On October 2, 2017, the Pan-STARRS 1 telescope at Haleakala, Hawai‘i, discovered a 19.9 magnitude comet. Nothing unusual about that these days as this telescope now finds the lion’s share of such objects and most of them do not become bright enough to draw the attention of visual observers using small telescopes. However, this comet — C/2017 T2 (PANSTARRS) — was a little different as, at the time of its discovery, it was still 9.3 astronomical units (a.u.) from the Sun and 8.6 from Earth and not due to arrive at perihelion until May 4, 2020.
Despite its faint magnitude, to have reached even that brightness at such a vast distance means that, intrinsically speaking, C/2017 T2 must be a relatively luminous comet. Moreover, subsequent refinement of its orbital elements…