17. NEPTUNE

This is NEPTUNE. Distance to earth: 4.5 x 10^9 km or 4.5 billion km, but in spite of that, one succeeded to manage the VOYAGER with an accuracy of 40 km along the planet after a journey of 12 years. That led the American projectleader to the statement:" That is the heavenly equivalence of a golfball smashed in one blow into a hole in London!" Don't forget that it takes 4 hours for a lightsignal to arrive at the spacecraft and another 4 hours to get an answer!
Moreover, the Voyager is now, 2006, outside our solarsystem, arriving in the area called the "heliopause" where the solarwind is confronted with the interstellar dust, between 12 x 10^9 - 26 x 10^9 km.