Janus: The loneliest moon in the solar system

Chuck Bednar for redOrbit.com – @BednarChuck

A moon that shares its orbit with another moon seems like an unlikely candidate to be considered the “loneliest moon in the solar system,” but such is the case with Janus, scientists from NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California explained on Monday.

Janus has the same average distance from Saturn as its sister moon Epimetheus, but is alternately closer and farther away from the planet as the two moons change position about every four years. New images captured of Janus, which is 111 miles of 179 kilometers across, capture the moon at a distance of 1.6 million miles (2.5 million kilometers), the US space agency said.

The image was captured in visible light by the Cassini spacecraft on February 4, and shows the sunlit side of the rings from approximately 19 degrees above the ringplane. It was taken from a phase angle of 91 degrees and is nine miles (15 kilometers) per pixel in scale, they added.

More about Janus and Epimetheus

According to NASA’s Solar System Exploration profile of Janus, it and its neighboring moon are known as the planet’s Siamese twins due to their co-orbital condition. This phenomenon, which is also known as 1:1 resonance, initially baffled astronomers who were stunned that two separate moons could share a nearly-identical orbit around the planet without colliding.

The break-up of a single moon may have led to the formation of Janus and Epimetheus, and if so, this event would likely have taken place early on in the lifespan of the Saturn system, since both moons have ancient cratered surfaces, several soft edges, and grooves apparently indicating they received glancing blows from other objects. Combined, they also produce a faint ring.

Janus and Epimetheus have orbital radial distances from Saturn of approximately 94,100 miles or 151,500 kilometers, with one moon orbiting 31 miles (50 kilometers) higher and moving slower than the other. Due to the velocity difference, the inner moon catches up to the outer one in about four Earth years, and the gravity causing the two moons to change places and velocities.

Also known by astronomers as Saturn X and as S/1980 S1, Janus was discovered on December 15, 1966 by Audouin Dollfus, who proposed that it be named in honor of the Roman god of gates, doors, doorways, beginnings, and endings, NASA said. Three days later, Richard Walker made a similar observation, which is now credited as the discover of Epimetheus, but at the time was viewed by astronomers as two sighting of a single moon unofficially called “Janus.”

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