The Jet Propulsion Laboratory of the California Institute of Technology (Caltech), in Pasadena, California, is the premier U.S. organization for planetary exploration. Its origin, however, was in rocket research led by graduate student Frank Malina in the late 1930s. Successful tests of rockets to help aircraft take off in 1939 led eventually to its formal constitution as an Army facility in June 1944, managed under contract by Caltech and led by Malina.

For its first two decades, JPL's specialty was rockets and missiles. During World War II, JPL developed a variety of unguided rockets, expanding to guided missiles during the 1950s. Its engineers developed two guided missile systems for the Army, Corporal and Sergeant. These were complete, mobile weapon systems, and in the process of developing them, JPL created a new engineering discipline known as systems engineering.

The 1957 launch of Sputnik led to a revolution at JPL. The lab had cooperated with the Army Ballistic Missile Agency to launch a small satellite on one of the Army's Jupiter-C rockets. This became America's first artificial satellite, Explorer 1, on 31 January 1958. The Army transferred JPL's facilities and Caltech contract to NASA in December 1958. This university arrangement, unique among NASA facilities, caused occasional friction among all three organizations, as NASA at times desired more direct control over JPL, and at other times, such as in the 1970s and 1980s, allowed JPL to expand its contracts with other agencies such as the Army and the Department of Energy.

In 1959, NASA assigned JPL the lead role in planetary exploration, along with the Ranger lunar impact probes and the Surveyor landers. While ultimately successful, a series of failures on these two programs led to strengthening of JPL's project management and of NASA oversight. JPL's first planetary success was Mariner 2, which flew by Venus in 1962. Perhaps the two most dramatic missions of this early period were Mariner 4, whose images showed a cratered, Moon-like Mars, and Mariner 9, which showed dramatic images of volcanic cones and massive canyons, with tantalizing evidence that water might have once flowed on the Martian surface but had since disappeared. JPL's last Mariner mission, 1974's Mariner 10, was the only spacecraft, as of 2007, to explore Mercury. Because of its planetary role, JPL also became the builder and manager of the Deep Space Network that communicates with planetary spacecraft.

With the planetary exploration budget shrinking in the 1970s, the lab diversified into Earth science and astronomy. JPL developed specialties in atmospheric chemistry, physical oceanography, geophysics, and infrared astronomy. In addition to its spacecraft-building role, the lab developed expertise in remote-sensing instruments. JPL carried out four highly productive missions during this period. Its twin Voyagers carried out flybys of Jupiter and Saturn, and Voyager 2 explored Uranus and Neptune. JPL built the orbiters for NASA Langley Research Center's two Viking missions to Mars and operated the spacecraft.

New missions essentially ceased until 1989, when JPL sent Galileo and Magellan to Jupiter and Venus, respectively. JPL's diversification continued, as it developed an integrated combat management system for the Army, the All-Source Analysis System.

The arrival of Magellan at Venus in 1990 began JPL's resurgence in planetary exploration. Magellan produced the first complete map of the planet's shrouded surface. JPL followed it with Galileo and Cassini missions to Jupiter and Saturn and a host of missions to Mars, including successes such as Mars Pathfinder, Mars Global Surveyor, and Mars Exploration Rovers, and disappointments such as Mars Observer and Mars Polar Lander, whose failure was attributed to overzealous cutbacks in budgets and testing. JPL sent missions to examine the interior of cometary nuclei and to collect particles of the solar wind and return them to Earth. It built cameras to repair the Hubble Space Telescope and orbited infrared and ultraviolet telescopes. JPL developed important missions to Earth, including the first global ocean circulation measurements and new weather prediction instruments.

See also: Deep Space Probes, Planetary Science, Systems Engineering, United States Army