What is the most distant object from the Earth that a spacecraft has visited to date?

Question

What is the most distant object from the earth that a spacecraft visited has visited so far? What was the mission and when did it happen?

Answer 1

New horizons visited a few Kuiper Belt Objects on New Years day. Those are the furthest objects that were programmed to reach an object. The mission was simply to record data, which is still being processed and has not been released to the public. The object was named Ultima Thule. It looks like a poorly constructed snowman made of rock, laying on its side.