Today’s anniversary is the discovery of Neptune‘s second largest moon, Proteus, found by analysis of Voyager 2 snapshots taken over a period of time leading up to June 16th 1989. So, while June 16th isn’t the actual discovery date, it’s as close as we’re likely to get. It is thought that Proteus wasn’t formed at the same time as Neptune, but is a by-product of the capture of Triton.
Proteus is approximately 418 km in diameter (about 260 miles) and orbits Neptune close to the equatorial plane at a distance of a little over 117,000 km. But aside from this, and the fact that it is dark and heavily cratered, almost nothing else is known about it.
Proteus is named after a shape-changing sea god, son of Poseidon, the Greek god whose Roman equivalent is Neptune. Neptune’s moons are generally named after children or other associates of Poseidon (Triton, for example, was his other son).