We performed follow-up measurements of this object while it was still on the PCCP webpage.

Stacking of 14 unfiltered exposure, 120 seconds each, obtained remotely on 2021, January 6.4 from H06 (iTelescope network, New Mexico) through a 0.43 m f/4.5 Reflector + CCD, shows that this object is a comet with a diffuse coma about 7" in diameter.

Our confirmation image (click on it for a bigger version; made with TYCHO software by D. Parrott):

M.P.E.C. 2021-A99, assigns the following nearly parabolic orbital elements to comet C/2021 A1: T 2022 Jan. 3.3; e= 0.99; Peri. =  225.09; q = 0.6;  Incl.= 132.68

This comet has excellent brightness prospects for December 2021. In fact, before the perihelion on January 3, 2022, at a distance of 0.6 AU, comet C/2021 A1 (Leonard) will pass just 0.233 AU from Earth on December 12, 2021 and it will have an exceptionally close pass of Venus at 0.028 AU on December 18, 2021.

Comet C/2021 A1 (Leonard) could potentially become a naked-eye object. Below you can see a graph generated using the software Orbitas and showing the predicted magnitude (in red) versus the maximum height (for Northern Hemisphere). (as always with comets, the future magnitudes reported here are only indicative). 

Below a graph showing the orbit diagram (made using the JPL Small-Body Database Browser) of comet C/2021 A1 (Leonard) and its position on December 18, 2021.

by Adriano Valvasori & Ernesto Guido