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The mass ratio of Charon to Pluto is a basic parameter describing the binary system and is necessary for determining the individual masses and densities of these two bodies. Previous measurements of the mass ratio have been made, but the solutions differ significantly (Null et al., 1993; Young et al., 1994; Null and Owen, 1996; Foust et al., 1997; Tholen and Buie, 1997). We present the first observations of Pluto and Charon with a well-calibrated astrometric instrument--the fine guidance sensors on the Hubble Space Telescope. We observed the motion of Pluto and Charon about the system barycenter over 4.4 days (69% of an orbital period) and determined the mass ratio to be 0.122 [+ or -] 0.008 which implies a density of 1.8 to 2.1 g [cm.sup.-3] for Pluto and 1.6 to 1.8 g [cm.sup.-3] for Charon. The resulting rock-mass fractions for Pluto and Charon are higher than expected for bodies formed in the outer solar nebula, possibly indicating significant postaccretion loss of volatiles. Keywords: Pluto; Charon

This paper presents new results about Triton's atmospheric structure from the analysis of all ground-based stellar occultation data recorded to date, including one single-chord occultation recorded on 1993 July 10 and nine occultation lightcurves from the double-star event on 1995 August 14. These stellar occultation observations made both in the visible and in the infrared have good spatial coverage of Triton, including the first Triton central-flash observations, and are the first data to probe the altitude level 20-100 km on Triton. The small-planet lightcurve model of J. L. Elliot and L. A. Young (1992, Astron. J. 103, 991-1015) was generalized to include stellar flux refracted by the far limb, and then fitted to the data. Values of the pressure, derived from separate immersion and emersion chords, show no significant trends with latitude, indicating that Triton's atmosphere is spherically symmetric at [approximately]50-km altitude to within the error of the measurements; however, asymmetry observed in the central flash indicates the atmosphere is not homogeneous at the lowest levels probed ([approximately]20-km altitude). From the average of the 1995 occultation data, the equivalent-isothermal temperature of the atmosphere is 47 [+ or -] 1 K and the atmospheric pressure at 1400-km radius ([approximately]50-km altitude) is 1.4 [+ or -] 0.1 [micro]bar. Both of these are not consistent with a model based on Voyager UVS and RSS observation in 1989 (D. F. Strobel, X. Zhu, M. E. Summers, and M. H. Stevens, 1996, Icarus 120, 266-289). The atmospheric temperature from the occultation is 5 K colder than that predicted by the model and the observed pressure is a factor of 1.8 greater than the model. In our opinion, the disagreement in temperature and pressure is probably due to modeling problems at the microbar level, since measurements at this level have not previously been made. Alternatively, the difference could be due to seasonal change in Triton's atmospheric structure.

The radius of the planet Pluto and its atmospheric composition are determined by photometric observations of the occultation of star P8 by Pluto on Jun 9, 1988. The haze model is used as the basis for determining the parameters and the thermal gradient model is used to explain the obtained results. The radius of the planet is 1195 kilometers. The atmosphere of Pluto is uniform all over the planet.