Enter Comet S3 Erasmus: A Bright Comet For November

Comet C/2020 S3 Erasmus ‘may’ end out the cometary cavalcade for 2020.

Ready for one more comet for 2020? Thus far, this year has been a memorable one for comet watchers, with a steady stream of binocular fuzzball comets, crowned by the amazing sight of naked eye comet C/2020 F3 NEOWISE this past summer. 2020 cometary alumni has also included comets F8 SWAN, P1 NEOWISE, and M3 ATLAS. Now, newly discovered Comet C/2020 S3 Erasmus takes center stage.

Discovered on the night of September 17th, 2020 by astronomer Nicolas Erasmus during the ATLAS (Asteroid Terrestrial-impact Last Alert System) sky survey, Comet S3 Erasmus has the potential to be a decent binocular comet from early November into mid-December 2020… but you’ll have to act soon to see it in the early dawn sky before it gets lost in the Sun’s glare.

The comet reaches perihelion in mid-December, just inside the orbit of Mercury. S3 Erasmus is on an 1,800-year orbit give or take 300 years, inclined 20 degrees relative to the ecliptic plane. Like so many comets prior to the advent of the telescope, S3 Erasmus went unnoticed during its last passage through the inner solar system, two millennia ago.

The path of Comet S3 Erasmus through the inner solar system. Credit: NASA/JPL

The Prospects: Now’s the time to catch the comet, as it sits high to the southeast in the pre-dawn sky. In early November, the comet shares the same patch of pre-dawn sky with the inner planets Mercury and Venus. The comet will lose elevation through late November into December, as it approaches the Sun and perihelion.

The slim waning crescent Moon sits very near the comet on the morning of December 13th and actually occults (passes in front of) the comet for northwestern Canada, though the event is unfortunately unobservable, as it transpires under daytime skies.

The dawn path of Comet S3 Erasmus from November through mid-December. Credit: Starry Night.

After that, its brief January 2021 perihelion will be a bashful one, as the comet rounds the bend and speeds away from us on the far side of the Sun. At its closest, the comet will be moving about 75’ (1.25 degrees) per day. And though we lose the comet low in the dawn sky towards Christmas, the joint NASA/ESA sunwatching Solar Heliospheric Observatory spacecraft SOHO will pick up the view from its sunward Lagrange L1 vantage point, as the comet crosses the LASCO C2 field of view from December 18th to December 31.

The path of Comet S3 Erasmus through the field of view of SOHO’s LASCO C2 imager. Credit: Created using Starry Night planetarium software.

As of writing this, Comet S3 Erasmus is overperforming slightly, shining at magnitude +9 “with a bullet.” Here’s a month-by-month breakdown of celestial dates with destiny for Comet C/2020 S3 Erasmus:

(Note: ‘passes near’ denotes passes less than a degree apart, unless otherwise noted)


5-Crosses into the constellation Crater the Cup

13-Passes near the +5 magnitude star Eta Crateris

14-Crosses into the constellation of Corvus the Crow

15-Passes near (2’) the +2.6 magnitude star Gienah (Gamma Corvi)

19-Reaches closest Earth approach, at 1.038 Astronomical Units (AU) distant

21-Crosses into the constellation of Virgo the Maiden

28-Crosses into the constellation of Hydra the Sea Serpent

30-Crosses into the constellation of Libra the Scales

The celestial path of Comet S3 Erasmus through mid-December. Credit: Starry Night


1-May top out at magnitude +6

9-Crosses into the constellation of Scorpius the Scorpion

10-Passes near the +2.3 magnitude star Dschubba (Delta Scorpii)

12-Reaches perihelion at 0.396 Astronomical Units (AU) from the Sun

12-Passes near the globular cluster Messier 80

13-Crosses into the constellation Ophiuchus the Serpent Bearer

13-Occulted by the Moon (in the daytime for NW Canada)

14-Crosses the ecliptic plane northward

18-Passes near the +4.4 magnitude star Xi Ophiuchi

18-Enters the field of view of SOHO’s LASCO C3 viewer

22-Crosses into the constellation of Sagittaurius the Archer

23-Transits the open cluster Messier 23

24-Passes 5 degrees from the Sun

25-Crosses the galactic plane southward

26-Passes near the open star cluster Messier 18

27-Passes between the open star cluster Messier 25 (2 degrees) and the Omega Nebula Messier 17 (1.75 degrees)

31-Exits the field of view of SOHO’s LASCO C3 coronagraph imager

31-Enters the constellation of Scutum the Shield

The light curve for Comet S3 Erasmus. Credit: adapted from Seiichi Yoshida’s Weekly Information About Bright Comets.


2- Reenters the constellation of Sagittarius the Archer

11-Enters the constellation of Aquila the Eagle

14-Enters the constellation of Capricornus the Sea Goat

20-Reenters the constellation of Aquila the Eagle

23- Enters the constellation Aquarius the Water-Bearer

31-May drop below +10th magnitude

Observing comets is as simple as sweeping the suspect field at low power with binoculars, and looking for a fuzzy compact ‘star’ that refuses to snap into focus. I always find that around +6th to +8th magnitude, comets look lots like some of the better globulars in the Messier catalog. Near its maximum in December, Comet S3 Erasmus may sprout a tiny spiky tail. If it over-performs, the comet may just nudge its way into naked eye territory.

When it comes to comets, you just never know, unless you get out and look. Be sure to check out Comet S3 Erasmus, for a fine coda to the comets of 2020.

Lead image credit: Comet S3 Erasmus from October 28th courtesy of Michael Jäger, taken with an 11″ RASA telescope plus a modified Nikon Z50 camera.