NASA/JPL/Space Science Institute

NASA’s Jet Propulsion Laboratory (JPL) recently published an image of Janus, Saturn’s small irregularly-shaped moon. The image was acquired by the Saturn-bound Cassini spacecraft. It shows impact craters of various sizes covering the rugged surface of potato-shaped Janus (179 km across).

This view looks toward the southern hemisphere of Janus. The moon's south pole is at center. The image was obtained with Cassini’s narrow-angle camera on 26 May 2008, using an infrared filter. Cassini was at a distance of approximately 186,000 km from Janus. Image scale is 1 km per pixel.

Epimetheus, a small co-orbital moon of Janus
Credit: NASA/JPL/Space Science Institute

Intriguingly, Janus orbits Saturn every 0.7 days, at a distance of approximately 150,000 km. Janus and its co-orbital moon, Epimetheus (116 km across), represent a unique dynamical case in the Solar System. The orbital radii of Janus and Epimetheus differ only slightly. Their orbital velocities are thus very nearly equal and, faster moon (the closer to Saturn) slowly overtakes the other. As the moons approach each other, they exchange orbit; the result is to boost the faster moon into a higher orbit and to drop the slower moon to a lower orbit. This exchange occurs about every four years.

Janus was discovered by the French astronomer Audouin Dollfus in 1966. In Roman mythology, Janus was the god of gates and doorways, depicted with two faces looking in opposite directions. Janus is also the root of the English word "January".

Further Reading

The Cassini-Huygens Mission to Saturn
http://saturn.jpl.nasa.gov/home/index.cfm
The Eight Planets
www.nineplanets.org/

Aymen Mohamed Ibrahem
Senior Astronomy Specialist