This is a 4-second exposure  of asteroid 19 Fortuna, one of three made
in the F555W filter with the Planetary Camera at 12:50 UT on 10
September 1993.   The image scale is 0.044 arcseconds per pixel, and
the field seen here was extracted from the 800x800 field of camera
PC6.

Fortuitously, Fortuna was passing about 7 arcsecnds from a 13-magnitude
field star (GSC 6316.01954).  The telescope tracked the motion of the
asteroid at a (geocentric) rate of  0.00071 arcseconds per second.  The
differential motion was clearly seen between the separate exposures
which were 6 minutes apart, but was too small to have a noticeable
effect on the stellar image during the 4-second exposure.

Also fortuitously, the peak-pixel count was about the same for the
asteroid and the field star.  In this reproduction the contrast has
been set to emphasize the inner core of the point-spread function for
each object, and the extended haze due to the spherical aberration of
HST's primary mirror is only dimly seen.

Note that the star appears roughly as a 2.x2 pixel square, nicely
demonstrating the 0.08 arcsecond FWHM resolution of a 94-inch
diffraction-limited telescope in space.  It is also clear that Fortuna
is semi-resolved, four or five pixels in diameter.

Fortuna is a C-type asteroid in the inner main belt, believed from
ground-based thermal radiometry to be about 225 km in diameter.
Because of current pointing constraints of HST, Fortuna could not be
observed at opposition but at a solar elongation angle of about 128
degrees. At the time of the exposure it was 1.547 AU or about
231,000,000 km distant from the earth, and was expected to show an
apparent diameter of 0.20 arcseconds or 4.5 PC pixels.  The image shows
the expected size, and looks quite round at this resolution.


This work was done under Hubble Space Telescope program 4521.  The
Principal Investigator is B. H. Zellner of Computer Sciences
Corporation.  Co-Investigators are C. T. Kowal and E. N. Wells of CSC,
A. Storrs of STScI, and D. Tholen of the University of Hawaii.