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

On June seventh, NASA’s Jupiter-orbiting Juno spacecraft will carry out a shut flyby of Ganymede, passing about 1,000 kilometers (600 miles) from the icy moon’s floor. This flyby locations the mission alongside seven different planetary spacecraft which have photographed the moon over the previous few a long time, rising our understanding of this distinctive world.

Part of the aim of the flyby is to start adjusting Juno’s orbit in anticipation of its prolonged mission, introduced final January. But the flyby can even permit Juno to check a few of Jupiter’s massive moons from shut vary, and Ganymede is up first. This icy world is the most important moon within the photo voltaic system, larger even than the planet Mercury, with a tenuous environment and, distinctive amongst moons, its personal magnetic area. It additionally in all probability has a subsurface ocean, although it’s not as accessible because the one on Jupiter’s Europa or Saturn’s Enceladus.

Photography was by no means supposed to be a prime goal of the Juno mission, which focuses as an alternative on measuring Jupiter’s magnetic fields, gravity, and environment. But Juno nonetheless carries a public-outreach digital camera referred to as JunoCam, which has been extremely profitable. It has supplied wealthy imagery of Jupiter’s cloud tops and occasional distant views of the Galilean satellites, all downlinked as uncooked photos that amateurs then processed into lovely panoramas.

Now, JunoCam can have a probability to offer extra detailed photographs of Ganymede. Before the brand new photographs arrive, let’s take a take a look at the historical past of Ganymede’s exploration up up to now.

17th Century: Discovery

The incomparable Italian astronomer Galileo Galilei found Ganymede — together with Io, Europa, and Callisto — in 1610. A collection of winter evenings discovered Galileo aiming one of many world’s first telescopes at Jupiter, discovering three, and later 4 small dots close to the planet. Galileo was initially intrigued as a result of these “stars,” as he initially referred to as them in his notes, had been in a straight line close to Jupiter’s equator. Within a few weeks, punctuated by the occasional cloudy nights (some issues by no means change!), the reality turned apparent: These celestial our bodies had been orbiting the bigger planet.

1610–1973: Earth-based Observations

Unlike Jupiter, the moons had been too far-off for floor particulars to be resolved even in massive telescopes. At finest, observers might make some very slight albedo and colour measurements. So what might probably be realized about Ganymede and its companions once they appeared as nothing greater than factors of sunshine, transferring from night time to nighttime across the large planet?

By analyzing Ganymede’s orbit, it was attainable to approximate its mass and density, by which astronomers might in flip infer details about the moon’s composition. Spectroscopy additionally supplied clues.

But typically these clues had been deceptive. An astronomy textbook from 1901, for instance, feedback 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 inside oceans, each of which additionally scale back the majority density, weren’t but thought-about.

In the top, even cautious observations might solely go to date. There had been nonetheless no clues to the character of Ganymede’s geography and floor particulars — was it a cratered world? Did it have maria just like the Moon? Or one thing new completely? The similar 1901 textbook laments that whereas “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 and 11

That all modified with the area age. In 1973, the unmanned Pioneer 10 spacecraft returned the primary photographs of Ganymede. The finest of those photographs confirmed Ganymede’s main floor geography for the primary time, together with particulars as small as 400 km throughout. The photographs had been a little fuzzy and of pretty low decision however much better than what had been achieved beforehand. Pioneer 11 additionally returned some photographs of Ganymede in 1974. Together, the spacecraft refined measurements of Ganymede’s mass and composition.

Pioneer 10 photographed Ganymede on July 1, 1973. NASA1979: Voyager 1 and a pair of

A actual breakthrough got here in 1979, when the dual Voyager spacecraft made shut flybys of Ganymede. No longer seen as a level of sunshine or a fuzzy disk, the photographs from the Voyager probes present Ganymede as a dramatic world in its personal proper.

Voyager 1 took this image of Ganymede, Jupiter’s (and the photo voltaic system’s) largest satellite tv for pc, on March 5, 1979 from a vary of 253,000 kilometers (151,800 miles). NASA / JPLMapping a lot of the moon’s floor, the Voyagers returned detailed photographs of Ganymede, exhibiting that the darkish areas had been closely cratered and that the lighter topography had been reworked by tectonic exercise. The nature of the floor — a mixture of ice and rock suffering from cracks and craters — turned clear finally. The probes additionally noticed that Ganymede has polar ice caps. Earth from area is usually referred to as the “blue marble;” from shut vary, Ganymede seems as one thing of a “gray marble.”

Voyager 2 colour photograph of Ganymede.NASA / JPL1996: Galileo

The extremely profitable, though considerably malfunction-plagued Galileo probe reached Jovian orbit in 1995 and carried out six flybys of Ganymede over the subsequent few years. It obtained even nearer views of the moon than the Voyagers had: During one flyby, it handed a mere 225 km above Ganymede’s icy floor — flying greater than 50 occasions nearer than Voyager 2.

Natural colour view of Ganymede from the Galileo spacecraft throughout its first encounter with the satellite tv for pc.NASA / JPLGalileo’s photographs confirmed plains, mountains, and valleys, and a few areas of rugged terrain disrupted by tectonic motion. The probe additionally found that Ganymede has a magnetic area — the one moon within the photo voltaic system with that distinction. Much of our present understanding of Ganymede comes from this mission.

Complex tectonism is obvious on this picture of Ganymede’s floor.NASA / JPL / Brown University 2001: Cassini

The fabulous Cassini spacecraft handed by Jupiter on its strategy to Saturn in 2001. Although it didn’t journey as near Ganymede as different probes earlier than it, Cassini did snap a handful of aesthetically interesting photographs of the massive moon juxtaposed towards Jupiter’s turbulent cloudtops.

Cassini photographed the comparatively tiny Ganymede juxtaposed towards large Jupiter.PDS / OPUS2007: New Horizons

While en path to Pluto, New Horizons took benefit of Jupiter’s sturdy gravitational area to spice up its trajectory; on the similar time, the probe captured some wonderful photographs of the Jovian system, included a few distant photographs of Ganymede. Infrared spectral photographs stuffed in massive gaps on compositional maps of Ganymede’s floor.

This is New Horizons’ finest picture of Ganymede, taken with the spacecraft’s Long Range Reconnaissance Imager digital camera on February 27, 2007, from a vary of three.5 million kilometers (2.2 million miles).NASA / JHUAPL / SWRI2016–current: Juno

And that brings us to Juno, NASA’s present mission at Jupiter. Even although exploration of the Jovian moons was not a main focus of the mission, Juno has repeatedly photographed Ganymede, Europa, and Io, albeit from appreciable vary. The JunoCam has even given us our first views of Ganymede’s north polar area, due to Juno’s distinctive orbit round Jupiter.

These photographs the JIRAM instrument aboard NASA’s Juno spacecraft took on Dec. 26, 2019, present the primary infrared mapping of Ganymede’s northern frontier. NASA / JPL-Caltech / SwRI / ASI / INAF / JIRAMIn addition to visible knowledge, spectra taken throughout a 2019 Juno flyby enabled astronomers to map the distribution of water ice in Ganymede’s north polar areas and revealed the potential presence of magnesium salts, ammonia, and carbon dioxide on the floor. This area is inaccessible from Earth-based observations, so Juno’s explorations are invaluable to scientists.

June 7, 2021: Juno’s First Close Flyby

Now, with Juno quick approaching Ganymede for one more shut-vary research of the moon, new photographs and spectra will assist additional our understanding of this fascinating world. Once once more, anybody with an curiosity is invited to “ride along” with the probe and consider Ganymede from the angle of a planetary spacecraft as knowledge is downlinked. What will we see this time within the new pictures? What new sights await? Stay tuned to seek out out!


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