List of proposed missions to the outer planets

The list of proposed missions to the outer planets is a listing of concept studies for an uncrewed or crewed mission to the gas giants. Proposed missions to gas giants are typically based on engineering and scientific assessments of technological capabilities at the time of study. These proposals are usually associated with high-budget space agencies like NASA. Mission profiles may include strategies such as flybys, landers, or other types of system encounters aimed at exploring a gas giant and its moons. The missions that were launched can be found in the list of missions to the outer planets.

History In February 1969, NASA approved two spacecraft missions under the Pioneer program, managed by Ames Research Center (ARC), to explore Jupiter. In 1970, NASA granted a contract to the TRW Company of Redondo Beach, California, to construct the spacecraft. NASA initially directed the TRW Company to develop the Pioneer spacecraft to ensure that future missions could withstand the intense radiation belts of Jupiter. Each spacecraft was equipped with 11 instruments to conduct close-up studies of Jupiter and interplanetary space during transit. Pioneer 10 was launched in 1972 from Cape Canaveral Launch Complex 36A, with the goal of exploring Jupiter, its moons, magnetic field, and radiation belts. During its closest approach on 3 December, it passed within 82,178 mi (132,253 km) of Jupiter, collecting data on the planet and its moons, ultimately transmitting over 500 images by 2 January 1974. In 1973, Pioneer 11 launched from Cape Canaveral Launch Complex 36B as a backup to the Pioneer 10 spacecraft. By May 1974, the mission trajectory was adjusted to include a gravity assist from Jupiter, redirecting it toward Saturn. On 2 December 1974, Pioneer 11 made a close flyby of Jupiter, passing beneath the planet's southern pole before being propelled toward Saturn through its northern pole. On 1 September 1979, Pioneer 11 passed within 13,000 mi (21,000 km) of Saturn's cloud tops.

In August 1977, Voyager 2 launched from Cape Canaveral Space Force Station to explore Jupiter, Saturn, Uranus, and Neptune during a rare 175-year planetary alignment. The following month, Voyager 1 was launched from the same location. In March 1979, Voyager 1 made its close approach to Jupiter, capturing detailed images of the planet and its moons, with Voyager 2 conducting its flyby four months later. In November 1980, Voyager 1 flew by Saturn, using a gravity assist to explore Titan before traveling north out of the ecliptic plane. Voyager 2 followed with its own Saturn flyby nine months later, in August 1981, using a gravity assist to set a course for Uranus. In January 1986, Voyager 2 became the first spacecraft to visit Uranus, discovering 10 new moons, 2 new rings, and revealing a magnetic field that was both tilted 55 degrees off its axis and off-center from the planet's core. In August 1989, Voyager 2 conducted its final flyby of a planet, passing by Neptune and its moon Triton, revealing its active geology, including geysers of nitrogen gas. The gravity assist from Neptune sent Voyager 2 traveling south of the ecliptic plane. In October 1989, the Galileo spacecraft was launched from Kennedy Space Center's Complex 39B. Designed to study Jupiter, its moons, and its surrounding environment, Galileo was the first spacecraft to orbit an outer planet. The mission also included encounters with the asteroids 951 Gaspra and 243 Ida. On 7 December 1995, the Galileo spacecraft reached Jupiter after gravitational assist flybys of Venus and Earth. After eight years in Jupiter's orbit, Galileo was intentionally destroyed in Jupiter's atmosphere on 21 September 2003, to avoid contaminating potentially habitable moons. The next orbiter to visit Jupiter was NASA's Juno, which arrived on 5 July 2016. In October 1990, the Ulysses spacecraft was launched from Kennedy Space Center's Complex 39B on a mission to study the Sun at all latitudes. To accomplish this, Ulysses needed to achieve an orbital inclination of about 80°, which required a significant change in heliocentric velocity. Since the energy required for this inclination change was beyond the capabilities of any available launch vehicle, mission planners used a gravity assist from Jupiter. In February 1992, Ulysses passed close to Jupiter, utilizing its gravity to alter its trajectory and propel it into a high-inclination orbit around the Sun. This enabled the spacecraft to study the Sun's polar regions. Given its distance from the Sun during the mission, Ulysses could not rely on solar panels for power. Instead, it was equipped with a General Purpose Heat Source Radioisotope Thermoelectric Generator (RTG).

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