What is a Trojan Moon?
Trojan moons, such as the Telesto moon, are unique in the way that they share orbits with other moons. Learning about them would mean understanding the Lagrange points and the Trojans in general.
There are a lot of exciting relationships among astronomical bodies in the solar system. Some of them are in a co-orbital configuration with other objects. It means that they orbit from a very similar distance from the parent object.
Depending on the point of libration, co-orbital objects fall into several categories. Some follow a horseshoe orbit while other bodies, with about similar masses, exchange orbits. The third class of co-orbital bodies is called the trojans. They are situated in the stable areas of the Lagrange points.
The Lagrange Points
There are five Lagrange points, L1, L2, L3, L4, and L5, located close to two large astronomical bodies in orbit. These points are also called Lagrangian points or libration points.
These five points can be found in relationships between a planet and the Sun or even a moon and its planet. The gravitational forces exerted by these two bodies are balanced out at such points. Because of this, the objects near them are in sync with the movement of the orbiting planet or moon. These bodies never collide because of being locked in this synchronous dance.
The first three points L1, L2, and L3 are on the same line as the two main bodies. However, they are not stable and objects can be easily displaced in such regions. The other two areas, L4 and L5, are stable points. This is where we can see the trojans.
The Stable Lagrange Points
L4 is the leading stable point. It stays 60 degrees ahead of the bigger orbiting body. The other stable point, L5, on the other hand, stays 60 degrees behind it.
Because they are stable, these two points tend to pull asteroids and other objects close to them. It is the reason why we can two big swarms of asteroids in the leading and trailing stable points of the planet Jupiter.
Did You Know?
It is estimated that there about one million Jupiter trojans that measure around 1 km.
Jupiter’s leading swarm is called the “Greek Camp” while the trailing group is called the “Trojan Camp.”
Trojans were also discovered in the orbits of other planets like Neptune, which has nearly 30 of them, Mars, and Uranus. Even the Earth also has a trojan asteroid called 2010 TK7. Large asteroids can also have their own trojans and it is believed that trojan planets also exist!
Trojan moons are in the same system as the trojan asteroids. Only this time, we take a look at a planet-moon system where the moon, of course, orbits the planet. The objects in the stable points of this moon are not minor planets but smaller natural satellites. They orbit the planet relative to the bigger moon, staying either 60 degrees ahead of or behind it.
The Telesto Moon
Telesto Moon: Facts and Figures
All About the Name
- Pronunciation – /təˈlɛstoʊ/
- Behind the name – Telestō (An Oceanid in Greek mythology)
- Adjective – Telestoan / Telestoian
- Other designation/s – Saturn XIII / Tethys B
- Provisional designation – S/1980 S 13
- Discoverer/s –
- B. Smith
- H. Reitsema
- S. Larson
- J. Fountain
- Discovery date – April 8, 1980
- Parent Planet – Saturn
- Orbital period – 1.887802 day/s (45 hours)
- Average Orbit Distance – 294, 619 km
- Mean Orbit Velocity – 40,865.9 km/h
- Orbital Eccentricity – 0.0002
- Orbital Inclination – 1.19° (to Saturn’s equator)
- Dimensions – 32.6 ± 1.0 km × 23.6 ± 0.6 km × 20 ± 0.6 km
- Equatorial radius – 12.4 ± 0.4 km
- Equatorial circumference – 77.9 km
- Volume – 7,986 km3
- Mass – 4,046,404,769,362,420 kg
- Surface area – 1,932.21 km2
- Density – 0.5 g/cm3
- Surface gravity – 0.002 m/s2
- Rotation – Synchronous
- Axial tilt – Zero
- Apparent Magnitude – 18.7
Telesto Moon Features
Telesto is a natural satellite of Saturn. We can also call it Saturn XIII. It was discovered in 1980 through ground-based observations.
Telesto can be found in the wide E ring of Saturn between the major moons Enceladus and Dione. It is considered a co-orbital moon as it shares an orbit with the moon Tethys, a body made mostly of water ice. What makes these objects extraordinary is that they maintain a harmonious relationship with each other, never colliding. But how so?
The main moon of the said orbit is the bigger, Tethys. A collision between the two is nothing to worry about because Telesto remains 60° ahead of Tethys’s in its leading Lagrangian point or L4. Because of this, it is called a trojan moon as it has the same orbital arrangement as the trojan asteroids. Another designation for Telesto is Tethys B.
The moon Calypso also has the same relationship as Telesto. However, it lies in the trailing Lagrangian point or L5 of Tethys. Together with Telesto and Calypso, the other pair of Saturn’s trojan moons, Helene and Polydeuces, can be found sharing an orbit with the larger moon Dione.
Did You Know?
Saturn is the only planetary system known to have trojan moons.
Telesto was discovered on April 8, 1980, through telescopic observations here on Earth. It was observed by a team of four astronomers, Bradford A. Smith, Harold Reitsema, Stephen M. Larson, and John W. Fountain. They used a coronagraphic imaging system when they discovered the moon.
Telesto’s mean radius is roughly 12.4 km or about 7.7 miles. Its dimensions measure approximately 32.6 km × 23.6 km × 20 km. It is larger than Calypso, the trailing trojan of Tethys, whose equatorial radius is 10.7 km.
The largest body of the three, Tethys, is 43 times bigger than the trojan Telesto.
Based on its smooth surface, Telesto is believed to be made up of fine icy material the size of dust. This composition is likely similar to the ring shepherd moon, Pandora, which possibly has a porous and icy body.
Other properties of this moon such as temperature and albedo are still to be determined.
Telesto’s orbit is inclined to Saturn’s equator by about 1.19°. With a semi-major axis of 294,619 km, this small moon orbits Saturn with the same average distance as Tethys and Calypso.
Telesto moves around Saturn in a prograde motion, that is, in the same direction that the gas giant rotates. It takes for this moon nearly two Earth days or about 45.3 hours to completely circle the ringed planet.
Telesto acts as the leading trojan of Tethys, staying 60 degrees ahead in its leading Lagrangian point. It is not known to form any ring near its orbit nor contribute particles in the E ring’s formation, as the icy moon Enceladus is mostly responsible for that.
What Does Telesto Look Like?
The moon Telesto has an irregular shape which makes it look like a potato. A Cassini flyby in 2005 revealed that it has a smooth surface. There are hardly any small impact craters observed on this moon, unlike most of Saturn’s other satellites.
The monochrome image, as shown above, was taken on December 25, 2005. The Cassini spacecraft was about 20,000 kilometers away from Telesto at the time. Using a narrow-angle camera, the probe captured the image of the tiny moon with a scale of 118 meters per pixel.
Regional color differences were observed on Telesto’s surface. The differences in the size of ice particles on this moon’s surface are thought to be responsible for this variation.
Behind the Name
Upon discovery, Tethys’s leading trojan moon was given the temporary designation S/1980 S 13. The permanent name Telesto (Τελεστώ) was awarded to it in 1983. It means “success” in English.
The name was based on a character from Greek mythology, just like how the other Saturnian moons were named.
The Oceanid Telesto
Telesto was one of the three thousand Oceanids. The parents of these water nymphs were no other than the Titans Tethys and Oceanus. Their brothers, who also are great in number, are called the Potamoi.
Telesto and the rest of her family are associated with bodies of water. Her father, Oceanus, was the great river encircling the world. Tethys, her mother, who was also a sister of her father, was a great goddess of the sea. The Potamoi personified the great world’s great rivers.
Telesto and her sisters were said to be the personification of springs. The Oceanids were mentioned in the Greek epic poem Argonautica. The Argonauts pleaded with the nymphs to guide and show them a spring of water when they were stranded in a Libyan desert.
With her name being synonymous with “success,” Telesto was hailed as the personification of “divine blessing.” She is often depicted as someone wearing the traditional clothing in ancient Greece called the peplos.
- Telesto moon: https://en.wikipedia.org/wiki/Telesto_(moon)
- What it looks like: https://solarsystem.nasa.gov/resources/12987/a-closer-look-at-telesto-monochrome/
- Lagrange points: https://en.wikipedia.org/wiki/Trojan_(celestial_body)
- Lagrange points gif: https://en.wikipedia.org/wiki/Lagrange_point