Either of two US space probes. Voyager 1, launched on 5 September 1977, passed the planet Jupiter in March 1979, and reached Saturn in November 1980. Voyager 2 was launched earlier, on 20 August 1977, on a slower trajectory that took it past Jupiter in July 1979, Saturn in August 1981, Uranus in January 1986, and Neptune in August 1989. Like the Pioneer probes, the Voyagers are on their way out of the Solar System.

The Voyagers are currently the only operational probes at or near the edge of interstellar space. Their tasks include helping scientists to locate the heliopause, the boundary at which the influence of the Sun gives way to the forces exerted by other stars. The probes are still sending back measurements from their lowest-power instruments, such as the magnetometers and particle detectors. Both Voyager probes have only 69.63 kilobytes of total memory, but the crafts are programmed to write over old data once it is transmitted back to Earth.

Both Voyagers also carry coded recordings called ‘Sounds of Earth’, intended to enlighten any other civilizations that might find them.

Voyager 2 was not intended to visit Uranus and Neptune, but scientists were able to reprogram its computer to take it past those planets. Voyager 2 passed by Neptune at an altitude of 4 800 km, and its radio signals took 4 hours 6 minutes to reach Earth.

In December 2017, Voyager 1 fired a pair of thrusters for the first time in 37 years, from a distance of over 20 billion kilometres from Earth.

Photo montage of the gas giants from the Voyager 'grand tour'. Bottom to top, with increasing distance from the inner Solar System, are Jupiter, Saturn, Uranus, and Neptune.

Credit: NASA