A new comet has suddenly appeared in the southern hemisphere evening sky
and it is currently listed on the PCCP as SWAN25B.

https://www.minorplanetcenter.net/iau/NEO/pccp_tabular.html

The comet was first noted in SWAN data by Vladimir Bezugly on September
11 and was confirmed in terrestrial images by Martin Masek using the
FRAM camera in Argentina. We now have a 40 hour arc of decent astrometry
from southern hemisphere sites and, forcing e=1, I get a perihelion of
0.50 au on Sept 10 (see the elements attached from FINDORB). This orbit
is still pretty unreliable but it indicates that the comet will make a
close approach to the Earth on October 17 when it is at a decent
elongation (76 deg). How bright it will be at that time is anybody’s
guess. At present it is in Virgo and images show that it has a tail
several degrees in length and a magnitude of around 8.

The attached plots are based on the current, uncertain, orbit and the
predicted magnitude is just a guess. The observability plot shows the
elevation of the comet at evening nautical twilight for various
latitudes. At 50N we don’t get a chance to see it until early October
but it will be a good target for observers with access to telescopes
further south.

The elongation plot shows that it has been within 30 deg of the Sun
since late July. That fact that it wasn’t picked up by surveys earlier
this year when it was at a large elongation implies that it has
brightened rapidly and so it will possibly fade rapidly as well.

I’ll update the plots when we have a bit more astrometry but, if you
have access to telescopes in the southern hemisphere, please monitor
this object and submit observations to the section.

The attached ephemeris is based on the short-arc orbit.

Nick.