Beginning in the mid-1970s, a series of more extended interplanetary missions with expanded sets of scientific instruments built upon the know-how gained from earlier scouting missions to Mercury, Venus and Mars.

Twin Viking orbiters, designed and built by JPL, reached Mars in 1976. They deployed successful landers and returned more than 50,000 photographs that mapped 97 percent of Mars' surface over the following four years in orbit.

Another pair of versatile twins, Voyager 1 and Voyager 2 began a grand tour of the outer solar system in 1977. They visited Jupiter and Saturn, finding surprises such as active volcanoes on Jupiter's moon Io and "spokes" in Saturn's rings. Voyager 2's continuing route skimmed near Uranus and Neptune. The mission is still active with both spacecraft headed for the boundary region where the bubble dominated by our Sun cedes way to interstellar space.

To follow up on some of Voyager's intriguing discoveries, NASA launched Galileo in 1989 to orbit Jupiter, and Cassini in 1997 to orbit Saturn. Galileo will finish up its tour of Jupiter's system in 2003 and Cassini reaches Saturn in 2004.

The Magellan mission used radar to map nearly the entire surface of Venus during more than four years in orbit around that cloud-cloaked planet beginning in 1990.

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