THE CHARON PLUTO MASS-RATIO FROM MKO ASTROMETRY

Using the UH 2.2-m telescope, we obtained CCD images of Pluto as it pa ssed through a single field of 10 stars during 6 nights ot 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, Astron. J. 105, 2319-2335) of 19,405 +/- 86 km. Our semimajor axis implies a system mass of (14.32 +/- 0.1 3) x 10(24) g. From the motions of Pluto and Charon around their baryc enter, 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 cm-3 . 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 discu ssed. (C) 1994 Academic Press, Inc.