Voyager is either of two United States space probes launched in 1977 to Jupiter and beyond. The two crafts continue to provide valuable information.
Information gathered by the Voyager probes forms the basis of the modern study of Jupiter, Saturn, Uranus, and their satellites, rings, and magnetic fields (regions where magnetic force can be detected). The probes discovered nearly two dozen natural satellites. They also found evidence of geologic activity on two previously known moons—volcanoes on Jupiter’s moon Io and icy geysers on Neptune’s moon Triton. The mission also discovered numerous craters on most of the satellites, an ancient record of intense bombardment by meteoroids and comets. Scientists used Voyager data to calculate the density of 17 satellites and to determine the composition of the atmosphere of Saturn’s moon Titan.
The National Aeronautics and Space Administration (NASA) launched Voyager 1 on Sept. 5, 1977. The probe made its closest approach to Jupiter on March 5, 1979, encountered Saturn on Nov. 12, 1980, then headed toward interstellar space (the space between the stars). Voyager 2, launched on Aug. 20, 1977, made its closest approach to Jupiter on July 9, 1979, Saturn on Aug. 26, 1981, Uranus on Jan. 24, 1986, and Neptune on Aug. 25, 1989, then traveled toward interstellar space.
The Voyagers carried identical sets of scientific instruments. One instrument measured the strength, shape, and direction of the planets’ magnetic fields. Another studied waves traveling through plasma trapped within the fields. Plasma consists of electrically charged atoms as well as electrons that are not parts of atoms.
Three devices measured the quantities and speeds of these charged particles. Five instruments measured ultraviolet rays, visible light, infrared rays, and radio waves given off by the planets and their satellites, rings, and plasma. Also, as the two crafts moved behind each planet, the planet’s atmosphere and rings blocked the radio signals transmitted by the Voyagers in ways that revealed details of their structure.
The Voyagers’ radio receivers and their particle and magnetism detectors were still operating in the early 2000’s. Scientists monitored their data in hope of detecting the heliopause, where interstellar space begins. In late 2004, Voyager 1 crossed a shock wave called the termination shock, becoming the first craft to reach the region of space that lies just inside the heliopause. The crossing occurred at a distance of about 8.7 billion miles (14 billion kilometers) from the sun. In 2007, Voyager 2 crossed the shock in a different area and at a distance of about 7.8 billion miles (12.6 billion kilometers) from the sun. By detecting the shock at different distances from the sun, the two craft confirmed scientists’ belief that the solar system is not perfectly round. In 2012, Voyager 1 became the first human-made craft to fly beyond the heliopause and enter interstellar space. Voyager 2 reached interstellar space in late 2018. 
See also Porco, Carolyn C.; Space exploration (Exploring interstellar space).
