Comets Crawl through the Skies at Dawn and Dusk

After the summertime spectacle of Comet NEOWISE (C/2020 F3) the sky quickly emptied of easy-to-see comets for Northern Hemisphere observers. But in recent weeks a whole new crew has assembled to enliven the nights. There are currently four comets within range of amateur telescopes — periodic comet 156P/Russell-LINEAR in the evening sky and Comets ATLAS (C/2020 M3), Erasmus (C/2020 S3), and NEOWISE (C/2020 P1) visible in the early morning hours.

Let's start with Comet ATLAS, the brightest of the quartet. Discovered at magnitude 19 on June 27, 2020, by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) search program it's now about magnitude 8.5 and speeding northward from Lepus toward Orion.

Although the comet rises around midnight you'll see it best around 4 a.m. when it culminates in the south. I observed ATLAS with my 15-inch Dob on October 16th from a semirural sky and was immediately struck by how big and fluffy it appeared, like a big wad of cotton candy.

I estimated a coma diameter of 11′ with a degree of condensation (DC) of 3 on a scale from 0 (diffuse coma of uniform brightness) to 9 (stellar or disk-like appearance). Increasing the magnification to 245× uncovered a near-stellar pinpoint at center, the so-called false or pseudo-nucleus. A comet's true nucleus is tiny and well-hidden within a self-made cocoon of dust.

While the comet's overall magnitude is relatively bright, its diffuse appearance and large diameter make it look dimmer than you might expect — especially if you're battling light pollution. That said, I've heard of several recent binocular sightings, including one made by Comet Hale-Bopp co-discoverer Alan Hale of New Mexico who spotted it in his 10×50s on October 15th.

During my observation I discovered that a Swan Band filter — designed to pass the green light of diatomic carbon in gas-rich comets — clearly enhanced ATLAS's contrast and visibility. The comet should remain around 8th magnitude through early December as it climbs steadily northward from Lepus through Orion and into Auriga. Your best viewing opportunities are now through October 29th before the Moon returns and again from November 7–29.

Perihelion occurs on October 25th when Comet ATLAS will pass 1.27 a.u. (190 million kilometers) from the Sun, followed by its closest approach to Earth on November 14th at a distance of 53.6 million kilometers. ATLAS glides within ½° of Mu (μ) Leporis on October 27th and 1° east of Rigel on November 4th.

After you've successfully spotted ATLAS, swing your scope east to Hydra to find Comet Erasmus (C/2020 S3). Nicolas Erasmus, an astronomer at the South African Astronomical Observatory, discovered the comet at magnitude ~18.5 on September 20th this year. It's brightened quickly and currently glows around magnitude 11. During its plunge to a December 13th perihelion (0.4 a.u.) the comet could reach magnitude 7–8 by late November.

Right now, it's well placed in Hydra and Sextans just before the start of morning twilight, but its elongation is steadily decreasing. By late next month Comet Erasmus will become more challenging, standing just a fist high in southern Virgo at the start of dawn. Hopefully its declining altitude will be offset by a concurrent increase in brightness, helping to keep it in view even into early December before it's lost in the solar glare.

On October 16th I easily swept up Comet Erasmus at low magnification (64×) as a soft, milky patch of light glowing at magnitude 11.5. Its diffuse coma spanned 3′ with a DC of 3. Like for ATLAS, the Swan Band filter nicely boosted the comet's visibility.

Comet Erasmus travels through galaxy-rich territory during the next month, providing several opportunities for photogenic pairings with deep-sky objects. Perhaps the most interesting appulse occurs on November 17th when it passes just ½° north of the bright planetary nebula NGC 4361 in Corvus. At year's end the comet moves into the evening sky and fades rapidly.

Comet NEOWISE (C/2020 P1) will arrive on the scene this week just days after its October 20th perihelion when it approached the Sun at a distance of around 0.34 a.u. NEOWISE is a small comet but assuming it survives perihelion could be as bright as magnitude 8.

Due to its small elongation from the Sun you'll need a location with a clear view to the east-northeast, toward Virgo. Time your observation to the start of morning twilight or about 90 minutes to 2 hours before sunrise to catch this NEOWISE relation (by name only!). The comet will fade as it hurries north into Boötes.

We head over to the evening sky for our final comet, 156P/Russell-LINEAR. Located in southern Aquarius in late October it creeps steadily to the northeast, crossing into Pisces on November 15th and remains in that constellation through the end of the year.

I scoped it out in Sculptor on October 12th at ~13.0 magnitude with a small, well-condensed central coma that measured about 0.5′ across (DC=5). After scrutiny with averted vision I detected a fainter outer coma that increased the total diameter to 1.5′. Other observers have also noticed these two distinct dichotomies — a dense, bright inner coma set within a much larger (up to 5′), low surface brightness outer coma.

Comet 156P/Russell-LINEAR should brighten to magnitude 12 by early November and then slowly fade. If you have an 8-inch or larger telescope and reasonably dark skies give this one a try. The comet's visibility improves as it moves steadily northward during its run.

Because 156P is already near maximum brightness well before its November 17th perihelion there's speculation it may be undergoing an outburst. As evidence, Michael Mattiazzo's photo from October 12th reveals a pseudo-nucleus off-centered from the coma, something observed routinely during outbursts of Comet 29P/Schwassmann-Wachmann.

Comet 156P/Russell-LINEAR was originally discovered by K. S. Russell at Siding Spring Observatory (Australia) on September 3, 1986, then rediscovered twice in 1993 and 2000 as an asteroid by the LINEAR survey until finally receiving its designation as a periodic comet. You can read more about the comet's convoluted history in Gary Kronk's Cometography.

I've prepared maps of the three fainter comets that you'll find by clicking on the links below. Stars are plotted to magnitude 9 and dates are 0h UT, so remember to subtract 4 hours for Eastern Daylight; 5 for Central; 6 for Mountain; and 7 for Pacific and change the date accordingly. You'll find additional maps for each comet at astro.vanbuitenen.nl and more details at the Comet Observation Database and Seiichi Yoshida's Bright Comets.

156P/Russell-LINEAR
C/2020 S3 (Erasmus)
C/2020 P1 (NEOWISE)

Like autumn leaves in the wind, comets are sailing across the sky again. Grab an instrument and try to catch one . . . or two or three or four.