NASA’s Juno Space Probe Zooms by Largest Moon in the Solar System

The flyby will be the closest any spacecraft has flown near Ganymede in over two decades

An image of Jupiter's largest moon, Ganymede. The images are geologic maps of the moon taken by NASA's Voyager 1 and 2 spacecraft and NASA's Galileo spacecraft.
Streaked across Ganymede's surface are bright light-colored regions of ridges and grooves that overlap darker-colored terrains along the icy shell. The textured scar-like areas suggest that the moon's surface underwent extreme geologic changes over time. USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech

NASA's Juno space probe will zoom by Jupiter's largest moon Ganymede this week, collecting new data about the moon for the first time in two decades. The spacecraft, which has been orbiting planet Jupiter since 2016, will soar 645 miles above Ganymede's icy surface at 43,200 miles per hour.

The flyby will give researchers a better understanding of the enormous moon's water-ice crust and magnetic field to help prepare for future missions to Jupiter, reports NPR's Joe Palca.

"Juno carries a suite of sensitive instruments capable of seeing Ganymede in ways never before possible," said Scott Bolton, Juno's principal investigator at the Southwest Research Institute in San Antonio in a statement. "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."

The Jovian moon is as fascinating as it is mysterious. The natural satellite is larger than the planet Mercury, and it's the only moon in our solar system with a magnetic field, reports Kim Lyons for the Verge. Ganymede's dense, iron-rich core produces the magnetic field, which creates visible ribbons of glowing auroras around its north and south poles. Surrounding the core is a spherical shell of rock and a 497-mile-thick ice shell that envelops and makes up the moon's surface. In 1996, the Hubble Space Telescope found evidence of a thin layer of oxygen-rich atmosphere trapped in its icy surface. While the atmosphere is too thin to support life as we know it, researchers suspect that there could have been life-producing conditions on Ganymede and other icy moons at some point, NPR reports.

Juno's flyby will be the closest any spacecraft has been to the frozen Jovian moon since NASA's Galileo space probe zipped by Ganymede in 2000. Before 2000, NASA's twin Voyager probes observed the moon in 1979. Aboard Juno are several instruments designed to photograph the moon and gather more data on its composition and icy shell. The tools include three different cameras, various radio instruments, an Ultraviolet Spectrograph (UVS), Microwave Radiometer (MWR), and the Jovian Infrared Auroral Mapper (JIRAM), reports Meghan Bartels for Space.com. Juno will begin collecting data three hours before it arrives at its closest approach of Ganymede.

Streaked across Ganymede's surface are bright light-colored regions of ridges and grooves that overlap darker-colored terrains along the icy shell. The textured scar-like areas suggest that the moon's surface underwent extreme changes over time. There's even some evidence that an ocean lies beneath Ganymede's surface, CNN reports. Scientists will use the microwave radiometer to identify what the lighter and darker patches on the moon are made of and how the moon maintains its frozen shell, per Space.com.

"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 in a statement. "[The microwave radiometer] 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 Juno mission's JunoCam, which has previously taken gorgeous images of the swirly gas giant, Jupiter, will also take photos of the planet's largest moon—but will need to be quick. The JunoCam will only have 25 minutes to snap five photos as the probe speeds by Ganymede, Space.com reports. Researchers will compare the images to those taken by the Voyager probes and Galileo spacecraft.

The Juno spacecraft will use the opportunity to make another flyby of Jupiter to help researchers plan future Jovian System missions. The missions include NASA's Europa Clipper and the European Space Agency's Jupiter Icy moons Explorer (JUICE) mission. Both missions focus on searching for life on icy moons and making detailed observations of their surfaces, CNN reports.