Its orbit is also very eccentric, being only 12 million miles from Neptune at perineptune, and 48 million miles from Neptune at aponeptune, and highly inclined, making an angle of about 45 degrees with the rotational plane of the planet, with a backwards (or retrograde) relative to the rotational direction of the planet. It has the largest orbit of any known moon, orbiting Neptune at an average distance of just over 30 million miles.
STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington.Neso (NEE-soh) is a small irregular satellite (a satellite with a distant, often highly inclined, eccentric or retrograde orbit) of Neptune. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). They maintain the peace by never getting too close." "Naiad and Thalassa have probably been locked together in this configuration for a very long time, because it makes their orbits more stable. "We are always excited to find these co-dependencies between moons," said Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, California, and a co-author of the new paper. Researchers used the observations to compute their mass and, thus, their densities - which were close to that of water ice. The work also provides the first hint about the internal composition of Neptune's inner moons. "Only later, after its orbital tilt was established, could Naiad settle into this unusual resonance with Thalassa."īrozović and her colleagues discovered the unusual orbital pattern using analysis of observations by NASA's Hubble Space Telescope. "We suspect that Naiad was kicked into its tilted orbit by an earlier interaction with one of Neptune's other inner moons," Brozović said. So how did they end up together - but apart? It's thought that the original satellite system was disrupted when Neptune captured its giant moon, Triton, and that these inner moons and rings formed from the leftover debris. They are two of Neptune's seven inner moons, part of a closely packed system that is interwoven with faint rings. Naiad and Thalassa are small and shaped like Tic Tacs, spanning only about 60 miles (100 kilometers) in length. Neso, the farthest-flung of them, orbits in a wildly elliptical loop that carries it nearly 46 million miles (74 million kilometers) away from the planet and takes 27 years to complete. Some orbit in the opposite direction their planets rotate othersswap orbits with each otheras if to avoid collision. Some of those moons formed alongside their planets and never went anywhere others were captured later, then locked into orbits dictated by their planets. "There are many different types of 'dances' that planets, moons and asteroids can follow, but this one has never been seen before."įar from the pull of the Sun, the giant planets of the outer solar system are the dominant sources of gravity, and collectively, they boast dozens upon dozens of moons. "We refer to this repeating pattern as a resonance," said Marina Brozović, an expert in solar system dynamics at NASA's Jet Propulsion Laboratory in Pasadena, California, and the lead author of the new paper, which was published Nov. This up, up, down, down pattern repeats every time Naiad gains four laps on Thalassa.Īlthough the dance may appear odd, it keeps the orbits stable, researchers said.
An observer sitting on Thalassa would see Naiad in an orbit that varies wildly in a zigzag pattern, passing by twice from above and then twice from below. In this perpetual choreography, Naiad swirls around the ice giant every seven hours, while Thalassa, on the outside track, takes seven and a half hours. Every time it passes the slower-moving Thalassa, the two are about 2,200 miles (3,540 kilometers) apart. But they never get that close to each other Naiad's orbit is tilted and perfectly timed. The two are true partners, orbiting only about 1,150 miles (1,850 kilometers) apart. Orbital dynamics experts are calling it a "dance of avoidance" performed by the tiny moons Naiad and Thalassa.
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