Mohawk Valley Astronomical Society

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Among the Galileans

by Perry Pezzolanella

On August 5, 2011, Juno blasted off on a mission to Jupiter. Arriving on July 4, 2016, Juno settled into orbit to begin exploring Jupiter’s turbulent atmosphere and ferocious magnetic field. A sticky valve prohibited the spacecraft from settling into a 14-day orbit but ended up in an elongated 53-day orbit. This problem actually benefits Juno as it spends less time exposed to the intense radiation and can explore years longer than planned. The mission was to end during February 2018 but was extended to July 2021.

The elongated polar orbit prohibited Juno from approaching Jupiter’s Galilean Moons: Callisto, Ganymede, Europa, and Io. The planet was the main focus of the mission anyhow. The recent approval to extend Juno’s mission to September 2025 will make it possible to visit three of these moons 16 times: 2 visits to Ganymede, 3 to Europa, and 11 to Io. Juno has a public outreach camera called JunoCam. Images from Juno are downloaded into a database and shared with anyone who wishes to process them. This idea has yielded some of the most beautifully detailed images of Jupiter’s rippling clouds, scalloped belts, swirling cyclones, and the Great Red Spot.

The moon encounters will begin with a planned flyby of Ganymede on June 7, 2021 from a distance of only 650 miles. Europa will first be encountered on October 16, 2021, with a breathtakingly close flyby occurring on September 29, 2022 from a distance of only 190 miles! The first Io flyby occurs on July 5, 2022 from a distant 52,700 miles with a pair of close flybys occurring on December 30, 2023 and February 3, 2024 around 900 miles from the volcanically active surface, safely above the highest plumes. The flybys of the moons will trim Juno’s orbit around Jupiter from 53 days to 33 days.

Juno will study the geological features of Ganymede, its rarified oxygen atmosphere, suspected aurora, and its weak magnetic field. It will also use its ice penetrating radar to determine the thickness of the icy crust and the characteristics of the ocean beneath. Europa will be explored for signs of geysers and transient plumes along with the thickness of the icy crust. Juno will image and monitor Io’s erupting volcanoes, flowing lava, and measure its heat flow. Juno will also study the surface composition of minerals and ices of the moons and measure the radiation field among the moons. Callisto will be too far away for Juno to study.

Hopefully, Juno will be able to carry out this extended mission to the end, but there is risk. Juno is solar powered and the increasing accumulation of radiation throughout its lifetime, especially during close flybys of Io where radiation is intense, will degrade the solar panels with time. If loss of control of Juno is anticipated, it will be programmed to impact Jupiter to avoid possibly contaminating Europa with any earthly microbes. This was done with Galileo at Jupiter in 2003 and Cassini at Saturn in 2017.

Juno will be gathering vital data for future missions and its journey among the Galileans will compliment those missions. Next up is the launch of JUICE (Jupiter Icy Moons Explorer) during June 2022. This ESA (European Space Agency) mission will arrive at Jupiter in 2029 and go into orbit around Ganymede for at least two years starting in 2032. NASA plans to launch the Europa Clipper in 2024 with arrival at Jupiter in 2030. It will fly close by Europa at least 44 times as it orbits Jupiter. NASA is also planning the Io Volcano Observer mission and if approved, it could launch as soon as 2026 flying closely by Io at least nine times in the early 2030s while orbiting Jupiter. Even Callisto may not be ignored as China is seriously considering a Jupiter-Callisto orbiter that would first orbit Jupiter and then Callisto with the hope of deploying a lander to Callisto’s surface. It could launch in 2029 and arrive in 2035 if approved.

The “Golden Age of Jovian Exploration” may already be under way. Juno started it all by pioneering the way for an armada of sophisticated spacecraft that will make awesome discoveries that will stretch the imagination!