Why Did NASA's Pioneer Spacecraft Mysteriously Slow Down?

Before Voyager 1 and 2 explored the outer solar system, Pioneer 10 and 11 paved the way. Launched in 1972 and 1973, respectively, these spacecraft were the first to transit the asteroid belt and the first to make close observations of Jupiter (both Pioneer 10 and 11) and Saturn (Pioneer 11). Like their successors, the Voyagers and New Horizons, both Pioneers are past the orbit of Pluto and will continue speeding outward from the center of the solar system.