Nereid
| Nereid | |
|---|---|
 |
|
| Nereid on a photo taken by the Voyager 2 spacecraft | |
| Central body | Neptune |
| Properties of the orbit | |
| Major semi-axis | 5,513,787 km |
| Periapsis | 1,374,587 km |
| Apoapsis | 9,652,987 km |
| eccentricity | 0.7507 |
| Orbit inclination to the equator of the central body | 32.55 ° |
| Orbit inclination to the Laplace plain | 7.090 ° |
| Orbit inclination to the ecliptic | 5.07 ° |
| Orbital time | 360.13619 d |
| Mean orbital velocity | 0.934 km / s |
| Physical Properties | |
| Albedo | 0.155 |
| Apparent brightness | 19.7 mag |
| Medium diameter | 340 ± 50 km |
| Dimensions | ≈ 3.1 · 10 19 kg |
| Medium density | 1.5 g / cm 3 |
| Sidereal rotation | 0.48 |
| Acceleration of gravity on the surface | 0.07 m / s 2 |
| Escape speed | 160 m / s |
| Surface temperature | ≈ −223 ° C / 50 K |
| discovery | |
| Explorer | |
| Date of discovery | May 1, 1949 |
| Remarks | Most eccentric orbit of a planetary moon |
Nereid or Nereid (also Neptune II ) is the ninth innermost and third largest moon (of 14 known moons) of the planet Neptune . It has the most eccentric orbit of a planetary moon in the solar system .
Discovery and naming
Nereid was discovered by Gerard Peter Kuiper on May 1, 1949 . It was the second known Neptune moon and was only discovered 103 years after this due to its dark surface and about eight times smaller diameter compared to the by far largest Neptune moon Triton . The discovery was made on photographic plates made with the McDonald Observatory's 82-inch telescope . Kuiper suggested the name Nereid in his discovery report.
The moon was named after the 50 or so nereids , sea nymphs from Greek mythology . They are the beautiful daughters of the wise old man Nereus , who was said to be prophecy and constant metamorphosis, and his wife, the Oceanid Doris .
Track properties
Orbit
Nereid orbits Neptune on a prograde , highly elliptical orbit between 1,353,600 and 9,623,700 km from its center (major orbit half-axis 5,513,400 km or 222,653 Neptune radii). The orbital eccentricity is 0.7507; this is the most eccentric known orbit of a moon in the solar system. No irregular outer moon of the gas planets of the outer solar system comes so close to its planets. The orbit is inclined 32.55 ° to the equator of Neptune.
The orbit of the closest moon Triton is on average 5,159,000 km away from Nereid's orbit; due to the high orbital eccentricity, Nereid Triton comes close to about 1 million km. The distance of the nearest outer moon Halimede averages about 10 million km.
The irregular orbit could indicate that Nereid was originally an object of the Kuiper Belt that was captured by the gravitational force of Neptune. Another explanation would be that Nereid's unusual orbit can be traced back to gravitational disturbances from the largest Neptune moon Triton .
Nereid orbits Neptune in 360 days, 3 hours 16 minutes and 7 seconds, a nearly earthy year .
rotation
Nereid rotates around its own axis in about 11 ½ hours (11.52 ± 0.14 hours).
Physical Properties
Nereid has an average diameter of 340 km, making it the third largest Neptune moon. It is also the largest of the irregular satellites in the solar system. On the images of the Voyager-2 probe, the moon appears elongated, so it is not spherical.
Nereid appears gray in the spectrum. Spectrally, Nereid moves between the Uranus moons Titania and Umbriel , which indicates a surface composition of water ice - which was identified in 1998 by Michael E. Brown's group - and spectrally neutral material. In terms of spectrum, Nereid is more similar to Proteus than Triton and is noticeably different from the asteroids of the outer solar system such as the centaurs Pholus , Chiron or Chariklo , which indicates a formation in the Neptune system rather than a captured asteroid.
The total area is around 360,000 km 2 and thus roughly corresponds to the area of Germany . The mean surface temperature is estimated to be −223 ° C (about 50 K ).
The nearest outer moon, Halimede , may be a fragment of Nereid that was created by a collision.
exploration
Before the Neptune flyby of the Voyager 2 space probe in the summer of 1989, Nereid could only be observed through earth-based telescopes and its orbital elements and their brightness could be determined.
Since during the flyby of Voyager 2 no close-up reconnaissance was possible due to the priority of a nearby Triton passage, Nereid was passed between April 20 and August 19, 1989 at a minimum distance of 4,652,880 km on August 24. It was possible to take 83 pictures in resolutions from 43 to 800 km. Although hardly any surface details could be made out in the pictures, the size and color as well as the higher albedo compared to the other small Neptune moons could be determined.
Since the flyby, the Neptune system has been studied intensively by earth-based observations as well as the Hubble space telescope .
Web links
- IAU: Natural Satellites Ephemeris Service IAU: Orbital parameters and ephemeris of irregular moons
- NASA: Planetary Satellite Physical Parameters NASA: Physical Parameters (English)
- NASA: Planetary Satellite Mean Orbital Parameters NASA: Orbital Parameters (English)
- NASA: Neptunian Satellite Fact Sheet NASA: List of Parameters
- Scott S. Sheppard: Neptune's Known Satellites List with parameters of the Neptune system (English)
- USGS: Planet and Satellite Names and Discoverers Names of planets and satellites
- Polish moon page: Nereid description and further links (English)
- Gerard P. Kuiper: The second satellite of Neptune. In: PASP . 61, 1949, pp. 175-176.
- Matthew J. Holman, JJ Kavelaars: The short rotation period of Nereid. (PDF; 141 kB). (English)
- Michael E. Brown et al: Detection of water ice on Nereid. (PDF). (English)