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M.R. Voelzke, O.C. Winter (UNESP - Brazil)
Although approximately two dozen comets are observed to come to perihelion every year, only a small fraction of these comets is extensively studied. Observational data sets for direct measurements of nuclear comet dimensions are scarce. Only for comet P/Halley we do have extensive and in situ data, including resolved images of the nucleus (Keller et al. 1986; Keller et al. 1987). Unlike an oblate spheroid, the most probable position of the rotation axis is not immediately obvious. The situation about the nuclear rotation is further complicated by the lack of information on the structure and mass distribution in the comet's interior, so that the moments of inertia about the three axes can only crudely be estimated. An irregular rigid body with unequal moments of inertia is known to freely precess if it is spinning about an axis that is oblique to its principal axes of inertia. Sekanina (1987) and Julian (1987) modeled the comet P/Halley nucleus by a dynamically-equivalent, torque-free, rigid, homogeneous ellipsoid with principal axes of unequal lengths, l1 > l2 = l3 and l1 > l2 > l3, respectively. In both cases the motion is assumed to be unaffected by jet-induced torques, i.e., they did not consider the nongravitational effects that occur when comets have outgassing in its active areas. Giotto spacecraft images showed that comet P/Halley has some active areas (Keller et al. 1986; Keller et al. 1987) but these images do not cover the whole surface. Therefore, it is unknown the number of active areas, their exact location and how is the variation in their activity with the cometary rotation. This work presents the initial results of numerical simulations for the evolution of rotational states of near-prolate cometary nuclei over one perihelion passage under jet-outgassing (dust and gas) torques. Initially the authors present a simple, first-approximation model with only one jet at the extremity on the semimajor axis to understand the amount of delay or advance that these nongravitational forces make on the cometary period of rotation. Then, it is increased the number of jets and they are distributed along the cometary surface to look for the contribution from the torque effects by outgassing, which would possibly violate the assumption of free precession. The rotational states, under sublimation-induced torques, strongly depend on the location of active areas on the nucleus. - Keller, H.U., Arpigny, C., et al., 1986, Nature, 321, 320 - Keller, H.U., Delamere, W.A., et al., 1987, Astron. Astrophys., 187, 807 - Julian, W.H., 1987, Nature, 326, 57 - Sekanina, Z., 1987, Nature, 325, 326
The author(s) of this abstract have provided an email address for comments about the abstract: voelzke@feg.unesp.br