NASA’s Juno spacecraft has finally captured the elusive auroral signature of Callisto, the most distant of Jupiter’s four largest moons—completing a decades-long quest to map the electromagnetic footprints each moon leaves on the gas giant’s atmosphere.

Jupiter’s auroras are the most dazzling in the solar system, glowing near its poles as charged particles interact with the planet’s powerful magnetic field. Unlike Earth’s auroras, Jupiter’s are shaped not only by solar winds but also by its moons. These interactions form distinct “satellite footprints”—electromagnetic trails etched into Jupiter’s upper atmosphere.

Before this breakthrough, only three Galilean moons—Io, Europa, and Ganymede—had confirmed auroral imprints. Callisto’s remained hidden, often masked by Jupiter’s brighter main auroral oval and too faint for previous instruments to detect.

During Juno’s 22nd orbit in September 2019, a rare alignment allowed the spacecraft to cross the magnetic field line connecting Callisto to Jupiter. Simultaneously, a high-density solar stream pushed Jupiter’s auroral oval toward the equator, revealing a faint but unmistakable signature from Callisto.

This discovery not only confirms that all four Galilean moons contribute to Jupiter’s auroral landscape but also deepens our understanding of how planetary magnetospheres interact with their satellites. Juno’s data offers a rich sample of the particles, waves, and magnetic fields involved—opening new doors for space weather research and planetary science.