Saturn's natural satellites
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Saturn_sat.jpg
Saturn has 48 confirmed natural satellites.
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Introduction
Saturn is currently known to have 48 moons (plus 2 unconfirmed), many of which were discovered very recently. However, the precise number of Saturn's moons will never be certain as the orbiting chunks of ice in Saturn's rings are all technically moons, and it is difficult to draw a distinction between a large ring particle and a tiny moon.
- Before the Space Age, 9 moons were known to orbit Saturn.
- In 1980, the Voyager space probes discovered 9 more moons in the inner Saturnian system.
- A survey starting in late 2000 found 12 new moons orbiting Saturn at a great distance in orbits that suggest they are fragments of larger bodies captured by Saturn's gravitational pull (Nature vol. 412, p.163-166).
- The Cassini mission, which arrived at Saturn in the summer of 2004, discovered three small moons in the inner Saturnian system. In addition three other moons in the F Ring are suspected, two of which remain unconfirmed. This increased the suspected number of moons to 37.
- On November 16, 2004, Cassini scientists announced that the structure of Saturn's rings indicates the presence of several more moons orbiting within the rings, but only one (S/2005 S 1) has been visually confirmed so far [1] (http://www.space.com/scienceastronomy/saturn_update_041116.html).
- On May 3, 2005, astronomers using the Mauna Kea Observatory announced the discovery of 12 more small outer moons [2] (http://www.ifa.hawaii.edu/~jewitt/saturn2005.html)[3] (http://www.planetary.org/news/2005/saturn_12newmoons_0503.html).
- On May 6, 2005, the Cassini imagining team announced the discovery of a small moon orbiting within the rings, S/2005 S 1.
The latest announcement thus brings the total number of confirmed moons to 48 (excluding the two unconfirmed moons S/2004 S 4 and 6 in the F Ring).
The spurious satellite Themis, "discovered" in 1905, does not exist.
Table of known moons
| Name | Diameter (km) | Mean orbital radius (km) | Orbital period (d) | Position | Discovered | ||
|---|---|---|---|---|---|---|---|
| XVIII | Pan | pan' | 20 | 133,583 | 0.576(1) | in Encke Division | 1990 |
| S/2005 S 1 | ~7 | 136,505(8) | 0.59537(1) | in Keeler Gap | 2005 | ||
| XV | Atlas | at'-lus | 33 (37 × 34 × 27) | 137,670 | 0.603(1) | outer A Ring shepherd | 1980 |
| XVI | Prometheus | proe-mee'-thee-us | 106 (148 × 100 × 68) | 139,350 | 0.614(1) | inner F Ring shepherds | 1980 |
| S/2004 S 6(2) | ~5 | 140,000 | 0.61 | 2004 | |||
| S/2004 S 4(2) | ~5 | 140,100 | 0.619(1) | 2004 | |||
| S/2004 S 3 | ~5 | 140,580 | 0.62095 | outer F Ring Shepherds | 2004 | ||
| XVII | Pandora | pan-dor'-a | 87 (110 × 88 × 62) | 141,520 (3) | 0.6285(7) | 1980 | |
| XI | Epimetheus | ep'-i-mee'-thee-us | 119 (138 × 110 × 110) | 151,422(7) | 0.6956(1) | co-orbitals | 1980 |
| X | Janus | jay'-nus | 179 (194 × 190 × 154) | 151,472(7) | 0.6960(1) | 1966 | |
| I | Mimas | mye'-mus | 397 (418 × 392 × 382) | 185,404(3) | 0.942422(4) | 1789 | |
| XXXII | Methone | me-thoe'-nee | 3 | 194,000 | 1.01(1) | 2004 | |
| XXXIII | Pallene | pa-lee'-nee | 4 | 211,000 | 1.14(1) | 2004 | |
| II | Enceladus | en-sel'-a-dus | 499 (512 × 494 × 490) | 237,950(3) | 1.370218(4) | In the thick of E ring | 1789 |
| XIII | Telesto | te-les'-toe | 23 (30 × 25 × 15) | 294,619(3) | 1.887802(4) | leading Tethys trojan | 1980 |
| III | Tethys | tee'-this | 1060 (1072 × 1056 × 1052) | 1684 | |||
| XIV | Calypso | ka-lip'-soe | 21 (30 × 16 × 16) | trailing Tethys trojan | 1980 | ||
| XII | Helene | hel'-e-nee | 33 (36 × 32 × 30) | 377,396(3) | 2.736915(4) | leading Dione trojan | 1980 |
| IV | Dione | dye-oe'-nee | 1120 | 1684 | |||
| XXXIV | Polydeuces | pol'-ee-dew'-seez | 3.5 | trailing Dione trojan | 2004 | ||
| V | Rhea | ree'-a | 1528 | 527,108(5) | 4.518212(5) | 1672 | |
| VI | Titan | tye'-tun | 5151 | 1,221,930(3) | 15.94542 | 1655 | |
| VII | Hyperion | hye-peer'-ee-on | 292 (370 × 280 × 226) | 1,481,010(3) | 21.27661 | 1848 | |
| VIII | Iapetus | eye-ap'-i-tus | 1436 | 3,560,820 | 79.3215(1) | 1671 | |
| XXIV | Kiviuq | kee'-vee-oek | ~16 | 11,333,200(5) | 450.444(5) | Inuit group | 2000 |
| XXII | Ijiraq | ee'-ye-raak | ~12 | 11,372,000(5) | 452.760(5) | 2000 | |
| IX | Phoebe | fee'-bee | 220 (230 × 220 × 210) | 12,944,300 | -549.834(1,6) | Norse group | 1899 |
| XX | Paaliaq | paw'-lee-aak | ~22 | 14,923,800(5) | 680.667(5) | Inuit group | 2000 |
| XXVII | Skathi | skaadh'-ee | ~8 | 15,576,200(5) | -725.784(4,6) | Norse (Skathi) Group | 2000 |
| XXVI | Albiorix | al'-bee-or'-iks | ~32 | 16,401,600(5) | 784.226(5) | Gallic group | 2000 |
| S/2004 S 11 | - | ~6 | 16,898,400(5) | 820.130(5) | Inuit group | 2004 | |
| XXVIII | Erriapo | air'-ee-ap'-oe | ~10 | 17,408,700(5) | 857.556(5) | Gallic group | 2000 |
| XXIX | Siarnaq | see'-ar-naak | ~40 | 17,905,700(5) | 894.542(5) | Inuit group | 2000 |
| S/2004 S 13 | - | ~6 | 18,056,300(5) | -905.848(4,6) | Norse group | 2004 | |
| XXI | Tarvos | tar'-vus | ~15 | 18,160,200(5) | 913.685(5) | Gallic group | 2000 |
| XXV | Mundilfari | moon'-dil-fair'-ee | ~7 | 18,360,100(5) | -928.806(4,6) | Norse group | 2000 |
| S/2004 S 17 | - | ~4 | 19,099,200(5) | -985.453(4,6) | 2004 | ||
| XXXI | Narvi | nar'-vee | ~7 | 19,370,700(5) | -1006.541(4,6) | 2003 | |
| S/2004 S 15 | - | ~6 | 19,372,200(5) | -1006.659(4,6) | Norse (Skathi) group | 2004 | |
| S/2004 S 10 | - | ~6 | 19,618,400(5) | -1025.908(4,6) | Norse group | 2004 | |
| XXIII | Suttungr | soot'-oong-ur | ~7 | 19,666,700(5) | -1029.703(4,6) | 2000 | |
| S/2004 S 12 | - | ~5 | 19,905,900(5) | -1048.541(4,6) | Norse group | 2004 | |
| S/2004 S 18 | - | ~7 | 19,958,700(5) | -1052.722(4,6) | Norse (Skathi) group | 2004 | |
| S/2004 S 9 | - | ~5 | 20,290,800(5) | -1079.099(4,6) | Norse (Skathi) group | 2004 | |
| S/2004 S 14 | - | ~6 | 20,303,300(5) | -1080.099(4,6) | Norse group | 2004 | |
| S/2004 S 7 | - | ~6 | 20,576,700(5) | -1101.989(4,6) | 2004 | ||
| XXX | Thrymr | thrim'-ur | ~7 | 20,810,300(5) | -1120.809(4,6) | 2000 | |
| S/2004 S 16 | - | ~4 | 22,610,700(5) | -1269.362(4,6) | 2004 | ||
| XIX | Ymir | ee'-mur | ~18 | 23,174,600(5) | -1317.137(4,6) | 2000 | |
| S/2004 S 8 | - | ~6 | 23,608,900(5) | -1354.342(4,6) | 2004 | ||
- (1) Computed from the semi-major axis using the IAU-MPC Natural Satellites Ephemeris Service µ value
- (2) It is not yet clear if these are real satellites or merely persistent clumps within the F Ring
- (3) Computed from the period using the IAU-MPC Natural Satellites Ephemeris Service µ value
- (4) Source: NASA (http://exp.arc.nasa.gov/downloads/celestia/data/solarsys.ssc)
- (5) Source: IAU-MPC Natural Satellites Ephemeris Service (http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html)
- (6) Negative orbital periods indicate a retrograde orbit around Saturn (opposite to the planet's rotation)
- (7) Source: NASA/JPL (http://sse.jpl.nasa.gov/planets/profile.cfm?Object=Saturn&Display=Moons)
- (8) Source: [4] (http://www.nasa.gov/mission_pages/cassini/media/cassini-051005.html)
Grouping the moons
Although the borders may be somewhat nebulous, Saturn's moons can be divided into six groups.
The ring shepherds
Shepherd satellites are moons that orbit within, or just beyond, a planet's ring system. They have the effect of sculpting the rings: giving them sharp edges, and creating gaps between them. Saturn's shepherd moons are Pan, S/2005 S 1, Atlas, Prometheus, Pandora, S/2004 S 3, in addition to the unconfirmed moons S/2004 S 4 and S/2004 S 6.
The co-orbitals
Janus and Epimetheus, are co-orbital moons. These two moons are of roughly equal size and have orbits with only a few kilometer's difference in diameter, close enough that they would collide if they attempted to pass each other. Instead of colliding, however, their gravitational interaction causes them to swap orbits every four years. See Epimetheus' article for a more detailed explanation of this arrangement.
The inner large moons
The innermost large moons of Saturn orbit within its tenuous E Ring. They are Mimas, Enceladus, Tethys and Dione.
Two recently discovered tiny moons also orbit within this group: Methone and Pallene. So too do the co-orbital moons that form a group of their own (see below).
The Trojan moons
Trojan moons are another kind of co-orbitals. Like other co-orbitals, they are a feature unique to the Saturnian system. They are moons that orbit at exactly the same distance from Saturn as another moon, but at such a distance from the other moon that they never collide. Tethys has two tiny co-orbitals Telesto and Calypso, and Dione has also two, Helene and Polydeuces. All four of these moons orbit in the larger moons' Lagrangian points, one in each point.
The outer large moons
Saturn's largest moons all orbit beyond its E Ring and can thus be considered a distinct group. They are Rhea, Hyperion (which is relatively small and very irregular), Titan and Iapetus.
The Inuit group
The Inuit group are five outer moons that are similar enough in their distances from Saturn and their orbital inclinations that they can be considered a group. They are Kiviuq, Ijiraq, Paaliaq, Siarnaq, and S/2004 S 11.
The Norse group
The Norse group are 18 outer moons that are similar enough in their distance from Saturn and their orbital inclination that they can be considered a group. They are Phoebe, Skathi, Narvi, Mundilfari, Suttungr, Thrymr, Ymir, S/2004 S 7 through S/2004 S 10, and S/2004 S 12 through S/2004 S 18. All of these moons orbit Saturn in a retrograde direction.
The Gallic group
The Gallic group are three outer moons that are similar enough in their distance from Saturn and their orbital inclination that they can be considered a group. They are Albiorix, Erriapo and Tarvos.
Naming notes
Some asteroids share the same names as moons of Saturn: 55 Pandora, 106 Dione, 577 Rhea, 1809 Prometheus, 1810 Epimetheus, 4450 Pan.
See also
References
- Saturnian Satellites Fact Sheet (http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html) – NSSDC
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