Epimetheus is an
inner satelliteIn astronomy, an inner moon is a natural satellite following a prograde, low inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet...
of
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
. It is also known as
Saturn XI. It is named after the mythological
EpimetheusIn Greek mythology, Epimetheus was the brother of Prometheus , a pair of Titans who "acted as representatives of mankind"...
, brother of
PrometheusIn Greek mythology, Prometheus is a Titan, the son of Iapetus and Themis, and brother to Atlas, Epimetheus and Menoetius. He was a champion of human-kind known for his wily intelligence, who stole fire from Zeus and gave it to mortals...
.
Epimetheus occupies essentially the same orbit as the moon
JanusJanus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies essentially the same orbit as the moon Epimetheus...
. Astronomers assumed that there was only one body in that orbit, and accordingly had difficulty determining their orbital characteristics. Observations were photographic and spaced widely apart in time, so that while the presence of two objects was not obvious, the observations were difficult to reconcile with a reasonable orbit.
Audouin DollfusAudouin Charles Dollfus is a French astronomer and aeronaut,specialist in studies of the solar system and discoverer of Janus, a moon of Saturn.- Astronomical Career and Research :...
observed a moon on December 15, 1966, which he proposed to be named "Janus".
Epimetheus is an
inner satelliteIn astronomy, an inner moon is a natural satellite following a prograde, low inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet...
of
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
. It is also known as
Saturn XI. It is named after the mythological
EpimetheusIn Greek mythology, Epimetheus was the brother of Prometheus , a pair of Titans who "acted as representatives of mankind"...
, brother of
PrometheusIn Greek mythology, Prometheus is a Titan, the son of Iapetus and Themis, and brother to Atlas, Epimetheus and Menoetius. He was a champion of human-kind known for his wily intelligence, who stole fire from Zeus and gave it to mortals...
.
Discovery
Epimetheus occupies essentially the same orbit as the moon
JanusJanus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies essentially the same orbit as the moon Epimetheus...
. Astronomers assumed that there was only one body in that orbit, and accordingly had difficulty determining their orbital characteristics. Observations were photographic and spaced widely apart in time, so that while the presence of two objects was not obvious, the observations were difficult to reconcile with a reasonable orbit.
Audouin DollfusAudouin Charles Dollfus is a French astronomer and aeronaut,specialist in studies of the solar system and discoverer of Janus, a moon of Saturn.- Astronomical Career and Research :...
observed a moon on December 15, 1966, which he proposed to be named "Janus". On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus. However, at the time, it was believed that there was only one moon, unofficially known as "Janus", in the given orbit.
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. This was confirmed in 1980 by
Voyager 1The Voyager 1 spacecraft is a 722-kilogram robotic space probe of the outer Solar System and beyond, launched September 5, 1977. It remains operational, currently pursuing its extended mission to locate and study the boundaries of the Solar System, including the Kuiper belt and beyond...
, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.
Epimetheus received its name in 1983. The name Janus was approved by the
IAUThe International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
at the same time, although the name had been used informally since Dollfus proposed it shortly after the 1966 discovery.
Orbital relationship between Epimetheus and Janus
Epimetheus and
JanusJanus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies essentially the same orbit as the moon Epimetheus...
are
co-orbitalCo-orbital moons are natural satellites that orbit at the same distance from their parent planet as each other. Only the Saturn system is known to have co-orbital moons, and it has three sets....
: Janus's mean orbital radius from Saturn is currently only 50 km less than that of Epimetheus. This is smaller than either moon's diameter. In accordance with
Kepler's laws of planetary motionIn astronomy, Kepler's three laws of planetary motion are:#The orbit of every planet is an ellipse with the sun at a focus.#A line joining a planet and the sun sweeps out equal areas during equal intervals of time....
, the closer
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
is completed more quickly. As the inner moon catches up to the outer moon, their mutual gravitational attraction boosts the inner moon's momentum and saps the outer moon's momentum. With this added velocity, the inner moon's distance from Saturn and
orbital periodThe orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
are increased, and vice versa for the outer moon. The timing and magnitude of the momentum exchange is such that the moons "trade" orbits, never approaching closer than about 10,000 km. The exchange takes place about once every four years; the last close approach was on January 21, 2006, the next will be in 2010. At that time, Janus's orbital radius will increase by ~20 km, while Epimetheus's decreases by ~80 km; Janus's orbit is less affected because it is 4 times more massive than Epimetheus. As far as it is currently known, this arrangement is unique in the
solar systemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
.
The orbital relationship between Janus and Epimetheus can be understood in terms of the
circular restricted three-body problemThree-body problem has two distinguishable meanings:# In an extended modern sense, a three-body problem is a class of problems in classical or quantum mechanics that model the motion of three particles...
, as a case in which the two moons (the third body being Saturn) are similar in size to each other. Other examples of the three-body problem include the
Lagrangian pointThe Lagrangian points , are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
s, Trojan asteroids and Trojan moons, the
"horseshoe" orbitA horseshoe orbit appears when a viewer on an orbiting body watches the movement of another orbiting body, whose orbit is skinnier , but has about the same period...
of
Cruithne3753 Cruithne is an asteroid in orbit around the Sun in 1:1 orbital resonance with that of the Earth. It is a periodic inclusion planetoid orbiting the Sun in an apparent horseshoe orbit...
with respect to Earth, and potentially dozens of other objects in similar orbits.
Physical characteristics
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 high
albedoThe albedo of an object is the extent to which it diffusely reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure...
, it seems likely that Epimetheus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.
The south pole shows what might be the remains of a large impact crater covering most of this face of the moon, and which could be responsible for the somewhat flattened shape of the southern part of Epimetheus.
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.
Ring
A faint 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 the Cassini spacecraft in 2006. The ring has a radial extent of about 5000 km. Its source are particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.
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