NASA’s Juno Spacecraft Gets a Close Look at Ganymede - Jupiter’s Largest Moon
This image of the Jovian moon Ganymede was obtained by the JunoCam imager aboard NASA’s Juno spacecraft during its June 7, 2021, flyby of the icy moon. At the time of closest approach, Juno was within 645 miles (1,038 kilometers) of its surface – closer to Jupiter’s largest moon than any other spacecraft has come in more than two decades. This image is a preliminary product – Ganymede as seen through JunoCam’s green filter. Juno is a spin-stabilized spacecraft (with a rotation rate of 2 rpm), and the JunoCam imager has a fixed field of view. To obtain Ganymede images as Juno rotated, the camera acquired a strip at a time as the target passed through its field of view. These image strips were captured separately through the red, green, and blue filters. To generate the final image product, the strips must be stitched together and colors aligned. At the time this preliminary image was generated, the “spice kernels” (navigation and other ancillary information providing precision observation geometry) necessary to properly map-project the imagery were not available. The red and blue filtered image strips were also not available. When the final spice kernel data and images from the two filters are incorporated, the images seams (most prevalent on lower right of sphere) will disappear and a complete color image will be generated. (Image Credit: NASA, JPL-Caltech, SwRI, MSSS)
NASA’s Juno Spacecraft Gets a Close Look at Ganymede - Jupiter’s Largest Moon
Ganymede, Jupiter’s largest moon, is bigger than the planet Mercury and is the only moon in the solar system with its own magnetosphere – a bubble-shaped region of charged particles surrounding the celestial body.
On Monday, June 7, 2021 at 1:35 p.m. EDT (10:35 a.m. PDT), NASA’s Juno spacecraft came within 645 miles (1038 kilometers) of the surface of Ganymede. The flyby was the closest a spacecraft has come to the solar system’s largest natural satellite since NASA’s Galileo spacecraft made its close approach back on May 20, 2000. Along with striking imagery, the solar-powered spacecraft’s flyby yielded insights into the moon’s composition, ionosphere, magnetosphere, and ice shell.
“Juno carries a suite of sensitive instruments capable of seeing Ganymede in ways never before possible,” said Juno Principal Investigator Scott Bolton of the Southwest Research Institute in San Antonio. “By flying so close, we will bring the exploration of Ganymede into the 21st century, both complementing future missions with our unique sensors and helping prepare for the next generation of missions to the Jovian system – NASA’s Europa Clipper and ESA’s [European Space Agency’s] JUpiter ICy moons Explorer [JUICE] mission.”
Juno’s science instruments began collecting data about three hours before the spacecraft’s closest approach. Along with the Ultraviolet Spectrograph (UVS) and Jovian Infrared Auroral Mapper (JIRAM) instruments, Juno’s Microwave Radiometer’s (MWR) peered into Ganymede’s water-ice crust, obtaining data on its composition and temperature.
“Ganymede’s ice shell has some light and dark regions, suggesting that some areas may be pure ice while other areas contain dirty ice,” said Bolton. “MWR will provide the first in-depth investigation of how the composition and structure of the ice varies with depth, leading to a better understanding of how the ice shell forms and the ongoing processes that resurface the ice over time.” The results will complement those from ESA’s forthcoming JUICE mission, which will look at the ice using radar at different wavelengths when it becomes the first spacecraft to orbit a moon other than Earth’s Moon in 2032.
Signals from Juno’s X-band and Ka-band radio wavelengths were used to perform a radio occultation experiment to probe the moon’s tenuous ionosphere (the outer layer of an atmosphere where gases are excited by solar radiation to form ions, which have an electrical charge).
“As Juno passes behind Ganymede, radio signals will pass through Ganymede’s ionosphere, causing small changes in the frequency that should be picked up by two antennas at the Deep Space Network’s Canberra complex in Australia,” said Dustin Buccino, a signal analysis engineer for the Juno mission at JPL. “If we can measure this change, we might be able to understand the connection between Ganymede’s ionosphere, its intrinsic magnetic field, and Jupiter’s magnetosphere.”
Normally, Juno’s Stellar Reference Unit (SRU) navigation camera is tasked with helping keep the Jupiter orbiter on course, but during the flyby it will do double duty. Along with its navigation duties, the camera – which is well shielded against radiation that could otherwise adversely affect it – will gather information on the high-energy radiation environment in the region near Ganymede by collecting a special set of images.
“The signatures from penetrating high-energy particles in Jupiter’s extreme radiation environment appear as dots, squiggles, and streaks in the images – like static on a television screen. We extract these radiation-induced noise signatures from SRU images to obtain diagnostic snapshots of the radiation levels encountered by Juno,” said Heidi Becker, Juno’s radiation monitoring lead at JPL.
The Advanced Stellar Compass camera, built at the Technical University of Denmark, was used to count very energetic electrons that penetrate its shielding with a measurement every quarter of a second.
The JunoCam imager, conceived to bring the excitement and beauty of Jupiter exploration to the public, has provided an abundance of useful science as well during the mission’s almost five-year tenure at Jupiter. For the Ganymede flyby, JunoCam collected images at a resolution equivalent to the best from Voyager and Galileo. The Juno science team will scour the images, comparing them to those from previous missions, looking for changes in surface features that might have occurred over four-plus decades. Any changes to crater distribution on the surface could help astronomers better understand the current population of objects that impact moons in the outer solar system.
Due to the speed of the flyby, the icy moon will – from JunoCam’s viewpoint – go from being a point of light to a viewable disk then back to a point of light in about 25 minutes. So that’s just enough time for five images.
“Things usually happen pretty quick in the world of flybys, and we have two back-to-back next week. So literally every second counts,” said Juno Mission Manager Matt Johnson of JPL. “We are going to race past Ganymede at almost 12 miles per second (19 kilometers per second). Less than 24 hours later we’re performing our 33rd science pass of Jupiter – screaming low over the cloud tops, at about 36 miles per second (58 kilometers per second). It is going to be a wild ride.”
On June 7, 2021, NASA’s Juno spacecraft flew closer to Jupiter’s ice-encrusted moon Ganymede than any spacecraft in more than two decades. Less than a day later, Juno made its 34th flyby of Jupiter, racing over its roiling atmosphere from pole to pole in less than three hours. Using the spacecraft’s JunoCam imager, the mission team has put together this animation to provide a “starship captain” point of view of each flyby.
“The animation shows just how beautiful deep space exploration can be,” said Scott Bolton, principal investigator for Juno from the Southwest Research Institute in San Antonio. “The animation is a way for people to imagine exploring our solar system firsthand by seeing what it would be like to be orbiting Jupiter and flying past one of its icy moons. Today, as we approach the exciting prospect of humans being able to visit space in orbit around Earth, this propels our imagination decades into the future, when humans will be visiting the alien worlds in our solar system.”
The 3:30-minute-long animation begins with Juno approaching Ganymede, passing within 645 miles (1,038 kilometers) of the surface at a relative velocity of 41,600 mph (67,000 kph). The imagery shows several of the moon’s dark and light regions (darker regions are believed to result from ice sublimating into the surrounding vacuum, leaving behind darkened residue) as well as the crater Tros, which is among the largest and brightest crater scars on Ganymede.
It takes just 14 hours, 50 minutes for Juno to travel the 735,000 miles (1.18 million kilometers) between Ganymede and Jupiter, and the viewer is transported to within just 2,100 miles (3,400 kilometers) above Jupiter’s spectacular cloud tops. By that point, Jupiter’s powerful gravity has accelerated the spacecraft to almost 130,000 mph (210,000 kph) relative to the planet.
Among the Jovian atmospheric features that can be seen are the circumpolar cyclones at the north pole and five of the gas giant’s “string of pearls” – eight massive storms rotating counterclockwise in the southern hemisphere that appear as white ovals. Using information that Juno has learned from studying Jupiter’s atmosphere, the animation team simulated lightning one might see as we pass over Jupiter’s giant thunderstorms.
The camera’s point of view for this time-lapse animation was generated by citizen scientist Gerald Eichstadt, using composite images of Ganymede and Jupiter. For both worlds, JunoCam images were orthographically projected onto a digital sphere and used to create the flyby animation. Synthetic frames were added to provide views of approach and departure for both Ganymede and Jupiter.
As planned, the gravitational pull of the giant moon has affected Juno’s orbit, resulting in the reduction of its orbital period from 53 days to 43 days. The next flyby of Jupiter, the 35th of the mission, is scheduled for July 21, 2021.
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