Want to stay on top of all the space news? Follow @universetoday on TwitterAdrastea, also known as Jupiter XV, is the second innermost, by distance, and the smallest of the four inner moons of Jupiter. The moon was first discovered by Voyager 2 in 1979. It was the first natural satellite discovered by an interplanetary spacecraft. Adrastea is one of the few moons in the solar system known to orbit its planet in less than the length of that planet’s day.
Adrastea orbits at the edge of Jupiter’s main ring and is thought to be the main contributor of material to the rings of Jupiter. Despite observations made in the 1990s by the Galileo spacecraft, very little is known about the moon’s physical characteristics outside its size and the fact that it is tidally locked to Jupiter. What is known is that Adrastea has an irregular shape and measures 20×16×14 km across. This makes it the smallest of the four inner moons. The bulk, composition and mass of Adrastea are not known, but assuming that its mean density is like that of Amalthea(around 0.86g/cm3 its mass can be estimated at about 2×1015kg. Amalthea’s density implies that the moon is composed of water ice and has a porosity of 10–15%. It is thought that Adrastea may be similar.
No surface details of this moon are known because of the low resolution of available images.
Adrastea was discovered by David Jewitt and G. Edward Danielson in Voyager 2 photographs taken on July 8, 1979, and received the designation S/1979 J1. It appeared only as a dot. Soon after its discovery, two more of the inner moons (Thebe and Metis) were observed in the images taken earlier by Voyager 1. The Galileo spacecraft was able to determine the moon’s shape in 1998, but the images were too poor for further details to be determined. Adrastea was officially named, in 1983, after the Greek nymph Adrastea, who was the daughter of Zeus and Ananke.
Adrastea is thought to be the largest contributor of material in Jupiter’s rings. This material appears to consist primarily of material that is ejected from the surfaces of the inner moons by meteorite impacts. It seems that Adrastea contributes the most of the source of ring material, because the densest ring is located within the moon’s orbit. The exact extent of visible ring material depends on the phase angle of the images: in forward-scattered light Adrastea is firmly outside the Main Ring, but in back-scattered light there appears to also be a narrow ringlet outside Adrastea’s orbit.