Pioneer 10

Pioneer 10, initially called Pioneer F, was a NASA probe that launched in 1972 and became the first spacecraft to reach Jupiter. It was also the first to cross the asteroid belt and one of five human-made objects to reach the speed needed to exit the Solar System. NASA’s Ames Research Center in California ran the mission, and TRW Inc. built the craft.

The probe had a hexagonal body with a 2.74-meter parabolic antenna and spun for stability around that antenna’s axis. Four radioisotope thermoelectric generators supplied power, giving 155 watts at launch. It lifted off on March 3, 1972, at 01:49 UTC (March 2 local time) atop an Atlas-Centaur rocket from Cape Canaveral’s Launch Complex 36A. From July 15, 1972, to February 15, 1973, it became the first spacecraft to travel through the asteroid belt. It started imaging Jupiter on November 6, 1973, from 25 million kilometers away, sending back over 500 pictures. Its closest pass to Jupiter came on December 3, 1973, at 132,252 kilometers. The mission studied the asteroid belt, Jupiter’s environment, solar wind, cosmic rays, and the outer heliosphere. The final signal arrived on January 23, 2003, after the RTGs lost enough power to run the transmitter. At that point, the probe was about 80 astronomical units from Earth.

In the 1960s, Gary Flandro at NASA’s Jet Propulsion Laboratory suggested the Planetary Grand Tour, using a rare alignment of outer planets. Though the Voyager program later carried that out, NASA decided in 1964 to test key parts by sending two probes outward. The Outer Space Panel, led by James A. Van Allen, built the scientific case for exploring the outer planets. Goddard Space Flight Center proposed two “Galactic Jupiter Probes” to cross the asteroid belt and study Jupiter. They were planned for launch in 1972 and 1973 during windows that opened for a few weeks every 13 months; missing those windows would need much more fuel. NASA approved the mission in February 1969. Before launch, the probes were called Pioneer F and Pioneer G, later renamed Pioneer 10 and 11. They were part of the Pioneer program, a series of uncrewed U.S. missions from 1958 to 1978. Pioneer 10 and 11 were the first in that program meant for the outer Solar System. Their main goals were to study the interplanetary medium beyond Mars, examine the asteroid belt, check hazards for spacecraft passing through, and explore Jupiter and its surroundings.

Over 150 experiments were proposed. The final instrument set, chosen through meetings in the 1960s and finished by early 1970, was to image Jupiter and its moons, make polarimetric, infrared, and ultraviolet observations, detect asteroids and meteoroids, measure charged particles, magnetic fields, plasma, cosmic rays, and zodiacal light. Radio tracking during Jupiter occultation would probe the planet’s atmosphere, and precision tracking would improve mass estimates of Jupiter and its moons. NASA Ames, not Goddard, managed the project under Charles F. Hall, chosen for its experience with spin-stabilized craft. The mission needed a small, lightweight, magnetically clean probe for interplanetary space, using hardware proven on Pioneer 6 through 9. In February 1970, NASA gave TRW Inc. a $380 million contract without competitive bidding to meet the schedule. B. J. O’Brien and Herb Lassen led the assembly team, which took an estimated 25 million man-hours. One TRW engineer joked the craft was guaranteed for two years of flight, and if anything failed, they’d repair it free. To meet the original schedule, the first probe would have launched between February 29 and March 17 to reach Jupiter in November 1974. The plan later shifted to a December 1973 arrival, avoiding conflicts with other Deep Space Network missions and a period when Earth and Jupiter were on opposite sides of the Sun. Pioneer 10’s flyby path was chosen to maximize science on Jupiter’s radiation, even though some systems were expected to suffer damage. The closest approach, about three Jupiter radii from the planet’s center, was seen as the minimum safe distance for survival while giving a clear view of the sunlit side. A backup, Pioneer H, sits in the National Air and Space Museum’s “Milestones of Flight” gallery. Many aspects of Pioneer 10’s mission shaped the Voyager program’s planning and design.