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Firing off a string of snapshots like a sports photographer at a
NASCAR race, NASA's Hubble Space Telescope captured a rare look at
three of Jupiter's largest moons parading across the banded face of the
gas-giant planet: Europa, Callisto, and Io.
These so-called Galilean satellites (named after the 17th century
scientist Galileo Galilei, who discovered them with a telescope)
complete orbits around Jupiter ranging from 2 days to 17 days in
duration. 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 Hubble image on the left (unlabeled at top left, labeled at
bottom left) shows the beginning of the event, which took place on
January 24, 2015. From left to right the moons Callisto and Io are above
Jupiter's cloud tops. The shadows from Europa (not seen in the image),
Callisto, and Io are strung out from left to right.
Near the end of the event, approximately 42 minutes later (right-side
image), Europa has entered the frame at lower left. Slower-moving
Callisto is above and to the right of Europa. Fastest-moving Io is
approaching the eastern limb of the planet; its shadow is no longer
visible on Jupiter. Europa's shadow is toward the left side of the
image, and Callisto's shadow to the right. (The moons' orbital
velocities are proportionally slower with increasing distance from the
Missing from the sequence is the moon Ganymede, which was outside
Hubble's field of view and too far from Jupiter in angular separation
to be considered part of the conjunction.
The moons in these photos have distinctive colors. 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.
The images were taken with Hubble's Wide Field Camera 3 in visible light.
For additional information, contact: Ray Villard Space Telescope Science Institute, Baltimore, Md. 410-338-4514 firstname.lastname@example.org
Zolt Levay Space Telescope Science Institute, Baltimore, Md. 410-338-4907 email@example.com