By Scott Dance The Baltimore Sun BALTIMORE — Three and a half years after giving humanity its first close-up view of Pluto, and almost 13 years after launching from Earth, the New Horizons spacecraft will explore another new frontier: a reddish hunk of rock and ice known as Ultima Thule. The object — or, perhaps, pair of objects (it's so far away astronomers aren't sure) — is thought to be a pristine