Hubble captures rare triple-moon conjunction

NASA's Hubble Space Telescope has captured the rare occurrence of three of Jupiter's largest moons - Europa, Callisto and Io - racing across the banded face of the gas-giant planet.

These so-called Galilean moons, named after the 17th century scientist Galileo Galilei who discovered them with a telescope, complete orbits around Jupiter with durations ranging from two days to 17 days.

They can commonly be seen transiting the face of Jupiter and casting shadows onto its cloud tops.

However, seeing three moons transiting the face of Jupiter at the same time is rare, occurring only once or twice a decade, the US space agency said in a statement.

The images were taken with Hubble's Wide Field Camera 3 in visible light.

The moons in these photos have distinctive colours.

The ancient cratered surface of Callisto is brownish; the smooth icy surface of Europa is yellow-white; and the volcanic, sulfur-dioxide surface of Io is orange.

The apparent "fuzziness" of some of the shadows depends on the moons' distances from Jupiter.

The farther away a moon is from the planet, the softer the shadow because the shadow is more spread out across the disk.