Planetary astronomer Mark Showalter is rabid about rings. While everyone knows that Saturn has a spectacular ring system, it’s often forgotten that Jupiter, Uranus and Neptune are also encircled by rings, albeit ones that are fainter and narrower. Each of these systems interacts closely with a family of small, inner moons. Dr. Showalter works on some of NASA’s highest-profile missions to the outer planets, including Cassini, now orbiting Saturn, and New Horizons, which flew past Jupiter en route to its 2015 encounter with Pluto. Known for his persistence in planetary image analysis, his work on the earlier Voyager mission led to his discovery of Jupiter’s faint, outer “gossamer” rings and Saturn’s tiny ring-moon, Pan.
Dr. Showalter was recently granted three more years to study the system of rings and moons orbiting Uranus with the Hubble Space Telescope. Then, this summer, he discovered a new moon orbiting Neptune.
Designated S/2004 N 1, the Neptune moon was previously overlooked by astronomers due to its small size and the speed at which the tiny body orbits the planet — one time every 23 hours. The finding makes S/2004 N 1 the 14th known moon of Neptune and the smallest, estimated to be just 14 miles across. It is so small and so dim, that it’s about 100 million times fainter than the faintest star that can be seen with the naked eye — a body so miniscule that it escaped even the eagle eyes of NASA’s Voyager 2 spacecraft, which flew past Neptune in 1989, surveying the planet’s system of moons and rings.
The discovery is significant since astronomers studying Neptune’s moons previously thought that the smallest moons orbit closer to the planet. However, tiny S/2004 N 1 is located between Neptune’s second- and third-largest moons.
Using a method similar to one often employed by action photographers, Dr. Showalter’s intuition, creativity and doggedness led him to discover the moon, a tiny, white dot about 65,400 miles from Neptune, in Hubble images taken between 2004 and 2009.
“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 in a statement released by NASA. “It’s the same reason a sports photographer tracks a running athlete — the athlete stays in focus, but the background blurs. The procedure I devised predicts where any given moon ought to move from one image to the next, and then combines the images with a ‘twist’ that compensates for the expected motion. It was only when I expanded my analysis out to regions well beyond Neptune’s ring system that an extra little dot turned up, over and over again. In less than a week, we went from our first detection to ten.”
In addition to helping to discover Saturn’s moon Pan, Dr. Showalter discovered Mab and Cupid, two moons of Uranus (and got to name them, after two Shakespeare characters); and Styx and Kerberos, two moons of Pluto. The images he used for his latest discovery have been in the public domain for years, so “anyone could have discovered this,” he says. But nobody else did until this astronomer, armed with curiosity and persistence, saw the tiny, white dot.
Dr. Showalter has been leading a team of astronomers studying rings and moons since 2002. For the next few years, he hopes that this work will illuminate the subtle interactions at play within a dense pack of moons and rings that circle Uranus. The system shows signs of chaotic motion, which means that the orbits are slightly unpredictable and collisions between moons can occur on time scales as short as one million years. That may sound like a long time, but it is astonishingly short for a system that is more than 4 billion years old.
Find out who we chose as our Rebel of the Week last week: http://info.cunyba.gc.cuny.edu/blog/bid/338856/Who-is-CUNY-BA-s-Rebel-of-the-Week-Sept-30-2013