from july to september 1989, "voyager 2" discovered six new moons of neptune. of these, proteus is the second most massive neptunian moon. it has only one quarter of 1% of the mass of triton. neptune's closest four moons, naiad, thalassa, despina, and galatea, orbit close enough to be inside neptune's rings. the next farthest out, larissa was originally discovered in 1981 when it had covered up a star. the moon was credited for causing neptune's ring arcs when "voyager 2" observed neptune in 1989. five new unusual moons discovered between 2002 and 2003 were announced in 2004. the latest moon, hippocamp, was discovered from examining hubble telescope images on 16 july 2013.