Juno’s mission of NASA approaches the moon Io of Jupiter.

NASA’s Juno spacecraft will fly by Jupiter’s volcanic moon Io on Tuesday, May 16, and then shortly thereafter the gas giant itself. The Jovian moon’s flyby will be its closest yet, at an altitude of about 22,060 miles (35,500 kilometers). Now in the third year of its expanded mission to investigate the interior of Jupiter, the solar-powered spacecraft will also explore the ring system where some of the gas giant’s inner moons reside.

So far, Juno has made 50 flybys of Jupiter and also collected data during close encounters with three of the four Galilean moons — the icy worlds Europa and Ganymede, and the fiery Io.

“Io is the most volcanic celestial body we know of in our solar system,” said Scott Bolton, Juno principal investigator of the Southwest Research Institute in San Antonio. “By observing them over time over multiple passes, we can watch how volcanoes vary — how often they erupt, how bright and hot they are, whether they are associated with a group or alone, and whether the shape of the lava flow changes.”

Slightly larger than Earth’s moon, Io is a world in perpetual agony. Not only is the solar system’s largest planet forever tugged in by its gravity, so are its Galilean siblings – Europa and the solar system’s largest moon, Ganymede. The result is that Io is constantly being stretched and compressed, actions associated with the formation of lava seen erupting from many volcanoes.

While Juno was designed to study Jupiter, its many sensors additionally provided a wealth of data about the planet’s moons. Together with JunoCam’s visible light imager, the spacecraft’s JIRAM (Jovian InfraRed Auroral Mapper), SRU (Stellar Reference Unit) and MWR (Microwave Radiometer) will study Io’s volcanoes and how volcanic eruptions interact with Jupiter’s powerful magnetosphere and aurora borealis.

“We’re entering another amazing part of the Juno mission as we get closer and closer to Io on successive orbits,” Bolton said. “This 51st orbit will give us our closest look yet at this tortured moon.” “Our next flybys in July and October will bring us closer, leading to our double encounters with Io in December this year and February next year, when we fly within 1,500 kilometers of its surface. All of these flybys provide stunning views of the volcanic activity of this amazing moon. The data must be amazing.”

Half a century in Jupiter

During its flyby of Jupiter, Juno zoomed low over the planet’s cloud tops – close to about 2,100 miles (3,400 kilometers). The spacecraft approaches the planet from above the North Pole and exits from the south during these flybys, uses its instruments to probe beneath the mysterious cloud cover, and studies inside Jupiter and the aurora borealis to learn more about the planet’s origins, structure, atmosphere, and magnetosphere.

Juno has been orbiting Jupiter for more than 2,505 Earth days and has flown more than 510 million miles (820 million km). The spacecraft reached Jupiter on July 4, 2016. The first science flyby occurred 53 days later, and the spacecraft continued in that orbital period until its flyby of Ganymede on June 7, 2021, which reduced the orbital period to 43 days. Europa Flyby’s flight on September 29, 2022 reduced the orbital period to 38 days. After Io’s next two flybys, on May 16 and July 31, Juno’s orbital period will remain constant at 32 days.

“Io is only one of the celestial bodies still under Juno’s microscope during this extended mission,” said Matthew Johnson, acting Juno project manager, of NASA’s Jet Propulsion Laboratory in Southern California. “In addition to constantly changing our orbit to allow new perspectives on Jupiter and flying low over the planet’s night side, the spacecraft will also thread the needle between some of Jupiter’s rings to learn more about their origin and formation.”