C/2020 S3 (Erasmus) was discovered by Nicolas Erasmus of the Asteroid Terrestrial-Impact Last Alert System (ATLAS) on 17 September 2020. The comet reached a perihelion distance of 0.399 au on 12.66 December 2020. According to the IAU Minor Planet Center the original reciprocal semimajor axis was 0.004996, or a semimajor axis of 200 au indicating that it is definitely not dynamically new and was likely last through the inner solar system ~2600 years in the past. The Solar Wind ANisotropies (SWAN) camera on the Solar and Heliospheric Observatory (SOHO) spacecraft, located in a halo orbit around the Earth-Sun L1 Lagrange point, makes daily full-sky images of hydrogen Lyman-alpha. Water production rates were determined from the SWAN hydrogen Lyman-alpha brightness and its spatial distribution. The water production rate rose surprisingly rapidly before perihelion, varying as the heliocentric distance, r in au, with an exponent of r-5 to a maximum value on 5 December 2020, a few days before perihelion of 2.4 x 1029 s-1. Because of observational constraints it was observed again starting 19 January 2021 and until 13 February when the water production rate decreased very slowly from ~7 x 1028 to ~4 x 1028 s-1. At a comparable heliocentric distance of ~1.2 au the production rate was a factor of 5 larger after perihelion than before. Since it is not a dynamically new comet the extreme difference in behavior before and after perihelion is likely due to rather large seasonal effects.
Support is acknowledged from NASA grant 80NSSC18K1005 from the Solar System Observations Program, the Finnish Meteorological Institute (FMI), and from CNRS and CNES.