ASTROMETRY OF MINOR PLANETS USING THE FINE GUIDANCE SENSORS OF THE HUBBLE
SPACE TELESCOPE

Hemenway, P.D.1, Duncombe, R.L.1, Benedict, G.F.1, Franz, O.2, Fredrick, L.3,
Jefferys, W.1, Shelus, P.1, van Altena, W.4,  Whipple, A.1, 
Lalich, A.1,  Lubinow, A.5, Johnson, A.1, Nelan, E.1,  Storrs A.5 

1) Univ of Texas at Austin
2) Lowell Obs.
3) Univ. Virginia
4) Yale Univ.
5) Space Tele. ScI.

Accurate (milliarcsec, rms) astrometry of selected minor planets has
been a goal of the Hubble Space Telescope (HST) Astrometry Science Team
since its inception in 1977.  The uses of such observations 
include measuring an accurate plate scale within the Fine Guidance Sensor
(FGS), measuring minor planets at crossing points for coordinate system
studies, and measuring minorplanets directly with respect to Hipparcos Stars
to relate the Dynamical and Hipparcos Reference Frames at the mas level of
accuracy.  

In July, 1994, we observed the minor planet 770 in fine lock with 
FGS III (the Astrometer), in an attempt to obtain an accurate plate scale.
Only 4 of 15 observations were obtained, mainly due to an HST "safing", but
the FGS obtained clear Fine Lock on a moving solar system target.  The
observations are discussed and the observational capabilities are analysized.