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, growing our understanding of this distinctive world.
Part of the aim of the flyby is to start adjusting Juno’s orbit in anticipation of its extended mission, announced last January. But the flyby can even enable Juno to review a few of Jupiter’s giant moons from shut vary, and Ganymede is up first. This icy world is the biggest moon within the solar system, greater even than the planet Mercury, with a tenuous ambiance and, distinctive amongst moons, its personal magnetic subject. It additionally most likely 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 meant to be a prime goal of the Juno mission, which focuses as an alternative on measuring Jupiter’s magnetic fields, gravity, and ambiance. But Juno nonetheless carries a public-outreach digicam known 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 footage that amateurs then processed into stunning panoramas.
Now, JunoCam can have a likelihood to supply extra detailed photos of Ganymede. Before the brand new photos arrive, let’s take a take a look at the historical past of Ganymede’s exploration up up to now.
seventeenth Century: Discovery
The incomparable Italian astronomer Galileo Galilei found Ganymede — together with Io, Europa, and Callisto — in 1610. A sequence 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 known as them in his notes, have 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 grew to become apparent: These celestial our bodies have been orbiting the bigger planet.
1610–1973: Earth-based Observations
Unlike Jupiter, the moons have been too distant for floor particulars to be resolved even in giant telescopes. At finest, observers might make some very slight albedo and shade measurements. So what might probably be discovered about Ganymede and its companions after they appeared as nothing greater than factors of sunshine, transferring from night time to nighttime across the large planet?
By inspecting 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 generally these clues have 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 cut back the majority density, weren’t but thought of.
In the top, even cautious observations might solely go to this point. There have 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 fully? The identical 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 space age. In 1973, the unmanned Pioneer 10 spacecraft returned the primary photos 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 photos have been a little fuzzy and of pretty low decision however much better than what had been achieved beforehand. Pioneer 11 additionally returned some photos of Ganymede in 1974. Together, the spacecraft refined measurements of Ganymede’s mass and composition.
1979: Voyager 1 and a couple 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.
Mapping a lot of the moon’s floor, the Voyagers returned detailed photos of Ganymede, displaying that the darkish areas have 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 affected by cracks and craters — grew to become clear ultimately. The probes additionally noticed that Ganymede has polar ice caps. Earth from space is usually known as the “blue marble;” from shut vary, Ganymede seems as one thing of a “gray marble.”
The extremely profitable, though considerably malfunction-plagued Galileo probe reached Jovian orbit in 1995 and carried out six flybys of Ganymede over the following 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 instances nearer than Voyager 2.
Galileo’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 subject — the one moon within the solar system with that distinction. Much of our present understanding of Ganymede comes from this mission.
The fabulous Cassini spacecraft handed by Jupiter on its way 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 big moon juxtaposed in opposition to Jupiter’s turbulent cloudtops.
2007: New Horizons
While en path to Pluto, New Horizons took benefit of Jupiter’s sturdy gravitational subject to spice up its trajectory; on the identical time, the probe captured some wonderful photographs of the Jovian system, included a few distant photos of Ganymede. Infrared spectral images filled in large gaps on compositional maps of Ganymede’s floor.
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, because of Juno’s distinctive orbit round Jupiter.
In 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 useful to scientists.
June 7, 2021: Juno’s First Close Flyby
Now, with Juno quick approaching Ganymede for one more close-range research of the moon, new photos 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 examine Ganymede from the attitude 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 search out out!