Amalthea

Identity and discovery

Amalthea was discovered by Edward Emerson Barnard on 9 September 1892. It is the largest of the small inner group comprising Metis, Adrastea, Amalthea and Thebe. Its historical designation was Jupiter V and its NAIF code is 505. Software keys in the atlas are not official identifiers.

Shape and size

Its shape is irregular. Galileo images show craters, hills and valleys. JPL’s physical table gives a volume-equivalent spherical radius of 83.50 ± 3.00 km. NASA’s overview cites 83.5 ± 2.4 km: the difference between published uncertainties remains explicit, without turning it into incompatible sizes or choosing an average.

Density and structure

JPL gives a mean density of 1.0111 ± 0.1213 g/cm³ and GM = 0.16456 ± 0.00867 km³/s². GM is a gravitational parameter, not a mass in kilograms; density derives from volume and the mass obtained from GM. These values replace the old figures of 1.8 g/cm³ and 7.17 × 10¹⁸ kg here. A 2005 NASA/JPL description interpreted Galileo findings as indicating abundant ice and a rubble-pile structure. That historical interpretation is not a direct measurement of pure ice or a determined percentage.

Orbit and rings

JPL’s JUP365 mean elements give a semimajor axis of 181,400 km and a period of 0.499918 days, in the Laplace plane at epoch 2000-01-01.5 TDB. NASA summarizes the period as 0.498 days and describes rotation that keeps the same side facing Jupiter. The figures retain separate attribution and are not used as exact current ephemerides. Amalthea and Thebe supply material to the Gossamer ring.

Exploration

According to NASA/JPL, Galileo passed about 99 miles from Amalthea on 5 November 2002. It was the mission’s last moon flyby. The image composite published in 2005 combines Voyager and Galileo observations; publication date is not the acquisition date of every image. Its escape-velocity model illustrates effects of low density and rapid rotation.

Open questions

NASA’s overview proposes electrical currents or tidal stresses as possible explanations for the apparent excess heat: these are hypotheses, not mechanisms established in this profile. The ice-rich interpretation prompted the possibility that Amalthea did not form in its current orbit. It does not establish a migration path, a unique formation chronology or a resolved internal composition.