Since the first flyby of Jupiter in December 1973, when NASA’s Pioneer 10 spacecraft flew past the gas giant, Jupiter has been the subject of numerous flybys and close encounters by pioneering Earth probes. Pioneer 11 made its closest approach a year after Pioneer 10 in December 1974, and Voyager 1 and Voyager 2 flew past Jupiter in 1979.
But it wasn’t until NASA’s Galileo spacecraft approached the Jupiter system in December 1995 that the gas giant finally got its chance to spend some extended time in the spotlight. Galileo became the first spacecraft to orbit Jupiter, making important discoveries about the gas giant and its moons. The spacecraft spent nearly eight years in Jupiter’s orbit, uncovering its many secrets, from a possible subsurface ocean on Jupiter’s icy moon Europa to volcanoes on Io. At the end of its stay in September 2003, Galileo deliberately crashed into the gas giant to prevent collisions and subsequent contamination with Jupiter’s moons.
The NASA-ESA Ulysses mission used Jupiter for several gravity assists in 1992 and 2004 on its way to the Sun. NASA’s Cassini-Huygens spacecraft also made a brief visit to Jupiter on its way to Saturn. Cassini made its closest approach to Jupiter in December 2000 and took about 26,000 images of the gas giant. Then, in February 2007, NASA’s New Horizons spacecraft visited Jupiter on its way to Pluto and captured spectacular nighttime auroras and lightning near the planet’s poles.
The Jupiter system is currently being explored by NASA’s Juno mission, which arrived in orbit in July 2016. Juno is only the second spacecraft to orbit Jupiter and is studying its composition, atmosphere, gravity, and magnetic field. The mission’s goal is to learn more about the origin and evolution of Jupiter and the solar system.
The European Space Agency’s Jupiter icy moons explorer JUICE successfully launched on April 14, 2023, at 8:14 a.m. ET (12:14 GMT) from Europe’s Kourou Spaceport in French Guiana. But it will take nearly eight years to reach the solar system’s largest planet. The mission will explore Jupiter and its three Galilean moons: Ganymede, Callisto, and Europa.
NASA’s Europa Clipper mission launched in October 2024. The mission will focus on Jupiter’s icy moon Europa and will investigate whether the unusual world has habitable conditions.
Here we will look at all the missions that have visited Jupiter’s vicinity.
Jupiter has been visited by numerous landmark missions, with more on the horizon. vjanez via Getty Images
Pioneer 10
NASA’s Pioneer 10 was the first spacecraft to reach Jupiter. The probe was launched on March 2, 1972, with an ambitious mission to study the asteroid belt, Jupiter’s atmosphere, and the outer solar system. About a year and a half after launch, Pioneer 10 reached the vicinity of Jupiter.
The probe flew within 200,000 kilometers of the gas giant’s turbulent atmosphere and measured Jupiter’s magnetosphere, radiation belts, magnetic field, and interior. Pioneer 10’s measurements of intense radiation in Jupiter’s environment were used to design the Voyager and Galileo spacecraft, which NASA said would study the gas giant in even greater detail.
Pioneer 10 during final assembly. TRW/NASA
Pioneer-11
A sister ship to its predecessor, Pioneer 11 was launched on April 6, 1973. The spacecraft became the second to fly through the asteroid belt, passing Jupiter in December 1974.
Pioneer 11 took many detailed images of Jupiter’s Great Red Spot and the planet’s poles, and measured the mass of its moon Callisto. In July 1979, Pioneer 11 became the first spacecraft to approach Saturn.
Voyager 1
Voyager 1 was launched on September 5, 1977, and flew past Jupiter in March 1979. The spacecraft took more than 18,000 images of the gas giant and its moons.
The first images of Jupiter obtained by Voyager 1 were transmitted to Earth in April 1978, when the probe was 266 million kilometers from Earth. To NASA’s surprise, Voyager 1 discovered a thin ring around the giant planet in March 1979. It also discovered two new moons, Thebe and Metis. Voyager 1 also transmitted detailed images of Jupiter’s large Galilean moons (Io, Europa, Ganymede, and Callisto), as well as Amalthea.
Among Voyager 1’s discoveries was the presence of volcanic activity on Io, showing that, like planets, moons may have active interiors.
In 2012, Voyager 1 entered interstellar space, becoming the first man-made object to do so. The spacecraft is currently about 23.5 billion kilometers (14.6 billion miles) from our planet, making it the farthest spacecraft from Earth. According to NASA’s Jet Propulsion Laboratory, Voyager 1 is currently moving through space at about 17 kilometers per second (38,000 miles per hour).
A photograph of Jupiter taken by Voyager 1. Jupiter’s moon Io passes in front of a gas cloud. The photograph was taken on January 17, 1979, from a distance of 47 million kilometers (29 million miles). NASA
Voyager 2
Voyager 2 was launched on August 20, 1977, about two weeks before its twin, Voyager 1. However, the probe reached the vicinity of Jupiter later than its twin, arriving there in April 1979.
The arrival of Voyager 2 allowed NASA to take valuable comparative images of Jupiter and its moons. The spacecraft captured changes in the Great Red Spot and also looked at the surfaces of some moons in greater detail.
Voyager 2 photographed many of Jupiter’s moons. Among its most impressive finds were images of the icy moon Europa. The probe took detailed images of cracks in the icy moon from a distance of 205,996 km and found no changes in the altitude of the moon’s surface. Voyager 2 also photographed Adrastea, an exceptionally small moon with a diameter of about 30.5 km at its closest approximation.
In 2018, Voyager 2 followed its sibling into interstellar space.
Galileo
The Galileo spacecraft was the first mission to orbit Jupiter. It launched on October 18, 1989, and entered orbit around the gas giant in December 1995. While in orbit, Galileo dropped an atmospheric probe that measured the temperature, wind speed, and pressure in Jupiter’s clouds as it descended.
The Galileo mission was expanded to study Jupiter’s moons: Io, Europa, Ganymede, and Callisto. The spacecraft revealed many secrets about these moons, including the presence of a salty ocean beneath Europa’s surface, and an iron core and magnetic field on Ganymede.
With Galileo running low on fuel, NASA deliberately sent the spacecraft on a suicide mission to Jupiter on September 21, 2003. The agency said the sacrifice was necessary to protect Europa, whose subsurface ocean could harbor life.
The Galileo spacecraft became the first mission to orbit Jupiter. NASA
Ulysses
The Ulysses spacecraft was launched on October 6, 1990, as part of a joint NASA-ESA project to study the Sun in more detail. The probe flew past Jupiter in February 1992, using a gravity assist to enter a polar orbit around the Sun. During its flyby of Jupiter, Ulysses collected useful data on the planet’s intense magnetic field and radiation environment.
Cassini-Huygens
The Cassini–Huygens probe (a mission of NASA, ESA and the Italian Space Agency) is best known for its exploration of Saturn, which the spacecraft entered orbit in 2004. However, on its way to the ringed planet, Cassini flew past Jupiter in December 2000, three years after its launch in October 1997.
The probe’s high-resolution cameras captured 26,000 stunning images of Jupiter’s atmosphere during its months-long flyby. The images have helped scientists revise their understanding of the red-and-white bands of gas around the planet.
The Huygens probe landed on the surface of Saturn’s moon Titan in 2005, and the Cassini probe ended its mission by diving into Saturn’s atmosphere on September 15, 2017, yielding valuable scientific data.
The Cassini-Huygens spacecraft entered orbit around Saturn in 2004. NASA/JPL
New Horizons
The New Horizons spacecraft was launched on January 19, 2006, and became the first spacecraft to visit the dwarf planet Pluto in July 2015. New Horizons flew past Jupiter from January to May 2007.
Visiting Jupiter was an integral part of the New Horizons mission, as the giant planet’s gravity provided a unique opportunity to launch the craft to Pluto. During its five-month flyby, New Horizons refined calculations of the orbits of Jupiter’s inner moons and took the first space photographs of the planet’s Little Red Spot.
Among the first images New Horizons took were of Io, Jupiter’s volcanic moon. The spacecraft captured the sharpest images ever taken of Io’s Tvashtar volcano, capturing giant volcanic sediments.
Juno
The Juno mission is only the second spacecraft designed to study the solar system’s largest planet. The probe was launched on August 5, 2011, and reached Jupiter on July 4, 2016.
Currently in its extended mission, Juno is the most distant planetary orbiter ever. Juno is studying the planet in detail to give scientists a better understanding of the gas giant’s weather, magnetic environment, and formation history.
Among Juno’s most significant scientific discoveries are the first clear, close-up images of Jupiter’s north pole; the fact that the gas giant’s magnetic field is much stronger than anyone expected; and the discovery that the cyclone at the center of Jupiter’s north pole is surrounded by eight others.
Juno was launched on August 5, 2011, and is currently studying Jupiter. NASA/JPL-Caltech
JUICE
The Jupiter Icy Moons Explorer (JUICE) mission is a European Space Agency (ESA) mission to explore Jupiter and its three icy moons: Europa, Callisto, and Ganymede. During the mission, the spacecraft will orbit Ganymede, the largest moon in the Solar System, and will become the first probe to orbit a moon of a planet other than Earth.
The mission successfully launched from Europe’s Kourou Spaceport in French Guiana on April 14, 2023. It will take JUICE about eight years to reach the gas giant.
JUICE joins NASA’s Europa Clipper, which was launched after JUICE but will be the first to arrive in Jupiter’s vicinity.
About four years after arriving in the Jupiter system, JUICE will enter orbit around Ganymede to conduct an extensive study of the giant moon.
The European Space Agency’s JUICE probe explores Jupiter’s moon Europa. ESA/ATG MediaLab
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Europa Clipper
NASA’s Europa Clipper mission is set to explore Jupiter’s icy moon Europa and determine whether its subsurface ocean could harbor life. The $5 billion spacecraft is set to launch on October 6, 2024.
If all goes according to plan, the probe will reach Jupiter on April 11, 2030, where it will enter a long looping orbit around the gas giant.
Europa Clipper, like NASA’s previous Juno mission, will travel in an elliptical orbit that will spend most of its time away from Jupiter and the planet’s intense radiation. Clipper will approach Europa and then move away, over and over again, making nearly 50 flybys of the oceanic moon. This will allow the spacecraft to fly over a different part of Europa each time, allowing it to survey its entire surface.
On January 7, 1610, Italian astronomer Galileo Galilei was peering through his newly improved 20x homemade telescope at the planet Jupiter. He noticed three other points of light near the planet, initially mistaking them for distant stars. After observing them for several nights, he noticed that they seemed to be moving in the wrong direction relative to the background stars, remaining close to Jupiter but changing their positions relative to each other. Four days later, he observed a fourth point of light near the planet with the same unusual behavior. By January 15, Galileo had correctly concluded that he had discovered four moons orbiting Jupiter, providing compelling evidence for Copernicus’s theory that most celestial bodies do not orbit Earth.
A page from Galileo’s notebook about his observations of Jupiter’s moons. University of Michigan Special Collections Library
In March 1610, Galileo published his discoveries of Jupiter’s moons and other celestial observations in a book called Siderius Nuncius (Starry Messenger). As the discoverer, Galileo had the right to name Jupiter’s moons. He suggested naming them after his Medici patrons, and astronomers called them the Medici stars for much of the 17th century, although in his notes Galileo designated them by the Roman numerals I, II, III, and IV, in order of their distance from Jupiter. Astronomers still refer to the four moons as the Galilean satellites in honor of their discoverer.
In 1614, German astronomer Johannes Kepler proposed naming the moons after mythological figures associated with Jupiter: Io, Europa, Ganymede, and Callisto. However, his idea did not catch on for more than 200 years. Scientists did not discover any new moons of Jupiter until 1892, when American astronomer E.E. Barnard discovered a fifth moon of Jupiter, Amalthea, significantly smaller than the Galilean moons and orbiting closer to the planet than Io. It was the last moon in the solar system to be discovered visually; all subsequent discoveries have been made using photography or digital imaging. To date, astronomers have identified 95 moons of Jupiter.
Painting by Giuseppe Bertini (1858) depicts Galileo demonstrating his telescope to the Doge of Venice. gabrielevanin.it
While each of the Galilean moons has unique features, such as Io’s volcanoes, Callisto’s heavily cratered surface, and Ganymede’s magnetic field, scientists have focused more attention on Europa because of the tantalizing possibility that it might be habitable. In the 1970s, NASA’s Pioneer 10 and 11 and Voyager 1 and 2 spacecraft took increasingly detailed images of the larger moons, including Europa, as they flew past Jupiter. The images showed that Europa has the smoothest surface of any object in the solar system, indicating a relatively young crust, and one of the brightest of all the moons, indicating a highly reflective surface. These features have led scientists to hypothesize that Europa is covered in an icy crust floating on an underground salty ocean. They also suggested that tidal heating caused by Jupiter’s gravity reforms the surface ice layer in melting and freezing cycles.
More detailed observations by NASA’s Galileo spacecraft, which orbited Jupiter from 1995 to 2003 and made 11 close approaches to Europa, have shown that extensive linear structures on its surface may indicate tidal or tectonic activity. Reddish-brown material along cracks and in patches elsewhere on the surface may contain salts and sulfur compounds brought up from beneath the crust and modified by radiation. Observations by the Hubble Space Telescope and reanalysis of Galileo images have revealed possible jets emanating from beneath Europa’s crust, supporting this hypothesis. Although the exact composition of this material is unknown, it could provide clues to whether Europa is habitable.
Galileo’s Two Telescopes. National Geographic
Future robotic explorers of Europa could answer some of the unanswered questions about Jupiter’s unique moon. NASA’s Europa Clipper spacecraft launched in October 2024 on a 5.5-year journey to Jupiter. After arriving in 2030, the spacecraft will orbit the giant planet and make 49 flybys of Europa over the course of its four-year mission. Managed by the Jet Propulsion Laboratory in Pasadena, California, and the Johns Hopkins University Applied Physics Laboratory in Baltimore, Maryland, Europa Clipper will carry nine instruments, including imaging systems and radar, to better understand the structure of the icy crust. Data from Europa Clipper will complement information from the European Space Agency’s Jupiter Icy Moon Explorer (JUICE) spacecraft. Launched in April 2023, JUICE will first orbit Jupiter in 2031 and then Ganymede in 2034. The spacecraft also plans to conduct additional studies of Europa in conjunction with Europa Clipper. Both spacecraft should significantly increase our knowledge of Europa and potentially uncover new secrets.
More detailed views of different parts of Europa’s terrain from Galileo. NASA
How to See Jupiter’s Moons with Your Own Eyes
With the right equipment, you can see Jupiter and its four Galilean moons. Jupiter can reach a magnitude of -2.94. At this brightness, the gas giant can be seen with the naked eye, binoculars, or a small telescope. (On the magnitude scale used by astronomers, smaller numbers correspond to brighter objects. For example, at its brightest, Venus has a magnitude of about -4.6.) To see the Galilean moons, you’ll need good binoculars (at least 10x magnification) or a small telescope.
