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J.N. Spitale, R. Greenberg (LPL)
The Yarkovsky effect, in which a body is accelerated by the reaction to thermal reradiation, may play a key role in the orbital evolution of asteroids and near-Earth objects. To evaluate the acceleration under various conditions, a three-dimensional finite-difference solution to the heat equation is applied to homogeneous, spherical bodies with 1-, 10- and 100m diameters. This approach employs neither the linearization, the plane-parallel approximation, nor the assumption of fast rotation used in earlier work (Rubincam, JGR 100:1685 1995, JGR 103:1725 1998; Vokrouhlicky, AJ 116:2032 1998), allowing exploration of a wide range of orbital elements and physical properties. Our work agrees with earlier results in the regimes where their approximations are valid. We investigate both the "seasonal" (obliquity = \pi/2) and "diurnal" (obliquity = 0) extremes of the Yarkovsky effect, as well as the general case of intermediate obliquity. The numerical approach is applied to large eccentricities, where we find that the semimajor axis and eccentricity can change much faster than for circular orbits. For such orbits, the orientation of the rotation axis with respect to the direction of pericenter is critical in determining the evolution. For certain orientations, the seasonal Yarkovsky effect can cause both the semimajor axis and eccentricity to increase, contrary to previous expectations that the semimajor axis should always decrease (\emph{op. cit.} Rubincam, Vokrouhlicky). An important implication of this work is that the Yarkovsky effect may remove an asteroid from resonance once its eccentricity has been sufficiently excited, but before the eccentricity grows so large that the asteroid is lost. This mechanism might significantly increase the efficiency for delivery of asteroids from the main belt to the terrestrial planets via the strong resonances. This work is funded by NASA PG&G grant NAG5-3631.