|
|
 |
| Abstract |
| Rotational Properties of Asteroids, Comets and TNOs | Alan W. Harris
Petr Pravec
| Space Science Institute
Ondrejov Observatory
| | At the second ACM conference in Uppsala 20 years ago, I noted that there appears to be a clear separation of fast and slow rotators among asteroids smaller than ~100 km diameter, but I offered no hypotheses to explain these groups. In the last few years, just since the last ACM meeting, it has become clear that the explanation of the spin characteristics of smaller asteroids is the torque from solar radiation and re-radiation, the so-called YORP effect. This effect leads to either spin-up or spin down, hence the bimodal spin distribution of smaller asteroids, and also creates spin axis alignments that essentially rule out any other explanation. YORP is sufficient to explain the very slow rotations of moderately large asteroids (e.g. 253 Mathilde) and is also sufficient to spin up asteroids as large as ~10 km diameter to the point of rotational instability, as seen in the spin barrier among rubble-pile asteroids in the 1-10 km diameter range. It is strong enough to accelerate very small monolithic asteroids in only a few million years to rotation periods of only a few minutes. In the present environment collisions are insufficient to significantly alter the spins of normal asteroids, but are sufficient to excite tumbling motion of the slow rotators and keep them from despinning totally to sun-synchronous rotation. Comets have a similar bimodal distribution of spins, including slow and tumbling rotation. Gas jetting is the likely explanation for this distribution among comets, suggesting that the bimodal spins of NEAs may indicate a large fraction of dead comets among NEAs. This is not supported by taxonomy or orbital dynamics. YORP gets us out of that dilemma, but leaves unanswered just how many NEAs are dead comets. To date, only about 15 TNO rotations are known. There are no very long spin periods among them, not even Sedna, although not much else can be inferred from such small numbers. | | Presenting author: | Alan Harris |
|
|
|
|
