How many US missions to other solar system objects have there been?
None. There are telescopes that can see other solar systems, like the Hubble telescope. However, the furthest missions have only been to the outer planets. The two voyager spacecraft, sent to Jupiter in 1977, have kept on travelling and are now the furthest objects from earth. To send something to another solar system is practically impossible, given the great distances. At the speeds we can travel, even to get to the edge of our own solar system would take thousands of years. The best we can do in terms of other solar systems is observe them from here or from the telescopes like Hubble.
