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 planet.)
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.
Credits
NASA, ESA, and the Hubble Heritage Team (STScI/AURA)| About The Object | |
|---|---|
| Object Name | Jupiter |
| Object Description | Planet |
| Dimensions | Jupiter has a diameter of roughly 88,789 miles (142,984 km) at the equator. |
| About The Data | |
| Data Description | The image was created from Hubble data from the following proposal: PI: Z. Levay (STScI), K. Noll (NASA/GSFC), M. Mutchler, J. Mack, L. Frattare, C. Christian, M. Livio, and S. Meyett (Hubble Heritage Team, STScI/AURA), A. Roman (STScI), A. Simon (NASA/GSFC), M.H. Wong (University of California, Berkeley), and R. Landis (NASA/JSC). |
| Instrument | HST>WFC3/UVIS |
| Exposure Dates | January 24, 2015 |
| Filters | 395 nm, 502 nm, and 631 nm |
| About The Image | |
| Color Info | These images are composites of separate exposures acquired by the WFC3 instrument on the Hubble Space Telescope. Several filters were used to sample narrow wavelength ranges. The color results from assigning different hues (colors) to each monochromatic (grayscale) image associated with an individual filter. In this case, the assigned colors are: Blue: F395N (395 nm) Green: F502N (502 nm) Red: F631N (631 nm) |
| Compass Image |  |
| About The Object | |
|---|---|
| Object Name | A name or catalog number that astronomers use to identify an astronomical object. |
| Object Description | The type of astronomical object. |
| R.A. Position | Right ascension – analogous to longitude – is one component of an object's position. |
| Dec. Position | Declination – analogous to latitude – is one component of an object's position. |
| Constellation | One of 88 recognized regions of the celestial sphere in which the object appears. |
| Distance | The physical distance from Earth to the astronomical object. Distances within our solar system are usually measured in Astronomical Units (AU). Distances between stars are usually measured in light-years. Interstellar distances can also be measured in parsecs. |
| Dimensions | The physical size of the object or the apparent angle it subtends on the sky. |
| About The Data | |
| Data Description |
|
| Instrument | The science instrument used to produce the data. |
| Exposure Dates | The date(s) that the telescope made its observations and the total exposure time. |
| Filters | The camera filters that were used in the science observations. |
| About The Image | |
| Image Credit | The primary individuals and institutions responsible for the content. |
| Publication Date | The date and time the release content became public. |
| Color Info | A brief description of the methods used to convert telescope data into the color image being presented. |
| Orientation | The rotation of the image on the sky with respect to the north pole of the celestial sphere. |
