4979 Otawara: flyby target of the Rosetta mission

An international observing campaign was organized to determine the physical and chemical characteristics of asteroid 4979 Otawara, which is the first target of the Rosetta mission (flyby on July 10, 2006). Knowledge of the ph ysical parameters of the flyby targets is required for both refinement of t he design of the spacecraft and the instrument payload, and optimization of the mission trajectory and scenarios. We present the results of observatio ns obtained from December, 1998 through March, 1999. The spectral classific ation of 4979 Otawara could be either a pyroxene and/or olivine-rich S-type asteroid or a V-type asteroid, a member of the Vesta dynamical family. Fur ther observations are needed in order to discriminate between the two spect ral types. The synodic rotation period of Otawara is P-syn = 2.107 +/- 0.00 5 hr. The lower limit for the axial ratio of the enveloping ellipsoid is a/ b greater than or equal to 1.3. The circular effective radius is 2.0 or 1.3 km in the case of an S-type or a V-type asteroid, respectively. A lower li mit on its density is obtained: rho(min) greater than or equal to 1.9 g cm( -3) if we assume that Otawara is an aggregate or rubble pile object. Howeve r, if Otawara is a single solid body, no constraint can be set on its densi ty. 4979 Otawara is a small, fast rotating asteroid (FRA) and hence, will b e a particularly interesting target to be studied from a spacecraft, since no fast rotator has been visited yet.