Janus (moon) - Discovery and Orbit

Discovery and Orbit

The following is a summary. For more detailed information about Janus and Epimetheus's unusual shared orbit, see Epimetheus.

Janus occupies practically the same orbit as the moon Epimetheus. This caused some confusion for astronomers, who assumed that there was only one body in that orbit, and for a long time struggled to figure out what was going on. It was eventually realized that they were trying to reconcile observations of two distinct objects as a single object.

The discovery of Janus is attributed to its first observer: Audouin Dollfus, on December 15, 1966. The new object was given the temporary designation S/1966 S 2. Previously, Jean Texereau had photographed Janus on October 29, 1966 without realising it; Dollfus named it at the same occasion. On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus.

Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Voyager 1 confirmed this in 1980. (See Co-orbital moon for a more detailed description of their unique arrangement.)

Janus was observed on subsequent occasions and given different provisional designations. It was observed by the Pioneer 11 probe when it passed near Saturn on September 1, 1979: three energetic particle detectors observed its "shadow" (S/1979 S 2.) Janus was observed by Dan Pascu on February 19, 1980 (S/1980 S 1,) and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on the 23rd (S/1980 S 2.)

All of these people thus share, to various degrees, the title of discoverer of Janus.