He says glare from Jupiter is so strong it would hide anything smaller than 3 kilometers across. “We have searched very deeply for objects near Themisto, and have found nothing else to date,” says Sheppard. It orbits 7.5 million kilometers (4.6 million miles) from Jupiter, roughly halfway between Callisto at 1.9 million km and the group of prograde moons starting at 11 million km. In the yawning gap between Himalia and the Galilean moons, there’s only one moon known: Themisto, a 9-kilometer object discovered by Elizabeth Roemer and Charles Kowal in 1975 but not recovered until 2000. Searches for prograde moons outside these groups turned up nothing. The new discoveries added two of Himalia’s current tally of nine, and one of Carpo’s duo. The prograde objects outside the Galilean moons fall into two groups: the nine moons of the Himalia group orbit 11 to 12 million km from Jupiter, and the more distant duo in the Carpo group at 17 million km. Three missions are in the works for the Jupiter system: the European Space Agency's Jupiter Icy Moon Explorer (JUICE), scheduled for launch in April NASA’s Europa Clipper, set for launch late next year and a Chinese mission being considered for the 2030s. Five were found before 2000, and only eight more have been discovered since then.īesides the interest in their origins, these prograde moons could make suitable targets for a flyby from an upcoming mission. “The reason is that they are closer to Jupiter and the scattered light from the planet is tremendous,” he says. They’re harder to find than the more distant retrograde moons, though, says Sheppard. Three of the newly discovered moons are in among 13 others that orbit in a prograde direction and lie between the large, close-in Galilean moons and the far-out retrograde moons. Only five of all the retrograde moons are larger than 8 kilometers (5 miles) Sheppard says the smaller moons probably formed when collisions fragmented larger objects. Jupiter probably captured these moons, as evidenced by their retrograde orbits, opposite in direction to the inner moons. Nine of the 12 are among the 71 outermost Jovian moons, whose orbits are more than 550 days. New MoonsĪll of the newly discovered moons are small and far out, taking more than 250 days to orbit Jupiter. If we could count all moons measuring at least 3 kilometers across, “Saturn would have more moons than all the rest of the solar system,” says Brett Gladman (University of British Columbia, Canada), who helped identify the new Saturnian objects but was not involved in the Jovian observations. (The fragments have not been tracked carefully enough to count as moons yet, though.) The more numerous Saturnian objects might have come from a collision that disrupted a larger moon a few hundred million years ago. A search for objects with sizes down to about 3 kilometers across that are moving along with the gas giants found three times more near Saturn than near Jupiter. However, while Jupiter may have the most moons for now, Saturn might catch up. The new finds put Jupiter’s lunar family count well ahead of Saturn’s 83 confirmed moons. (Note: The number of moons in this diagram is not up to date.) This top-down diagram shows the orbits of moons around Jupiter: Purple denotes the Galilean moons, yellow for Themisto, blue for the Himalia group, cyan and green for Carpo and Valetudo, respectively, and red for far-out retrograde moons. Other data from Sheppard’s observations even enabled recovery of the last “missing” Jovian moon, S/2003 J 10 the newest observations extended the track of its orbit to 18 years. The MPC's orbital calculations confirm the new objects are in orbit around Jupiter. The discoveries bring the list of Jovian moons to 92, a hefty 15% increase from the previous tally of 80. More publications are expected, says Scott Sheppard (Carnegie Institution for Science), who recently submitted observations of the Jovian system taken between 20. Since December 20th, the Minor Planet Center (MPC) has published orbits for 12 previously unreported moons of Jupiter. The biggest planet in the solar system now has the largest family of moons.
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