SATURN'S NATURAL SATELLITES

Saturn has 60 confirmed 'natural satellites', plus three hypothetical moons.

Contents

Introduction

Table of known moons

Grouping the moons

Ring shepherds

Co-orbitals

Inner large moons

Trojan moons

Outer large moons

Irregular satellites

Inuit group

Norse group

Gallic group

Naming notes

See also

References

Introduction

Saturn is currently thought to have sixty-three moons, many of which were discovered very recently, including three particularly un-confirmed, hypothetical moons. However, a precise number of moons can never be given, as there is no objective dividing line between the anonymous orbiting fragments that form Saturn's ring system and the larger objects that have already been named as moons.
Before the advent of telescopic photography, eight moons of Saturn were discovered by direct observation using an optical telescope:

★ Titan, discovered in 1655 by Christiaan Huygens;

★ Tethys, Dione, Rhea and Iapetus (the "Sidera Lodoicea") discovered 1671-1684 by Giovanni Domenico Cassini;

★ Mimas and Enceladus, discovered 1789 by William Herschel;

★ Hyperion, discovered 1848 by W.C. Bond, G.P. Bond and Lassell.

★ The spurious satellite Chiron, "discovered" in 1861, is now known not to exist.
The use of long-exposure photographic plates made it possible to discover additional moons:

★ Phoebe was the first satellite discovered by telescopic photograph in 1899 by W.H. Pickering.

★ Themis, "discovered" in 1905, also was later proven not to exist.

★ In 1966, the satellites Janus and Epimetheus were observed, but not confirmed, and it was not realized that there were two distinct moons sharing an orbit.
The study of the outer planets has since been revolutionized, first by the use of unmanned space probes, and then by advances in telescopy:

★ From 1980, when the first of the ''Voyager'' space probes arrived at Saturn, to 1990, analysis of Voyager images revealed eight more moons in the inner Saturnian system. The last discovered was Pan.

★ A survey starting in late 2000 found thirteen 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, pp. 163–166).

★ The ''Cassini'' mission, which arrived at Saturn in the summer of 2004, discovered three small moons in the inner Saturnian system as well as three suspected but unconfirmed moons in the F Ring. This increased the total to thirty-seven moons, confirmed and unconfirmed.

★ 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, Daphnis, has been visually confirmed so far (its confirmation was announced on May 6, 2005).[1].

★ On May 3, 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons [2] [3].

★ On June 30, 2006, astronomers using the Subaru 8.2 m telescope announced the discovery of nine more small outer moons [4].

★ On April 13, 2007, S/2007 S 1 was announced.

★ On May 1, 2007, S/2007 S 2 and S/2007 S 3 were announced.

★ On July 18, 2007, Anthe was announced.

Table of known moons

The Saturnian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in light purple. Titan, which is planetary in size, has darker highlighting. The irregular (captured) moons are indicated in grey: light grey for prograde satellites, darker grey for retrograde satellites.

Order	Name (spheroidal moons in bold) (Pronunciation key)			Image	Diameter (km)	Semi-major axis (km)	Orbital period (d)	Inclination (°) (to Saturn's equator)	Position	Discovered
1	XVIII	Pan			30 (35 × 35 × 23) [1]	133,584 [2]	+0.57505 2	0.001°	in Encke Division	1990
2	XXXV	Daphnis			6 − 8	136,505 2	+0.59408 2	≈ 0°	in Keeler Gap	2005
3	XV	Atlas			31 (46 × 38 × 19) 1	137,670 2	+0.60169 2	0.003°	outer A Ring shepherd	1980
4	XVI	Prometheus			86 (119 × 87 × 61)	139,380 2	+0.61299 2	0.008°	inner F Ring shepherd	1980
★		S/2004 S 6 [3]			~3−5	140,130 2	+0.61801 2		uncertain objects around the F-Ring	2004
★		S/2004 S 4 [4]			~3−5	~140,100 [5]	+0.619 [6]			2004
★		S/2004 S 3 3			~3−5	~140,300 [7]	~ +0.62			2004
5	XVII	Pandora			81 (103 × 80 × 64) 1	141,720	+0.62850	0.050°	outer F Ring Shepherd	1980
6	XI	Epimetheus			113 (135 × 108 × 105) 1	151,422	+0.69433	0.335°	co-orbitals	1980
7	X	Janus			179 (193 × 173 × 137) 1	151,472	+0.69466	0.165°		1966
8	I	'Mimas'			397 (415 × 394 × 381) [8]	185,404 [9]	+0.942422 [10]	1.566°		1789
9	XXXII	Methone			3	194,440 2	+1.00957 2	0.007°		2004
10	XLIX	Anthe			~2	197,700	1.03650	0.1°		2007
11	XXXIII	Pallene			4	212,280 2	+1.15375 2	0.181°		2004
12	II	'Enceladus'			504 (513 × 503 × 497) 8	237,950 9	+1.370218 10	0.010°	In the thick of E ring	1789
13	III	'Tethys'			1066 (1081 × 1062 × 1055) 8	294,619 9	+1.887802 10	0.168°		1684
13a	XIII	Telesto			24 (29 × 22 × 20) 1			1.158°	leading Tethys trojan	1980
13b	XIV	Calypso			21 (30 × 23 × 14) 1			1.473°	trailing Tethys trojan	1980
16	IV	'Dione'			1123 (1128 × 1122 × 1121) 8	377,396 9	+2.736915 10	0.002°		1684
16a	XII	Helene			33 (36 × 32 × 30)			0.212°	leading Dione trojan	1980
16b	XXXIV	Polydeuces			3.5 [11]			0.177°	trailing Dione trojan	2004
19	V	'Rhea'			1529 (1535 × 1525 × 1526) 8	527,108 [12]	+4.518212 12	0.327°		1672
20	VI	'Titan'			5151	1,221,930 9	+15.94542	1.634°		1655
21	VII	Hyperion			292 (360 × 280 × 225)	1,481,010 9	+21.27661	0.568°		1848
22	VIII	'Iapetus'			1472 (1494 × 1498 × 1425) 8	3,560,820	+79.3215 6	7.570°		1671
23	XXIV	Kiviuq			~16	11 294 800 12	+448.16 12	49.087°	Inuit group	2000
24	XXII	Ijiraq			~12	11 355 316 12	+451.77 12	50.212°		2000
25	IX	Phoebe			220 (230 × 220 × 210)	12 869 700	−545.096[13]	173.047°	Norse group	1899
26	XX	Paaliaq			~22	15 103 400 12	+692.98 12	46.151°	Inuit group	2000
27	XXVII	Skathi			~8	15 672 500 12	−732.52 1013	149.084°	Norse (Skathi) Group	2000
28	XXVI	Albiorix			~32	16 266 700 12	+774.58 12	38.042°	Gallic group	2000
29		S/2007 S 2	—		~6	16 560 000	−792.96	176.68°	Norse group	2007
30	XXXVII	Bebhionn			~6	17 153 520 12	+838.77 12	40.484°	Gallic group	2004
31	XXVIII	Erriapo			~10	17 236 900 12	+844.89 12	38.109°		2000
32	XLVII	Skoll			~6	17 473 800 9	−862.37 12	155.624°	Norse (Skathi) group	2006
33	XXIX	Siarnaq			~40	17 776 600 12	+884.88 12	45.798°	Inuit group	2000
34		S/2007 S 1	—		~7	17 910 600 [14]	+894.86 12	49.904°		2007
35		S/2004 S 13	—		~6	18 056 300 12	−905.85 1013	167.379°	Norse group	2004
36		S/2006 S 4	—		~6	18 065 700 9	−906.56 12	172.666°		2006
37	XLIV	Hyrrokkin			~8	18 168 300 9	−914.29 12	153.272°	Norse (Skathi) group	2006
38		S/2006 S 6	—		~6	18 556 900 9	−943.78 12	162.861°	Norse group	2006
39	XXI	Tarvos			~15	18 562 800 12	+944.23 12	34.679°	Gallic group	2000
40	XXV	Mundilfari			~7	18 725 800 12	−956.70 1013	169.378°	Norse group	2000
41		S/2006 S 1	—		~6	18 930 200 9	−972.41 12	154.232°	Norse (Skathi) group	2006
42		S/2004 S 17	—		~4	19 099 200 12	−985.45 1013	166.881°	Norse group	2004
43	XXXVIII	Bergelmir			~6	19 104 000 12	−985.83 1013	157.384°	Norse (Skathi) group	2004
44	XXXI	Narvi			~7	19 395 200 12	−1008.45 1013	137.292°	Norse (Narvi) group	2003
45	XXIII	Suttungr			~7	19 579 000 12	−1022.82 1013	174.321°	Norse group	2000
46	XLIII	Hati			~6	19 709 300 12	−1033.05 1013	163.131°		2004
47		S/2004 S 12	—		~5	19 905 900 12	−1048.54 1013	164.042°		2004
48	XL	Farbauti			~5	19 984 800 12	−1054.78 1013	158.361°	Norse (Skathi) group	2004
49	XXX	Thrymr			~7	20 278 100 12	−1078.09 1013	174.524°	Norse group	2000
50	XXXVI	Aegir			~6	20 482 900 12	−1094.46 1013	167.425°		2004
51		S/2007 S 3	—		~5	20 518 500	~ −1100	177.22°		2007
52	XXXIX	Bestla			~7	20 570 000 12	−1101.45 1013	147.395°	Norse (Narvi) group	2004
53		S/2004 S 7	—		~6	20 576 700 12	−1101.99 1013	165.596°	Norse group	2004
54		S/2006 S 3	—		~6	21 076 300 9	−1142.37 12	150.817°	Norse (Skathi) group	2006
55	XLI	Fenrir			~4	21 930 644 12	−1212.53 1013	162.832°	Norse group	2004
56	XLVIII	Surtur			~6	22 288 916 9	−1242.36 12	166.918°		2006
57	XLV	Kari			~7	22 321 200 9	−1245.06 12	148.384°	Norse (Skathi) group	2006
58	XIX	Ymir			~18	22 429 673 12	−1254.15 1013	172.143°	Norse group	2000
59	XLVI	Loge			~6	22 984 322 9	−1300.95 12	166.539°		2006
60	XLII	Fornjot			~6	24 504 879 12	−1432.16 1013	167.886°		2004

1. ''Physical characteristics and possible accretionary origins for Saturn's small satellites'', C.C. Porco et al., , , Bulletin of the American Astronomical Society, 2006
2. ''The orbits of Saturn's small satellites derived from combined historic and ''Cassini'' imaging observations'', J.N. Spitale et al, , , The Astronomical Journal, 2006
3. It is not yet clear if these are real satellites or merely persistent clumps within the F Ring
4. S/2004 S4 was most likely a transient clump − it has not been recovered since the first sighting.
5. PGJ Astronomie webpage (Gilbert Javaux). Note that the F ring is centered at ~140,180 km
6. Computed from the semi-major axis using the IAU-MPC Natural Satellites Ephemeris Service µ value
7. See references in S/2004 S 3
8. Source: Thomas ''et al.'' 2006
9. Computed from the period using the IAU-MPC Natural Satellites Ephemeris Service µ value
10. Source: NASA
11. Source: Porco ''et al.'' 2005
12. Source: IAU-MPC Natural Satellites Ephemeris Service
13. Negative orbital periods indicate a retrograde orbit around Saturn (opposite to the planet's rotation)
14. MPEC 2007-G38

Grouping the moons

Although the borders may be somewhat nebulous, Saturn's moons can be divided into eight groups.

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, Daphnis, Atlas, Prometheus, Pandora, in addition to the unconfirmed moons S/2004 S 4, S/2004 S 6 and S/2004 S 3.

Co-orbitals

Janus and Epimetheus are co-orbital moons. These two moons are of roughly equal size and have orbits with only a few kilometers 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.

Inner large moons

The innermost large moons of Saturn orbit within its tenuous E Ring. They are Mimas, Enceladus, Tethys and Dione.
Three recently discovered tiny moons also orbit within this group: Methone, Pallene and Anthe. So too do the co-orbital moons that form a group of their own (see below).

Trojan moons

Main articles: Trojan moon

Trojan moons are another kind of co-orbital. Like the 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 also has two, Helene and Polydeuces. All four of these moons orbit in the larger moons' L₄ or L₅ Lagrangian points, one in each point.

Outer large moons

Saturn's largest moons all orbit beyond its E Ring and can thus be considered a distinct group. They are Rhea, Titan, Hyperion (which is relatively small and very irregular), and Iapetus.

Irregular satellites

Irregular satellites are satellites with large-radius, inclined, and sometimes retrograde orbits, believed to have been acquired by the parent planet through a capture process.

Inuit group

Main articles: Saturn's Inuit group of satellites

The Inuit group are five prograde outer moons that are similar enough in their distances from Saturn and their orbital inclinations that they can be considered a group. They are Ijiraq, Kiviuq, Paaliaq, Siarnaq, and S/2007 S 1.

Norse group

Main articles: Saturn's Norse group of satellites

The Norse group are 29 retrograde outer moons that are similar enough in their distance from Saturn to be considered a group. They are Aegir, Bergelmir, Bestla, Farbauti, Fenrir, Fornjot, Hati, Hyrrokkin, Kari, Loge, Mundilfari, Narvi, Phoebe, Skathi, Skoll, Surtur, Suttungr, Thrymr, Ymir, S/2004 S 7, S/2004 S 12, S/2004 S 13, S/2004 S 17, S/2006 S 1, S/2006 S 3, S/2006 S 4, S/2006 S 6, S/2007 S 2, and S/2007 S 3. All of these moons orbit Saturn in a retrograde direction.

Gallic group

Main articles: Saturn's Gallic group of satellites

The Gallic group are four prograde 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, Bebhionn, Erriapo, and Tarvos.
The diagram illustrates the orbits of the irregular satellites of Saturn discovered so far¹. The eccentricity of the orbits is represented by the segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis. The satellites above the axis are prograde, the satellites beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's (gravitational influence) radius (~65 Gm for Saturn). Prograde groups: Inuit and Gallic and the retrograde Norse group are clearly identifiable (from top to bottom).
¹Named satellites are plotted in yellow; the unnamed satellites S/2004 Sxx (announced in 2005 and 2006) are plotted in white and S/2006 Sxx in grey.

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 ''Name conflicts of solar system objects''.

See also

★ Saturn's moons in fiction

★ Natural satellites of Mars Jupiter Uranus Neptune Pluto

★ Timeline of discovery of Solar System planets and their natural satellites

★ Naming of natural satellites

References

★ Saturnian Satellites Fact Sheet

★ Saturn's Rings by NASA's Solar System Exploration

★ USGS Astrogeology: Gazetteer of Planetary Names - Planetary Body Names and Discoverers