Epimetheus occupies essentially the same orbit as the moonJanus. Astronomers originally assumed that there was only one body in that orbit, disbelieving that two moons could share nearly identical orbits without eventually colliding.[7] Thus, there was difficulty in determining their orbital characteristics. Observations were photographic and spaced widely apart in time, so that while the presence of two objects was obvious, the observations were difficult to reconcile with a reasonable orbit.[8]
Audouin Dollfus observed a moon on 15 December 1966,[9] which he proposed to be named "Janus".[10] On 18 December,Richard Walker made a similar observation which is now credited as the discovery of Epimetheus.[11] However, at the time, it was believed that there was only one moon, unofficially known as "Janus", in the given orbit.[7]
Twelve years later, in October 1978,Stephen M. Larson andJohn W. Fountain realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits.[12] This was confirmed in 1980 byVoyager 1,[13] and so Larson and Fountain officially share the discovery of Epimetheus with Walker.[7] A moon that was probably Epimetheus appeared in twoPioneer 11 images and was designatedS/1979S1, though there is uncertainty because the two images were not enough to allow a reliable orbit to be calculated.[14]
Epimetheus received its name in 1983.[b] The name Janus was approved by theIAU at the same time, although the name had been used informally since Dollfus proposed it shortly after the 1966 discovery.[7]
Epimetheus (lower left) andJanus (right) seen on 20 March 2006, two months after swapping orbits. The two moons appear close only because offoreshortening; in reality, Janus is about 40,000 km farther fromCassini than Epimetheus.Rotating-frame depiction of thehorseshoe orbits of Janus and EpimetheusAnimation of Epimetheus's orbit – Rotating reference frame Saturn· Janus· Epimetheus
Epimetheus's orbit isco-orbital with that ofJanus. Janus'smean orbital radius from Saturn is, as of 2006 (as shown by the green color in the picture below), only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius.
In accordance withKepler's laws of planetary motion, the object which is closer to Saturn completes itsorbit more quickly. The orbit is completed only around 30 seconds more quickly, due to the small difference between the moons' distances from Saturn. Each day, the inner moon progresses 0.25° more around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum causes the inner moon's distance from Saturn and itsorbital period to increase while those of the outer moon are decreased.
The timing and magnitude of themomentum exchange is such that the moons effectively swap orbits, never approaching closer than about 10,000 km to each other. At each encounter Janus's orbital radius changes by ~20 km and Epimetheus's by ~80 km: Janus's orbit is less affected because it is four times the mass of Epimetheus. The exchange of orbits takes place approximately every four years; the last close approaches occurred in January 2006,[15] 2010, 2014, 2018, and 2022.
This type of orbit is sometimes referred to as ahorseshoe orbit, due to the shape of each moon's orbit, as seen from the perspective of the other moon. Some asteroids are known to have horseshoe orbits but this is the only known such orbital configuration for moons within theSolar System.[16]
The orbital relationship between Janus and Epimetheus can be understood in terms of thecircular restricted three-body problem, as a case in which the two moons (the third body being Saturn) are similar in size to each other.[17]
There are several Epimethean craters larger than 30 km in diameter, as well as both large and small ridges and grooves. The extensive cratering indicates that Epimetheus must be quite old. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites, but if this is the case the disruption must have happened early in the history of the satellite system. From its very low density and relatively highalbedo, it seems likely that Epimetheus is a very porous icy body.[7] There is considerable uncertainty in these values, however, and so this remains to be confirmed.[citation needed]
The south pole is dominated by a large, flat basin with a central peak that covers much of the moon's southern hemisphere, which may be the remains of a large impact crater.[7]
There appear to be two terrain types: darker, smoother areas, and brighter, slightly more yellowish, fractured terrain. One interpretation is that the darker material evidently moves down slopes, and probably has a lower ice content than the brighter material, which appears more like "bedrock". Nonetheless, materials in both terrains are likely to be rich in water ice.[18]
Afaint dust ring is present around the region occupied by the orbits of Epimetheus and Janus, as revealed by images taken in forward-scattered light by theCassini spacecraft in 2006. The ring has a radial extent of about 5000 km.[20] Its source are particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.[21][22]
Along with Janus, Epimetheus acts as ashepherd moon, maintaining the sharp outer edge of theA Ring in a 7:6orbital resonance. The effect is more obvious when the more massive Janus is on the resonant (inner) orbit.[16]
^Calculated from the standard gravitational parameterGM =(3.50806±0.00054)×10−2 km3·s–2 given by Lainey et al. (2023), divided by thegravitational constantG =6.6743×10−2 km3·kg–1·s–2.[5]
^ Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (confirms Janus, names Epimetheus, Telesto, Calypso) (mentioned inIAUC 3872:Satellites of Jupiter and Saturn 1983 September 30)
