Pluto was discovered in 1930. It is also the multiple system which has been known for the longest time with the discovery of its first satellite Charon in 1978. Because of Pluto's distance to the Sun, the system still has not completed a revolution since its discovery, hence an uncertain heliocentric distance. The difference between the different ephemeris available far exceeds the uncertainty needed for the mission New Horizons, that is 1,000 km. A new astrometric reduction of old photographic plates may be an efficient way to constrain it.