This illustration shows an imagined view of the JUICE (Jupiter Icy Moons Explorer) spacecraft with the planet Jupiter and its four largest moons. JUICE was developed by the European Space Agency (ESA).
Credit: ESA/ATG medialab/NASA/J. Nichols (University of Leicester)/Ganymede /JPL/University of Arizona/DLR

On Friday, April 14th, 2023, the European Space Agency (ESA) launched JUICE. Not a sweet fruit drink, but Jupiter Icy Moons Explorer (JUICE). JUICE is a space probe designed to study the planet Jupiter and its moons. The probe was launched from Guiana Space Center in Kourou, French Guiana, on the northeastern coast of South America.  The launch was delayed one day after lightning was detected in the area on Thursday morning. The spacecraft launched aboard an Ariane 5 rocket. JUICE separated from the rocket after thirty minutes and began its long journey.

Where is JUICE going? JUICE’s main targets are the moons Ganymede, Callisto, and Europa. Scientists think these moons have oceans of water under an outer covering of ice. Some experts believe these oceans might contain extraterrestrial life. JUICE will reach Jupiter in 2031 and attempt to enter orbit around Ganymede in 2034. Before attempting orbit, JUICE will conduct 35 planned flybys to search for magnetic signals.

Past space probes have taken measurements of magnetic fields and gravity around Jupiter’s moons. A magnetic field is the area around a magnetic object in which its influence can be felt. Scientists use such measurements to make informed guesses about the characteristics of any subsurface (underground) ocean on each moon. They can estimate the thickness of the ice crust and the amount of water in the ocean.

JUICE carries instruments to collect detailed observations of Jupiter and its moons. The spacecraft’s radio instruments and magnetometer will collect data on the environment surrounding the moons. Its spectrometer will identify chemicals in Jupiter’s atmosphere and on its moons. JUICE’s laser altimeter will use laser pulses to map the surfaces of the moons. The spacecraft’s powerful camera will be able to spot surface features as small as 8 feet (2.5 meters) across on Ganymede. JUICE will use radar to detect subsurface features.

JUICE’s main goal is to help scientists understand Ganymede, Callisto, and Europa and their oceans more completely and accurately. The spacecraft will also take some distant measurements of other moons. However, a probe launched by the National Aeronautics and Space Administration (NASA) in October 2024 might beat JUICE to Jupiter. The Europa Clipper is planned to launch from the SpaceX Falcon Heavy rocket, a much stronger rocket than the Ariane 5. The stronger launch will propel the Europa Clipper toward Jupiter at a much faster speed. The two missions are planning to work together to better understand Jupiter and its moons!

The Lucy spacecraft flies past the Trojan asteroid 617 Patroclus and Menoetius in this artist’s rendering.
Credit: NASA Goddard

It’s not every day that the National Aeronautics and Space Administration (NASA)  launches a new spacecraft. Their latest, named Lucy, was launched Oct. 16, 2021, from Cape Canaveral Space Force Station in Florida. Its mission? Explore the Trojan asteroid belt near the orbit of Jupiter. Lucy doesn’t have a crew, but the craft is equipped with powerful telescopes and cameras to gather information about the mysterious asteroids.

Lucy won’t be taking a direct path to Jupiter’s orbit, however. The spacecraft will use a series of gravity assists to help move it closer to its target. In a gravity assist, a spacecraft uses momentum gained from swinging by a planet to adjust its path and fling it toward a target.

After spending a year orbiting the sun, Lucy will return to Earth’s orbit for its first gravity assist in October 2022. This assist will set it on a course it toward Mars. In 2024, a second gravity assist will send Lucy towards the main asteroid belt, between the orbits of Mars and Jupiter. On this orbit, Lucy’s telescopes and cameras will capture images of asteroid 52246 Donaldjohanson. The probe should arrive at the Trojan asteroid belt by 2027.

Each of the asteroids in the Trojan belt is named after a different hero from the Greek epic the Iliad. The first Trojan Lucy will photograph is Eurybates in August 2027. Eurybates is unique because it has a smaller moon orbiting it, named Queta. Polymele will be photographed in September 2027, and Leucus and Orus will be photographed in April 2028 and November 2028, respectively. After that, Lucy will return to Earth for one more gravity assist. In 2033, Lucy will finish the mission by visiting the Trojan asteroids Patroclus and Menoetius. The spacecraft will remain in orbit around the asteroids, studying the asteroids for as long as it continues to function.

The Trojan asteroids were named after Greek myths, but what about Lucy? Usually, NASA uses acronyms for naming spacecraft. An acronym is a word that uses the first letter of each word in a phrase to create a new word, for example referring to the National Aeronautics and Space Administration as NASA.  In Lucy’s case, however, they named the craft after a fossilized skeleton found in 1974, in Ethiopia. Lucy wasn’t exactly a human – she represented a species of hominin called Australopithecus Afarensis. Hominins are a group of living things that includes human beings and early humanlike ancestors.

The expedition that found the skeleton Lucy was led by the anthropologist Donald Johanson and his student Tom Gray. An anthropologist is a scientist who studies humans and their close relatives. They were looking for fossils, and on Nov. 24, 1974, they found one of the most significant fossils to date. With 40 percent of her recovered, Lucy represents one of the most complete skeletons ever found from her time, dating to about 3.18 million years ago. When the team returned to their camp the night they found the first fossils, they celebrated their finding by listening to music and partying. One of the songs on repeat that night was “Lucy In The Sky With Diamonds,” a song by British rock band The Beatles. Pamela Alderman, a member of the team, gave the fossil hominid they found the nickname “Lucy” after the song. Just as Donald Johanson was the first to discover the fossil Lucy, the spacecraft Lucy will be the first to study asteroid 52246 Donaldjohanson, named in the anthropologist’s honor.

August 9, 2018

Astronomers at the Carnegie Institution for Science in Washington, D.C., recently discovered a new batch of moons orbiting Jupiter, the largest planet in our solar system. The new group of 12 moons—which bring’s Jupiter’s moon total to an astounding 79—includes an oddball, however: one is going “the wrong way.”

Ganymede is the largest of Jupiter’s moons. Astronomers recently found 12 new and much smaller moons orbiting Jupiter, bringing the planet’s solar system-leading total to 79. Credit: NASA

Led by astronomer Scott Sheppard, the Carnegie team had been looking for “Planet Nine,” a hypothetical major planet in the Kuiper belt, a band of objects in the outer regions of our solar system. In March 2017, Jupiter moved into the astronomers’ search area. The telescope the team was using was uniquely suited for finding small or faraway objects: it could block out light from larger nearby heavenly bodies. Sheppard took time away from Planet Nine to poke around Jupiter, and his curiosity was rewarded with the discovery of 12 new moons. The first two bodies orbit close to Jupiter, and were quickly confirmed as moons. The other 10 skew farther out from the mighty planet and were not announced as moons until July 17, 2018.

Callisto, another large moon of Jupiter, has a diameter of almost 3,000 miles (4,800 kilometers), many times the size of Jupter’s newly discovered moons. Credit: NASA

The new moons are small, some less than a mile (several hundred meters) in diameter. Their orbital characteristics tell scientists a lot about them. Nine of the 10 newest moons orbit in retrograde, meaning in the opposite direction of the rotation of Jupiter. This leads the astronomers to think they formed from objects captured by Jupiter’s hefty gravitational pull. Most moons form with their parent planet and have prograde orbits, meaning they orbit in the same direction as the host planet. But captured objects often have retrograde orbits. Sheppard’s team thinks these nine moons are parts of captured objects broken up by collisions over millions or billions of years.

The tenth of the newest moons, however—which Sheppard calls Valetudo, a great-granddaughter of the Roman mythological god Jupiter—travels in a prograde orbit. It dives through the orbits of the other nine moons, putting it on an eventual collision course. When moon finally meets moon, the impact will either destroy the bodies or make them even smaller.

June 26, 2017

Juno was the most powerful goddess of ancient Roman mythology. She was married to Jupiter, the king of the gods, and she was the queen. The Romans considered Juno a protector who would warn them of danger to their empire. Major shrines dedicated to Juno, Jupiter, and Minerva, the goddess of wisdom, stood on the Capitoline Hill, the religious center of ancient Rome. Juno’s shrine housed a flock of geese, which were sacred to the goddess. Legend claimed that the geese once saved Rome when their honking alerted the Romans to enemies sneaking up on the city.

A marble statue of Juno holding the golden apple of discord stands in a park in Sokyryntsi, Ukraine. Credit: © IMG Stock Studio/Shutterstock

Juno received special devotion from Roman women. She was the goddess of marriage, and her name may have been the source for the name of the month of June—still a popular month for marriages today. She was also the goddess of childbirth. In that role, she was called Juno Lucina, which means Juno Who Brings to Light.

Many stories about Juno come from Greek mythology. After the Romans came into contact with the Greeks, they identified Juno with Hera, the wife of Zeus, the king of the Greek gods. Although both Juno and Hera were goddesses of marriage, their own marriages were portrayed as rather stormy. Modern historians believe the reasons for this go back to ancient—very ancient—Greek history.

The hierarchy and worship of ancient Greek gods was not always as we understand it and label it today. People in different parts of ancient Greece often worshiped different gods, and only after cultures mixed were the religious traditions interwoven. In some areas, people had worshiped Hera for many generations before they ran into people who worshiped the sky god Zeus. Modern historians think the two religious traditions eventually merged, and this was expressed in mythical legends as a marriage between Hera and Zeus. The new people did not always get along, and thus neither did their gods. The legends further described other goddesses (and mortal women, too) as rivals for Zeus’s affections, and Hera came to be pictured as a jealous and vengeful wife. The Romans absorbed these stories into their own mythology, and they similarly portrayed Juno as a wife with many rivals who had to keep a watchful eye on her husband.

Juno’s watchful ways have not entirely disappeared, even today in the 21^st century. In 2011, NASA sent an unmanned spacecraft called Juno on a mission to Jupiter, the largest planet in our solar system. Juno arrived at Jupiter in 2016, and has since been observing the planet’s unique and often turbulent behavior. Juno remains as vigilant as ever.

June 9, 2017

Jupiter, our solar system’s largest planet, is slowly giving up its secrets. Last month, astronomers and planetary scientists of NASA’s Juno Mission to Jupiter released spectacular new images of the planet along with scientific papers fueled by the mission’s newly gathered data. The Juno probe has been in orbit around Jupiter since July 2016. NASA launched the spacecraft, named for the wife of the god Jupiter in Roman mythology, in August 2011. Juno is managed by the Jet Propulsion Laboratory in Pasadena, California.

Jupiter’s south pole, as seen by the Juno spacecraft from an altitude of 32,000 miles (52,000 kilometers). The oval features are massive cyclones up to 600 miles (1,000 kilometers) in diameter. Credit: NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles

Juno circles Jupiter in an unusual elongated (stretched out) orbit. Juno flies as far as 5 million miles (8 million kilometers) away from the planet and dives as close as 3,000 miles (5,000 kilometers) above Jupiter’s turbulent cloud tops. Rather than orbiting at or near the planet’s equator, Juno circles over Jupiter’s north and south poles. This orbit helps Juno avoid Jupiter’s radiation belt, which emanates from the planet’s waistline like an invisible ring. The intense radiation of this region would quickly destroy Juno’s electronic circuits and scientific instruments.

Spectacular storms cluster around Jupiter’s turbulent north pole. Credit: MSSS/SwRI/JPL-Caltech/NASA

Juno’s polar orbit has given scientists (and everyone else) new views of Jupiter. Unlike the rest of the planet’s atmosphere, which is famous for its banded structure (as we see in our familiar side view of the planet), Jupiter’s poles resemble bubbling cauldrons. Cyclonic storms many times larger than the largest hurricanes on Earth pockmark these regions, whose grayish-blue hues contrast with the tans, oranges, and reds found elsewhere on the planet. Scientists await further images to determine if these polar storms are short lived or if they may last years, decades, or even centuries—as has Jupiter’s famous and stormy Great Red Spot.

The familiar side view of Jupiter reveals its varied color bands and the Great Red Spot just south of the planet’s equator. Credit: Johns Hopkins U. APL/SWRI/NASA

One of Juno’s primary objectives is to study Jupiter’s internal makeup and determine if the planet has a solid, rocky core. The data returned so far suggest that Jupiter does indeed have a solid core, but that it is “fuzzy.” Data from future orbits of the probe will help scientists understand the makeup of the core and why it appears to have an undefined boundary.

Launched from Earth in 2011, the Juno spacecraft arrived at Jupiter in 2016. Credit: NASA/JPL

In addition to providing new looks at Jupiter, Juno has forced scientists to reconsider several previously believed notions of the giant planet. The probe has detected more ammonia in Jupiter’s atmosphere than was previously thought to be there. The ammonia is also spread unevenly, with much of it concentrating near Jupiter’s equator. Juno has discovered that the planet’s magnetic field is about as twice as strong as previously thought, and that the magnetism is “lumpy,” having stronger and weaker pockets. The magnetic field is also generated much closer to the planet’s surface than scientists had earlier guessed. Further measurements from Juno are needed to help understand how the field is generated. (Scientists think Earth’s magnetic field comes from the liquid core at our planet’s center.)

Juno still has more than 30 dives to collect information on Jupiter, providing numerous opportunities to further rewrite the book on our distant, gassy, and giant planetary neighbor.

August 11, 2016

It is going to storm tonight—but don’t worry, you won’t get wet. The Perseid meteor shower peaks in the evening hours tonight and early tomorrow morning. Sky watchers in the Northern Hemisphere may be able to see bright streaks flashing across the night sky. Thanks to the mighty influence of the planet Jupiter, the Perseids are more numerous than they’ve been for several years.

The Perseid meteor shower lights up the night sky each year in August.
Credit: NASA/JPL

Meteor showers occur when Earth travels through a stream of meteoroids. A meteoroid is a piece of matter that enters Earth’s atmosphere at a high speed. The rate at which meteors appear generally increases, peaks, and then decreases as Earth moves in and out of a stream. As seen from the ground, all meteors in a particular shower appear to come from the same direction in the sky. This direction is called the radiant of the shower.

The Perseids are an annual meteor shower caused by a dusty trail of debris left by Comet Swift-Tuttle. As the comet orbits the sun, it sheds rock and dust particles. As Earth travels through this ring of debris, meteoroids crash into the atmosphere. Their high speed heats up the air, causing it to glow. Almost all these meteoroids are too small to reach Earth’s surface and instead burn up in the atmosphere. The Perseids get their name because the radiant of the shower lies near the constellation Perseus.

Comet Swift-Tuttle leaves a wide swath of debris, so meteors have been visible for several weeks already and should be visible for several more. The frequency of meteor sightings is expected to peak tonight, however, as Earth passes through the thickest part of the debris cloud. Astronomers estimate that there may be as many as 200 visible meteors per hour! What’s the reason for this abnormally heavy shower? The intense gravity of Jupiter sometimes tugs at debris streams left by comets. In the case of this year’s Perseids, Jupiter has pulled the center of the stream closer to Earth’s orbit, nearly doubling the usual meteor shower.

Look toward Perseus tonight after sunset to catch the show (it is best seen when the constellation is directly overhead). If possible, get away from urban light pollution, but some shooting stars can be seen even near a city’s glow. Astronomers think the brightest meteors may even outshine the stars tonight—for only a fraction of a second, of course!

Other World Book articles:

• Astronomy
• Famous comets
• Stones From Space (a Special report)

July 5, 2016

Traditionally, Independence Day in the United States—yesterday, July 4—is celebrated with fireworks. This Fourth of July, the National Aeronautics and Space Administration (NASA) celebrated with fireworks of the deep space variety as its probe Juno fired its engine for 35 minutes to enter into orbit around Jupiter. After being launched in 2011, Juno has finally reached its destination.

Launched from Earth in 2011, the Juno spacecraft arrived at Jupiter on July 4, 2016, to study the giant planet from an elliptical polar orbit.
Credit: NASA/JPL

Jupiter is the largest planet in our solar system. It has a mass (amount of matter) that is greater than the masses of all the other planets in the solar system added together. Astronomers call Jupiter a gas giant because the planet consists mostly of hydrogen and helium, and it has no solid surface. Jupiter is named for the king of the gods in Roman mythology. Juno was Jupiter’s wife and queen of the gods.

Juno will study Jupiter from an elongated polar orbit. The probe will be 4,900 miles (7,200 kilometers) from Jupiter at the farthest point in its orbit, and dive to within 2,600 miles (4,200 kilometers) of the Jovian clouds, flying over the planet’s north and south poles. Mission scientists planned this orbit to keep Juno out of Jupiter’s radiation belts as much as possible. Jupiter emits a powerful magnetic field, much stronger than that produced by Earth. This strong magnetic field traps high-energy charged particles in belts around the planet. Any spacecraft entering these belts would be bombarded by huge doses of radiation. Such radiation is just as dangerous to electronics as it is to living things: it can damage computer chips and fry circuits.

Scientists shielded Juno’s most important electronics in a thick titanium case to protect them from radiation. But no spacecraft can stand exposure to such highly charged particles for long. NASA scientists do not expect the craft to survive much longer than its 20-month mission, even with its titanium vault and elliptical orbit. Consequently, they have already scheduled the probe to crash into Jupiter in February 2018.

Before it smashes into its object of study, Juno will probe the makeup of Jupiter. Scientists hope it will help determine whether Jupiter has a rocky core, and why the planet has a higher concentration of elements heavier than hydrogen and helium than the sun. Juno will also study Jupiter’s famous Great Red Spot, a storm that has been churning around the atmosphere for at least 300 years. Scientists think that answering these questions will help us better understand Jupiter’s history and the formation of our solar system. It may even give us a better idea of how other solar systems form and behave. Rather than a grand finale, Juno’s fireworks are the start of some amazing science!

November 26, 2014

A remastered image of Jupiter’s moon Europa released by NASA shows, for the first time, how this ice-covered body would appear to the human eye. The image is actually a mosaic of images taken by the Galileo spacecraft in the late 1990′s. The previous version of the image had been strongly enhanced with false color. The new image shows Europa in near-natural color.

Areas of Europa’s surface that appear blue or white contain relatively pure water ice. Reddish and brownish areas include higher amounts of other gases or solids. The polar regions, which are visible at the left and right of the image, are much bluer than the more equatorial latitudes. Scientists suspect this difference is due to differences in the size of the ice grains in these locations. (NASA/JPL-Caltech/SETI Institute)

Although Europa is one of the smoothest bodies in the solar system, its surface features include shallow cracks, valleys, ridges, pits, blisters, and icy flows. However, none of them extend more than a few hundred yards or meters upward or downward. In some places, huge sections of the surface have split apart and separated. The surface of Europa has few impact craters (pits caused by collisions with asteroids or comets). The splitting and shifting of the surface and disruptions from below have destroyed most of the old craters.

Europa is considered one of the likeliest places in the solar system for the existence of extraterrestrial life. Beneath its icy surface is an immense lake of salty liquid water that might be a home for living things.

February 14, 2014

The wildly complicated surface of Ganymede, the largest moon in the solar system, appears in colorful detail in a new map published by the United States Geological Survey. The map is based on images taken by NASA’s Voyager 1 and Voyager 2 spacecraft, which flew by Jupiter and its moons in 1979, and the Galileo probe, which orbited Jupiter from 1995 to 2003. The map is the first global geologic map of Ganymede, Jupiter’s seventh moon, or any other icy moon, for that matter.

Ganymede, also Jupiter’s largest moon, is bigger than the planet Mercury and almost as large as Mars. Ganymede is one of four Jovian moons discovered by the Italian scientist Galileo Galilei in 1610. From observations using Earth-based telescopes and orbiting spacecraft, scientists have long known that Ganymede’s icy surface is made up of almost equal amounts of dark and light material. A thin ocean of salty water may lay about 105 miles (170 kilometers) below the surface. The presence of water makes Ganymede and the other large moons of Jupiter—along with Mars—prime candidates in the search for extraterrestrial life.

The false colors in a new map of Ganymede reveal geologic features created during different times in the moon's history. (USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech)

The new false-color map, which was created by a team of scientists headed by Geoffrey Collins of Wheaton College in Massachusetts, provides the first solid evidence for three major geological periods in the moon’s history. During the oldest period, represented in brown, asteroids and comets bombarded the surface, leaving many impact craters. Forces released by the biggest impacts also created large cracks in the moon’s crust. Lighter colors on the map represent two somewhat younger periods. During one of these, changes in the temperature of the moon caused the surface to expand and crack. The new terrain (land) created by these tectonic forces was marked with parallel sets of ridges and valleys. During the third period, geologic activity declined. As the crust settled, new ridges and valleys appeared.

Additional articles in World Book:

• Callisto
• Europa
• Io
• Satellite (Satellites of the gas giants)
• Probing the Planets (a Special Report)
• The Thirty-Year Journey of Voyagers 1 & 2 (a Special Report)

October 15, 2013

It rains diamonds on Saturn and Jupiter. New atmospheric data about these gas giant planets led Kevin Baines–an astronomer at the University of Wisconsin-Madison and NASA’s Jet Propulsion Laboratory–and his colleague Mona Delitsky to conclude that carbon is abundant on Saturn and Jupiter in its most dazzling crystal form–diamonds. Massive lightning storms high in the atmospheres of both planets turn methane into soot. Soot consists chiefly of particles of carbon. As the soot falls through Saturn and Jupiter’s gaseous atmospheres, it hardens into chunks of graphite and then diamond, which fall like “hail stones.” “The bottom line is that 1,000 tons (907 metric tons) of diamonds a year are being created on Saturn,” stated Baines.

Diamonds are one of the three crystalline forms of pure, solid carbon. The other forms are graphite and fullerenes (shown as a Buckminsterfullerene and carbon nanotube). (World Book illustrations by Oxford Illustrators Limited and Bensen Studios)

Baines delivered his and Delitsky’s unpublished findings last week at the American Astronomical Society’s annual Division for Planetary Sciences meeting. Analyzing the latest temperature and pressure predictions for the planets’ interiors, the two scientists concluded that stable crystals of diamond “hail down over a huge region” of Saturn in particular. “It all begins in the upper atmosphere, in the thunderstorm alleys, where lightning turns methane into soot,” stated Baines. “As the soot falls, the pressure on it increases. And after about 1,000 miles (1,600 kilometers) it turns to graphite–the sheet-like form of carbon you find in pencils.” The falling graphite hardens into diamonds, the hardest substance found on Earth. However, the diamonds eventually fall into the planets’ hot cores and melt into a liquid sea. “Once you get down to those extreme depths, the pressure and temperature is so hellish, there’s no way the diamonds could remain solid,” explained Barnes. On Saturn and Jupiter–diamonds are not forever!