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S/2003 J4: Overview
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S/2003 J4 is considered a member of the Pasiphae group, a family of Jovian satellites which have similar orbits and are therefore thought to have a common origin.

Most or all of the Pasiphae satellites are thought to have begun as a single asteroid that, after being captured by Jupiter's gravity, suffered a collision which broke off a number of pieces. The bulk of the original asteroid survived as the moon called Pasiphae, and the other pieces became some or all of the other moons in the group.

All of the Pasiphae moons are retrograde, so they orbit Jupiter in the opposite direction from the planet's rotation. Their orbits are also eccentric (elliptical rather than circular) and highly inclined with respect to Jupiter's equatorial plane. All of these characteristics support the idea that the Pasiphae satellites began as one or more captured asteroids, rather than forming as part of the original Jupiter system.

Compared to Jupiter's other satellite groups, confidence is lower that all the moons in the Pasiphae group originated in a single collision, because of differences in color (varying from red to gray), and orbital eccentricity and inclination. Sinope, in particular, is suspected of starting out as an independent asteroid.

If Sinope does not belong in the Pasiphae group, then the individual moon called Pasiphae retains 99% of the mass of the original asteroid. If Sinope is included, Pasiphae still retains the lion's share: 87% of the original mass. None of the Pasiphae members is massive enough to pull itself into a sphere, so they are probably all irregularly shaped.

S/2003 J4 has a mean radius of 1 km, assuming an albedo of 0.04. At a mean distance of about 23.9 million km from Jupiter, the satellite takes about 755 Earth days to complete one orbit.

Discovery:
S/2003 J4 was discovered in February or March 2003 by Scott S. Sheppard, David C. Jewitt, Jan T. Kleyna, Yanga R. Fernandez, and Henry H. Hsieh at the Mauna Kea Observatory in Hawaii.

How S/2003 J4 Got its Name:
S/2003 J4 was so designated because it is a satellite (S) that was discovered in 2003, and was the 4th satellite of Jupiter (J) to be found that year.

Just the Facts
Orbit Size (semi-major axis):  14,868,170 miles
Mean Radius:  0.6 miles
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Last Updated: 9 Mar 2011