Juno

Jupiter, king of the gods in Roman mythology, was also god of the sky and storms; he once concealed himself in a veil of clouds to hide his misbehavior.  

His wife, Juno, queen of the gods, noticed this sudden fog and grew suspicious; she swooped down from the heavens and broke through the haze to investigate. She could see Jupiter’s true nature. 

Like its namesake, NASA’s Juno spacecraft – flying low across the clouds that encircle the planet – peers through storms and cyclones to unveil Jupiter’s secrets. 

The mission’s many discoveries have changed our view of Jupiter’s atmosphere and interior, revolutionizing our understanding of the planet, and of the solar system’s formation.  

Juno's principal goal is to understand the origin and evolution of Jupiter. Underneath its dense cloud cover, Jupiter safeguards secrets to the fundamental processes and conditions that likely governed our solar system as it formed. Our primary example of a giant planet, Jupiter can also help us understand planetary systems being discovered around other stars.  

Theories about solar-system formation all begin with the collapse of a giant cloud of gas and dust, or nebula, the bulk of which went into forming the infant Sun. Like the Sun, Jupiter is mostly hydrogen and helium, so it must have formed early, capturing most of the material left over after our star came to be. How this happened, however, is unclear.  

Unlike Earth, Jupiter's giant mass allowed it to hold onto its original composition; like a time capsule, it provides us with a way to trace our solar system's history. Deciphering the origin and evolution of Jupiter will help us understand the formation of the rest of the solar system, including how Earth came to be, and how it became a haven for life. 

Juno completed its primary mission by mid-2021, offering revelations about Jupiter’s interior structure, atmosphere (including polar cyclones, deep atmosphere, and auroras), and magnetosphere – the vast region around the planet influenced by Jupiter's uniquely intense magnetic field. Now Juno has pivoted to an extended mission, in which NASA’s most distant planetary orbiter expands on the discoveries it already made, while also exploring the full Jovian system. That includes the dust in the planet’s faint rings, and three of Jupiter’s four Galilean Moons – Ganymede, Europa, and Io – so named because in 1610 Italian astronomer Galileo Galilei was the first to identify the quartet of satellites (Callisto is the fourth – farther out than the other three, and not visited by Juno). 

With a distinctive silhouette, like a three-bladed fan spinning through space, the Juno orbiter would fill a basketball court. Its hexagonal main body measures about 11.5 feet high and wide (3.5 meters), and the “blades” – actually huge solar arrays – give it an overall width exceeding 66 feet (20 meters).  

Juno’s rotation keeps it stable in flight – the way a spinning top or gyroscope remains steady – as the spacecraft spins twice per minute for science operations, which it increased to five rotations per minute when it performed main-engine maneuvers. Also, Juno’s instruments are all fixed, so the rotating spacecraft gives each of them regular turns at their targets during close passes over Jupiter, a trip that takes about two hours from pole to pole. 

The instruments Juno carries in its toolbox: 

Juno is the first solar-powered spacecraft designed by NASA to operate at such a great distance from the Sun. Jupiter’s orbit is five times farther from the Sun than Earth’s, so the giant planet receives 4% as much sunlight as Earth does. To generate adequate power, Juno’s solar arrays needed to be quite large; each of its three solar arrays extend about 29.5 feet (9 meters) from the main body. But each expansive array was built as multiple hinged segments, so the panels could fold up before launch, and the spacecraft could fit into the payload fairing of its rocket. 

With Juno pioneering the use of solar power at the outer planets, NASA’s next mission to Jupiter will carry even larger arrays. Europa Clipper, set to launch in October 2024, will be powered by twin solar arrays, each about 46.5 feet (14.2 meters) long and 13.5 feet (4.1 meters) wide. In total, its wingspan extends more than 100 feet (30.5 meters) – greater than the height of a 10-story building. 

For many of Juno’s instruments to do their job, the spacecraft had to orbit closer to Jupiter than any previous mission. But the planet is surrounded by a powerful magnetic field that is 15 times more powerful than Earth's magnetic field, and in some places, more than 50 times more intense. It rotates with Jupiter and sweeps up charged particles, accelerating them to very high energies and creating intense radiation that bombards anything passing near the planet, including spacecraft.  

To avoid the highest levels of radiation in the belts surrounding Jupiter, Juno flies in long, looping orbits that approach the gas giant from the north. Over the north pole it dives between the radiation belts and the planet, at times descending to 2,200 miles (3,500 kilometers) above the cloud tops. That might sound like a great height, but if Jupiter were the size of a basketball, the equivalent distance would be less than one-quarter of an inch (0.6 centimeters). 

Juno traverses the planet and exits over the south pole, then zooms far away – beyond even the orbit of the Jovian moon Callisto – before Jupiter’s gravity pulls the spacecraft back for another pass. 

To shield its delicate instruments from the planet’s merciless radiation belts, Juno carries a first-of-its-kind electronics vault. The titanium enclosure at the center of the spacecraft, about the size of an SUV’s trunk and weighing 400 pounds (180 kilograms), protects Juno’s sensitive electronics from the barrage of high-energy particles pummeling the spacecraft. 

Among the tools Juno is carrying, JunoCam is especially noteworthy. In space exploration, where every ounce of weight, second of observation time, and byte of data transmitted is precious, this visible-light, color camera was included on the spacecraft explicitly to reach out to the public – whom the Juno science team is embracing, even deputizing. 

Early in the mission, they invited amateur astronomers to weigh in, debate, and help plan where JunoCam should focus next. And they’ve encouraged researchers, photographers, artists, and anyone else to take Juno’s raw images to study, process, and play with them.  

Some of the pictures – after citizen scientists adjusted color or contrast – revealed cloud heights and other features that were hard to distinguish in the muted raw images coming straight from the spacecraft. Another contributor stitched together images of individual cyclones near the north pole; the complete picture surprised researchers, showing a pinwheel of complementary storms twirling around a central vortex. Others had more whimsical takes on the JunoCam views, turning Jupiter’s swirls into artwork that echoes Monet or VanGogh, or adding dreamlike figures or stark geometrics. 

Juno began its circuitous five-year journey to Jupiter on Aug. 5, 2011, launching from Cape Canaveral Air Force Station on Florida’s Atlantic coast.  

With the four-ton probe and its wide solar arrays folded inside the nose, an Atlas V rocket thundered aloft at 12:25 p.m. EDT (26:25:00 UT) from Launch Complex 41 – the same pad where the Voyager and New Horizons spacecraft took flight, and where the Viking landers, and Curiosity and Perseverance rovers headed to Mars. 

Juno, likewise, set a course away from Earth toward the outer planets. It crossed the orbit of Mars, but didn’t continue onto Jupiter quite yet. Mission designers used the Sun’s gravity and a pair of main engine burns in September 2012 to guide Juno back toward Earth – an essential detour.  

Juno returned home 26 months after it left. But it didn’t stop.  

It dashed toward Earth, pulled in by gravity, then sped away with greater energy than before. In this maneuver, called a gravity assist, Juno borrowed some momentum from Earth – which itself is hurtling through space around the Sun at 18 miles per second (29 kilometers per second). This slingshot effect boosted Juno’s velocity by 16,330 mph (about 7.3 kilometers per second), giving the spacecraft sufficient energy to climb away from the inner solar system and make it out to Jupiter.  

Without the boost from gravity assist, Juno would have required a more powerful launch vehicle, or a more time-consuming voyage, to reach its destination. 

After a voyage of nearly five years and 1.7 billion miles (2.8 billion kilometers), Juno finally reached Jupiter and was scheduled to go into orbit on July 4, 2016. 

Now the Juno team had to hit the brakes, to give Jupiter’s gravity field a chance to ensnare the craft and bring it into orbit. The spacecraft needed to fire its main engine at precisely the right time and place. Too soon, it wouldn’t have enough fuel for a second chance. Too late, and Juno would speed past the planet, ending the science mission before it even began. 

During the Jupiter Orbit-Insertion phase, or JOI, the spacecraft had to complete an intricate series of critical steps on its own – turn away from the Sun, point in the correct direction relative to Jupiter, turn off unnecessary instruments that could interrupt the sequence, increase its rotation from 2 to 5 revolutions per minute (RPM) to help stabilize it, maintain communications with the enormous dish antennas of the Deep Space Network in Australia and California, and finally ignite the crucial 35-minute main-engine burn, to slow down Juno and guide it into orbit.  

Mission operators designed the spacecraft’s configuration and commands so it could complete the entire sequence all on its own, even if some things didn’t go as planned, even if it had to reboot itself and start over. Because of the distance from Earth, messages to and from Juno took 48 minutes to arrive. There was no time for last-minute corrections to speed or attitude.  

A tweet from NASA’s Juno Mission account at 11:54 p.m. EDT relayed the outcome: “Engine burn complete and orbit obtained. I’m ready to unlock all your secrets, #Jupiter. Deal with it.”  

Orbit insertion had gone flawlessly. After a five-year journey across billions of miles, the engine burn completed within one second of its predicted timing. Then Juno got to work.  

From its very first data-collection pass on Aug. 27, 2016, Juno revealed Jupiter as a complex, gigantic, turbulent world – even more than scientists had predicted. Polar cyclones as large as Earth, plunging storm systems that travel deep into the heart of the gas giant, and a mammoth, lumpy magnetic field far more powerful than the strongest magnetic field found on Earth, all tantalized scientists. 

Previous spacecraft visiting Jupiter traveled along a plane more or less even with its equator – the same view of the planet that’s visible from Earth. Juno was the first to fly over the north and south poles, a unique vantage point that accounts for many of the mission’s surprises.  

Scientists discovered that the massive auroras undulating at Jupiter’s poles seem to form by different processes than the northern lights and southern lights on Earth. Investigators also discovered that lightning storms, which on Earth are more common near the equator, are a polar phenomenon on Jupiter, where they drop softball-sized hailstones of ammonia and water. 

A technical concern arose early in the mission, however – it changed Juno’s timeline, and altered what it would accomplish during its time in orbit.  

Originally, the flight plan had Juno looping around Jupiter for a couple of 53-day orbits, then shrinking those orbits to 14 days for the remainder of the mission – spending about 15 months circling and studying the planet. But a problem in the main engine, with some valves not working as expected, troubled the Juno team; a misfire could prove disastrous. They decided to maintain the status quo of the longer orbits. 

Each flyby of Jupiter would give Juno’s instruments the same two-hour window to collect data, but they would come every seven weeks, instead of every two. 

The new plan meant a much longer schedule to complete Juno’s science goals. Then again, the wider orbits would allow for “bonus science” that wasn’t part of the original mission design.  

The spacecraft could further explore the far reaches of space dominated by Jupiter’s magnetic field. And after it completed its initial goals, and NASA extended the mission, the Juno team realized that the long orbits would evolve to eventually put the planet’s Galilean moons within reach. 

On June 7, 2021, during orbit 34, Juno flew within 652 miles of Ganymede, Jupiter’s largest moon (in fact, the largest moon in the solar system). It was the closest such encounter since the Galileo spacecraft flew by Ganymede in May 2000. 

Ganymede’s gravity altered Juno’s orbit, reducing its period from 53 days to 43 days and enabling a rendezvous with Europa in September 2022. That flyby further shrank Juno’s orbital period to 38 days, setting up encounters with Io in December 2023 and February 2024, which pushed Juno into its final orbital schedule, circling Jupiter every 33 days until end of mission. 

When the spacecraft launched, its primary mission was scheduled to end in 2017, but NASA pushed that back to July 2021, to incorporate time needed for the longer orbits. When Juno completed those tasks, the spacecraft remained healthy and continued sending back astonishing data and breathtaking imagery, so NASA extended its stay to 2025. 

The signature bands of multicolored clouds we see encircling Jupiter, called belts (the dark orange stripes) and zones (the light areas, in shades of gray), penetrate deep into the atmosphere. But how far? One of the most fundamental questions about the giant planet, scientists have debated that for decades. By measuring Jupiter’s gravity field, Juno discovered that the belts and zones – which are alternating east- and west-flowing jet streams – extend about 1,900 miles (3,000 kilometers) below the cloud tops.  

Scientists long thought Jupiter had a small, solid core surrounded by all its hydrogen and helium. Juno data suggests the core is much larger than expected, yet partially dissolved, with no clear separation from the hydrogen swirling around it, leading researchers to describe the core as dilute, or “fuzzy.”  

Under extreme pressure, gases can turn into their liquid form. Inside Jupiter the atmospheric pressure is so great, it compresses hydrogen gas not merely into liquid, but a fluid known as liquid metallic hydrogen. This exaggerated pressure makes the liquid hydrogen act like a metal conducting electricity, and scientists believe it’s the source of Jupiter’s uniquely intense magnetic field. Years of Juno measurements of Jupiter’s magnetic field point to that possibility, at a depth much shallower than expected.   

Before Juno arrived, the largest planet in our solar system still had vast areas unexplored, unknown, even unseen. Among other discoveries Juno made possible: 

The Juno team originally expected the craft would make 33 orbits – enough passes to compile a full map of the planet – completing its science objectives by October 2017. They would then steer it into the planet, to burn up in the atmosphere.   

In spite of all the mission team’s precautions while building and launching Juno, microbes from Earth could still have hitched a ride on the spacecraft, lying dormant, awaiting a new home. The self-destruct maneuver would have satisfied NASA’s planetary protection requirements, avoiding any chance Juno could crash into the Jovian moons Europa, Ganymede, or Callisto – ocean worlds, with Europa in particular considered a prime candidate in the search for life beyond Earth – and contaminate them with microscopic life from Earth.  

The Cassini mission, orbiting Saturn from 2004 to 2017, prompted similar concerns – protecting the integrity of Saturn’s moons Enceladus and Titan, also believed to contain possibly habitable environments. As a grand finale, the mission team plunged Cassini through the rings and into Saturn, collecting science until the end. 

The same fate befell NASA’s Galileo orbiter – Juno’s predecessor to Jupiter from 1995-2003, and the mission that discovered the possibility of a subsurface ocean on Europa. That hardy spacecraft endured more than four times its designed radiation exposure, but was running out of fuel when placed on a collision course with Jupiter in September 2003. The possibility of life existing on Europa compelled protection for that icy moon, as well as plans for future investigations.  

With Juno, however, the evolution of its flight path in the extended mission meant that it no longer posed a risk to contaminate these ocean worlds, now beyond its reach. The spacecraft will continue its exploration of Jupiter, its ring system, and moons through September 2025. Then Juno’s orbit will degrade naturally, and Jupiter’s gravity will pull the spacecraft in to be consumed in the atmosphere. 

In its extended mission, NASA’s most distant planetary orbiter continues its investigation of the solar system’s largest planet. And Juno’s work continues building on the discoveries of previous missions that visited Jupiter – Pioneer 10 and 11, and Voyager 1 and 2 in the 1970s, then the Galileo orbiter and its atmospheric probe, as well as those spacecraft who made key observations while en route to other targets (Ulysses, Cassini, and New Horizons). 

In turn, Juno is also preparing the way for future visitors, such as ESA’s Juice mission (JUpiter ICy moons Explorer), launched in April 2023 and scheduled to reach Jupiter in July 2031. Next to go, NASA’s Europa Clipper is scheduled to launch in October 2024, and enter orbit around Jupiter in April 2030. 

For Juno, exploring the full Jovian system, passing through the doughnut-shaped clouds of charged particles surrounding Europa, Io, and the other moons, testing the radiation environment, studying the dust in Jupiter’s faint rings – all this work and more will inform the mission designs, science priorities, observation strategies, and more for Europa Clipper, Juice, and others that will surely follow. 

Images from JunoCam – the dazzling views of Jupiter and its satellites, whether processed by amateur astronomers or artists – have sharpened our perspective of the giant planet, and blurred the divide between art and science. It’s just one way Juno has bridged disparate worlds. 

The mission inspired a partnership with Apple Music, which released a short film ahead of Jupiter Orbit Insertion. “Visions of Harmony: Inspired by NASA’s Mission Juno” featured a soundtrack composed by Academy Award winners Trent Reznor and Atticus Ross. At the same time, Apple released the compilation “Destination: Juno” with music tracks celebrating the mission, from artists including country star Brad Paisley, singer-songwriter Corinne Bailey Rae, rapper GZA of Wu Tang Clan, singer-songwriter QUIÑ, alternative band Weezer, and Mexican rock band Zoé. 

To mark Juno’s 50th orbit around Jupiter on April 8, 2023, the Juno team released a collage of 50 images spanning various aspects of the mission, and NASA paired with Google Arts and Culture to show off a gallery of images and video from the project. 

NASA has released several animations of JunoCam images, replicating the flight of Juno as it collected those shots, set to the music of Greek composer Vangelis (A "Flight" Over Jupiter - Mission Juno). The keyboardist won an Academy Award for his avant-garde “Chariots of Fire” film score in 1981, but the year before that he gained attention when Carl Sagan used his tune “Heaven and Hell” as a theme for “Cosmos: A Personal Voyage.” Vangelis’ 23rd and final studio album, released a few months before he died in 2022, was “Juno to Jupiter.” 

The Juno mission also tasked science educator Bill Nye with bringing technical details of the project down to Earth, in the video series “Why with Nye: The Cool Stuff You Need to Learn About NASA’s Mission to Jupiter.” 

Aboard the spacecraft itself, in addition to scientific instruments, Juno is carrying items of historical and educational significance. The Italian Space Agency provided a plaque depicting astronomer Galileo Galilei and handwritten text copied from his journal. In the January 1610 entry, he described what would later be known as the Galilean moons, Jupiter’s largest satellites – Io, Europa, Ganymede, and Callisto. Using a telescope he fashioned, Galileo spotted three points of light (and later a fourth) that stayed close to Jupiter, but didn’t move like the other stars. “It is evident that around Jupiter there are three moving stars invisible till this time to everyone,” he wrote. After a week of observation, Galileo realized they were moons – the first discovered beyond Earth. 

Also secured to the spacecraft, as part of a joint outreach program between NASA and the LEGO Group, Juno carries a set of three LEGO mini-figurines. Crafted in aluminum and shielded under a thermal blanket, they represent Galileo, who is carrying a telescope, the Roman god Jupiter (wielding a lightning bolt), and the goddess Juno, his wife (holding a magnifying glass, ready to investigate). 

NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the Juno mission for the principal investigator, Scott J. Bolton of Southwest Research Institute in San Antonio. Juno is part of NASA’s New Frontiers Program, which is managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington. Lockheed Martin Space in Denver built and operates the spacecraft.