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We celebrate their firsts and celebrate them as the first to fly to the outer fringes of the solar system.  Voyagers 1 & 2, launched in 1977, continue to return data, continue to probe the heliopause where the suns influence flutters and ceases.  Yet they were not the first to probe the outer solar system.  That honor belongs to two nearly forgotten pathfinders, Pioneers 10 & 11.

On this date, forty years ago, Pioneer 10 became the first probe to fly “beyond the planets.”  By that definition the tiny, 569-lb. spinning spacecraft became the first to leave the solar system.

The Pioneer — Jupiter program was approved by NASA in February 1969 and managed by the Ames Research Center outside of Mountain View, Calif.   Two identical spacecraft were built by TRW, each 9 ft. 6 in. long with a 9-ft.-diameter dish antenna.  The spacecraft maintained a spin for stabilization, keeping the antenna constantly pointed at Earth.

Pioneer 10 was launched by an Atlas-Centaur on March 2, 1972, on a 21-month first-ever voyage to Jupiter.  On July 15, 1972, it became the first spacecraft to enter the asteroid belt.  It emerged in February 1973 unscathed, proving that the belt was no barrier to deep-space exploration.   On Dec. 3, 1973, it became the first spacecraft to fly by Jupiter.  Plunging through the planet’s powerful radiation belts, it provided the first profile of Jupiter’s massive magnetosphere, and proved that a spacecraft could survive the soak of hard radiation.  Thus, it opened the way for the Voyagers to make close encounters with the planet.  It also was equipped with an imaging photopolarimeter which made spin-scan photos built from repeated scans of the planet as the probe revolved.  While not nearly as detailed as the Voyager images to follow, they were the first close-ups of Jupiter.

Pioneer 10 was followed by its twin, Pioneer 11, launched April 5, 1973.  It not only conducted a flyby of Jupiter on Dec. 3, 1974, but made the first gravity-assist boost to Saturn, again pathfinding for Voyager.  It flew by Saturn on September 1, 1979.

Pioneer 10 was expected to expire sometime  around 1980.  Simple and unsophisticated compared to Voyagers, it far exceeded expectations.  Powered by a radioisotope power generator, like the Voyagers, it continued to return data for years on the electromagnetic fields and particles in the space beyond Jupiter. 

At 8 a.m. EDT on June 13, 1983, Pioneer 10 passed beyond the orbit of Neptune, at the time further away from the sun than Pluto whose eccentric orbit dips within that of Neptune for a slice of its year.  The signal confirming its passage beyond took 4 hr.s 20 min. to reach Earth.  It had traveled 2.8 billion miles at this point.

Pioneer 10 continued to defy limits of its design.  Routine contact was maintained until March 31, 1997, cut for budgetary reasons, of all things.  Intermittent contact continued, NASA now and then checking in with the tiny probe.  On February 17, 1978, the speedier Voyager 1 overtook it to take the lead as the most distant probe.  

Last contact with Pioneer 10, it’s nuclear battery dying, was made on January 23, 2003.  It was 7.6 billion miles from Earth.