It is thought that Pluto's atmosphere will freeze as it moves further from the Sun. We know that Pluto was it's closest to the Sun in the 1990's. Is it known if the atmosphere will freeze prior to New Horizon's reaching it?
According to this paper, Pluto's atmosphere never collapses. Though it was handy that we thought it did at one time, since that helped to get New Horizons approved as a mission, in order to try to get there before the imminent collapse.