When NASA and the New Horizons team planned this mission to Pluto, speed was a paramount concern. Pluto made its closest approach to the Sun in 1989. Ever since, the icy dwarf planet has been heading into the solar system's outer reaches, growing colder with each passing year. If New Horizons didn't reach Pluto quickly, the planet's tenuous atmosphere might completely freeze onto the surface. And scientists wouldn't get another chance to observe the atmosphere until Pluto warmed up again in the 23rd century.