During its recent flyby of Earth, ESA's Jupiter Icy Moons Explorer (Juice) travelled through the zones of charged particles that surround our planet. These two zones are known as the Van Allen radiation belts. The inner belt is mostly full of energetic protons, and the outer belt is mostly full of energetic electrons. The region between the two belts is mostly empty.

The high levels of radiation in the Van Allen belts makes them very dangerous for electronics and humans, but they pale in comparison to Jupiter's own radiation belts. At Jupiter, extremely energetic electrons can get through even the thickest of shielding, so they could damage Juice's scientific instruments over time.

Juice carries a radiation monitor called RADEM to continuously measure the spacecraft's exposure to high-energy particles. RADEM forms part of a long-term plan to better understand radiation throughout the Solar System, and supplements Juice's Plasma Environment Package – a collection of sensors designed to measure charged particles around Jupiter and its icy moons.

Juice's flight through the Van Allen belts was RADEM's first big test in space. It passed with flying colours, successfully measuring electrons in the outer belt, then protons in the inner belt, then electrons again as it moved away from Earth. The blue and yellow dots indicate the intensity of electrons and protons that Juice measured; in both cases the intensity peaks as Juice goes through the densest part of the belt.

These observations demonstrated the capabilities of RADEM and provided a great opportunity to cross-calibrate the instrument with other spacecraft orbiting Earth in a well-known environment.

More about Juice's lunar-Earth flyby

More about Juice's instruments

[Image description: Black background with planet Earth at the centre. We see a spacecraft travelling around Earth from right to left. Concentric white circles surround Earth, indicating distance. It is possible to move a slider to switch between two images that both have all of the aforementioned elements. The left image shows a ring of white closely surrounding Earth; blue dots (protons) are plotted along Juice's trajectory, peaking where Juice travels through this white band. The right image shows a ring of white further from Earth; yellow dots (electrons) are plotted along Juice's trajectory, peaking in the two places where Juice travels through this white band.]