Nereid, or Neptune II, is the third-largest

moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

. It has the most eccentric orbit of all known moons in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

. It was the second moon of Neptune to be discovered, by Gerard Kuiper in 1949.

Discovery and naming

Nereid was discovered on 1 May 1949 by Gerard P. Kuiper using photographic plates taken with the 82-inch telescope at the McDonald Observatory. He proposed the name in the report of his discovery. It is named after the

Nereid In Greek mythology, the Nereids or Nereides ( ; ; , also Νημερτές) are sea nymphs (female spirits of sea waters), the 50 daughters of the ' Old Man of the Sea' Nereus and the Oceanid Doris, sisters to their brother Nerites. They ofte ...

s, sea-nymphs of

Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...

and attendants of the god

. It was the second moon of Neptune to be discovered, and the last before the arrival of ''

Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...

'' (not counting a single observation of an occultation by

Larissa Larissa (; , , ) is the capital and largest city of the Thessaly region in Greece. It is the fifth-most populous city in Greece with a population of 148,562 in the city proper, according to the 2021 census. It is also the capital of the Larissa ...

in 1981).

Physical characteristics

Nereid is third-largest of Neptune's satellites, and has a

mean radius In applied sciences, the equivalent radius (or mean radius) is the radius of a circle or sphere with the same perimeter, area, or volume of a non-circular or non-spherical object. The equivalent diameter (or mean diameter) (D) is twice the equiva ...

of about , similar to

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

's moon Mimas. It is by far the largest normal irregular satellite known, having about two-thirds the mass of all irregular moons combined. ( Triton is much larger, but is very unusual for an irregular satellite.) Nereid also accounts for about 98% of the mass of Neptune's entire irregular satellite system altogether (excluding Triton), which is similar to the situation of Phoebe at Saturn (the second-largest normal irregular moon in the Solar System). The shape of Nereid is unknown. Since 1987 some photometric observations of Nereid have detected large (by ~1 magnitude) variations of its brightness, which can happen over years and months, but sometimes even over a few days. They persist even after a correction for distance and phase effects. On the other hand, not all astronomers who have observed Nereid have noticed such variations. This means that they may be quite chaotic. To date, there is no credible explanation of the variations, but if they exist, they are likely related to the rotation of Nereid. Nereid's rotation could be either in the state of forced

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...

or even chaotic rotation (like Hyperion) due to its highly elliptical orbit. In 2016, extended observations with the Kepler space telescope showed only low-amplitude variations (0.033 magnitudes). Thermal modeling based on infrared observations from the Spitzer and Herschel space telescopes suggests that Nereid is only moderately elongated with a maximum

aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...

of 1.3:1, which disfavors forced precession of the rotation. The thermal model also indicates that the surface roughness of Nereid is very high, likely similar to the Saturnian moon Hyperion. Spectrally, Nereid appears neutral in colour and water ice has been detected on its surface. Its spectrum appears to be intermediate between

Uranus Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...

's moons Titania and

Umbriel Umbriel () is the third-largest moon of Uranus. It was discovered on October 24, 1851, by William Lassell at the same time as neighboring moon Ariel. It was named after a character in Alexander Pope's 1712 poem '' The Rape of the Lock''. Umb ...

, which suggests that Nereid's surface is composed of a mixture of water ice and some spectrally neutral material. The spectrum is markedly different from

minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...

s of the outer solar system,

centaur A centaur ( ; ; ), occasionally hippocentaur, also called Ixionidae (), is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse that was said to live in the mountains of Thessaly. In one version o ...

s Pholus,

Chiron In Greek mythology, Chiron ( ; also Cheiron or Kheiron; ) was held to be the superlative centaur amongst his brethren since he was called the "wisest and justest of all the centaurs". Biography Chiron was notable throughout Greek mythology for ...

and Chariklo, suggesting that Nereid formed around Neptune rather than being a captured body. Halimede, which displays a similar gray neutral colour, may be a fragment of Nereid that was broken off during a collision.

Orbit and rotation

Nereid

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

in the prograde direction at an average distance of , but its high

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

of 0.749 takes it as close as and as far as . The unusual orbit suggests that it may be either a captured

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

Kuiper belt The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...

object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon Triton. If the latter is true, it may be the only survivor of Neptune's original (pre- Triton capture) set of regular satellites. In 1991, a rotation period of Nereid of about 13.6 hours was determined by an analysis of its light curve. In 2003, another rotation period of about was measured. However, this determination was later disputed, and other researchers for a time failed to detect any periodic modulation in Nereid's light curve from ground-based observations. In 2016, a clear rotation period of 11.594 ± 0.017 hours was determined based on observations with the

Kepler space telescope The Kepler space telescope is a defunct space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...

Exploration

The only spacecraft to visit Nereid was ''

'', which passed it at a distance of between 20 April and 19 August 1989. ''Voyager 2'' obtained 83 images with observation accuracies of to . Prior to ''Voyager 2s arrival, observations of Nereid had been limited to ground-based observations that could only establish its intrinsic brightness and

orbital elements Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same o ...

. Although the images obtained by ''Voyager 2'' do not have a high enough resolution to allow surface features to be distinguished, ''Voyager 2'' was able to measure the size of Nereid and found that it was grey in colour and had a higher

albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

than Neptune's other small satellites. If selected, The Arcanum mission would do a flyby of Nereid before its primary purposes of orbiting Neptune and observing Triton.

Notes

References