# Do some comets spin? If so, how fast?

The GIF below is copied from my question earlier What might a CN filter be in the context of comet watching? Is it showing dust, or gas, or something else? where I'd said:

In this post on the website for the Comet Wirtanen Observing Campaign at wirtanen.astro.umd.edu, there's mention of a CN filter being used to observe the comet, and this cool GIF.

It certainly looks like the comet is spinning; it reminds me of images of the density of the solar wind as active areas on the sun "spray" out matter as the Sun rotates, or the patterns discussed in Why is one of these two concurrent fuel-dump spirals blue?.

Question: This leeds me to wonder if I'm just imagining things, or if some comets really do spin? If so, how fast have they been observed to rotate?

## 1 Answer

Yes, comets spin although measuring it can be tricky due to the coma and outgassing from the nucleus. It's easiest to measure the rotation period when the comet is inactive near aphelion although this is more difficult as the comet is fainter, necessitating a large amount of large (>4m) telescope time which is difficult to obtain.

Searching the JPL Small Body Database (select Object Kind=Comets, set Limit by object characteristics using rot_per (h) greater than 0, add rotation period (h) to the Output Fields) should produce a list of 21 comets with measured rotation periods:

The initial rotation is likely primordial since the pre-solar nebula the comets formed from would have some initial rotation from the galactic rotation which can be increased as the proto-comets contracted during formation. The spin rate of the nucleus can be changed by the torques caused by the activity-driven outgassing from the nucleus; this was observed for Comet 67P from in-situ measurements during the ESA Rosetta mission (Keller et al. 2015) and also have been observed to change in rotation from remote observations (abstract from Bodewits et al 2018 for Comet 41P)

• I think a broader question would be - does anything in the universe not spin? Other than tidally locked moons/planets, which arguably still do, just at the same rate as their orbital period. An object with 0 angular momentum would seem to be quite the exception - it'd be very unlikely for all forces to completely zero out. Feb 4, 2021 at 15:25
• The ICRS by definition... 😂 Feb 4, 2021 at 16:00
• (Maybe I should've restricted that to individual physical objects...) Feb 4, 2021 at 16:32