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Ganymede, or Jupiter III, is the largest and most massive natural satellite of Jupiter, and in the Solar System. Despite being the only moon in the Solar System with a substantial magnetic field , it is the largest Solar System object without a substantial atmosphere.
Its largest moon Charon, named after the ferryman who took souls across the River Styx, is more than half as large as Pluto itself, and large enough to orbit a point outside Pluto's surface. In effect, each orbits the other, forming a binary system informally referred to as a double-dwarf-planet .
Jupiter's magnetic field is the strongest and second-largest contiguous structure in the Solar System, generated by eddy currents within the fluid, metallic hydrogen core. The solar wind interacts with the magnetosphere , extending it outward and affecting Jupiter's orbit.
Although Himalia is the sixth-largest moon of Jupiter, it is the fifth-most massive. Amalthea is only a few km bigger, but less massive. Resolved images of Himalia by Cassini have led to a size estimate of 150 km × 120 km (93 mi × 75 mi), while ground-based estimates suggest that Himalia is large, with a diameter around 170 km (110 mi).
This diagram compares the orbital elements and relative sizes of the largest four members of the Himalia group. The horizontal axis illustrates their average distance from Jupiter, the vertical axis their orbital inclination, and the circles their relative sizes. This diagram illustrates all the irregular satellites of Jupiter.
The Galilean moons are named after Galileo Galilei, who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610; [2] they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter Amalthea in 1892. [3]
The largest of these may have a hydrostatic-equilibrium shape, but most are irregular. Most of the trans-Neptunian objects (TNOs) listed with a radius smaller than 200 km have " assumed sizes based on a generic albedo of 0.09" since they are too far away to directly measure their sizes with existing instruments.
The seven largest natural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together.