Saturn's natural satellites

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Saturn_system.jpg

The Saturnian System (photographic montage)

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Saturn Rings and Satellites. ESA

Saturn has 48 confirmed natural satellites.

Contents

1 Introduction

2 Table of known moons

3 Grouping the moons

3.1 The ring shepherds
3.2 The co-orbitals
3.3 The inner large moons
3.4 The Trojan moons
3.5 The outer large moons
3.6 The Inuit group
3.7 The Norse group
3.8 The Gallic group

4 Naming notes

5 See also

6 References

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].
On May 3, 2005, astronomers using the Mauna Kea Observatory announced the discovery of 12 more small outer moons [2][3].
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 (Pronunciation key)			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⁾	outer F Ring Shepherds	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⁾	co-orbitals	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⁾	Inuit group	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)	Norse group	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 � 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 � value
⁽⁴⁾ Source: NASA
⁽⁵⁾ Source: IAU-MPC Natural Satellites Ephemeris Service
⁽⁶⁾ Negative orbital periods indicate a retrograde orbit around Saturn (opposite to the planet's rotation)
⁽⁷⁾ Source: NASA/JPL
⁽⁸⁾ Source: [4]

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.

References

Saturnian Satellites Fact Sheet – NSSDC

Saturn's natural satellites

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see also: Rings of Saturn | Cassini-Huygens | Themis