…see first, second, and third posts…
The COIAS program (centered around the Subaru Telescope and its HSC instrument) aims to discover asteroids swept up by the wide field of the HSC. That it does, certainly: the effort has a few thousand asteroids to its name. Now, project staff announce, COIAS has discovered/named two comets as well:
https://subarutelescope.org/en/news/topics/2025/04/23/3547.html
Urakawa, S. COIAS Achieves the First Comet Discovery and Names Three Asteroids!
Subaru’s Hyper Suprime-Cam, or HSC, was custom-built to see the maximum field of view from the 8-meter-class telescope. The result is a sky survey both deep and wide: wide, from the expansive field, and deep, from the light-gathering power of such a big aperture. COIAS, short for Come On! Impacting ASteroids, is then an organized campaign to sift through HSC images, looking for chance asteroids caught on clear nights. Somehow, everyone else missed faint comets C/2015 K7 and P/2016 P5.
The comets are now credited to the program, as “C/2015 K7 (COIAS)” and “P/2016 P5 (COIAS).” C/2015 K7 is a long-period comet, like many others. It is rather faint due to its high perihelion distance: the closest point in its orbit that it will ever get to the Sun (or, perihelion) is just not that close to the Sun. C/2015 K7 never gets that warm and active. Hence, a Subaru-sized telescope was needed to not only see it at all, but to detect its faint coma and tail, to identify it as a comet.
Comet P/2016 P5 is a bit more interesting. It is trapped within Jupiter’s orbit, also not getting close to the Sun, but not getting that far, either. This resembles the Hilda family of asteroids, making P/2016 P5 a “quasi-Hilda” object. The Hilda asteroids (?) are bodies in the outer Main Belt, trapped in a 2:3 resonance: every time Jupiter makes two orbits around the Sun, the Hildas make three laps of the Sun. Exactly three. This timing keeps the Hildas stable. One astronomer hypothesized that P/2016 P5 was previously a longer-period comet, but that it fell into the quasi-Hilda state. Alternately, P/2016 P5 may have been more of a true Hilda, but somehow perturbed outward (such as by comet jetting), and now in the process of destabilizing. The faintness of P/2016 P5 is then due to a combination of weak solar heating, and a composition closer to asteroids- even outer Main Belt asteroids, thought to be icier than those from the inner Belt. In other words, a potential “Main Belt” comet barely in the Main Belt.