List of Past Planetary Astronomy Lunches (PALS) : 01-Jan-2000 to 01-Jun-2000

Date:   Thursday 22-Jun-00
Speaker:   Dr. Michael Efroimsky (Harvard U.)
Title:  Inelastic Dissipation in a Freely Rotating Body. Applications to Interstellar-Dust Astrophysics and to Tumbling Asteroids and Comets

Asteroids, comets, cosmic-dust granules, spacecrafts, as well as whatever other freely spinning body dissipate energy when they rotate about any axis different from a principal one. We show that the internal dissipation takes place not so much at the frequency of body's precession but rather at the second and higher harmonics. In other words, this simple mechanical system provides an exellent example of an extreme non-linerity. Moreover, it turns out that in some situations the inelastic dissipation takes place at frequencies LOWER than the precession frequency (we call this "exotic nonlinearity").

Earlier estimates by our predecessors ignored the non-linearity as well as some other relevant subtleities. These oversights resulted in four to six order underestimating the efficiency of the precession-relaxation process. The new, more rigorous, treatment has been applied to the research of cosmic-dust alignment and to the analysis of asteroidal and cometary wobble. These results may find another application in the work on damping of rotating spacecrafts.

Regarding the asteroidal and cometary wobble, estimates show that the presently available angular resolution of spacecraft-based instruments makes it possible to observe relaxation of cometary precession within a year-long or, probably, even a several-month-long span of time. In case of asteroids, the typical times are very sensitive to their inner structure (rubble pile or solid chunk).

One of the applications of this research is the following: if one day we manage to get statistics of the excited rotators in the mainbelt, then our knowledge of the relaxation effectivenes will enable us to estimate the frequency of collisions, data most relevant for our civilisation safety.

The other, more immediate application is this: from our knowledge of the precession-damping rate of comets, we will be able to make conclusions on their mechanical properties and inner structure.

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