The next planetary first will be our last. After traveling for 10 years and crossing more than 5 billion kilometers, NASA's New Horizons spacecraft will flash by Pluto in a matter of hours, and an age of exploration will be over. Does Pluto hold geological wonders like those the Voyager 2 probe found in 1989 on Neptune's youthful moon Triton, a close relation of Pluto's? Or will Pluto present an eons-old, crater-pocked face like the one that Voyagers 1 and 2 found on Jupiter's big moon Callisto? It all depends on how much heat has been generated and retained inside Pluto since it formed.