Pioneer probes(pÿ-ŏ-neer ) A series of US Solar-System probes. Pioneers 1–3 were lunar probes intended for launch in 1958, Pioneer 2 suffered launch failure, and Pioneers 1 and 3 fell short of the Moon but did measure the extent of the Van Allen radiation belts. Pioneer 4 began to orbit the Sun after overshooting the Moon and was followed into solar orbit in 1960 by Pioneer 5, which returned data about the solar wind and solar flares.
Pioneer 6, launched in 1965, was joined in solar orbit by Pioneer 7 in 1966, Pioneer 8 in 1967, and Pioneer 9 in 1968. Together they formed a network of ‘solar weather stations' between 0.75 and 1.12 AU from the Sun. Monitoring solar activity and the solar wind, they provided early warnings of solar flares during the Apollo program.
Pioneer 10, launched on Mar. 3 1972, made the first flyby of Jupiter at a distance of 130 300 km on Dec. 4 1973, after becoming the first probe to traverse the asteroid belt. It returned photographs of three of the Galilean satellites of Jupiter (Europa, Ganymede, and Callisto), as well as of the Jovian atmosphere and the Great Red Spot. Other experiments measured the strength and extent of Jupiter's radiation belts. Pioneer 11, launched on a similar mission on Apr. 5 1973, passed 42 940 km below Jupiter's south pole on Dec. 3 1974; it reached a top speed of 171 000 km per hour, faster than any previous man-made object. Nearly five years later it flew past Saturn, making its closest approach, 20 900 km above the clouds, on Sept. 1 1979. It twice crossed the plane of Saturn's rings. The probe, renamed Pioneer Saturn, returned 440 pictures and much new information on the planet, its satellites, and its rings. Pioneers 10 and 11 are heading out of the Solar System. Pioneer 10's mission was officially ended in 1997 but it continued to be tracked occasionally until its power source decayed below the point where its signal could be detected. The last attempt to contact Pioneer 10 was made on Feb. 7 2003. The last signals from Pioneer 11 were received on Sept. 30 1995. Since then the motion of the Earth has taken it out of alignment with Pioneer 11's radio antenna and no further communication is scheduled. Both Pioneers 10 and 11 carry messages from Earth that an alien civilization may decipher. Meanwhile, Pioneer 10 is heading out of the Solar System in the direction of the star Aldebaran in Taurus, while Pioneer 11 will pass near one of the stars in Aquila in about 4 million years.