Juno makes its first ultra-close flyby of the volcano-covered moon Io

On Saturday NASA's Juno spacecraft, which has been orbiting Jupiter for the better part of a decade, made its closest flyby of the innermost moon in the Jovian system.

The spacecraft came to within 930 miles (1,500 km) of the surface of Io, a dense moon that is the fourth largest in the Solar System. Unlike a lot of moons around Jupiter and Saturn, which have surface ice or subsurface water, Io is a very dry world. It is also extremely geologically active. Io has more than 400 active volcanoes and is therefore an object of great interest to astronomers and planetary scientists.

Images from the December 30 flyby were posted by NASA over the New Year holiday weekend, and they provide some of the clearest views yet of this hell-hole world. The new data will help planetary scientists determine how often these volcanoes erupt and how this activity is connected to Jupiter's magnetosphere—Io is bathed in intense radiation from the gas-giant planet.

To date, Juno has mostly observed Io from afar as the spacecraft has made 56 flybys of Jupiter, studying the complex gas giant in far greater detail than ever before. Since arriving in the planetary system in July 2016, Juno has previously gotten to within several thousand miles of the moon. Juno will make another close flyby of Io on February 3, 2024, and this will allow scientists to compare changes on the moon's surface over a short period.

Since its launch on an Atlas V rocket, Juno has performed very well while operating in the Jovian system, surviving extended operations in the harsh radiation of the planet. This is a significant challenge for any spacecraft bound for Jupiter, which must carry radiation-hardened instruments, including its cameras.

"The cumulative effect of all that radiation has begun to show on JunoCam over the last few orbits," said Ed Hirst, project manager of Juno at NASA’s Jet Propulsion Laboratory in Southern California. "Pictures from the last flyby show a reduction in the imager’s dynamic range and the appearance of ‘striping’ noise. Our engineering team has been working on solutions to alleviate the radiation damage and to keep the imager going."

Eventually, the radiation will win, so NASA has a disposal planned for Juno before it ceases to be operational. Originally, the space agency planned to end the vehicle's life in 2018, but because Juno has been such a survivor as it has probed the largest planet in the Solar System, the spacecraft is now planned to operate until September 2025.

At that point, however, it will descend into Jupiter's atmosphere to burn up in order to not contaminate any of the planet's moons with any stray Earth microbes on board, unlikely though that may be.