Rotation of Cometary Nuclei (original) (raw)

Rotation of Cometary Nuclei [and Discussion]

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1984

Asymmetric comas and repetitive appearances of structures in the heads and tails of comets are used to infer nuclei rotation periods. However, periodic behaviour of optically-thick expanding comas or of ion plasma production may contribute spurious results. The spin periods of comets are longer than those of asteroids, ranging generally over 10-100 h and above probable limits for gravitational escape. The periods show a flatter distribution, which may reflect an accretional rather than collisional fragmentation history. Arguments for spin-up with age due to sublimating gases are weak; the converse is possible and spin-down due to preferential escape of particles from equatorial regions appears likely.

Evolution of Comet Nucleus Rotation

Icarus, 2002

The secular evolution of comet nucleus rotation states subject to outgassing torques is studied. The dynamical model assumes that the nucleus inertia ellipsoid is axially symmetric. The outgassing torques acting on the surface are modeled using standard cometary activity formulae. The general rotational equations of motion are derived and separately averaged over the fast rotational dynamics terms and the comet orbit. Special cases where the averaging assumptions cannot be applied are evaluated separately. The modification of the comet orbit due to comet outgassing is neglected. Resulting from this analysis is a system of secular differential equations that describes the dynamics of the comet nucleus angular momentum and rotation state. We find that the qualitative secular evolution of the rotation state is controlled by a single parameter that combines parameters related to the comet orbit and parameters related to the nucleus surface geometry and activity. From this solution, we find qualitatively different evolutionary paths for comet nuclei whose entire surface is active, as compared to nuclei with only a single active region. For surface activity models between these extremes, we show that certain evolutionary paths are more likely than others. Additionally, our solution indicates that a comet nucleus' rotational angular momentum will tend to increase over time, potentially contributing to the observed phenomenon of comet nucleus splitting. c 2002 Elsevier Science (USA)

The influence of reactive torques on comet nucleus rotation

2003

Reactive torques, due to anisotropic sublimation on a comet nucleus surface, produce slow variations of its rotation. In this paper the secular effects of this sublimation are studied. The general rotational equations of motion are averaged over unperturbed fast rotation around the mass center (Euler-Poinsot motion) and over the orbital comet motion. We discuss the parameters that define typical properties of the rotational evolution and discover different classifications of the rotational evolution. As an example we discuss some possible scenarios of rotational evolution for the nuclei of the comets Halley and Borrelly.

Long-term simulations of the rotational state of small irregular cometary nuclei

Astronomy and Astrophysics, 2003

We have performed simulations of the long-term evolution of the spin state of small active comet nuclei (1 km and 0.6 km) in the orbit of 46P/Wirtanen under the effect of the torque of the nongravitational force. A total of 46 combinations of irregular shapes and activity patterns have been simulated. We observe typical changes of the spin period of 0.01−10 h at each perihelion passage during the simulations, depending on the initial spin period and on the temporal evolution of the spin state. The direction of the angular momentum also changes by 0.1 to several tens of degrees per orbit. These changes are not always associated with an observable excitation of the spin state. While the nucleus gets to excited spin states in some simulations, it remains in a pure spin state during several tens of orbits in others. Therefore, even small and very active nuclei like 46P/Wirtanen could remain in their spin state of minimum energy (pure spin) during at least several tens of perihelion passages. We find that, for the parameters used in our simulation, a drastic increase in spin period to ∼200 h is required before the excitation of the spin state can occur. Further results and their consequences for the observations of rotational parameters of cometary nuclei are thoroughly described in the text.

Further Investigation of Changes in Cometary Rotation

The Astronomical Journal, 2018

Samarasinha & Mueller (2013) related changes of cometary rotation to other physical parameters for four Jupiter family comets defining a parameter X, which is approximately constant within a factor of two irrespective of the active fraction of a comet. Two additional comets are added to this sample in this paper and the claim of a nearly constant parameter X for these six comets is confirmed, albeit with a larger scatter. Taking the geometric mean of X for all the comets above excluding 2P/Encke (as X for each comet was determined with respect to that of 2P/Encke), the expected changes in the rotation periods for a sample of 24 periodic comets are derived. We identify comets from this sample that are most likely to show observationally detectable changes in their rotation periods. Using this sample and including the six comets used to determine X, we find a correlation between the parameter ζ (i.e. the total water production per unit surface area per orbit approximated by that inside of 4 au) and the perihelion distance q; specifically we derive ζ ∝ q −0.8 and provide a theoretical basis for this in Appendix A. This relationship between ζ and q enables ready comparisons of activity due to insolation between comets. Additionally, a relationship between the nuclear radius R and the rotation period P is found. Specifically, we find that on average smaller nuclei have smaller rotation periods compared to the rotation periods of larger nuclei. This is consistent with expectations for rotational evolution and spin-up of comet nuclei, providing strong observational evidence for sublimation-driven rotational changes in comets.

Evolution of the Rotational State of Irregular Cometary Nuclei

Earth Moon and Planets, 2002

A simplified thermal model has been used to calculate thenon-gravitational forces acting on small irregular nuclei in the orbit ofComet 46P/Wirtanen. The torque of thenon-gravitational force has beencalculated and the Euler equations have been solved in order toinvestigate the rotational evolution of several irregular nuclei duringa single orbital step. Several initial spin axis orientations andactivity patterns on their surfaces have been considered. The nucleiconsidered have a mean radius of 1 km and their inertia moments havebeen calculated assuming a homogeneous bulk density of 500 kg/m3. Inall the simulations, the initial spin period is 6h and the nucleiinitially rotate around their shortest axis. Under these assumptions,significant changes in the angular momentum and in spin period have beenobtained in all the simulations, but the nucleus is found to practicallyremain in its spin state of lowest energy during the entire orbitalperiod.

Rotation and activity of comets

Advances in Space Research, 2007

We explore how nuclear rotation and activity can be used as effective probes of the gross nuclear structure and therefore of the interior of comets. We present a model of nuclear activity and discuss that in the context of how activity and rotation can control the present day size distribution of active short period comets. We argue that there is a real paucity of sub-km comets when compared with what one expects based on the size distribution of the known Kuiper Belt Objects.

An updated rotation model for Comet 9P/Tempel 1

Icarus

a b s t r a c t Observations from the second encounter of Comet 9P/Tempel 1 by the Stardust-NExT spacecraft provide an improved shape model and rotational pole for the nucleus (Thomas, P.C. et al. [2012]. Icarus 222, 453– 466) that allows us to greatly improve our knowledge of its rotational evolution beyond that outlined earlier in Belton et al. (Belton, M.J.S. et al. [2011]. Icarus 213, 345–368). Model light curves are shown to fit observations at both perihelia with a single pole direction indicating that polar precession during a single perihelion passage is small. We show that the rotational phasing associated with observations taken far from perihelion in the previous work was incorrectly assessed by approximately half a cycle leading us to a significant reassessment of the evolution of the non-gravitational torques acting on the nucleus. We present an updated spin rate profile (torque model) for the 2005 perihelion passage and show that retardation of the spin rate well befor...

On the rotation of comet Borrelly’s nucleus

Celestial Mechanics and Dynamical Astronomy, 2008

We consider the secular effect of outgassing torques on the rotation of a comet nucleus. An averaging method is applied to obtain evolutionary equations which allow us to study the long-term variations in the nucleus spin state. Since the spin axis direction of 19P/Borrelly's nucleus is close to the line of apsides direction, a simplified version of these equations can be written to analytically study the most important qualitative effects. In particular, a correlation between the drift of the rotation axis direction and the possible spin up/spin down of the nucleus is revealed.

A rapid decrease in the rotation rate of comet 41P/Tuttle-Giacobini-Kresák

Nature, 2018

Cometary outgassing can produce torques that change the spin state of the cometary nucleus, which in turn influences the evolution and lifetime of the comet. If these torques increase the rate of rotation to the extent that centripetal forces exceed the material strength of the nucleus, the comet can fragment. Torques that slow down the rotation can cause the spin state to become unstable, but if the torques persist the nucleus can eventually reorient itself and the rotation rate can increase again. Simulations predict that most comets go through a short phase of rapid changes in spin state, after which changes occur gradually over longer times. Here we report observations of comet 41P/Tuttle-Giacobini-Kresák during its close approach to Earth (0.142 astronomical units, approximately 21 million kilometres, on 1 April 2017) that reveal a rapid decrease in rotation rate. Between March and May 2017, the apparent rotation period of the nucleus increased from 20 hours to more than 46 hou...