Just discovered November (while we were feasting): a new Main-Belt comet. A typical comet comes out of the outer Solar System, then warms and activates when it reaches the inner Solar System. Not Main-Belters. These “comets” reside in the asteroid belt all the time, yet somehow show activity (mass loss- some sort of coma and/or tail, even faint ones):
M.P.E.C. 2025-X48 Issued 2025 December 5, 16:49 UT Minor Planet Center
Initial orbit: semi-major axis (a) 2.5441492 AU, eccentricity (e) 0.1275281
Electronic Telegram No. 5644, 2025 December 6 Central Bureau for Astronomical Telegrams
New orbit solution: a = 2.5732660 AU, e = 0.1863048
It was the 2006 announcement of a third Main-Belt comet that precipitated the term “Small Solar System Body” (along with Pluto). By 2006, the discovery of 118401 LINEAR (then known by its temporary designation 1999 RE70) made three such objects, enough to form a group with “statistics” (such as they are). The fact that three were found, without particularly looking, is a strong sign that there are more, should we actually start a search. Indeed, the MBC discoveries kept coming… like this one.
The implications were strong too, though not unprecedented and surprising. Main-Belt comets show that volatiles (likely including water) exist in the asteroid belt, and the line between asteroids and comets is no bright line at all. In the Main Belt, depending on exactly how far it is to the Sun, volatiles have to be shielded by a top layer (here called the mantle), thicker in the inner Belt, thinner to perhaps negligible in the outer Belt, and maybe nothing at all for Hilda objects. Combined with the whole Pluto debate (precipitated by the discovery of the Kuiper Belt), the IAU met in 2006 and created a new category, Small Solar System Object. SSSOs no longer draw any distinction between “active” and “inactive”, when a (bona fide) comet might be inactive for multiple reasons, and a (volatile-rich) asteroid might just happen to be inactive while you’re pointing your telescope at it.
I’ll throw out a note of caution: it’s early days. The object was discovered, and discovered to be active, just over a week ago. Continued reports may show (after weeks to a few months) that the “coma” is dissipating and disappearing. In that case, the likely explanation is that the “activity” was a meteor impact, freeing and ejecting particles. Of course, yet another caveat is that the “temporary activity” might still be a volatile release- a strike by an impactor may be what’s needed to puncture a mantle, expose interior ices to sunlight, and trigger true cometary activity. We’ll just have to wait.
M.P.E.C. 2025-X49 Issued 2025 December 6, 01:38 UT
semi-major axis a 2.5736337 AU, eccentricity e 0.1868003
By the way, we’ve confirmed at least some MBCs are true comets (such as it is), not just lots of impacts in the Belt. Turning the James Webb telescope to an MBC, the spectrum of water was confirmed. That, and a good amount of them did not, in fact, dissipate after weeks to a few months.
Thanks, Luke!