This material has been adapted from the New Horizons web site.
The primary science goals of the NEW HORIZONS mission are to characterize the global geology and morphology of Pluto and Charon, to map the surface composition of Pluto and Charon, and to characterize the neutral atmosphere of Pluto and its escape rate (NASA AO, 2001 [NASAAO2001]; Stern & Spencer, 2004 [STERN&SPENCER2004A]).
The New Horizons spacecraft trajectory was designed to have as early an arrival time at Pluto as practicable.
There are two reasons why the New Horizons science team wanted to reach Pluto and Charon quickly. The first has to do with the Pluto atmosphere: Since 1989, Pluto has been moving farther from the Sun, getting less heat every year [LUNINEETAL1995]. As Pluto gets colder scientists expect its atmosphere will freeze out, so the team wanted to arrive while there is a chance to see a thicker atmosphere.
The second reason is to map as much of Pluto and Charon as possible.
As New Horizons approaches and flies by the Pluto system, parts of Pluto or Charon will be in constant darkness, and, the later the flyby, the more of Pluto and Charon that will be unlit.