This week's Planetary Radio features Bruce Betts on"The Meaning of LIFE" -- Phobos LIFE, that is. It's a cool project that the Society is contributing to the Russian Phobos-Grunt mission; it'll carry microbes to Mars orbit and back, and is designed to study the long-term effects of exposure to the space environment on DNA and other important proteins. On "Questions and Answers" I answered the question:"After going to all the trouble of sending New Horizons to Pluto why just pass by and not go into orbit?"
I researched my answer on the New Horizons mission FAQ page.
Nearly a decade will have elapsed between New Horizons' launch and its close approach to Pluto on July 14, 2015. The flyby will happen so quickly that only part of the currently sunlit hemisphere of Pluto will be mapped at high resolution. This seems like a lost opportunity, but there's no other way.
Other spacecraft have achieved orbit at nearer planets because they started out with trajectories that more closely matched their goals, and the bigger planets have much more mass than Pluto does, so the planets' gravity was used to brake spacecraft into orbit.
The New Horizons team has another excellent reason for not going into orbit. Flying past Pluto means that New Horizons will travel onward to encounter two or hopefully more other, smaller members of the Kuiper belt, helping to place Pluto in context and advance our understanding of the dimmest reaches of the solar system.