style="font-size:larger;" | Thalassa
Naiad or Thalassa as seen by Voyager 2
(smearing has caused excessive elongation) |
| Discovery |
| Discovered by |
Richard J. Terrile Richard John Terrile is a Voyager scientist who discovered several moons of Saturn, Uranus, and Neptune. He works for the NASA's Jet Propulsion Laboratory....
and Voyager Imaging Team |
| Discovered in |
September 1989 |
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....
al characteristics |
| Epoch In astronomy, an epoch is a moment in time used as a reference for the orbital elements of a celestial body. Typically, the epoch is either the moment an observation was made or the moment for which a prediction was calculated.-Epoch versus equinox:...
18 August 1989 |
| Semi-major axis In geometry, the semi-major axis is used to describe the dimensions of ellipses and hyperbolae.- Ellipse :The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
|
50 075 ± 1 km The kilometre , symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second....
|
| Eccentricity |
0.0002 ± 0.0002 |
| Orbital period The 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...
|
0.31148444 ± 0.00000006 d |
InclinationInclination in general is the angle between a reference plane and another plane or axis of direction.- Orbits :The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
|
0.21 ± 0.02° (to Neptune equator)
0.21° (to local Laplace planeThe Laplace plane or Laplacian plane of a planetary satellite, named after its discoverer Pierre-Simon Laplace , is a mean or reference plane about whose axis the instantaneous orbital plane of a satellite precesses....
)
|
Is a satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
of |
NeptuneNeptune is the eighth planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and...
|
| Physical characteristics |
| Dimensions |
108×100×52 km |
Mean radiusIn classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter....
|
41 ± 3 km |
MassIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
|
(based on assumed density) |
| Mean density The density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....
|
~1.2 g/cm3 (estimate) |
| Rotation period |
assumed synchronous In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
|
Axial tiltIn astronomy, axial tilt is the angle between an object's rotational axis and a line perpendicular to its orbital plane. The angle is measured between the line perpendicular to object's orbital plane and object's rotational axis passing through north pole at which the planet appears to rotate...
|
~zero presumably |
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...
(geometricThe geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle to that of an idealized flat, fully reflecting, diffusively scattering disk with the same cross-section....
) |
0.09 |
Surface temp.In physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
|
~51 K The kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
mean (estimate) |
| Atmosphere |
none |
( , or as in Greek
Θάλασσα), also known as
Neptune IV, is the
second innermost 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
NeptuneNeptune is the eighth planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and...
.
Thalassa was named after a daughter of
AetherAether originally was the personification of the "upper sky", space and heaven, in Greek mythology.The term aether, æther or ether may also refer to one of the following:...
and
HemeraIn Greek mythology Hemera was the personification of day and one of the Protogenoi or primordial deities. She is the goddess of the daytime and, according to Hesiod, the daughter of Erebos and Nyx . Hemera is remarked upon in Cicero's De Natura Deorum, where it is logically determined that Dies ...
from
Greek mythologyGreek mythology is the body of myths and legends belonging to the ancient Greeks concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
.
"Thalassa" is also the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
word for "
seaA sea is any large amount of water filled with animals such as crabs, whales, sharks, and fish, but there is inconsistency as to its precise definition and application. Most commonly, a sea may refer to a large expanse of saline water connected with an ocean, but it is also used sometimes for a...
".
Thalassa was discovered sometime before mid-September, 1989 from the images taken by the
Voyager 2The Voyager 2 spacecraft is an unmanned interplanetary space probe launched on August 20, 1977. Identical in form to its sister Voyager program craft Voyager 1, Voyager 2 followed a slower trajectory that allowed it to be kept in the ecliptic so that it could be sent to Uranus and Neptune by means...
probeA robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...
. It was given the temporary designation
S/1989 N 5. The discovery was announced (IAUC 4867) on September 29, 1989, but the text only talks of "25 frames taken over 11 days", giving a discovery date of sometime before September 18. The name was given on 16 September 1991.
Thalassa is irregularly shaped and shows no sign of any geological modification. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from
TritonTriton is the largest moon of the planet Neptune, discovered on October 10, 1846 by William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2700 km in diameter, it is the seventh-largest...
soon after that moon's capture into a very eccentric initial orbit. Unusually for irregular bodies, it appears to be roughly disk-shaped.
Since the Thalassian orbit is below Neptune's
synchronous orbitA synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...
radius, it is slowly spiralling inward due to tidal decceleration and may eventually impact Neptune's atmosphere, or break up into a
planetary ringA planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most spectacular planetary rings known are those around Saturn, but the other three gas giants of the solar system possess ring systems of their own.Recent reports have...
upon passing its
Roche limitThe Roche Limit , sometimes referred to as the Roche Radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...
due to tidal stretching. Relatively soon after, the spreading debris may impinge upon
DespinaDespina , also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon and Demeter....
's orbit.
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