NASA’s Juno spacecraft is set to make its closest flyby of Jupiter’s moon Io on December 30, which will be the closest any spacecraft has been to the moon in over two decades. The spacecraft will come within about 930 miles (1,500 kilometers) of Io’s surface, allowing Juno instruments to collect a large amount of data. The Juno science team aims to study the variations in Io’s volcanoes by combining data from this flyby with previous observations. They will examine how often the volcanoes erupt, their brightness and temperature, changes in lava flow, and the connection between Io’s activity and charged particles in Jupiter’s magnetosphere.
Another close flyby of Io is scheduled for February 3, 2024, where Juno will again approach within approximately 930 miles (1,500 kilometers) of the moon’s surface.
Previously, Juno has been monitoring Io’s volcanic activity from distances ranging from about 6,830 miles (11,000 kilometers) to over 62,100 miles (100,000 kilometers), providing the first views of the moon’s north and south poles. The spacecraft has also conducted close flybys of Jupiter’s icy moons, Ganymede and Europa.
During the December and February flybys, Juno will investigate the source of Io’s extensive volcanic activity, the possibility of a magma ocean beneath its crust, and the significance of tidal forces from Jupiter. The spacecraft, which is now in its third year of the extended mission to explore the origin of Jupiter, will also examine the planet’s ring system where some of its inner moons reside. All three cameras aboard Juno will be active during the Io flyby. The Jovian Infrared Auroral Mapper (JIRAM) will capture heat signatures emitted by volcanoes and calderas on the moon’s surface. The Stellar Reference Unit will obtain the highest-resolution image of the surface to date, while the JunoCam imager will capture visible-light color images.
JunoCam was included on the spacecraft for public engagement and was designed to operate for up to eight flybys of Jupiter. However, the cumulative effects of radiation have started to impact JunoCam in recent orbits, resulting in reduced dynamic range and noise in the images. The engineering team has been working on solutions to mitigate radiation damage and keep the imager functioning.
After assessing and studying the issue, the Juno team adjusted the spacecraft’s trajectory to include seven additional distant flybys of Io in the extended mission plan. Following the close flyby on February 3, Juno will encounter Io every other orbit, with each orbit becoming more distant. The first distant flyby will be at an altitude of about 10,250 miles (16,500 kilometers) above Io, and the last will be at about 71,450 miles (115,000 kilometers).
The flyby of Io on December 30 will cause Juno’s orbit around Jupiter to decrease from 38 days to 35 days, and it will reduce further to 33 days after the February 3 flyby. Additionally, Juno’s new trajectory will result in Jupiter blocking the sun from the spacecraft for approximately five minutes during perijove, when the orbiter is closest to the planet. This will be the first time Juno encounters darkness since its flyby of Earth in October 2013, but the duration will not affect its overall operation significantly.
From now until the end of its extended mission in late 2025, Juno will encounter solar eclipses during every close flyby of Jupiter, except for the perijove on February 3. Starting in April 2024, the spacecraft will conduct a series of occultation experiments to probe Jupiter’s upper atmosphere using its Gravity Science experiment, providing valuable information about the planet’s shape and interior structure.