An amateur’s observations of Saturn’s satellite Janus

2020 August 2

Mike Foulkes

In these difficult times during the COVID-19 virus pandemic, I am sure that some BAA members have been using the lockdown period to catch up on a number of astronomical tasks.

During 2017, Polish amateur Milosz Leszewicz from Gdynia in Poland (Figure 1) visited Barbados to undertake some planetary imaging using a 356mm Schmidt–Cassegrain and ASI 290MM camera. He has recently been making use of the lockdown time to reprocess some of his Saturn images.

A series of images taken between 06:51 and 09:20 UT on 2020 Apr 25 were reprocessed to show several of Saturn’s satellites, plus some field stars. Inspection of all these images revealed an unidentified faint object lying just outside the outer edge of Ring A. Over the time period, this object appeared to be rotating around the planet but with a more rapid motion than that of the other identified satellites. This rapid motion is well shown in an animation made up from the images.

Two processed images taken during this sequence at 06:51 and 08:19 UT are shown in Figures 2 & 3, respectively. Each of these figures shows the satellites Tethys, Enceladus and Mimas plus the unidentified object. North is at the top in both figures. These figures also show diagrams of the predicted positions of several of the satellites, derived from the online NASA Planetary Data System software.

As can be seen from the figures, the position of the unidentified object corresponds to that of the satellite Janus, with very good agreement. The same results were obtained when examining all of the other images in this sequence. Further, images taken on 2020 Apr 9 show a similar object, also matching the predicted position of Janus.

In many references, Janus is cited as having a typical opposition magnitude of +14.5. This, coupled with its orbit lying close to the outer edge of Ring A, means that it is a difficult object to detect. Indeed, Janus was only discovered by Audouin Dollfus at the Pic Du Midi Observatory on 1966 Dec 15, during a period when the rings were close to edge-on and their glare was considerably reduced. Subsequent predictions of the satellite’s position were discordant with observations. However in 1978, Stephen Larson and John Fountain reassessed the observations. This analysis suggested that the observations were better fitted by there being two satellites in a similar orbit. This was later confirmed by spacecraft, with the second small satellite named Epimetheus.

Janus is not considered to be an object visible to amateurs. This detection of the satellite is a very good achievement, particularly as this occurred when the ring inclination to Earth was near to its maximum.