It’s the closest any spacecraft has got to the solar system’s most volcanic celestial body.
From a mere 22,060 miles/35,500 kilometers away NASA’s spacecraft Juno took these images of Io (pronounced “eye-oh”) on May 17, 2023.
That was mere hours before a rare eclipse of Jupiter by the Moon was visible from North America, an event captured by astrophotographers.
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Io is slightly larger than Earth’s moon and is being constantly tugged by the gravity of Jupiter, but also the other giant moons. Consequently, it spews lava from volcanoes that pockmark its ever-changing surface.
“Io is the most volcanic celestial body that we know of in our solar system,” said Scott Bolton, Juno principal investigator from the Southwest Research Institute in San Antonio. “By observing it over time on multiple passes, we can watch how the volcanoes vary – how often they erupt, how bright and hot they are, whether they are linked to a group or solo, and if the shape of the lava flow changes.”
Juno isn’t the first NASA spacecraft to image Io—it’s been imaged by Voyager, Cassini on its way to Saturn and the Pluto-bound New Horizon on flybys, and by Jupiter orbiter Galileo—but these images are the best yet.
“We are entering into another amazing part of Juno’s mission as we get closer and closer to Io with successive orbits. This 51st orbit will provide our closest look yet at this tortured moon,” said Bolton before the flyby.
After imaging Io, Juno went on to dip close to Jupiter’s north pole and photograph the giant planet, exiting over the south pole to return to a more distant position on what is its elliptical orbit of the planet.
Its JunoCam imager has a 58º field of view and captures two megapixel images, but the raw files it sends back are complex since the spacecraft is spinning as it takes photos. So the images you see here are painstakingly amalgamated and processed, not by NASA scientists but by volunteer citizen scientists.
In recent months JunoCam has suffered from problems with radiation and also with overheating, though this is the third perijove in a row that has performed faultlessly.
It was on its 51st perijove—close flyby—having been orbiting Jupiter since 2016—and can get as close as 2,100 miles/3,400 kilometers.
During that time it’s also conducted close flybys of Europa in 2022, a target for astrobiologists searching for life off-Earth, and Ganymede in 2021, Jupiter’s bigger-than-Mercury moon (though not Callisto, who’s turn will come). In April the European Space Agency’s new JUICE mission took-off to orbit Ganymede for nine months from 2034 to become the first spacecraft ever to orbit a moon other than Earth’s.
However, more and even closer encounters with Io are coming. “Our upcoming flybys in July and October will bring us even closer, leading up to our twin flyby encounters with Io in December of this year and February of next year, when we fly within 1,500 kilometers of its surface,” said Bolton. “All of these flybys are providing spectacular views of the volcanic activity of this amazing moon. The data should be amazing.”
The spacecraft currently orbits Jupiter every 38 days, but after this flyby of Io and another on July 31, its orbital period will remain fixed at 32 days.
Juno has a suite of instruments to measure Jupiter’s interior and auroras to learn more about the planet’s origins, structure, atmosphere and magnetosphere.
However, next for Juno is an examination of Jupiter’s tenuous rings. “As well as continuously changing our orbit to allow new perspectives of Jupiter and flying low over the nightside of the planet, the spacecraft will also be threading the needle between some of Jupiter’s rings to learn more about their origin and composition,” said Juno’s acting project manager, Matthew Johnson of NASA’s Jet Propulsion Laboratory in Southern California.