Original: There have been many unmanned NASA space missions as well , including at least one that visited each of the other seven planets in the Solar System , and four missions ( Pioneer 10 , Pioneer 11 , Voyager 1 , and Voyager 2 ) that have left the Solar System .
Simplified: Four NASA spacecraft have left the Solar System , Voyager 1 , Voyager 2 , Pioneer 10 and Pioneer 11 .