Hubble captures rare mutual occultation of Ganymede and Io on October 28

NASA’s Astronomy Picture of the Day for Oct. 28, 2026 showcases a rare mutual occultation of Jupiter’s moons Ganymede and Io, captured by the Hubble Space Telescope as the two bodies aligned from Earth’s perspective. The image, released early Thursday, shows the larger Ganymede sliding across the face of the smaller Io, creating a brief, striking silhouette against the bright glare of Jupiter’s reflected light. The event occurred at 02:17 UT and was highlighted by NASA as a visual reminder of the dynamic dance of the gas giant’s satellite system.

Mutual occultations of Jupiter’s moons are uncommon because they require a precise alignment of the moons’ orbital planes with Earth’s line of sight, a condition that only repeats every few years during Jupiter’s equinox season. This year, a series of such events has been predicted, offering astronomers a valuable window to refine the moons’ orbital models. The October 28 occultation follows a similar event involving Europa and Io earlier in the month, underscoring a busy period for Jovian observations. The APOD feature notes that these alignments are not just photogenic; they provide critical data for improving navigation of future spacecraft that may orbit or fly past Jupiter.

The accompanying explanation, written by planetary scientist Dr. Lena Ortiz of the Southwest Research Institute, points out that the brief dimming of Io’s reflected light as Ganymede passes in front can be measured to within a few milliseconds, tightening estimates of the moons’ positions by tens of meters. Ortiz added that the high‑resolution Hubble image, taken with the Wide Field Camera 3, reveals subtle atmospheric scattering on Io’s surface, hinting at volcanic activity that may be temporarily obscured. The image also captures faint auroral glows on Jupiter’s limb, a reminder of the planet’s powerful magnetic environment.

Scientists plan to use the timing and photometric data from this occultation to update the ephemerides that guide missions such as ESA’s JUICE and NASA’s Europa Clipper. Public interest is expected to rise as the APOD post circulates on social media, encouraging amateur astronomers to watch for the next alignment later this year. The event underscores how routine observations can yield precise measurements that advance both scientific understanding and future exploration of the Jovian system.