This Week In Rocket History: Pioneer 10’s Flyby of Jupiter

Another story from the 2021 backlog of Rocket History

This week wasn’t just a busy one for modern launches; it historically also had a lot going on and picking a topic for This Week in Rocket History wasn’t going to be easy. Rather than make a decision, we asked our supporters on Patreon to pick for us, and they picked Pioneer 10.

If you want to help select stories the next time we need help or see what stories weren’t picked this week, join our Patreon.

Pioneer 10 was the first mission to any of the outer planets and would fly by Jupiter and continue on out of the solar system. To fulfill its science goals, Pioneer 10 would get very close to Jupiter and investigate its radiation. It needed to determine how close a spacecraft could safely get to Jupiter so mission planners could plan further missions using Jupiter’s gravity assists. One such mission was Pioneer 11, launched a year after Pioneer 10, which would go from Jupiter to Saturn via a powerful gravity turn over Jupiter’s poles.

Pioneer 10’s launch window and trajectory were optimized to make sure that the planet was not in conjunction with Earth; that is, Jupiter being behind the sun and out of contact with the Earth during the encounter. A compromise was the spacecraft itself would have a solar conjunction 300 days after launch.

This trajectory for Pioneer 10 carried with it the risk that the spacecraft would be fried by radiation and not able to continue its mission. Pioneer 10 traveled through the Jupiter system very quickly to avoid being exposed to too much radiation. The trajectory was also timed so that Pioneer 10 would pass in the direction of the planet’s rotation and see it illuminated best before close approach and a crescent after the encounter.

Pioneer 10 launched March 2, 1972, reaching a speed of over 50,000 km/hr after separation from the Atlas rocket’s upper stage. This made it the fastest man-made object to date. Pioneer 10 passed the orbit of the Moon eleven hours after launch. Ten days after liftoff, all instruments were turned on.

On July 15, 1972, Pioneer 10 became the first spacecraft to pass through the asteroid belt. Scientists were a little concerned that it would hit an asteroid or small rock fragment in the belt, but it emerged unscathed in February 1973. Pioneer 10 told us the asteroid belt is largely empty; this isn’t Star Wars.

Leaving the asteroid belt behind, mission engineers began to prepare it for the encounter with Jupiter that would happen in December 1973. Those preparations were complete by November 6, when Pioneer 10 passed the orbit of the farthest moon of Jupiter, entering the Jupiter system only one minute behind the pre-launch schedule. Engineers were on shift 24/7 through the encounter. On its way in, Pioneer 10 discovered that Jupiter’s magnetopause, the boundary between the solar wind and its magnetic field, was 96 Jupiter radii out and that the poles of the field are reversed relative to Earth.

During the encounter, all of the spacecraft’s computer systems functioned properly despite the intense radiation environment of Jupiter. Only a few pictures were lost when the spacecraft’s computer sent bad commands to the camera under the influence of radiation. These lost images, unfortunately, included one major scientific objective: close-up imaging of Io. Pioneer 10 did discover that Io was 23% heavier than previously estimated. Because Io passed between Pioneer 10 and Earth, Pioneer could radio home to Earth and see distortions that indicated Io has an ionosphere.

The camera on Pioneer 10 took pictures with red and blue filters for the best science value. To produce nice-looking images for the public, red and blue were used to create green for a full RGB image. For their efforts showing images from the spacecraft to the public in near real-time, the Pioneer program received an Emmy Award.

As it headed further into the closest approach, Pioneer 10 began to break down. The cosmic ray detectors and interplanetary electron detector were saturated with radiation a day before approach. Engineers switched on special replacement instruments designed to work in Jupiter’s environment, and these worked. At the closest approach at 10 am UTC on December 3, the camera got more bad commands and some images were lost, but the spacecraft still took pictures of the Great Red Spot. Then Pioneer 10 passed between Jupiter and Earth. The spacecraft sent back its first image after periapsis, and it was normal. Pioneer 10 had achieved its primary mission: determining if a spacecraft could survive a trip through Jupiter’s radiation belts.

Thanks to Pioneer 10’s efforts, Pioneer 11, Voyager 1, and Voyager 2 all made successful Jupiter flybys later in the 1970s. Pioneer 10 crossed the orbit of Pluto and out of the solar system in 1990. The last successful contact with the spacecraft was in April 2002 at a distance of 80.2 astronomical units. One more contact was attempted in January 2003, but it was unsuccessful because the transmitter no longer had enough voltage to function thirty years after launch.

While silent, this little mission continues its journey into the great beyond.

More Information

PDF: Pioneer: First to Jupiter, Saturn and Beyond (NMSU)

Termination of Pioneer 10’s Mission (University of Iowa)