Pioneer 10 (1972): The First Spacecraft to Reach Jupiter

Pioneer 10 was the first spacecraft to cross the asteroid belt and fly past Jupiter. Launched by NASA on 2 March 1972 (US time), it reached the giant planet in December 1973 and returned the first close-up images and measurements of it. It then became the first human-made object set on a path out of the solar system, carrying a plaque meant for anyone who might one day find it.

Background

By the late 1960s spacecraft had visited the Moon, Venus and Mars, including Mariner 4 in 1965. Jupiter was a much harder target. It lies about five times farther from the Sun than Earth, and between them lies the asteroid belt, which some scientists feared might be dense enough with dust and debris to destroy a passing probe. Jupiter was also known to have intense radiation belts.

NASA approved two identical probes, Pioneer F and G, to scout the way. The project was managed by the Ames Research Center in California, and the spacecraft were built by TRW. Because sunlight is weak so far out, each probe carried four small radioisotope thermoelectric generators, which produce electricity from the heat of decaying plutonium-238.

The flight

Pioneer 10 was launched from Cape Kennedy on an Atlas-Centaur rocket with an extra solid-fuel stage. It left Earth faster than any spacecraft before it and passed the Moon’s orbit in about 11 hours.

The probe entered the asteroid belt in July 1972 and emerged safely in February 1973. Its detectors found far fewer dangerous particles than some had feared, showing that the belt could be crossed and opening the way for later missions to the outer planets.

The Jupiter encounter

Pioneer 10 made its closest approach to Jupiter on 3 December 1973 (US time; early on 4 December in Universal Time), passing about 130,000 kilometres above the cloud tops. Its imaging instrument scanned the planet line by line as the spacecraft spun, producing about 500 images, including the first close views of Jupiter and pictures of some of its large moons.

The radiation was stronger than expected. Several instruments suffered errors, and a few planned images were lost, but the probe survived. Its measurements of Jupiter’s magnetic field, radiation belts and atmosphere guided the design of later spacecraft, which were given extra shielding.

The Pioneer plaque

At the suggestion of journalists and scientists, NASA agreed to attach a small message to the spacecraft. Carl Sagan and Frank Drake designed it in a few weeks, and the artist Linda Salzman Sagan drew the figures.

The gold-anodised aluminium plaque, about 23 by 15 centimetres, shows a nude man and woman in front of an outline of the spacecraft for scale. A diagram shows the solar system and the probe’s path from the third planet. A starburst of lines locates the Sun relative to 14 pulsars, using their pulse rates as a kind of cosmic clock, and a symbol for hydrogen supplies the unit of measurement.

The plaque drew criticism at the time. Some objected to the nudity, while others argued that the figures and the waving hand were too culturally specific to be understood by aliens. The same idea was later expanded into the Golden Records carried by the Voyager probes.

After Jupiter

Jupiter’s gravity flung Pioneer 10 onto a path out of the solar system. On 13 June 1983 it crossed the orbit of Neptune, which at that time was the outermost planetary orbit because Pluto was temporarily closer to the Sun.

NASA ended the official science mission in 1997 but continued to track the probe occasionally. As its power supply decayed, the signal weakened; the last useful data arrived in April 2002, and the final faint contact came on 23 January 2003. Its twin, Pioneer 11, launched in April 1973, went on to make the first flyby of Saturn.

Myths and facts

Small unexplained slowdowns in both Pioneer probes, known as the Pioneer anomaly, once prompted speculation about new physics. Studies published in 2011 and 2012 concluded that the effect was caused by heat radiating unevenly from the spacecraft itself.

It is also sometimes said that Pioneer 10 has already left the solar system. It has passed every planet, but the Sun’s gravity and the vast, distant Oort cloud of comets extend much farther, so whether it has “left” depends on which boundary is used.

Timeline

• 1972, 2 March: Pioneer 10 is launched from Cape Kennedy (3 March in Universal Time).
• 1972, July: The probe enters the asteroid belt.
• 1973, February: Pioneer 10 emerges from the asteroid belt undamaged.
• 1973, 3 December: Closest approach to Jupiter (US time).
• 1983, 13 June: Pioneer 10 crosses the orbit of Neptune.
• 2003, 23 January: NASA receives the last faint signal from the spacecraft.

Why it matters

Pioneer 10 proved that spacecraft could cross the asteroid belt and survive Jupiter’s radiation, clearing the way for Voyager, Galileo and later missions to the outer planets. Its plaque was humanity’s first deliberate message sent beyond the solar system.