Comet ISONs Hyperbolic Love and Antimatter comets, Dafuq!?

Observations of Halley's comet by the Vega and Gioto spacecraft, as well as ground based studies in the 1980's showed two nuclear rotation periods of this potato-shaped comet nucleus. A 2.2 - 2.4 day period, and a 7.4 day period. Not enough is really known about other comets to unambiguously determine a rotation period, but their periods are most likely the same...to within an order of magnitude!!!!

Rotation

Asteroids and comets rotate, but not exactly like the Earth. Because Earth is a sphere, its mass is distributed relatively evenly, so it rotates smoothly. Asteroids and comets aren’t uniformly shaped, so their rotation can be more of a tumble. NASA equates their rotation to the spin you see on a badly thrown football. The direction of rotation can differ for each individual asteroid or comet.

3.4. Individual Comets

At present, the entire spin state of a cometary nucleus is known only in the case of three comets, 1P/Halley, and Hyakutake (C/1996 B2). However, even in the case of 1P/Halley, some controversy exists. For 19 other comets, reasonably good estimates of spin periodicities are available (Table 1). For some of these comets, information on the rotational angular momentum vector is available but sometimes the sense of rotation is not known. For other comets only periodicities are known. Comets 19P/Borrelly and 1P/Halley are unique in that they both were the subjects of successful space mission encounters. In both cases, different, but critical, imaging data for the determination of the spin state were acquired. Hyakutake (C/1996 B2) is unique because of its
close perigee during its 1996 apparition and the concomitant rapid change and the wide range of viewing geometry. Hale-Bopp (C/1995 O1) was special because of its highly structured coma.

In addition, its intrinsic brightness yielded an extended apparition that allowed a wide range of observing geometries. The fact that information on the spin state of most other comets is limited underscores the observational and interpretational difficulties that must be faced in spin determinations as discussed elsewhere in section 3.

3.4.1. Comets for which the spin state is approximately determined.

1P/Halley: The spin state of this comet has been the subject of many investigations with conflicting results. The Samarasinha et al.: Rotation of Cometary Nuclei 291 LAM spin state listed in Table 1 provides a satisfactory explanation of time variations in all major groundbased datasets as well as a viable interpretation of spacecraft encounter data; however, disagreements on the spin state still persists. In this model, which is also consistent with independent investigations by Samarasinha and A’Hearn, the long axis of the nucleus precesses around the rotational angular momentum vector once every 3.69 d at an angle of 66°. At the same time, the long axis executes a rotation about itself once every 7.1 d. This fulfills the observational requirement that the aspect of the nucleus return to roughly the same position every 7.4 d as seen by the Sun. Prior to the spacecraft encounters in 1986, the investigations of Sekanina and Larson and others suggested a nucleus in a PA spin state with a period near 2 d. Images from three distinct viewing geometries and timings during the encounters of the Vega 1, Vega 2, and Giotto spacecraft allow for several possible spin states depending on the interpretation of the images. One such interpretation, strongly advocated by the Vega investigators, yields a spin state with a dominant 2.2-d period. However, other interpretations yield results that are related to a 7.4-d periodicity and in which a 2.2-d period is absent. That several interpretations of the spacecraft imaging data are possible is primarily due to uncertainties introduced by the defocused state of the Vega 1 camera and the particular shape of the nucleus. There were subsequent attempts at correcting the defocused Vega 1 images, but there is no consesus among different groups of researchers on the effectiveness of the results.