Pioneer 11 at 40

Forty years ago, on April 5, 1973, a small, ambitious spacecraft launched from Cape Canaveral, heading towards the third-brightest point of light in the night sky. Following in the footsteps of its sister craft, Pioneer 10, Pioneer 11 was intended as a backup for the dangerous mission. A single additional instrument, a Flux-Gate Magnetometer, was the only difference between Pioneer 11 and the craft that had already become the first human-made object to leave the inner solar system and was well on its journey to the first and most massive of the gas giant planets, Jupiter.

By the beginning of 1974, Pioneer 10’s journey to Jupiter had proved to be an unmitigated success - the had craft sustained no damage through the asteroid belt, little lasting radiation damage during it’s encounter with Jupiter and had returned far greater volumes of scientific data than expected. After Pioneer 10 successfully survived the Jovian encounter, Pioneer 11 was retargeted mid-flight to include another planetary encounter. The science team at NASA's Ames Research Center in California decided not simply to duplicate Pioneer 10’s mission, but to build upon it, directing the small craft to use Jupiter’s massive gravitational pull as a slingshot to propel the craft at a significantly increased velocity (just as had been done to propel Pioneer 10 out the solar system) to the next – and arguably most beautiful – planet in our system, Saturn.

After some pressure from the Voyager team at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who wanted Pioneer to test the path that Voyager would follow several years later, a decision was made to travel much closer to Jupiter than Pioneer 10. This more risky path was opposed by some of the Pioneer team, but on December 2, 1974, Pioneer 11 passed only 42,000 km (compared to Pioneer 10’s 200,000 km) above Jupiter’s cloud tops. Pioneer 11’s route took the spacecraft over the poles of the planet to avoid the intense radiation belts around Jupiter’s equator. This allowed the first mapping of the planet’s polar regions and sent the craft hurtling through space at a record speed of 172,800 km/h. During its encounter, Pioneer 11 managed to take the most detailed images of the Great Red Spot and calculated the mass of the moon Calisto.
 Because of Pioneer 11’s ambiguous status as a combination science and engineering demonstration mission, a heated debate broke out in the NASA community about what route it would take past Saturn. The famous Voyager missions were launched a full two years before Pioneer 11 reached Saturn and were already heading towards Jupiter through what was now known to be virtually risk-free asteroid belt. The Voyager spacecraft were much more sophisticated and cost many times more than the relatively simple Pioneer probes. Voyager 2’s “Grand Tour” of the Solar System (which involved visiting Jupiter, Saturn, Uranus and Neptune) required that the spacecraft travel through Saturn’s outer A ring in order to get close enough to use gravitational assist to propel it to Uranus.

Scientists at the time were unsure of the makeup of the rings and were worried, as they were for Pioneer 10’s journey through the asteroid belt, that the spacecraft might be impacted and destroyed by objects in the ring. The material in the rings (now known to be mainly comprised of water ice) needed, on average, to be smaller than 1 mm, in which case they would be unlikely to damage the spacecraft, or larger than 1 cm so that they would be spaced far enough apart to allow a spacecraft to pass through them. If they fell between these two sizes, then a lethal impact was nearly inevitable.