Here's a photo-op pretty enough to tempt any astronomer. With a series of well-timed shots, NASA's Hubble Space Telescope has managed to catch three of Jupiter's biggest moons whizzing across its surface in the same frame.
This Jovian lunar gathering, which took place Jan. 24, is a relatively rare event -- it happens only once or twice a decade. Hubble had to snap a series of images in fairly quick succession to capture the brief moment when all three could be seen hovering over the colorfully banded planet.
The three moons, Europa, Callisto and Io, are three of the four Galilean satellites -- so named because astronomer Galileo Galilei discovered them in 1610. (The largest, Ganymede, was off-screen and too far to be considered part of the conjunction.) These four moons are Jupiter's largest, and among the largest bodies in the solar system.
In this image, the Jovian moons are traveling from the bottom left toward the top right as they cross Jupiter’s midriff, and their shadows are cast far in front of them; near the beginning of conjunction, Callisto’s shadow looks like it’s almost directly beneath Io.
The Hubble images also showcase in a single frame how different each of these moons are. Io, the leading moon at the top right, is an orange-yellow world, riddled with active volcanoes. Callisto, second in line, is ancient, cratered and brownish in color. Europa, the trailing moon in the bottom left, is a yellow-white, reflecting the ice covering its frozen exterior. (Europa has come to the forefront recently as one of the "water-worlds" in our solar system that may hide a subsurface ocean, and hold the potential for life.)
The shadows give away how far the moons are from the surface: A sharper shadow means the moon is close to Jupiter’s thick cloud tops, and a fuzzier one means it’s farther away, as the shadow spreads out over larger distances. Io, the innermost Galilean satellite, has a shadow like a clean circle; Callisto, the farthest of the four major moons, leaves a much fuzzier outline.
Jupiter's Galilean satellites have a special place in the history of astronomy: showing that tiny celestial bodies were orbiting larger ones directly contradicted the Aristotelian view of the heavens, which held that all bodies -- the planets, the moon, even the sun -- circled the Earth. More than two decades later, Galileo was put on trial during the Roman Inquisition for presenting a "Copernican" view of the solar system. He was placed under house arrest, where he remained until his death.