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M. Bird (Radioastronomisches Institut, Universität Bonn), M. Paetzold (Inst. Geophysik, Univ. Koeln, Germany), E.A. Marouf (San Jose State U.), B. Haeusler (Inst. Raumfahrttechnik, Univ. Bundeswehr, Neubiberg, Germany), M.K. Bird (Radioastron. Inst., Univ. Bonn, Germany)
Among the many scientific objectives of the Rosetta Radio Science Investigations (RSI) are determinations of the dielectric properties, small-scale roughness, and rotational state of the nucleus by means of a bistatic radar experiment. In the specific case of study, the radio subsystem transmitter located on board the Rosetta spacecraft beams circularly polarized radio signals toward the nucleus surface of comet P/Wirtanen. Part of the impinging radiation, most of which will likely be polarized in the sense opposite to that of the incident signal, is reflected from the surface and scattered toward a receiver at a ground station on Earth. One goal of this work is to verify the feasibility of the RSI bistatic radar experiment at comet P/Wirtanen by applying the radar equation to compute the expected signal-to-noise ratio for the specific radiometric and geometric parameters. The results indicate echo detectability over a broad range of spacecraft-comet-Earth angles, provided the spacecraft is within about 20 km of the comet's surface. During the Rosetta mission orbiting phase following rendezvous, the spacecraft will be moving slow enough that its Doppler variations will be almost negligible over the duration of the estimated comet rotation period (6-8 hours). Characteristic temporal variations in the total radio signal power, as well as distinct signatures in the center frequency and width of the echo spectra, would be produced by the fast rotation of the nonspherical nucleus. By properly timing the execution date of the investigation, it may be possible to derive estimates of the nucleus rotation and precession rates within a single tracking pass.