"The moon, designated S/2004 N 1, is estimated to be no more than 12 miles across, making it the smallest known moon in the Neptunian system. It is so small and dim that it is roughly 100 million times fainter than the faintest star that can be seen with the naked eye. It even escaped detection by NASA's Voyager 2 spacecraft, which flew past Neptune in 1989 and surveyed the planet's system of moons and rings.
Mark Showalter of the SETI Institute in Mountain View, Calif., found the moon July 1, while studying the faint arcs, or segments of rings, around Neptune. 'The moons and arcs orbit very quickly, so we had to devise a way to follow their motion in order to bring out the details of the system,' he said. 'It's the same reason a sports photographer tracks a running athlete -- the athlete stays in focus, but the background blurs.'
The method involved tracking the movement of a white dot that appears over and over again in more than 150 archival Neptune photographs taken by Hubble from 2004 to 2009."
FYI, Neptune's moons are named for minor water deities in Greek mythology.