![]() Spectroscopy also provided clues.īut sometimes those clues were misleading. 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?īy 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. At best, observers could make some very slight albedo and color measurements. Unlike Jupiter, the moons were too far away for surface details to be resolved even in large telescopes. 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. Galileo was initially intrigued because these “stars,” as he originally called them in his notes, were in a straight line near Jupiter’s equator. 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. The incomparable Italian astronomer Galileo Galilei discovered Ganymede - along with Io, Europa, and Callisto - in 1610. Before the new images arrive, let’s take a look at the history of Ganymede’s exploration up to this point. Now, JunoCam will have a chance to provide more detailed images of Ganymede. 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. But Juno nevertheless carries a public-outreach camera called JunoCam, which has been highly successful. 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. It also probably has a subsurface ocean, though it’s not as accessible as the one on Jupiter’s Europa or Saturn’s Enceladus. 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. But the flyby will also allow Juno to study some of Jupiter’s large moons from close range, and Ganymede is up first. Part of the purpose of the flyby is to begin adjusting Juno’s orbit in anticipation of its extended mission, announced last January. ![]() 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. 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. Read and see more in NASA's press release, and see additional images as they come in in the JunoCam archive. June 8, 2021: NASA has just posted new images from Juno's flyby showing the striated and cratered surface of the ice-covered moon: This image of the dark side of Ganymede was obtained by Juno’s Stellar Reference Unit navigation camera during its June 7, 2021, flyby of the moon. ![]() For both worlds, JunoCam images were orthographically projected onto a digital sphere, and synthetic frames were added between actual images to make the motion appear smoother and provide views of approach and departure for both Ganymede and Jupiter. Note: this animation contains real images as well as modifications for a better video. July 14, 2021: NASA has released an epic video of Juno's recent flyby past Ganymede and then, a day later, Jupiter. ![]()
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