S/2009 S 1

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S/2009 S 1
PIA11665 moonlet in B Ring cropped.jpg
Cassini image of S/2009 S 1 as a bright dot casting a long shadow over Saturn's B Ring
Discovered byCassini Imaging Team
Discovery date2009
Orbital characteristics[2]
117000 km
Eccentricity≈ 0.000
0.4715 d (11.32 h)
Inclination≈ 0.0
Satellite ofSaturn
Groupouter B ring moonlet
Physical characteristics
Mean diameter
0.3 km[3]
assumed synchronous

S/2009 S 1 is a moonlet embedded in the outer part of Saturn's B Ring, orbiting 117,000 km (73,000 mi) away from the planet. The moonlet was discovered by the Cassini Imaging Team during the Saturnian equinox event on 26 July 2009, when the Cassini spacecraft imaged the moonlet casting a 36 km (22 mi)-long shadow onto the B Ring.[4] With a diameter of 300 m (1,000 ft), it is most likely a long-lived solid body, which would count it as the smallest and innermost known moon of Saturn.[4]


A wider perspective of the image above. S/2009 S 1 is in the center. The Cassini Division is on the right.

S/2009 S 1 was first identified by the Cassini Imaging Team led by Carolyn Porco,[5] in a single image taken by the Cassini spacecraft approximately 296,000 km (184,000 mi) from Saturn on 26 July 2009 11:30 UTC.[4][6] The moonlet was discovered during Saturn's 2009 equinox, when it cast an approximately 36 km (22 mi)-long shadow on the planet's B ring.


Based on the shadow's width, the Cassini Imaging Team infer a diameter of 300 m (1,000 ft) for S/2009 S 1.[7] The presence of a shadow suggests that the S/2009 S 1 is most likely a solid body large enough to have existed since the formation of the B ring.[3] This particular moon is one of the smallest moons of Saturn that has been directly imaged.[4]

S/2009 S 1 is located about 650 km (400 mi) interior to the edge of the B ring, corresponding to a radial distance of 116,914 ± 17 km (72,647 ± 11 mi) from the center of Saturn. The moonlet protrudes 150 m (500 ft) above the B ring plane, which has a vertical thickness of 5 m (16 ft) for comparison.[3]

Although it is embedded in the B ring, S/2009 S 1 does not appear to produce extensive, propeller-shaped disturbance features unlike the propeller moonlets in Saturn's A ring. This may be because the B ring is very dense at the moonlet's location, which would hinder the formation of visible propeller gaps around the moonlet.[4][3][8]

See also[edit]


  1. ^ "Planetary Satellite Discovery Circumstances". Jet Propulsion Laboratory. Retrieved 28 March 2022.
  2. ^ Williams, David R. (15 October 2019). "Saturnian Satellite Fact Sheet". Space Science Data Coordinated Archive. NASA. Retrieved 28 March 2022.
  3. ^ a b c d Spitale, J. N.; Porco, C. C. (December 2010). "Detection of Free Unstable Modes and Massive Bodies in Saturn's Outer B Ring". The Astronomical Journal. 140 (6): 1747–1757. arXiv:0912.3489. Bibcode:2010AJ....140.1747S. doi:10.1088/0004-6256/140/6/1747. S2CID 117319280.
  4. ^ a b c d e "A Small Find Near Equinox". Cassini Solstice Mission. Jet Propulsion Laboratory. 7 August 2009. Archived from the original on 9 August 2009. Retrieved 2 January 2012.
  5. ^ "Cassini Imaging Science Team". Cassini Imaging Central Laboratory for OPerationS (CICLOPS). Retrieved 2 January 2012.
  6. ^ "Image of Saturn-rings". Solar System Exploration. NASA. 26 July 2009. Archived from the original on 27 November 2020. Retrieved 28 March 2022.
  7. ^ Porco, C. & the Cassini Imaging Team (2 November 2009). Green, D. W. E. (ed.). "S/2009 S1". IAU Circular. 9091 (1). Bibcode:2009IAUC.9091....1P. Archived from the original on 11 June 2011. Retrieved 2 January 2012.
  8. ^ Michikoshi, Shugo; Kokubo, Eiichiro (May 2011). "Formation of a Propeller Structure by a Moonlet in a Dense Planetary Ring". The Astrophysical Journal Letters. 732 (2): 4. arXiv:1104.0462. Bibcode:2011ApJ...732L..23M. doi:10.1088/2041-8205/732/2/L23. S2CID 18889201. L23.