During the first 4 years of HST's operation, Io has been observed frequently. Ultraviolet observational capabilities of HST, combined with the chance to gain high resolution images of part of the visible hemisphere of Io as Galileo approaches Jupiter in late 1995, provide a framework for understanding this volcanic satellite, which displays the highest level of activity in the Solar System.
Spatially resolved ultraviolet images of Io obtained with the FOC
( Paresce et al., 1992), significant differences in surface
morphology when visual
images are compared with Voyager images ( Sartoretti et al.,
1994), detection of atmospheric SO
gas absorption in the ultraviolet
( Sartoretti et al., 1994), spatially resolved atmospheric
emissions of neutral S and O which increase in brightness as Io enters eclipse
by Jupiter, and detection of [O II] 2417 
emission from the Io plasma torus
are among the new observations that have been obtained ( McGrath,
1994).
At closest approach to the earth, a pixel (CCD sample) spans about 125 km when the WFPC-2 is used in PC mode, yielding about 30 pixels across the diameter of Io, which subtends about 2.5 arc sec. Elimination of the spherical aberration will greatly benefit this active observational program.