Kanehekili Patera from Galileo

Due to the nature of its orbit, although Galileo made several close approaches to Io, some parts of its surface were never seen except from a great distance.  Some of these areas were covered by Voyager 1's close flyby, but others were not.  The are centered around Kanehekili, the volcano which can be seen erupting above the limb at about four o'clock, is one such place. A major eruption was seen here earlier this year.  Hopefully a new mission will be able to fill in this gap.

This image was taken on May 6, 1997, during Galileo's eighth orbit around Jupiter.

Processed image Copyright Ted Stryk, Raw Data Courtesy NASA/JPL

Nereid from Voyager

When planning began for Voyager 2's 1989 encounter with Neptune, the planet had two known moons - Triton and little Nereid, a small, distant moon that, like Triton, is likely captured.  The flyby didn't go well for little Nereid.  First, it takes nearly 360 days to orbit Neptune - almost a year! - and was not in a convenient place for Voyager to meet it (unless Neptune itself and Triton were sacrificed).  Second, it would be dethroned as Neptune's second largest moon by Proteus, a newly discovered moon that would receive much better coverage. 

Voyager 2 came no closer than 4.7 million kilometers from Nereid.  All that could be made out is that it is a somewhat spherical (if a bit lumpy), 340-km in diameter little world with a relatively low albedo -15%.  It rotates in 11 hours, as determined by light curve data, further supporting the idea that it is captured (not being tidally locked is unusual for a moon). 

Here are Voyager 2's two best views.  The first shows the lumpy little world in a gibbous phase as Voyager approached on August 21, 1989, and the second shows a crescent on August 24 - Nereid was "in front" of Neptune, so closest approach was earlier than the August 25 closest approach of Neptune.
 

Yet another world cries out for exploration!

Processed images Copyright Ted Stryk, Raw Data Courtesy NASA/JPL