020 M3 (ATLAS) maintains its predicted gradual fade, then it should shine at around magnitude +9 at the beginning of December.

Judging by current levels of activity, don’t expect this version of ATLAS to be anywhere near as thrilling as Comet C/2020 F3 (NEOWISE), but decent comets like this are not that common so when one does appear don’t miss your chance to see it.