Pluto and Charon

Explanation: Pluto and Charon, from artist David A. Hardy's book with Sir Patrick Moore, Futures: 50 Years in Space. "Charon is bluer than Pluto and appears grey, while the surface of Pluto is reddish, so the two are of different composition. Pluto is the only 'planet' to rotate synchronously with the orbit of its satellite, so that each always sees the same face of the other."

Pluto

Image credit: David A. Hardy.