The giant planet Jupiter was "photobombed" by three of its own moons in dramatic Hubble Space Telescope images released by astronomers.
Hubble captured the rare event -- three of the giant planet's biggest moons crossing in front of Jupiter's face together only happens once or twice in a decade -- last month, they report.
In two images taken 40 minutes apart the moons Europa, Io and Callisto were caught passing in front of their planetary host at the same time, throwing their shadows on Jupiter's colorful bands of clouds.
While Jupiter possesses more moons than any other planet in the solar system -- 67 at last count -- the four largest are known as the Galilean moons, after their discoverer Galileo who saw them through his early telescope in the 17th century.
The four -- Europa, Io, Callisto and Ganymede-- circle Jupiter in their separate orbits, so it is rare to see them all reach points in those orbits that put them all in front of Jupiter from the viewpoint of Earth.
Ganymede didn't make the party this time, being outside of Hubble's field of view as the space telescope's Wide Field Camera 3 took the images in natural light on January 23.
The Galilean moons -- which can be seen from Earth with a small telescope or even a good pair of binoculars -- have long intrigued astronomers.
Europa and Callisto are ice-covered bodies that might be hiding subsurface oceans beneath their frozen surfaces. Many scientists believe Europa might even be home to extreme microbial life similar to what is found deep within icy Arctic waters on Earth.
Io is thought to be the most volcanically active inhabitant of our solar system.
Ganymede is the largest moon in all of the solar system, and is even larger than the planet Mercury. Thought to be equal parts rock and ice, it also may have liquid oceans beneath its surface, astronomers say.
The three moons starring in the Hubble images all have their own distinctive colors; Europa with its smooth frozen surface is a yellowish white, Io appears orange due to a surface of volcanic sulphur, while Callisto's much-cratered surface -- one of the oldest in the solar system -- gives it a brown hue.
An unmanned mission to Europa will receive funding if NASA's new budget gets congressional approval, so the intriguing moon may go from starring in Hubble long shots to its very own cosmic close-ups.
