Venturing to the Outer Solar System: Pioneer 10 and 11 and the Technology of Long Duration Space Exploration

The Pioneer 10 and 11 space probes were the first missions to investigate Jupiter and Saturn. Pioneer 11, like Pioneer 10, used Jupiter’s gravitational field to alter its trajectory radically. It passed close to Saturn and then it followed an escape trajectory from the solar system.

As the first attempt to send robotic probes to any part of the outer solar system, in 1964 NASA scientists first conceived of what became Pioneer 10 and 11, missions that undertook a “windshield tour” of Jupiter and Saturn as they headed out toward the interstellar medium. Although severe budgetary constraints prevented starting the project until the fall of 1968 and forced a somewhat less ambitious effort, Pioneer 10 was launched on March 3, 1972. It arrived at Jupiter on the night of  December 3, 1973, and although many were concerned that the spacecraft might be damaged by intense radiation discovered in Jupiter’s orbital plane, the spacecraft survived, transmitted data about the planet, and continued on its way out of the solar system, away from the center of the Milky Way galaxy.

In 1973 NASA launched Pioneer 11, providing scientists with their first close-up view of Jupiter. The close approach and the spacecraft’s speed of 107,373 mph, by far the fastest ever reached by an object launched from Earth, hurled Pioneer 11 1.5 billion miles across the solar system toward Saturn, encountering the planet’s south pole within 26,600 miles of its cloud tops in December 1974. By 1990 Pioneer 11 had officially departed the solar system, and it ended its mission on September 30, 1995, when the last transmission from the spacecraft was received.

Earth received Pioneer 10’s last, very weak signal on January 22, 2003. The last time a Pioneer 10 contact actually returned telemetry data was April 27, 2002. At last contact, Pioneer 10 was 7.6 billion miles from Earth, or eighty-two times the nominal distance between the Sun and the Earth. At that distance, it takes more than eleven hours and twenty minutes for the radio signal, traveling at the speed of light, to reach the Earth. It will continue to coast silently as a ghost ship into interstellar space, heading generally for the red star Aldebaran, which forms the eye of the constellation Taurus (The Bull). Aldebaran is about sixty-eight light years away. It will take Pioneer 10 more than two million years to reach it. “From Ames Research Center and the Pioneer Project, we send our thanks to the many people at the Deep Space Network (DSN) and the Jet Propulsion Laboratory (JPL), who made it possible to hear the spacecraft signal for this long,” said Pioneer 10 Flight Director David Lozier at the time of the last contact.

Both Pioneer 10 and 11 were remarkable space probes, stretching from a 30-month design life cycle into a mission of more than 20 years and returning useful data not just about the Jovian planets of the solar system but also about some of the mysteries of the interstellar universe. In spite of this, these missions have been treated in the historical literature mostly as precursors for the much more famous Voyager 1 and 2 missions launched later in the 1970s.

Even so, the these two probes “pioneered”—double entendre intended—much of the technology that made outer solar system exploration possible. This included long-lived space systems, radiation hardened electronics, more robust and capable propulsion systems, more effective data transmission and reception communication technologies, and operational techniques that could be sustained over an indefinite period of time. Most important, they employed radioisotope power systems (RPS) that provided electrical power to the spacecraft for years far beyond the effective range possible for the use of solar energy systems. This allowed a mission duration of years rather than a few months.

While every study of solar system exploration mentions Pioneer 10 and 11, little is known about them, their origins, their evolution, and the results of their missions. Only one book of substance relates solely to this effort, Mark Wolverton’s The Depths of Space: The Pioneers Planetary Probes (Joseph Henry Press, 2004), but there is more to work needed. What is to be Done?