Voyager Probes Launched

In 1977, NASA launched Voyager 2 and Voyager 1, two robotic spacecraft built to explore the outer solar system. Voyager 2 left Cape Canaveral, Florida, on August 20, and Voyager 1 followed on September 5, on a faster path. Together they became the first spacecraft to visit Uranus and Neptune, returned detailed images of Jupiter and Saturn, and went on to become the first human-made objects to enter interstellar space.

The mission grew from a rare alignment of the outer planets. In the late 1970s Jupiter, Saturn, Uranus and Neptune were positioned so that a spacecraft could use each planet's gravity to bend its path and gain speed on the way to the next, a technique called gravity assist. Mission planners, building on calculations from the 1960s, recognized that such an arrangement would not recur for roughly 175 years. An earlier, more ambitious plan for a "Grand Tour" was scaled back for budget reasons, and the Voyagers were designed principally to study Jupiter and Saturn, though engineers hoped Voyager 2 could continue onward.

Both probes were built at NASA's Jet Propulsion Laboratory and powered by radioisotope thermoelectric generators rather than solar panels, since sunlight is faint in the outer solar system. Each carried cameras, magnetic field instruments, and sensors for charged particles and plasma. Voyager 1 passed Jupiter in 1979 and Saturn in 1980, revealing active volcanoes on the moon Io and a dense atmosphere around Titan. Voyager 2 flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989, and remains the only spacecraft to have visited the two ice giants. Among its findings were new moons and rings at Uranus and geysers on Neptune's moon Triton.

Each spacecraft also carries a Golden Record, a gold-plated copper disc holding sounds and images of Earth, including greetings in many languages, music from various cultures, and natural sounds. A committee led by astronomer Carl Sagan assembled the contents. The record is less a serious attempt at contact than a time capsule, since the chance of anyone finding it is extremely small and it would take tens of thousands of years to approach another star.

After their planetary encounters, the probes kept traveling. Voyager 1 crossed the heliopause, the edge of the Sun's influence, in 2012, and Voyager 2 did so in 2018, based on measurements of the surrounding plasma. Their instruments have gradually been shut down to conserve dwindling power, but they still transmit data from distances of many billions of miles. The Voyagers reshaped scientific understanding of the giant planets and their moons, and they remain among the most enduring achievements of space exploration.