View Full Version here: : Asteroid Spin Rates and Collisions

07-01-2011, 10:02 PM
I think I may be wearing the patience of my professional mentors rather thin as I am continually questioning them about their premise that collisions do not play a part in accounting for slow spinning asteroids. The paradigm seems to be that infrequent large impacts spin asteroids up while frequent small collisions slow the spin down such that an equilibrium exists.

This does not seem logical to me. I'm great with logic but not with science and I realise the natural world does not abide by logic so does anyone care to shed some light on this for me?


07-01-2011, 10:11 PM
I should point out that I am not against the idea that collisions play no part. There is other 'evidence' that it doesn't. For example one might expect to see a relationship between an asteroids orbital elements and its periodicy. Slow rotators would be expected to have more opportunities to collide with other objects. I did a 3D log plot of the 3600 U>=2 targets Diameter, Spin Frequency, a and e and found a uniform distribution.

The only non uniform distribution was related to Diameter and a - the larger the semi major axis the larger the diameter of the asteroid in the dataset - but this is to be expected as an observational bias given that the magnitude of a target decreases with distance and amateurs (who do the bulk of the period determinations) have limits to achieve the required SNR for precision photometry.


08-01-2011, 10:58 AM
Although waring their patience thin I did get a quick response.... The paradigm is based on averages only, "individual" results will vary.


08-01-2011, 11:25 AM

The other thought is to do with the 'initial' conditions.
With something as big as some of the ones you're looking at, the inertial mass is pretty big, so whatever the dynamics of the big ones was initially, it would continue until something perturbs it. Given that they drift, and that gravitational and magnetic fields exist everywhere, (to some degree), throughout the solar system, all of these phenomena can, and will, influence these objects.

What we see of their motion in the present, is clearly a combination of the inertial state plus just about every non-inertial force known. The combined influence over billions of years could result in anything (and seems to be confirmed in the distributions you're referencing) !

Perhaps you're looking too hard to explain something which has no single supporting evidence in the present day. (?)


08-01-2011, 12:14 PM
The presumed initial state is p=8-10h with 3h and 48h being the expected limits.

Yes, thats the averaging after modeling done by the pros that accounts for dispersion in the 8-48h range. YORP accounts for those <8h and tumbling dampening accounts for some of the slow to very slow rotators but not for the non tumbling slow rotators and ultra slow rotators.

True. But given that the modeling using all of the above can't account for the Slow/Ultra slow rotators, there must be something(s) that has been missed/overlooked!