Jupiter is a difficult world to explore. It lies hundreds of millions of miles from Earth, possesses an enormous magnetic field and is surrounded by intense radiation belts that can damage spacecraft electronics. Yet missions to the Jupiter system have produced some of the most important discoveries in planetary science.
Since Pioneer 10 made the first close encounter in 1973, spacecraft have revealed Jupiter as a turbulent gas giant surrounded by rings, powerful radiation belts and a remarkable family of moons. Some of those moons contain active volcanoes or hidden oceans, making the Jupiter system one of the most compelling destinations in the solar system.
1. Pioneer 10 Made the First Close Encounter With Jupiter
NASA’s Pioneer 10 became the first spacecraft to visit Jupiter when it flew past the planet in December 1973.
The mission entered largely unexplored territory. Scientists did not yet know whether a spacecraft could safely cross the asteroid belt or survive the intense radiation surrounding Jupiter. Pioneer 10 successfully accomplished both.
At closest approach it passed about 130,000 kilometers (81,000 miles) above Jupiter’s cloud tops. Its instruments studied the planet’s atmosphere, magnetic field and radiation environment while returning the first close-up spacecraft observations of the giant planet.
Pioneer 10 also provided early spacecraft observations of several of Jupiter’s large moons and demonstrated that missions into the outer solar system were possible.
2. Pioneer 11 Revealed Just How Dangerous Jupiter’s Radiation Could Be
Pioneer 11 followed in December 1974, passing substantially closer to Jupiter than its predecessor.
One of the most important lessons from the Pioneer missions concerned Jupiter’s enormous radiation belts. The charged-particle environment proved more intense than mission designers had expected and affected spacecraft instruments.
Those measurements became extremely valuable when engineers designed later spacecraft. Any mission spending significant time near Jupiter would need to protect sensitive electronics against radiation far more powerful than that encountered around Earth.
Pioneer 11 survived its Jupiter encounter and used the planet’s gravity to redirect itself toward Saturn, which it reached in 1979.
3. Voyager 1 and 2 Transformed Our View of the Jupiter System
Voyager 1 reached Jupiter in March 1979, followed by Voyager 2 in July. Their cameras provided vastly more detailed views of the planet and its major moons than had previously been possible.
The spacecraft studied Jupiter’s Great Red Spot and other atmospheric systems, investigated its magnetic environment and discovered that the planet is surrounded by a faint ring system.
They also discovered previously unknown small moons and transformed scientists’ understanding of the four large Galilean moons: Io, Europa, Ganymede and Callisto.
The Voyager encounters demonstrated that the Jupiter system was not simply a planet accompanied by inert satellites. It was a complex miniature planetary system containing worlds with radically different geology.
4. Voyager Discovered Active Volcanoes on Io
One of the greatest surprises came when Voyager 1 images revealed enormous plumes rising above Io.
The plumes were volcanic eruptions. It was the first time active volcanoes had been discovered on another world.
Io’s extraordinary activity is powered primarily by tidal heating. The moon is continually flexed by Jupiter’s immense gravity and by gravitational interactions with neighboring moons. Friction generated inside Io produces enough heat to drive extensive volcanism.
Voyager also revealed intriguing terrain on Europa. Its bright icy surface contained long fractures and relatively few large impact craters, providing early clues that something unusual was happening beneath the moon’s surface.
5. Jupiter’s Gravity Sent Ulysses Over the Sun’s Poles
Not every spacecraft visiting Jupiter was primarily a Jupiter mission.
The joint ESA-NASA Ulysses spacecraft was designed to investigate the Sun, but reaching the required orbit presented a problem. From Earth, it was extremely difficult to send a spacecraft directly into an orbit passing over the Sun’s poles.
Instead, Ulysses traveled outward to Jupiter and flew past the planet in February 1992. Jupiter’s gravity dramatically changed the inclination of the spacecraft’s orbit, sending it far above and below the plane in which the planets travel around the Sun.
During the encounter, Ulysses also collected valuable measurements of Jupiter’s enormous magnetosphere.
6. Galileo Became the First Spacecraft to Orbit Jupiter
NASA’s Galileo arrived at Jupiter in December 1995 and became the first spacecraft to enter orbit around the giant planet.
Unlike the brief Pioneer and Voyager flybys, Galileo could investigate the Jupiter system repeatedly. During almost eight years in orbit it made numerous close encounters with Io, Europa, Ganymede and Callisto.
Among its most important discoveries was strong evidence that Europa contains a salty liquid-water ocean beneath its icy surface. Galileo also found evidence of an internal ocean at Ganymede and discovered that Ganymede possesses its own magnetic field.
The possibility of large subsurface oceans fundamentally changed the scientific importance of Jupiter’s icy moons. Europa in particular became one of the leading places to investigate potentially habitable environments beyond Earth.
7. Galileo Sent a Probe Directly Into Jupiter’s Atmosphere
Galileo did not travel to Jupiter alone. Before the main spacecraft entered orbit, it released an atmospheric probe on a collision course with the planet.
On December 7, 1995, the probe plunged into Jupiter’s atmosphere at extremely high speed. After surviving the initial entry, it deployed a parachute and transmitted measurements back to the Galileo orbiter for about 58 minutes.
The probe directly measured temperature, pressure, winds and atmospheric composition while descending about 150 kilometers (95 miles) into Jupiter’s atmosphere.
It remains the first spacecraft to have directly sampled the atmosphere of one of the outer giant planets.
8. Cassini Produced a Remarkable Portrait of Jupiter
The Cassini spacecraft encountered Jupiter in late 2000 while traveling toward its primary destination, Saturn.
Jupiter provided Cassini with a gravity assist, but the encounter also became a major scientific observing campaign. Cassini spent months studying Jupiter and returned thousands of images of the planet and its surroundings.
Its observations allowed scientists to study the structure and movement of Jupiter’s atmosphere in exceptional detail. Cassini and Galileo were also able to observe the Jupiter system at the same time from different locations, providing an unusual opportunity to combine measurements from two spacecraft.
After leaving Jupiter, Cassini continued toward Saturn, where it entered orbit in 2004.
9. New Horizons and Juno Continued the Exploration of Jupiter
New Horizons encountered Jupiter in 2007 while traveling toward Pluto. The giant planet’s gravity increased the spacecraft’s speed and shortened its journey into the outer solar system.
The encounter was also an important rehearsal for Pluto. New Horizons tested its instruments and observing procedures while studying Jupiter’s atmosphere, rings, magnetosphere and moons. Its observations included spectacular views of enormous volcanic plumes erupting from Io.
NASA’s Juno spacecraft entered a highly elongated polar orbit around Jupiter on July 4, 2016. Its arrival marked the beginning of a very different investigation, focused on what lies beneath Jupiter’s visible clouds as well as the planet’s unusual polar regions.
Juno’s orbit was designed to take it close to the cloud tops for observations before carrying it far away from the most dangerous radiation.
Juno revealed Jupiter’s poles in unprecedented detail, including enormous cyclones arranged in striking patterns. Measurements of the planet’s gravity and magnetic fields also allowed scientists to investigate structures hidden far below the visible clouds.
Its extended mission broadened the investigation to Jupiter’s moons and rings, including close encounters with Ganymede, Europa and Io.
10. Europa Clipper and Juice Are Already on Their Way to Jupiter
The next era of Jupiter exploration has already begun, with two major spacecraft traveling toward the giant planet.
NASA’s Europa Clipper launched on October 14, 2024. Its primary target is Europa, where evidence points to a global saltwater ocean beneath the moon’s icy crust.
Europa Clipper is designed to investigate the thickness and structure of the ice, the composition of the surface and the nature of the hidden ocean. Rather than orbiting Europa directly inside Jupiter’s intense radiation environment, the spacecraft will orbit Jupiter and make dozens of close flybys of the moon.
ESA’s Jupiter Icy Moons Explorer (Juice) launched in April 2023. It is also traveling toward the Jupiter system, using a series of planetary gravity assists to reshape its orbit along the way.
Juice will study Jupiter as well as Europa, Callisto and Ganymede, three moons believed to contain subsurface oceans. It is scheduled to reach Jupiter in July 2031 and will eventually enter orbit around Ganymede, becoming the first spacecraft to orbit a moon other than Earth’s Moon.
Together, Europa Clipper and Juice will shift the focus of Jupiter exploration increasingly toward its icy moons and one of the biggest questions in planetary science: whether environments capable of supporting life exist in the dark oceans beneath their frozen surfaces.