The exploration of Jupiter has been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, , has continued with eight further spacecraft missions in the vicinity of Jupiter and two more en route. All but one of these missions were undertaken by the National Aeronautics and Space Administration (NASA), and all but four were flybys taking detailed observations without landing or entering orbit. These probes make Jupiter the most visited of the Solar System's outer planets as all missions to the outer Solar System have used Jupiter flybys. On July 5, 2016, spacecraft Juno arrived and entered the planet's orbit—the second craft ever to do so. Sending a craft to Jupiter is difficult due to large fuel requirements and the effects of the planet's harsh radiation environment.
The first spacecraft to visit Jupiter was Pioneer 10 in 1973, followed a year later by Pioneer 11. Aside from taking the first close-up pictures of the planet, the probes discovered its magnetosphere and its largely fluid interior. The Voyager 1 and Voyager 2 probes visited the planet in 1979, and studied its moons and the ring system, discovering the volcanic activity of Io and the presence of water ice on the surface of Europa. Ulysses, intended to observe the Sun's poles, further studied Jupiter's magnetosphere in 1992 and then again in 2004. The Saturn-bound Cassini probe approached the planet in 2000 and took very detailed images of its atmosphere. The Pluto-bound New Horizons spacecraft passed by Jupiter in 2007 and made improved measurements of its and its satellites' parameters.
The Galileo spacecraft was the first to have entered orbit around Jupiter, arriving in 1995 and studying the planet until 2003. During this period Galileo gathered a large amount of information about the Jovian system, making close approaches to all of the four large Galilean moons and finding evidence for thin atmospheres on three of them, as well as the possibility of liquid water beneath their surfaces. It also discovered a magnetic field around Ganymede. As it approached Jupiter, it also witnessed the impact of Comet Shoemaker–Levy 9. In December 1995, it sent an atmospheric probe into the Jovian atmosphere, so far the only craft to do so.
In July 2016, the Juno spacecraft, launched in 2011, completed its orbital insertion maneuver successfully, and is in orbit around Jupiter with its science programme ongoing, with goals to study its magnetosphere and atmosphere in depth.
The European Space Agency selected the L1-class JUICE orbiter mission in 2012 as part of its Cosmic Vision programme to explore three of Jupiter's Galilean moons, with a possible Ganymede lander provided by Roscosmos. JUICE was launched on April 14, 2023. The Russian lander did not materialize in the end.
NASA successfully launched another orbiter spacecraft, Europa Clipper, to study the moon Europa on October 14, 2024.
The Chinese National Space Administration planned to launch two Interstellar Express missions in 2024 on a flyby of Jupiter and Tianwen-4 around 2029 to explore the planet and Callisto.
Technical requirements
Flights from Earth to other planets in the Solar System have a high energy cost. It requires almost the same amount of energy for a spacecraft to reach Jupiter from Earth's orbit as it does to lift it into orbit in the first place. In astrodynamics, this energy expenditure is defined by the net change in the spacecraft's velocity, or delta-v. The energy needed to reach Jupiter from an Earth orbit requires a delta-v of about 9 km/s, compared to the 9.0–9.5 km/s to reach a low Earth orbit from the ground. Gravity assists through planetary flybys (such as by Earth or Venus) can be used to reduce the energetic requirement (i.e. the fuel) at launch, at the cost of a significantly longer flight duration to reach a target such as Jupiter when compared to the direct trajectory.Fischer, 1999, p. 44 Ion thrusters capable of a delta-v of more than 10 km/s were used on the Dawn spacecraft. This is more than enough delta-v to do a Jupiter fly-by mission from a solar orbit of the same radius as that of Earth without gravity assist.CRC Handbook of Chemistry and Physics, 64th EDITION, (C) 1983, page F-141
Jupiter has no solid surface on which to land, as there is a smooth transition between the planet's atmosphere and its fluid interior. Any probes descending into the atmosphere are eventually crushed by the immense pressures within Jupiter.
A major issue with sending probes to Jupiter is the amount of radiation to which a space probe is subjected, due to the harsh charged-particle environment around Jupiter (for a detailed explanation see Magnetosphere of Jupiter). For example, when Pioneer 11 made its closest approach to the planet, the level of radiation was ten times more powerful than Pioneers designers had predicted, leading to fears that the probes would not survive. With a few minor glitches, the probe managed to pass through the radiation belts, but it lost most of the images of the moon Io, as the radiation had caused Pioneer's imaging photo polarimeter to receive false commands. The subsequent and far more technologically advanced Voyager spacecraft had to be redesigned to cope with the radiation levels. Over the eight years the Galileo spacecraft orbited the planet, the probe's radiation dose far exceeded its design specifications, and its systems failed on several occasions. The spacecraft's gyroscopes often exhibited increased errors, and electrical arcs sometimes occurred between its rotating and non-rotating parts, causing it to enter safe mode, which led to total loss of the data from the 16th, 18th and 33rd orbits. The radiation also caused phase shifts in Galileo's ultra-stable quartz oscillator.
Flyby missions
Pioneer program (1973 and 1974)
The first spacecraft to explore Jupiter was Pioneer 10, which flew past the planet in December 1973, followed by Pioneer 11 twelve months later. Pioneer 10 obtained the first close-up images of Jupiter and its Galilean moons; the spacecraft studied the planet's atmosphere, detected its magnetic field, observed its radiation belts and determined that Jupiter is mainly fluid. Pioneer 11 made its closest approach, within some 43,000 km of Jupiter's cloud tops, on December 3, 1974. It obtained dramatic images of the Great Red Spot, made the first observation of Jupiter's immense polar regions, and determined the mass of Jupiter's moon Callisto. The information gathered by these two spacecraft helped astronomers and engineers improve the design of future probes to cope more effectively with the environment around the giant planet.
Voyager program (1979)
Voyager 1 began photographing Jupiter in January 1979 and made its closest approach on March 5, 1979, at a distance of 349,000 km from Jupiter's center. This close approach allowed for greater image resolution, though the flyby's short duration meant that most observations of Jupiter's moons, rings, magnetic field, and radiation environment were made in the 48-hour period bracketing the approach, even though Voyager 1 continued photographing the planet until April. It was soon followed by Voyager 2, which made its closest approach on July 9, 1979, 576,000 km away from the planet's cloud tops. The probes discovered Jupiter's rings, observed intricate vortices in its atmosphere, observed active volcanoes on Io, a process analogous to plate tectonics on Ganymede, and numerous craters on Callisto.
The Voyager missions vastly improved our understanding of the Galilean moons, and also discovered Jupiter's rings. They also took the first close-up images of the planet's atmosphere, revealing the Great Red Spot as a complex storm moving in a counter-clockwise direction. Other smaller storms and eddies were found throughout the banded clouds (see animation on the right). Two new, small satellites, Adrastea and Metis, were discovered orbiting just outside the ring, making them the first of Jupiter's moons to be identified by a spacecraft.(discovery) A third new satellite, Thebe, was discovered between the orbits of Amalthea and Io.
The discovery of volcanic activity on the moon Io was the greatest unexpected finding of the mission, as it was the first time an active volcano was observed on a celestial body other than Earth. Together, the Voyagers recorded the eruption of nine volcanoes on Io, as well as evidence for other eruptions occurring between the Voyager encounters.
Europa displayed a large number of intersecting linear features in the low-resolution photos from Voyager 1. At first, scientists believed the features might be deep cracks, caused by crustal rifting or tectonic processes. The high-resolution photos from Voyager 2, taken closer to Jupiter, left scientists puzzled as the features in these photos were almost entirely lacking in topographic relief. This led many to suggest that these cracks might be similar to ice floes on Earth, and that Europa might have a liquid water interior. Europa may be internally active due to tidal heating at a level about one-tenth that of Io, and as a result, the moon is thought to have a thin crust less than thick of water ice, possibly floating on a ocean.
Ulysses (1992, 2004)
On February 8, 1992, the Ulysses solar probe flew past Jupiter's north pole at a distance of 451,000 km. This swing-by maneuver was required for Ulysses to attain a very high-inclination orbit around the Sun, increasing its inclination to the ecliptic to 80.2 degrees. The giant planet's gravity bent the spacecraft's flightpath downward and away from the ecliptic plane, placing it into a final orbit around the Sun's north and south poles. The size and shape of the probe's orbit were adjusted to a much smaller degree, so that its aphelion remained at approximately 5 AU (Jupiter's distance from the Sun), while its perihelion lay somewhat beyond 1 AU (Earth's distance from the Sun). During its Jupiter encounter, the probe made measurements of the planet's magnetosphere. Since the probe had no cameras, no images were taken. In February 2004, the probe arrived again at the vicinity of Jupiter. This time the distance from the planet was much greater—about 120 million km (0.8 AU)—but it made further observations of Jupiter.
Cassini (2000)
In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever taken of the planet. It made its closest approach on December 30, 2000, and made many scientific measurements. About 26,000 images of Jupiter were taken during the months-long flyby. It produced the most detailed global color portrait of Jupiter yet, in which the smallest visible features are approximately across.
A major finding of the flyby, announced on March 5, 2003, was of Jupiter's atmospheric circulation. Dark belts alternate with light zones in the atmosphere, and the zones, with their pale clouds, had previously been considered by scientists to be areas of upwelling air, partly because on Earth clouds tend to be formed by rising air. Analysis of Cassini imagery showed that the dark belts contain individual storm cells of upwelling bright-white clouds, too small to see from Earth. Anthony Del Genio of NASA's Goddard Institute for Space Studies said that "the belts must be the areas of net-rising atmospheric motion on Jupiter, [so] the net motion in the zones has to be sinking".