The Charon-Pluto mass ratio from MKO astrometry.

L. A. Young, C. B. Olkin, J. L. Elliot, D. J. Tholen, and M. W. Buie. Icarus 108, 186-199 (1994) .


Using the University of Hawaii (UH) 2.2-m telescope, we obtained charge coupled device (CCD) images of Pluto as it passed through a single field of 10 stars during 6 nights of Charon's 6.4-day orbital period. From these data, Charon's orbital semimajor axis is found to be 19,460 +/- 58 km, which is consistent with the recent measurement by Null et al. (1993), of 19,405 +/- 86 km. Our semimajor axis implies a system mass of (14.32 +/- 0.13) x 10^24 g. From the motions of Pluto and Charon around their barycenter, we find that the ratio of Charon's mass to Pluto's is 0.1566 +/- 0.0035, indicating that the bodies both have densities near 2 g/cc. Our ratio is nearly twice that of Null et al., who find a ratio of 0.0837 +/- 0.0147. Possible reasons for the large discrepancy are discussed.

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