Original: It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than through telescopic photography.
Simplified: What Adrastea is made of and the mass of Adrastea are not known, but assuming that its mean density is like that of Amalthea (~ 0.86 g\/cm) its mass can be estimated at ~ 2 × 1015 kg.