A model for comet 81P Wild 2

[1] A model is presented for the nucleus of comet 81P/ Wild 2 and its activity, based on analysis of dust coma morphology, water and dust production, and nongravitational perturbations of the orbital motion. A four- apparition composite light curve is derived from similar to 500 magnitude estimates. There appear to be two major outgassing sources on the nucleus: one, near a rotation pole, sustains much of preperihelion activity; the other, on the opposite polar hemisphere and closer to the equatorial plane, accounts for the bulk of postperihelion activity. Both regions are active near perihelion. Their surface areas are estimated at 4.5 and 9.5 km(2), respectively. With the nucleus probably 4.0 to 6.3 km across, the total outgassing area makes up 11 to nearly 30% of the nucleus surface, unusually high for a short- period comet, yet consistent with relatively small perturbations of the orbital motion, driven by the momentum transferred to the nucleus by water sublimation. The absence of a more prominent nongravitational effect may largely be due to high dust mass loading of the gas flow, implying its subsonic initial speed. Indeed, the dustier, polar source appears to be the less efficient of the two in transferring the momentum to the nucleus. The comet's short history in the innermost solar system region could account for the relatively large outgassing fraction of the surface. At the Stardust encounter time, more than three months after perihelion, only the near- equatorial source ( the larger, less dusty one) is expected to be active.