50 years ago, Pioneer 11 set sail for the outer solar system

Pioneer 11 is just one of five spacecraft currently on a trajectory that will take it out of the solar system. And in some 4 million years, this robotic emissary from Earth, which sent its final transmission nearly 20 years ago, will silently pass near the star Lambda Aquilae, located some 125 light-years away in the constellation Aquila the Eagle.

However, Pioneer 11 was never meant for longevity. This quarter-ton, car-sized probe, brimming with 12 scientific instruments and powered by four plutonium generators, was a reserve player on NASA’s bench with limited candidacy for greatness. That greatness passed instead to its twin, Pioneer 10, launched in March 1972, which scored multiple empirical firsts: the first spacecraft to cross Mars’ orbit, the first to pass through the asteroid belt, and the first to reach the solar system’s largest planet, Jupiter.

In true second-child fashion, poor Pioneer 11 shadowed the lofty accomplishments of Pioneer 10. But Pioneer 11 still carried a trump card of its own.

If Pioneer 10 succeeded in its mission goals, Pioneer 11 would be redirected a half-billion miles deeper into space, making it the first spacecraft to visit the solar system’s second-largest planet, Saturn. Pioneer 11’s exploratory responsibilities there included probing Saturn’s glorious rings, which are comprised of trillions of particles (ranging from dust grains to house-sized boulders) and traverse million-mile-long racetracks around Saturn’s oblate waist.

The many mysteries of Saturn

For centuries, our forebears saw Saturn as a starlike point of light. The Romans named it for their god of harvests, a figure frequently depicted in classical art as elderly — the fabled Bringer of Old Age, as immortalized by Gustav Holst. Those associative aspects tying deity to planet may have arisen from Saturn’s slow apparent progress across the night sky.

Prior to the invention of the telescope, little more could be known. In 1610, Galileo Galilei noted Saturn’s odd oval shape and wondered if a pair of moons or two great ‘arms’ or ‘handles’ might sweep outward from the planet. “I do not know what to say,” Galileo confessed, “in a case so surprising, so unlooked-for and so novel.” Not until 1655 was Christiaan Huygens able to resolve a thin, flat rings encircling Saturn.

Yet the rings’ mystique lingered. Were they solid or liquid, astronomers pondered, and how did they remain stable over time?

In 1856, James Clerk Maxwell mathematically demonstrated that their observed stability was only attainable if they comprised “an indefinite number of unconnected particles.” But more than a century would pass before Pioneer 11 gave humanity its first chance to inspect the rings up close.

Pioneer 11 sets course for Jupiter and Saturn

At 9:11 P.M. EDT on April 5, 1973, with a throaty, guttural roar, an Atlas-Centaur rocket rose from Cape Canaveral’s Launch Complex 36B. Thunderstorms had recently struck Florida, but NASA took full advantage of a break in the weather. Fifteen minutes after launch, Pioneer 11 separated from the rocket’s upper stage before barreling away from Earth at a blistering 32,000 miles per hour (51,500 km/h).

But all was not well. An extendible boom holding one of Pioneer 11’s plutonium generators refused to budge; it needed to be jolted open by repeatedly firing the spacecraft’s thruster. Then Pioneer 11’s dust detector malfunctioned. It was an inauspicious start.

Nonetheless, Pioneer 11 passed the Moon in its first half-day of flight, and it crossed Mars’ orbit 12 weeks later. By April 1974, Pioneer 11 had traversed the 174-million-mile (280 million km) radial extent of the asteroid belt on its way to Jupiter.

But unlike its predecessor, Pioneer 10, which approached Jupiter’s equator, Pioneer 11 executed a south-to-north polar flyby, diving deep into Jupiter’s lethal radiation belts and venturing three times closer to the planet than Pioneer 10.

This was a risky maneuver. But the risk was outweighed by a hefty reward: Jupiter’s gravity accelerated Pioneer 11 to 108,000 mph (174,000 km/h), which helped sling it onward toward Saturn.

Before leaving the Jupiter system, however, Pioneer 11 swept 26,400 miles over the gas giant’s clouds on Dec. 3, 1974. This allowed it to map Jupiter’s polar extremities for the first time. It also observed the famous Great Red Spot, measured the size and shape of Jupiter’s magnetosphere, determined the mass of its four large Galilean moons, and revealed that Jupiter emits 1.9 times as much energy as it receives from sunlight.

Once beyond Jupiter, a pair of course correction maneuvers tweaked Pioneer 11’s trajectory for its looming date with Saturn. And on Aug 31, 1979, the spacecraft detected Saturn’s ‘bow shock’ — the earliest tangible trace of a magnetic field — before passing 13,000 miles (21,000 km) over the ringed planet’s butterscotch-hued clouds the next morning. Pioneer 11 revealed hydrogen to be Saturn’s principal gaseous constituent and measured atmospheric temperatures of –292 degrees Fahrenheit (–180 degrees Celsius).

Pioneer 11 also discovered Saturn’s tenuous ‘F-ring,’ which measures only a few hundred miles in radial extent and resembles a jumble of contorted strands shepherded into place by tiny moonlets. Pioneer 11 even came close to colliding with Epimetheus, missing the small, potato-shaped moon by a mere 2,500 miles (4,000 km). But it did distantly observe Saturn’s large moon Titan, revealing a featureless globe enshrouded by smog-like orange clouds.

Pioneer 11 eventually concluded its survey of Saturn in October 1979.

A decade after leaving the Saturn system, Pioneer 11 crossed the orbit of the distant planet Neptune, which circles the Sun at a distance of about 2.8 billion miles (4.5 billion km).

After its flyby of Neptune, Pioneer 11 continued gathering data as it ventured deeper into the solar system’s unexplored outer wastes. But with energy and power constraints now weighing on the old ship, the end was nigh.

The end is just the beginning

Pioneer 11’s final transmission came on Sept. 30, 1995, though engineering telemetry was intermittently received until November. By the time Pioneer 11 went silent, the probe was 4 billion miles (6.4 billion km) from Earth.

“This is the little spacecraft that could, a venerable explorer that has taught us a great deal about the solar system and, in the end, about our own innate drive to learn,” said then-NASA Administrator Dan Goldin when announcing the mission’s end after 22 years. “Pioneer 11 is what NASA is all about: exploration beyond the frontier.”

Today, for Pioneer 11, that frontier resides an estimated 10.2 billion miles (16.4 billion km) from Earth — or about 110 astronomical units (AU; 1 AU is equal to the average Earth-Sun distance). And every year that passes, the long-dead probe silently crosses another 219 million miles (352 million km) of uncharted space.

Should extraterrestrial intelligence ever stumble upon it, Pioneer 11 carries with it a gold anodized plaque, which is bolted to its antenna support struts to guard against erosion from interstellar dust.

Etched onto that plaque are schematic representations of hydrogen, the universe’s most prolific element, together with the periodicity of 14 pulsars to help pinpoint Pioneer 11’s point of origin and time since launch. The pioneer plaque also depicts the relative distance of the Sun from the galactic center and the general layout of the solar system’s planets.

But pride of place on the plaque goes to us humans: a naked male and female in an anatomical rendition of which Leonardo da Vinci would have been proud. Scaled against a depiction of the probe itself, the man raises his right hand, in part to display the fingers and opposable thumbs that helped build Pioneer 11, but also in greeting — a goodwill gesture, perhaps, from one side of the cosmos to the other.