"The most distant ever visited by a spacecraft from Earth"
It's called ellipsis. Your interpretation of the sentence is, I believe, correct.
I think there is an implicit object in the sentence: "The winning object could become the most distant object ever visited by a spacecraft from Earth."
That is called ellipsis:
the omission from speech or writing of a word or words that are superfluous or able to be understood from contextual clues.
[Reference: the New Oxford American Dictionary.]