1973

Pioneer 10 at Jupiter

Up until the 1970s, the robotic exploration of the solar system was confined to the inner solar system, and, even more specifically, to the exploration of the Moon, Venus, and Mars. Planetary scientists realized that gaining a complete understanding of the solar system would eventually require detailed, up-close exploration of the outer solar system. Thus the missions of the Pioneer 10 and 11 were born.

Pioneer 10, launched on March 2, 1972, and its twin spacecraft, Pioneer 11, launched on April 4, 1973, were nuclear-powered probes designed to study the properties of interplanetary space beyond Mars. Their data revealed that dust and micrometeors in the main asteroid belt do not pose much of a threat to future spacecraft. During the Jupiter flyby, Pioneer 10 got to within 125,000 miles (200,000 kilometers) of the cloud tops, and took a series of photographs revealing great detail in the planet’s atmosphere. Pioneer 10 continued its interstellar mission beyond Jupiter for thirty more years; the last signals were received from the probe in 2003.

Pioneer 10, still traveling at more than 7.5 miles (12 kilometers) per second and now more than 120 astronomical units (AU) from the Sun, is one of just five spacecraft that have been accelerated to escape velocity from our solar system. Three others, generally heading in the opposite direction, were also launched in the 1970s. Pioneer 11, last heard from in 1995, is currently at more than 99 AU and heading toward the center of the Milky Way. Voyager 2 is at more than 116 AU and is still sending back occasional fields and particles data from deep space. Voyager 1 holds the record, though, at more than 141 AU and still transmitting scientific data as it speeds outward at more than 10.5 miles per second (17 kilometers per second). The fifth mission to achieve escape velocity from the solar system is the NASA New Horizons probe, launched in 2006; it flew past Pluto in summer 2015 and is now more than 40 AU from the Sun.

Plaque carried by the Pioneer probes, with greetings and information from and about Earth.

Voyager 1 was the first of these robotic emissaries from Earth to actually leave the solar system. The spacecraft crossed the heliopause, the boundary where the Sun’s faint solar wind merges into the general background of interstellar space, in 2012.

Artist’s rendering of the Pioneer 10 spacecraft. The truss pointing down in this view is about 10 feet (3 meters) long.