The only important historical solar system object which has not yet been visited up close by satellite spacecraft is..
A) a comet or asteroid
d) saturn's moon titan
astronomy - bobpursley, Sunday, October 4, 2009 at 7:48pm
I don't know of any comets or asteroids which have had satellite spacecraft orbit them. Frankly, considering the masses of those, it would be quite a fete.
astronomy - jess, Sunday, October 4, 2009 at 7:54pm
astronomy - drwls, Sunday, October 4, 2009 at 8:09pm
Comets have been impacted by spacecraft, and the "tail" of Halley was sampled. At least asteroid have been photographed "up close", and so was Neptune, by Voyager 2. The answer to your question is Pluto.
for a detailed list of solar system probes
astronomy - Maciel, Monday, May 28, 2012 at 7:47pm
Assuming that a spacecraft lands on an asteroid and bores a hole perpendicular to its surface; it is noticed that the sun shines straight down to the bottom of the hole. Another hole is bored 10 miles "north' of the first hole at the same time, but it is noticed that the Sun makes a 20 degree angle to rod placed into that hole - what is the circumference and radius of that asteriod?