C/2020 S3 (Erasmus)

Lore & Background

The comet was first detected as an 18.5-magnitude object in four 30-second exposure images taken on 17 September 2020. Initially flagged as a potential near-Earth asteroid, Dr. Nicolas Erasmus identified a faint coma around it shortly before the discovery was announced. It became visible in predawn skies during its closest approach to Earth on 19 November 2020, when it was located in the constellation Hydra. Its maximum brightness was predicted to reach magnitude 3.0–4.0 at perihelion, but the comet's position too close to the Sun prevented ground-based viewing at that peak; it was observed only by SOHO and STEREO-A, and several observers captured it during the total solar eclipse on 14 December 2020.

Physical analysis between 13 and 23 November 2020 measured the comet's linear polarization, showing similarities to comets C/1989 X1 (Austin) and C/1996 B2 (Hyakutake). Models indicated a one-third decrease in Mg-rich silicate particles in the inner coma, suggesting a change in dust emissions from the nucleus. Chemical analysis revealed the coma was enriched in formaldehyde (H2CO), ammonia (NH3), and acetylene (C2H2), but depleted of methanol (CH3OH). During perihelion, the comet's tail exhibited wagging behavior due to interaction with a coronal mass ejection. Its water production rate reached a maximum of 3.5×10^29 sec−1 at that time, and when it reached 1.45 AU on its outbound journey, the rate was still seven times higher than the pre-perihelion estimate of about 1×10^28 sec−1. The nucleus is estimated to be 1.6–2.4 km in diameter.

Reader's Guide

C/2020 S3 (Erasmus) is notable for its discovery circumstances and the unusual observational challenges it presented. Discovered by Dr. Nicolas Erasmus after being flagged as a potential near-Earth asteroid, the comet's faint coma was key to its identification. Its peak brightness at magnitude 4 coincided with perihelion on 12 December 2020, but the comet's close angular proximity to the Sun made it invisible from ground-based telescopes at that time, limiting direct visual observation to space-based instruments like SOHO and STEREO-A. The total solar eclipse on 14 December 2020 provided a rare opportunity for several observers to photograph the comet. Physically, the comet showed distinctive polarization behavior similar to comets Austin and Hyakutake, and its chemical composition—enriched in formaldehyde, ammonia, and acetylene but depleted in methanol—offers insights into the diversity of cometary ices. The wagging of its tail during perihelion, caused by a coronal mass ejection, and the sustained high water production rate long after perihelion, indicate dynamic interactions with the solar wind and a possibly active nucleus. These characteristics make Erasmus a valuable object for studying comet–Sun interactions and the evolution of volatile release in non-periodic comets.

Did You Know?

• The comet was initially flagged as a potential near-Earth asteroid before Dr. Nicolas Erasmus identified its coma.
• Its tail exhibited wagging behavior during perihelion due to interaction with a coronal mass ejection.
• The comet's coma is enriched in formaldehyde, ammonia, and acetylene but depleted of methanol.
• Several observers captured the comet during the total solar eclipse on 14 December 2020.