Neptune’s new moon

A composite Hubble picture showing the location of the moon.

THE National Aeronautics and Space Administration’s (NASA) Hubble Space Telescope has discovered a new moon orbiting the distant blue-green planet Neptune. It is the planet’s 14th known moon and completes one revolution in 23 hours. Designated S/2004 N 1, it is estimated to be about 20 km across, making it the smallest known moon in the Neptunian system. It is also about 100 million times fainter than the faintest star that can be seen with the naked eye. It escaped detection by Voyager 2, which flew past Neptune in 1989. Mark Showalter of SETI Institute, California, discovered it on 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. It’s the same reason a 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. On a whim, Showalter looked far beyond the ring segments and noticed the white dot about 105,000 km from Neptune, located between the orbits of the moons Larissa and Proteus. The dot is S/2004 N 1.

R. Ramachandran