Estimated at no more than 12 miles across, the moon dubbed S/2004 N I is the smallest known moon in the Neptunian system, the space agency said Monday.
Small and dim -- roughly 100 million times fainter than the faintest star that can be seen with the naked eye -- the tiny moon even escaped detection by NASA's Voyager 2 spacecraft, which surveyed Neptune's system of moons and rings as it flew past in 1989.
Mark Showalter of the SETI Institute in Mountain View, Calif., discovered 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."
Showalter tracked the movement of a white dot that appeared in more than 150 archival Neptune photographs taken by Hubble from 2004 to 2009.
The moon, located between the orbits of the Neptunian moons Larissa and Proteus, completes one revolution around Neptune every 23 hours, he said.