Asteroid 4179 Toutatis: 1996 radar observations

We report initial results of daily delay/Doppler observations of Toutatis w ith the Goldstone 8510-MHz (3.5-cm) radar during Nov. 25-Dec. 3, 1996. Usin g the physical model of Toutatis derived from 1992 radar observations (Huds on and Ostro 1995, Science 270, 84-86) to analyze the new data, we obtain r efined estimates of the asteroid's orbit, spin state, and surface propertie s. The asteroid's centimeter-to-decameter surface characteristics are strik ingly uniform. The disc-integrated circular polarization (SC/OC) ratio mu(c ) averages 0.29 +/- 0.01 and is independent of rotational orientation at th e several percent level. Dual-polarization images reveal a slight drop in m u(c) at echo leading edges, which we interpret as the signature of a smooth surface component. The OC radar albedo averages 0.24 +/- 0.03; it depends on rotational orientation, as expected from the asteroid's angular scatteri ng behavior (limb-darkening slightly more than Lambertian). The OC albedo o f a sphere with Toutatis' radar properties would be 0.21, or three times th e lunar value. The radar properties and available nonradar constraints are consistent with Toutatis' surface having a smooth component that is at leas t 1/3 covered by rocks at least as large as the wavelength. if this S-class asteroid is mineralogically similar to stony-iron meteorites, then the smo oth surface component probably is regolith whose porosity resembles that of lunar soil. if the mineralogy is ordinary chondritic, then the smooth surf ace component is probably solid with not much more than a centimeter of ove rlying regolith. We report delay-Doppler astrometry referenced to the aster oid's center of mass (COM) for each day of our experiment. An orbit solutio n that incorporates those measurements as well as the radar astrometry repo rted by Ostro et al. (1995, Science 270, 80-83) and 588 optical astrometric observations from 1988 through March 1997 has weighted rms residuals of 0. 98 arcs, 0.10 Hz (1.8 mm s(-1) in radial velocity), and 0.49 mu s in time d elay (73 m in range). integration of that orbit into the past and future sh ows that Toutatis' pattern of close approaches to Venus, Earth, and Mars is highly asymmetric about the current epoch. The probability of the orbit in tersecting Earth is zero for at least the next six centuries. Toutatis will make its closest planetary approach since at least 1353 and until at least 2562 on Sep. 29, 2004, when the closest COM-to-COM separation of Earth and Toutatis will be 1,549,834 +/- 10 km (4.0 lunar distances). We use refined spin-state parameters and the 1995 shape model to generate "movies" that p redict the asteroid's rotational motion during its 2004 close approach, in geocentric and inertial frames. (C) 1999 Academic Press.