Juno’s Ganymede Flyby: A Giant Moon, a Long History – Sky & Telescope

On June 7th, NASA’s Jupiter-orbiting Juno spacecraft will perform a close flyby of Ganymede, passing about 1,000 kilometers (600 miles) from the icy moon’s surface. This flyby places the mission alongside seven other planetary spacecraft that have photographed the moon over the last few decades, increasing our understanding of this unique world.

Part of the purpose of the flyby is to begin adjusting Juno’s orbit in anticipation of its extended mission, announced last January. But the flyby will also allow Juno to study some of Jupiter’s large moons from close range, and Ganymede is up first. This icy world is the largest moon in the solar system, bigger even than the planet Mercury, with a tenuous atmosphere and, unique among moons, its own magnetic field. It also probably has a subsurface ocean, though it’s not as accessible as the one on Jupiter’s Europa or Saturn’s Enceladus.

Photography was never intended to be a prime objective of the Juno mission, which focuses instead on measuring Jupiter’s magnetic fields, gravity, and atmosphere. But Juno nevertheless carries a public-outreach camera called JunoCam, which has been highly successful. It has provided rich imagery of Jupiter’s cloud tops and occasional distant views of the Galilean satellites, all downlinked as raw pictures that amateurs then processed into beautiful panoramas.

Now, JunoCam will have a chance to provide more detailed images of Ganymede. Before the new images arrive, let’s take a look at the history of Ganymede’s exploration up to this point.

17th Century: Discovery

The incomparable Italian astronomer Galileo Galilei discovered Ganymede — along with Io, Europa, and Callisto — in 1610. A series of winter evenings found Galileo aiming one of the world’s first telescopes at Jupiter, discovering three, and later four small dots near the planet. Galileo was initially intrigued because these “stars,” as he originally called them in his notes, were in a straight line near Jupiter’s equator. Within a few weeks, punctuated by the occasional cloudy nights (some things never change!), the truth became obvious: These celestial bodies were orbiting the larger planet.

1610–1973: Earth-based Observations

Unlike Jupiter, the moons were too far away for surface details to be resolved even in large telescopes. At best, observers could make some very slight albedo and color measurements. So what could possibly be learned about Ganymede and its companions when they appeared as nothing more than points of light, moving from night to night around the giant planet?

By examining Ganymede’s orbit, it was possible to approximate its mass and density, by which astronomers could in turn infer information about the moon’s composition. Spectroscopy also provided clues.

But sometimes those clues were misleading. An astronomy textbook from 1901, for example, comments on the low densities of the Galilean satellites: “It has been surmised that these satellites are not solid bodies, like the earth and moon, but only shoals of rock and stone, loosely piled together and kept from packing into a solid mass by the action of Jupiter in raising tides within them.” Ice or interior oceans, both of which also reduce the bulk density, weren’t yet considered.

In the end, even careful observations could only go so far. There were still no clues to the nature of Ganymede’s geography and surface details — was it a cratered world? Did it have maria like the Moon? Or something new entirely? The same 1901 textbook laments that while “each of these satellites may fairly be considered a world in itself, their great distance from us makes it impossible . . . to see more upon their surfaces than occasional vague markings, which hardly suffice to show the rotations of the satellites upon their axes.

1973: Pioneer 10

That all changed with the space age. In 1973, the unmanned Pioneer 10 spacecraft returned the first images of Ganymede. The best of these photos showed Ganymede’s major surface geography for the first time, including details as small as 400 km across. The images were a little fuzzy and of fairly low resolution but far better than what had been achieved previously. Pioneer 11 also returned some images of Ganymede in 1974. Together, the spacecraft refined measurements of Ganymede’s mass and composition.

blurry image of Ganymede against blue background
Pioneer 10 photographed Ganymede on July 1, 1973.

1979: Voyager 1 & 2

A real breakthrough came in 1979, when the twin Voyager spacecraft made close flybys of Ganymede. No longer seen as a point of light or a fuzzy disk, the photos from the Voyager probes show Ganymede as a dramatic world in its own right.

a closeup of the corner of Ganymede shown against a black background.
Voyager 1 took this picture of Ganymede, Jupiter’s (and the solar system’s) largest satellite, on March 5, 1979 from a range of 253,000 kilometers (151,800 miles).

Mapping much of the moon’s surface, the Voyagers returned detailed images of Ganymede, showing that the dark regions were heavily cratered and that the lighter topography had been reworked by tectonic activity. The nature of the surface — a combination of ice and rock littered with cracks and craters — became clear at last. The probes also saw that Ganymede has polar ice caps. Earth from space is often called the “blue marble;” from close range, Ganymede appears as something of a “gray marble.”

jupiter's moon ganymede shown in grey and white, against a black background
Voyager 2 color photo of Ganymede.

1996: Galileo

The highly successful, although somewhat malfunction-plagued Galileo probe reached Jovian orbit in 1995 and performed six flybys of Ganymede over the next few years. It obtained even closer views of the moon than the Voyagers had: During one flyby, it passed a mere 225 km above Ganymede’s icy surface — flying more than 50 times closer than Voyager 2.

Ganymede in sepia tones against a black background
Natural color view of Ganymede from the Galileo spacecraft during its first encounter with the satellite.

Galileo’s photos showed plains, mountains, and valleys, and some regions of rugged terrain disrupted by tectonic action. The probe also discovered that Ganymede has a magnetic field — the only moon in the solar system with that distinction. Much of our current understanding of Ganymede comes from this mission.

close up of wavy lines on a grey background
Complex tectonism is evident in this image of Ganymede’s surface.
NASA / JPL / Brown University

2001: Cassini

The fabulous Cassini spacecraft passed by Jupiter on its way to Saturn in 2001. Although it didn’t travel as close to Ganymede as other probes before it, Cassini did snap a handful of aesthetically appealing photos of the large moon juxtaposed against Jupiter’s turbulent cloudtops.

close up of corner of jupiter with gaynmede in background
Cassini photographed the relatively tiny Ganymede juxtaposed against giant Jupiter.

2007: New Horizons

While en route to Pluto, New Horizons took advantage of Jupiter’s strong gravitational field to boost its trajectory; at the same time, the probe captured some excellent photos of the Jovian system, included a few distant images of Ganymede. Infrared spectral images filled in large gaps on compositional maps of Ganymede’s surface.

Ganymede shown in grey with white spots on it against a black background
This is New Horizons’ best image of Ganymede, taken with the spacecraft’s Long Range Reconnaissance Imager camera on February 27, 2007, from a range of 3.5 million kilometers (2.2 million miles).

2016–present: Juno

And that brings us to Juno, NASA’s current mission at Jupiter. Even though exploration of the Jovian moons was not a primary focus of the mission, Juno has repeatedly photographed Ganymede, Europa, and Io, albeit from considerable range. The JunoCam has even given us our first views of Ganymede’s north polar region, thanks to Juno’s unique orbit around Jupiter.

five images of ganymede shown partially illuminated and in infared light
These images the JIRAM instrument aboard NASA’s Juno spacecraft took on Dec. 26, 2019, provide the first infrared mapping of Ganymede’s northern frontier.
NASA / JPL-Caltech / SwRI / ASI / INAF / JIRAM

In addition to visual data, spectra taken during a 2019 Juno flyby enabled astronomers to map the distribution of water ice in Ganymede’s north polar regions and revealed the potential presence of magnesium salts, ammonia, and carbon dioxide on the surface. This region is inaccessible from Earth-based observations, so Juno’s explorations are valuable to scientists.

June 7, 2021: Juno’s First Close Flyby

Now, with Juno fast approaching Ganymede for another close-range study of the moon, new images and spectra will help further our understanding of this fascinating world. Once again, anyone with an interest is invited to “ride along” with the probe and view Ganymede from the perspective of a planetary spacecraft as data is downlinked. What will we see this time in the new photographs? What new sights await? Stay tuned to find out!


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