Natural satellite

Most of the 207 known natural satellites of the planets are irregular satellites. Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton are the seven largest and most massive natural satellites in the Solar System (see List of natural satellites § List). Triton is more massive than all smaller natural satellites combined.

A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as moons, a derivation from the Moon of Earth.

In the Solar System, there are six planetary satellite systems containing 207 known natural satellites altogether. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites: Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, and Eris.[1] As of November 2021, there are 442 other minor planets known to have natural satellites.[2]

A planet usually has at least around 10,000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter.[3] The Earth–Moon system is a unique exception in the Solar System; at 3,474 km (2,158 miles) across, the Moon is 0.273 times the diameter of Earth and about 1/80th of its mass.[4] The next largest ratios are the NeptuneTriton system at 0.055 (with a mass ratio of about 1 to 5000), the SaturnTitan system at 0.044 (with the second mass ratio next to the Earth-Moon system, 1 to 4250), the JupiterGanymede system at 0.038, and the UranusTitania system at 0.031. For the category of dwarf planets, Charon has the largest ratio, being 0.52 the diameter of Pluto.

Terminology

The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of De revolutionibus orbium coelestium in 1543. Until the discovery of the Galilean satellites in 1610 there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.

The first to use the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from the Latin word satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens.[5]

The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology.[5] The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has become linked primarily with artificial objects flown in space – including, sometimes, even those not in orbit around a planet.

Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with natural satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite (a proper noun), but not when referring to other natural satellites (common nouns).

Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites.[6][7][8] A few recent authors define "moon" as "a satellite of a planet or minor planet", and "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the Sun".[9][10][11]

Definition of a moon

Size comparison of Earth and the Moon

There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.[12]

The upper limit is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, because it depends on distance as well as relative mass.

Origin and orbital characteristics

Two moons: Saturn's natural satellite Dione occults Enceladus

The natural satellites orbiting relatively close to the planet on prograde, uninclined circular orbits (regular satellites) are generally thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary.[13][14] In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits.[15] The Moon[16] and possibly Charon[17] are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet.

Temporary satellites

The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon.[18][19] The only observed examples are 1991 VG, 2006 RH120, 2020 CD3.

2006 RH120 was a temporary satellite of Earth for nine months in 2006 and 2007.[20][21]

Tidal locking

Most regular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. This phenomenon comes about through a loss of energy due to tidal forces raised by the planet, slowing the rotation of the satellite until it is negligible.[22] The only known exception is Saturn's natural satellite Hyperion, which rotates chaotically because of the gravitational influence of Titan.

In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter's Himalia, Saturn's Phoebe, and Neptune's Nereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days.

Satellites of satellites

Artist impression of Rhea's proposed rings

No "moons of moons" or subsatellites (natural satellites that orbit a natural satellite of a planet) are currently known. In most cases, the tidal effects of the planet would make such a system unstable.

However, calculations performed after the 2008 detection[23] of a possible ring system around Saturn's moon Rhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow,[24] a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect rings around Rhea.[25]

It has also been proposed that Saturn's moon Iapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge.[26]

Trojan satellites

Two natural satellites are known to have small companions at both their L4 and L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called trojan moons, as their orbits are analogous to the trojan asteroids of Jupiter. The trojan moons are Telesto and Calypso, which are the leading and following companions, respectively, of the Saturnian moon Tethys; and Helene and Polydeuces, the leading and following companions of the Saturnian moon Dione.

Asteroid satellites

The discovery of 243 Ida's natural satellite Dactyl in the early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two comparably sized components.

Shape

The relative masses of the natural satellites of the Solar System. Mimas, Enceladus, and Miranda are too small to be visible at this scale. All the irregularly shaped natural satellites, even added together, would also be too small to be visible.

Neptune's moon Proteus is the largest irregularly shaped natural satellite; the shapes of Eris' moon Dysnomia and Orcus' moon Vanth are unknown. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded ellipsoids under hydrostatic equilibrium, i.e. are "round/rounded satellites". The larger natural satellites, being tidally locked, tend toward ovoid (egg-like) shapes: squat at their poles and with longer equatorial axes in the direction of their primaries (their planets) than in the direction of their motion. Saturn's moon Mimas, for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis. Methone, another of Saturn's moons, is only around 3 km in diameter and visibly egg-shaped. The effect is smaller on the largest natural satellites, where their own gravity is greater relative to the effects of tidal distortion, especially those that orbit less massive planets or, as in the case of the Moon, at greater distances.

NameSatellite ofDifference in axes
km
% of mean
diameter
MimasSaturn33.4 (20.4 / 13.0)8.4 (5.1 / 3.3)
EnceladusSaturn16.63.3
MirandaUranus14.23.0
TethysSaturn25.82.4
IoJupiter29.40.8
The MoonEarth4.30.1

Geological activity

Of the nineteen known natural satellites in the Solar System that are large enough to have lapsed into hydrostatic equilibrium, several remain geologically active today. Io is the most volcanically active body in the Solar System, while Europa, Enceladus, Titan and Triton display evidence of ongoing tectonic activity and cryovolcanism. In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their giant-planet primaries. (This mechanism would have also operated on Triton in the past, before its orbit was circularized.) Many other natural satellites, such as Earth's Moon, Ganymede, Tethys and Miranda, show evidence of past geological activity, resulting from energy sources such as the decay of their primordial radioisotopes, greater past orbital eccentricities (due in some cases to past orbital resonances), or the differentiation or freezing of their interiors. Enceladus and Triton both have active features resembling geysers, although in the case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also has hydrocarbon lakes. Also Io and Callisto have atmospheres, even if they are extremely thin.[27] Four of the largest natural satellites, Europa, Ganymede, Callisto, and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus may have localized subsurface liquid water.

Natural satellites of the Solar System

Euler diagram showing the types of bodies in the Solar System.

Of the objects within our Solar System known to have natural satellites, there are 76 in the asteroid belt (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers.[2] There are also 84 known natural satellites of trans-Neptunian objects.[2] Some 150 additional small bodies have been observed within the rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, none have yet been confirmed.

Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The giant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life.[28]

Among the objects generally agreed by astronomers to be dwarf planets, Ceres and Sedna have no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites; Styx, Nix, Kerberos, and Hydra.[29] Haumea has two natural satellites; Orcus, Quaoar, Makemake, Gonggong, and Eris have one each. The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system.

The seven largest natural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together. Similarly in the next size group of nine mid-sized natural satellites, between 1,000 km and 1,600 km across, Titania, Oberon, Rhea, Iapetus, Charon, Ariel, Umbriel, Dione, and Tethys, the smallest, Tethys, has more mass than all smaller natural satellites together. As well as the natural satellites of the various planets, there are also over 80 known natural satellites of the dwarf planets, minor planets and other small Solar System bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.

The following is a comparative table classifying the natural satellites in the Solar System by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and trans-Neptunian objects for comparison. The natural satellites of the planets are named after mythological figures. These are predominantly Greek, except for the Uranian natural satellites, which are named after Shakespearean characters. The nineteen bodies massive enough to have achieved hydrostatic equilibrium are in bold in the table below. Minor planets and satellites suspected but not proven to have achieved a hydrostatic equilibrium are italicized in the table below.

Mean
diameter
(km)
Satellites of planetsSatellites of dwarf planetsSatellites of
other
minor planets
Non-satellites
for comparison
EarthMarsJupiterSaturnUranusNeptuneOrcusPlutoHaumeaQuaoarMakemakeGonggongEris
12,000–13,000Earth
Venus
6,000–7,000Mars
4,000–6,000Ganymede
Callisto
TitanMercury
3,000–4,000MoonIo
Europa
2,000–3,000TritonEris
Pluto
1,000–2,000Rhea
Iapetus
Dione
Tethys
Titania
Oberon
Umbriel
Ariel
CharonMakemake
Haumea
Gonggong,
Quaoar
500–1,000EnceladusDysnomiaSedna, Ceres,
Salacia, Orcus,
Pallas, Vesta
many more TNOs
250–500Mimas
Hyperion
MirandaProteus
Nereid
VanthHiʻiakaVarda I Ilmarë
Salacia I Actaea
Lempo II Hiisi
10 Hygiea
704 Interamnia
87 Sylvia
47171 Lempo
and many others
100–250Amalthea
Himalia
Thebe
Phoebe
Janus
Epimetheus
Sycorax
Puck
Portia
Larissa
Galatea
Despina
NamakaS/2015 (136472) 1S/2005 (82075) 1
Sila–Nunam I
Ceto I Phorcys
Patroclus I Menoetius
Lempo I Paha
~20 more moons of TNOs
3 Juno
15760 Albion
5 Astraea
617 Patroclus
42355 Typhon
and many others
50–100Elara
Pasiphae
Prometheus
Pandora
Caliban
Juliet
Belinda
Cressida
Rosalind
Desdemona
Bianca
Thalassa
Halimede
Neso
Naiad
WeywotXiangliu (probably)90 Antiope I
Typhon I Echidna
Logos I Zoe
5 more moons of TNOs
90 Antiope
58534 Logos
253 Mathilde
and many others
25–50Carme
Metis
Sinope
Lysithea
Ananke
Siarnaq
Helene
Albiorix
Atlas
Pan
Ophelia
Cordelia
Setebos
Prospero
Perdita
Stephano
Sao
Laomedeia
Psamathe
Hippocamp
Hydra
Nix[30]
Kalliope I Linus1036 Ganymed
243 Ida
and many others
10–25Phobos
Deimos
Leda
Adrastea
Telesto
Paaliaq
Calypso
Ymir
Kiviuq
Tarvos
Ijiraq
Erriapus
Mab
Cupid
Francisco
Ferdinand
Margaret
Trinculo
Kerberos
Styx
762 Pulcova I
Sylvia I Romulus
624 Hektor I Skamandrios
Eugenia I Petit-Prince
121 Hermione I
283 Emma I
1313 Berna I
107 Camilla I
433 Eros
1313 Berna
and many others
< 1064 moons57 moonsSylvia II Remus
Ida I Dactyl
and many others
many

Visual summary

Solar System moons we have seen clearly
Ganymede - Perijove 34 Composite.png
Titan in true color.jpg
Callisto.jpg
Io highest resolution true color.jpg
Europa-moon-with-margins.jpg
Triton moon mosaic Voyager 2 (large).jpg
Ganymede
(moon of Jupiter)
Titan
(moon of Saturn)
Callisto
(moon of Jupiter)
Io
(moon of Jupiter)
Moon
(moon of Earth)
Europa
(moon of Jupiter)
Triton
(moon of Neptune)
Titania (moon) color cropped.jpg
PIA07763 Rhea full globe5.jpg
Voyager 2 picture of Oberon.jpg
Iapetus as seen by the Cassini probe - 20071008.jpg
Charon in True Color - High-Res.jpg
PIA00040 Umbrielx2.47.jpg
Ariel (moon).jpg
Titania
(moon of Uranus)
Rhea
(moon of Saturn)
Oberon
(moon of Uranus)
Iapetus
(moon of Saturn)
Charon
(moon of Pluto)
Umbriel
(moon of Uranus)
Ariel
(moon of Uranus)
Dione in natural light.jpg
PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg
PIA17202 - Approaching Enceladus.jpg
PIA18185 Miranda's Icy Face.jpg
Proteus (Voyager 2).jpg
Mimas Cassini.jpg
Hyperion true.jpg
Dione
(moon of Saturn)
Tethys
(moon of Saturn)
Enceladus
(moon of Saturn)
Miranda
(moon of Uranus)
Proteus
(moon of Neptune)
Mimas
(moon of Saturn)
Hyperion
(moon of Saturn)
Phoebe cassini.jpg
Larissa 1.jpg
PIA12714 Janus crop.jpg
Amalthea (moon).png
Puck.png
PIA09813 Epimetheus S. polar region.jpg
Thebe.jpg
Phoebe
(moon of Saturn)
Larissa
(moon of Neptune)
Janus
(moon of Saturn)
Amalthea
(moon of Jupiter)
Puck
(moon of Uranus)
Epimetheus
(moon of Saturn)
Thebe
(moon of Jupiter)
Prometheus 12-26-09a.jpg
PIA21055 - Pandora Up Close.jpg
Hydra Enhanced Color.jpg
Nix best view.jpg
Leading hemisphere of Helene - 20110618.jpg
Atlas (NASA).jpg
Pan by Cassini, March 2017.jpg
Prometheus
(moon of Saturn)
Pandora
(moon of Saturn)
Hydra
(moon of Pluto)
Nix
(moon of Pluto)
Helene
(moon of Saturn)
Atlas
(moon of Saturn)
Pan
(moon of Saturn)
Telesto cassini closeup.jpg
N00151485 Calypso crop.jpg
Phobos colour 2008.jpg
Deimos-MRO.jpg
Daphnis (Saturn's Moon).jpg
Methone PIA14633.jpg
Dactyl-HiRes.jpg
Telesto
(moon of Saturn)
Calypso
(moon of Saturn)
Phobos
(moon of Mars)
Deimos
(moon of Mars)
Daphnis
(moon of Saturn)
Methone
(moon of Saturn)
Dactyl
(moon of Ida)
Comparison of (a part of) Jupiter and its four largest natural satellites

History

See also

Moons of planets

Moons of dwarf planets and small Solar System bodies

References

  1. ^ "Planet and Satellite Names and Discoverers". International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN). Retrieved 27 January 2012.
  2. ^ a b c Wm. Robert Johnston (30 September 2018). "Asteroids with Satellites". Johnston's Archive. Retrieved 22 October 2018.
  3. ^ Canup, Robin M.; Ward, William R. (June 2006). "A common mass scaling for satellite systems of gaseous planets". Nature. 441 (7095): 834–839. Bibcode:2006Natur.441..834C. doi:10.1038/nature04860. ISSN 1476-4687. PMID 16778883. S2CID 4327454.
  4. ^ Glenday, Craig (2014). Guinness World Records 2014. p. 186. ISBN 978-1-908843-15-9.
  5. ^ a b "Early History – First Satellites". www.jpl.nasa.gov. Retrieved 8 February 2018.
  6. ^ Kenneth R. Lang. "The Cambridge Guide to the Solar System". 2011. p. 15. quote: "Any object that orbits a planet is now called a satellite, and a natural satellite is also now called a moon."
  7. ^ Therese Encrenaz, et al. "The Solar System". 2004. p. 30.
  8. ^ Tilman Spohn, Doris Breuer, Torrence Johnson. "Encyclopedia of the Solar System". 2014. p. 18.
  9. ^ David Andrew Weintraub. "Is Pluto a Planet?: A Historical Journey Through the Solar System". p. 65 quote: "... the general concept of a "moon" as a satellite of a planet and "planet" as a satellite of a star."
  10. ^ "Satellite". www.merriam-webster.com. Merriam Webster. Retrieved 16 November 2015.
  11. ^ Stillman, Dan (16 June 2015). "What Is a Satellite?". www.nasa.gov. NASA. Retrieved 16 November 2015.
  12. ^ F. Marchis, et al. (2005). "Discovery of the triple asteroidal system 87 Sylvia". Nature. 436 (7052): 822–24. Bibcode:2005Natur.436..822M. doi:10.1038/nature04018. PMID 16094362. S2CID 4412813.
  13. ^ Canup, Robin M.; Ward, William R. (30 December 2008). Origin of Europa and the Galilean Satellites. University of Arizona Press. p. 59. arXiv:0812.4995. Bibcode:2009euro.book...59C. ISBN 978-0-8165-2844-8.
  14. ^ D'Angelo, G.; Podolak, M. (2015). "Capture and Evolution of Planetesimals in Circumjovian Disks". The Astrophysical Journal. 806 (1): 29–. arXiv:1504.04364. Bibcode:2015ApJ...806..203D. doi:10.1088/0004-637X/806/2/203. S2CID 119216797.
  15. ^ Encyclopedia of the Solar System, page 366, Academic Press, 2007, Lucy-Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson
  16. ^ Canup, RM & Asphaug, E (2001). "Origin of the Moon in a giant impact near the end of the Earth's formation". Nature. 412 (6848): 708–12. Bibcode:2001Natur.412..708C. doi:10.1038/35089010. PMID 11507633. S2CID 4413525.
  17. ^ Stern, SA; Weaver, HA; Steffl, AJ; Mutchler, MJ; et al. (2006). "A giant impact origin for Pluto's small natural satellites and satellite multiplicity in the Kuiper belt". Nature. 439 (7079): 946–49. Bibcode:2006Natur.439..946S. doi:10.1038/nature04548. PMID 16495992. S2CID 4400037.
  18. ^ Camille M. Carlisle (30 December 2011). "Pseudo-moons Orbit Earth". Sky & Telescope.
  19. ^ Fedorets, Grigori; Granvik, Mikael; Jedicke, Robert (15 March 2017). "Orbit and size distributions for asteroids temporarily captured by the Earth-Moon system". Icarus. 285: 83–94. Bibcode:2017Icar..285...83F. doi:10.1016/j.icarus.2016.12.022.
  20. ^ "2006 RH120 ( = 6R10DB9) (A second moon for the Earth?)". Great Shefford Observatory. 14 September 2017. Archived from the original on 6 February 2015. Retrieved 13 November 2017.
  21. ^ Roger W. Sinnott (17 April 2007). "Earth's "Other Moon"". Sky & Telescope. Archived from the original on 2 April 2012. Retrieved 12 March 2018.
  22. ^ Barnes, Rory, ed. (2010). Formation and Evolution of Exoplanets. John Wiley & Sons. p. 248. ISBN 978-3527408962.
  23. ^ Jones, G. H.; et al. (2008). "The Dust Halo of Saturn's Largest Icy Moon, Rhea – Jones et al. 319 (5868): 1380 – Science" (PDF). Science. 319 (5868): 1380–1384. Bibcode:2008Sci...319.1380J. doi:10.1126/science.1151524. PMID 18323452. S2CID 206509814. Archived from the original (PDF) on 8 March 2018.
  24. ^ Jeff Hecht (6 March 2008). "Saturn satellite reveals first moon rings". New Scientist.
  25. ^ Tiscareno, Matthew S.; Burns, Joseph A.; Cuzzi, Jeffrey N.; Hedman, Matthew M. (2010). "Cassini imaging search rules out rings around Rhea – Tiscareno – 2010". Geophysical Research Letters. 37 (14): n/a. arXiv:1008.1764. Bibcode:2010GeoRL..3714205T. doi:10.1029/2010GL043663.
  26. ^ "How Iapetus, Saturn's outermost moon, got its ridge". 13 December 2010.
  27. ^ A moon with atmosphere | The Planetary Society
  28. ^ Woo, Marcus (27 January 2015). "Why We're Looking for Alien Life on Moons, Not Just Planets". Wired. Retrieved 27 January 2015.
  29. ^ "Hubble Discovers New Pluto Moon". ESA/Hubble Press Release. Retrieved 13 July 2012.
  30. ^ "How Big Is Pluto? New Horizons Settles Decades-Long Debate". NASA. 13 July 2015.

External links

All moons

Jupiter's moons

Saturn's moons

Media files used on this page

Solar System Template Final.png
Major Solar System objects. Sizes of planets and Sun are roughly to scale, but distances are not. This is not a diagram of all known moons – small gas giants' moons and Pluto's S/2011 P 1 moon are not shown.
Jupiter family.jpg
Montage of Jupiter and the Galilean satellites, Io, Europa, Ganymede, and Callisto, all photographed by Voyager 1.
PIA19856-PlutoCharon-NewHorizons-Color-20150714.jpg
PIA19856: A Binary Planet in Color


http://photojournal.jpl.nasa.gov/catalog/PIA19856


FILE DESCRIPTION:

Pluto and Charon are shown in enhanced color in this image, which is the highest-resolution color image of the pair so far returned to Earth by New Horizons. It was taken at 06:49 UT on July 14, 2015, five hours before Pluto closest approach, from a range of 150,000 miles (250,000 kilometers), with the spacecraft's Ralph instrument.

The image highlights the contrasting appearance of the two worlds: Charon is mostly gray, with a dark reddish polar cap, while Pluto shows a wide variety of subtle color variations, including yellowish patches on the north polar cap and subtly contrasting colors for the two halves of Pluto's "heart", informally named Tombaugh Regio, seen in the upper right quadrant of the image.

In order to fit Pluto and Charon in the same frame in their correct relative positions, the image has been rotated so the north pole on both Pluto and Charon is pointing towards the upper left. The image was made with the blue, red, and near-infrared color filters of Ralph's Multicolor Visible Imaging Camera, and shows colors that are similar, but not identical, to what would be seen with the human eye, which is sensitive to a narrower range of wavelengths.

The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built, and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. The Southwest Research Institute, based in San Antonio, leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Alabama.
243 ida crop.jpg
This color picture is made from images taken by the imaging system on the Galileo spacecraft about 14 minutes before its closest approach to asteroid 243 Ida on August 28, 1993. The range from the spacecraft was about 10,500 kilometers (6,500 miles). The images used are from the sequence in which Ida's moon was originally discovered; the moon is visible to the right of the asteroid. This picture is made from images through the 4100-angstrom (violet), 7560 A (infrared) and 9680 A (infrared) filters. The color is 'enhanced' in the sense that the CCD camera is sensitive to near infrared wavelengths of light beyond human vision; a 'natural' color picture of this asteroid would appear mostly gray. Shadings in the image indicate changes in illumination angle on the many steep slopes of this irregular body as well as subtle color variations due to differences in the physical state and composition of the soil (regolith). There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper right-hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance or composition of iron-bearing minerals in these areas. Ida's moon also has a deeper near-infrared absorption and a different color in the violet than any area on this side of Ida. The moon is not identical in spectral properties to any area of Ida in view here, though its overall similarity in reflectance and general spectral type suggests that it is made of the same rock types basically. These data, combined with study of further imaging data and more detailed spectra from the Near Infrared Mapping Spectrometer, may allow scientists to determine whether the larger parent body of which Ida, its moon, and some other asteroids are fragments was a heated, differentiated object or made of relatively unaltered primitive chondritic material.
Solar system.jpg
This is a montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune. The spacecraft responsible for these images are as follows:
  • the Mercury image was taken by Mariner 10,
  • the Venus image by Magellan,
  • the Earth and Moon images by Galileo,
  • the Mars image by Mars Global Surveyor,
  • the Jupiter image by Cassini, and
  • the Saturn, Uranus and Neptune images by Voyager.
  • Pluto is not shown as it is no longer a planet. The inner planets (Mercury, Venus, Earth, Moon, and Mars) are roughly to scale to each other; the outer planets (Jupiter, Saturn, Uranus, and Neptune) are roughly to scale to each other. PIA 00545 is the same montage with Neptune shown larger in the foreground. Actual diameters are given below:
  • Sun (to photosphere) 1,392,684 km
  • Mercury 4,879.4 km
  • Venus 12,103.7 km
  • Earth 12,756.28 km
  • Moon 3,476.2 km
  • Mars 6,804.9 km
  • Jupiter 142,984 km
  • Saturn 120,536 km
  • Uranus 51,118 km
  • Neptune 49,528 km
Crab Nebula.jpg
This is a mosaic image, one of the largest ever taken by NASA's Hubble Space Telescope, of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event in 1054 CE, as did, almost certainly, Native Americans.

The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second due to the neutron star's rotation. A neutron star is the crushed ultra-dense core of the exploded star.

The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844, using a 36-inch telescope. When viewed by Hubble, as well as by large ground-based telescopes such as the European Southern Observatory's Very Large Telescope, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of a star, 6,500 light-years away.

The newly composed image was assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.
The Earth seen from Apollo 17 with transparent background.png
"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972 by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometers (18,000 statute miles). It shows Africa, Antarctica, and the Arabian Peninsula.
He1523a.jpg
Author/Creator: ESO, European Southern Observatory, Licence: CC BY 4.0
Artist's impression of "the oldest star of our Galaxy": HE 1523-0901
  • About 13.2 billion years old
  • Approximately 7500 light years far from Earth
  • Published as part of Hamburg/ESO Survey in the May 10 2007 issue of The Astrophysical Journal
RocketSunIcon.svg
Author/Creator: Me, Licence: Copyrighted free use
SVG replacement for File:Spaceship and the Sun.jpg. A stylized illustration of a spaceship and the sun, based on the description of the emblem of the fictional Galactic Empire in Isaac Asimov's Foundation series ("The golden globe with its conventionalized rays, and the oblique cigar shape that was a space vessel"). This image could be used as a icon for science-fiction related articles.
Earth-moon.jpg
This view of the rising Earth greeted the Apollo 8 astronauts as they came from behind the Moon after the fourth nearside orbit. Earth is about five degrees above the horizon in the photo. The unnamed surface features in the foreground are near the eastern limb of the Moon as viewed from Earth. The lunar horizon is approximately 780 kilometers from the spacecraft. Width of the photographed area at the horizon is about 175 kilometers. On the Earth 240,000 miles away, the sunset terminator bisects Africa.
Europa-moon-with-margins.jpg
This image shows a view of the trailing hemisphere of Jupiter's ice-covered satellite, Europa, in approximate natural color. Long, dark lines are fractures in the crust, some of which are more than 3,000 kilometers (1,850 miles) long. The bright feature containing a central dark spot in the lower third of the image is a young impact crater some 50 kilometers (31 miles) in diameter. This crater has been named "Pwyll" after the figure from Welsh mythology. Europa is about 3,160 kilometers (1,950 miles) in diameter, or about the size of Earth's moon. This image was taken on September 7, 1996, at a range of 677,000 kilometers (417,900 miles) by the solid state imaging television camera onboard the Galileo spacecraft during its second orbit around Jupiter. The image was processed by Deutsche Forschungsanstalt fuer Luftund Raumfahrt e.V., Berlin, Germany.
Iapetus as seen by the Cassini probe - 20071008.jpg

Iapetus as seen by the Cassini probe.
Original NASA caption: Cassini captures the first high-resolution glimpse of the bright trailing hemisphere of Saturn's moon Iapetus.
This false-color mosaic shows the entire hemisphere of Iapetus (1,468 kilometers, or 912 miles across) visible from Cassini on the outbound leg of its encounter with the two-toned moon in Sept. 2007. The central longitude of the trailing hemisphere is 24 degrees to the left of the mosaic's center.
Also shown here is the complicated transition region between the dark leading and bright trailing hemispheres. This region, visible along the right side of the image, was observed in many of the images acquired by Cassini near closest approach during the encounter.
Revealed here for the first time in detail are the geologic structures that mark the trailing hemisphere. The region appears heavily cratered, particularly in the north and south polar regions. Near the top of the mosaic, numerous impact features visible in NASA Voyager 2 spacecraft images (acquired in 1981) are visible, including the craters Ogier and Charlemagne.
The most prominent topographic feature in this view, in the bottom half of the mosaic, is a 450-kilometer (280-mile) wide impact basin, one of at least nine such large basins on Iapetus. In fact, the basin overlaps an older, similar-sized impact basin to its southeast.
In many places, the dark material--thought to be composed of nitrogen-bearing organic compounds called cyanides, hydrated minerals and other carbonaceous minerals--appears to coat equator-facing slopes and crater floors. The distribution of this material and variations in the color of the bright material across the trailing hemisphere will be crucial clues to understanding the origin of Iapetus' peculiar bright-dark dual personality.
The view was acquired with the Cassini spacecraft narrow-angle camera on Sept. 10, 2007, at a distance of about 73,000 kilometers (45,000 miles) from Iapetus.
The color seen in this view represents an expansion of the wavelengths of the electromagnetic spectrum visible to human eyes. The intense reddish-brown hue of the dark material is far less pronounced in true color images. The use of enhanced color makes the reddish character of the dark material more visible than it would be to the naked eye.
This mosaic consists of 60 images covering 15 footprints across the surface of Iapetus. The view is an orthographic projection centered on 10.8 degrees south latitude, 246.5 degrees west longitude and has a resolution of 426 meters (0.26 miles) per pixel. An orthographic view is most like the view seen by a distant observer looking through a telescope.
At each footprint, a full resolution clear filter image was combined with half-resolution images taken with infrared, green and ultraviolet spectral filters (centered at 752, 568 and 338 nanometers, respectively) to create this full-resolution false color mosaic.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.
Thebe.jpg
This image of Thebe was taken by NASA's Galileo spacecraft on January 4, 2000, at a range of 193,000 kilometers.
Deimos-MRO.jpg
Color-enhanced image of Deimos, a moon of Mars, captured by the HiRISE instrument on the Mars Reconnaissance Orbiter on 21 Feb 2009. Cropped from source image.
Moon, Earth size comparison.jpg
Diameter comparison of the Moon and Earth.
Approximate scale is 29 km/px.
Titan in true color.jpg
Titan's atmosphere makes Saturn's largest moon look like a fuzzy orange ball in this natural color view from the Cassini spacecraft.

Titan's north polar hood is visible at the top of the image, and a faint blue haze also can be detected above the south pole at the bottom of this view.

This view looks toward the anti-Saturn side of Titan (3,200 miles, or 5,150 kilometers across). North is up. Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were obtained with the Cassini spacecraft wide-angle camera on Jan. 30, 2012 at a distance of approximately 119,000 miles (191,000 kilometers) from Titan. Image scale is 7 miles (11 kilometers) per pixel.
Prometheus 12-26-09a.jpg
This raw, unprocessed image of Prometheus was taken by Cassini on Dec. 26, 2009.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 26, 2009 at a distance of approximately 59,000 kilometers (36,000 miles) from Prometheus and at a Sun-Prometheus-spacecraft, or phase, angle of 19 degrees. Image scale is 351 meters (1,150 feet) per pixel.

The Cassini Equinox Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini Equinox Mission visit http://ciclops.org, http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

The original NASA image has been modified by cropping, doubling the linear pixel density, reducing brightness, and removal of some cosmic ray artifacts.
Hydra Enhanced Color.jpg
Color image of Hydra, image create using high resolution LORRI image and MVIC color data.
Io highest resolution true color.jpg
Original Caption Released with Image:

NASA's Galileo spacecraft acquired its highest resolution images of Jupiter's moon Io on 3 July 1999 during its closest pass to Io since orbit insertion in late 1995. This color mosaic uses the near-infrared, green and violet filters (slightly more than the visible range) of the spacecraft's camera and approximates what the human eye would see. Most of Io's surface has pastel colors, punctuated by black, brown, green, orange, and red units near the active volcanic centers. A false color version of the mosaic has been created to enhance the contrast of the color variations.

The improved resolution reveals small-scale color units which had not been recognized previously and which suggest that the lavas and sulfurous deposits are composed of complex mixtures (Cutout A of false color image). Some of the bright (whitish), high-latitude (near the top and bottom) deposits have an ethereal quality like a transparent covering of frost (Cutout B of false color image). Bright red areas were seen previously only as diffuse deposits. However, they are now seen to exist as both diffuse deposits and sharp linear features like fissures (Cutout C of false color image). Some volcanic centers have bright and colorful flows, perhaps due to flows of sulfur rather than silicate lava (Cutout D of false color image). In this region bright, white material can also be seen to emanate from linear rifts and cliffs.

Comparison of this image to previous Galileo images reveals many changes due to the ongoing volcanic activity.

Galileo will make two close passes of Io beginning in October of this year. Most of the high-resolution targets for these flybys are seen on the hemisphere shown here.

North is to the top of the picture and the sun illuminates the surface from almost directly behind the spacecraft. This illumination geometry is good for imaging color variations, but poor for imaging topographic shading. However, some topographic shading can be seen here due to the combination of relatively high resolution (1.3 kilometers or 0.8 miles per picture element) and the rugged topography over parts of Io. The image is centered at 0.3 degrees north latitude and 137.5 degrees west longitude. The resolution is 1.3 kilometers (0.8 miles) per picture element. The images were taken on 3 July 1999 at a range of about 130,000 kilometers (81,000 miles) by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft during its twenty-first orbit.

The Jet Propulsion Laboratory, Pasadena, CA manages the Galileo mission for NASA's Office of Space Science, Washington, DC.

This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at URL http://galileo.jpl.nasa.gov. Background information and educational context for the images can be found at URL http://www.jpl.nasa.gov/galileo/sepo.
PIA18185 Miranda's Icy Face.jpg
Uranus' icy moon Miranda is seen in this image from Voyager 2 on January 24, 1986. The Voyager project is managed for NASA by the Jet Propulsion Laboratory.
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(c) Kwamikagami at English Wikipedia, CC BY-SA 3.0
Pie chart of the masses of the moons of the Solar system. (The order of the moons reflects their order from the Sun, and then the order from their primary.) Only moons massive enough to be in hydrostatic equilibrium are graphed, as the other moons are insignificant at this scale. Even so, Mimas, Enceladus, and Miranda are too small to be visible.
Callisto.jpg
Bright scars on a darker surface testify to a long history of impacts on Jupiter's moon Callisto in this image of Callisto from NASA's Galileo spacecraft. The picture, taken in May 2001, is the only complete global color image of Callisto obtained by Galileo, which has been orbiting Jupiter since December 1995. Of Jupiter's four largest moons, Callisto orbits farthest from the giant planet. Callisto's surface is uniformly cratered but is not uniform in color or brightness. Scientists believe the brighter areas are mainly ice and the darker areas are highly eroded, ice-poor material.
Pan by Cassini, March 2017.jpg
View of the ring shepherd Saturnian moon of Pan, taken from above its northern hemisphere on 7 March 2017 by the Cassini spacecraft. This is a crop of an image by the CL1 and CL2 filters of the Narrow-Angle Imaging Science Subsystem camera, N00277980, that was received on Earth on 8 March.
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Global Color Mosaic of Triton, taken by Voyager 2 in 1989
Voyager 2 picture of Oberon.jpg
Original Caption Released with Image: This Voyager 2 picture of Oberon is the best the spacecraft acquired of Uranus' outermost moon. The picture was taken shortly after 3:30 a.m. PST on Jan. 24, 1986, from a distance of 660,000 kilometers (410,000 miles). The color was reconstructed from images taken through the narrow-angle camera's violet, clear and green filters. The picture shows features as small as 12 km (7 mi) on the moon's surface. Clearly visible are several large impact craters in Oberon's icy surface surrounded by bright rays similar to those seen on Jupiter's moon Callisto. Quite prominent near the center of Oberon's disk is a large crater with a bright central peak and a floor partially covered with very dark material. This may be icy, carbon-rich material erupted onto the crater floor sometime after the crater formed. Another striking topographic feature is a large mountain, about 6 km (4 mi) high, peeking out on the lower left limb. The Voyager project is managed for NASA by the Jet Propulsion Laboratory.
PIA07763 Rhea full globe5.jpg
This giant mosaic reveals Saturn's icy moon Rhea in her full, crater-scarred glory.

This view consists of 21 clear-filter images and is centered at 0.4 degrees south latitude, 171 degrees west longitude.

The giant impact basin Tirawa is seen above and to the right of center. Tirawa, and the even larger basin Mamaldi to its southwest, are both covered in impact craters, indicating they are quite ancient.

The bright, approximately 40-kilometer-wide (25-mile) ray crater seen in many Cassini views of Rhea is located on the right side of this mosaic (at 12 degrees south latitude, 111 degrees west longitude). See PIA07764 for a close-up view of the eastern portion of the bright, ray crater.

There are few signs of tectonic activity in this view. However, the wispy streaks on Rhea that were seen at lower resolution by NASA's Voyager and Cassini spacecraft, were beyond the western (left) limb from this perspective. In high-resolution Cassini flyby images of Dione, similar features were identified as fractures caused by extensive tectonism.

Rhea is Saturn's second-largest moon, at 1,528 kilometers (949 miles) across.

The images in this mosaic were taken with the Cassini spacecraft narrow-angle camera during a close flyby on Nov. 26, 2005. The images were acquired as Cassini approached the moon at distances ranging from 79,190 to 58,686 kilometers (49,206 to 36,466 miles) from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of about 19 degrees. Image scale in the mosaic is 354 meters (1,161 feet) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

The NASA image has been processed to enhance contrast and had black panels added to its borders.
Charon in True Color - High-Res.jpg
Three years after NASA's New Horizons spacecraft gave humankind our first close-up views of Pluto and its largest moon, Charon, scientists are still revealing the wonders of these incredible worlds in the outer solar system. Marking the anniversary of New Horizons' historic flight through the Pluto system on July 14, 2015, mission scientists released the highest-resolution color images of Pluto and Charon.

These natural-color images result from refined calibration of data gathered by New Horizons' color Multispectral Visible Imaging Camera (MVIC). The processing creates images that would approximate the colors that the human eye would perceive, bringing them closer to “true color” than the images released near the encounter.

This image was taken on July 14, 2015, from a range of 46,091 miles (74,176 kilometers). This single color MVIC scan includes no data from other New Horizons imagers or instruments added. The striking features on Charon are clearly visible, including the reddish north-polar region known as Mordor Macula.
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Amalthea, fifth largest moon of Jupiter
PIA09813 Epimetheus S. polar region.jpg

The Cassini spacecraft's close flyby of Epimetheus in December 2007 returned detailed images of the moon's south polar region.

The view shows what might be the remains of a large impact crater covering most of this face, and which could be responsible for the somewhat flattened shape of the southern part of Epimetheus (116 kilometers, or 72 miles across) seen previously at much lower resolution.

The image also shows two terrain types: darker, smoother areas, and brighter, slightly more yellowish, fractured terrain. One interpretation of this image is that the darker material evidently moves down slopes, and probably has a lower ice content than the brighter material, which appears more like "bedrock." Nonetheless, materials in both terrains are likely to be rich in water ice.

The images that were used to create this enhanced color view were taken with the Cassini spacecraft narrow-angle camera on Dec. 3, 2007. The views were obtained at a distance of approximately 37,400 kilometers (23,000 miles) from Epimetheus and at a Sun-Epimetheus-spacecraft, or phase, angle of 65 degrees. Image scale is 224 meters (735 feet) per pixel.

The Cassini–Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini–Huygens mission visit http://saturn.jpl.nasa.gov/home/index.cfm. The Cassini imaging team homepage is at http://ciclops.org.

The NASA image has been cropped.
Mimas Cassini.jpg
Description from NASA :

In this view captured by NASA's Cassini spacecraft on its closest-ever flyby of Saturn's moon Mimas, large Herschel Crater dominates Mimas, making the moon look like the Death Star in the movie "Star Wars."
Herschel Crater is 130 kilometers, or 80 miles, wide and covers most of the right of this image. Scientists continue to study this impact basin and its surrounding terrain (see PIA12569 and PIA12571).
Cassini came within about 9,500 kilometers (5,900 miles) of Mimas on Feb. 13, 2010. This mosaic was created from six images taken that day in visible light with Cassini's narrow-angle camera on Feb. 13, 2010. The images were re-projected into an orthographic map projection. This view looks toward the area between the region that leads on Mimas' orbit around Saturn and the region of the moon facing away from Saturn. Mimas is 396 kilometers (246 miles) across. This view is centered on terrain at 11 degrees south latitude, 158 degrees west longitude. North is up. This view was obtained at a distance of approximately 50,000 kilometers (31,000 miles) from Mimas and at a sun-Mimas-spacecraft, or phase, angle of 17 degrees. Image scale is 240 meters (790 feet) per pixel.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.
For more information about the Cassini-Huygens mission visit http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

The original NASA image has been cropped.
Leading hemisphere of Helene - 20110618.jpg
This raw, unprocessed image of Saturn's moon Helene was taken by Cassini on 18 June 2011 and received on Earth 20 June 2011.

Helene is a trojan moon of Dione. It leads Dione by 60 degrees in their shared orbit. The view looks toward the leading hemisphere of Helene (33 kilometers, 21 miles across). North on Helene is towards the top.

The camera was pointing toward Helene, and the image was taken using the CL1 and CL2 filters. The image has not been validated or calibrated. A validated/calibrated image will be archived with the Planetary Data System in 2012.

The Cassini Solstice Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colorado.

For more information about the Cassini Solstice Mission visit http://ciclops.org, http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

The original NASA image has been modified by cropping, sharpening and lightening shadows.
Euler diagram of solar system bodies.svg
Author/Creator: SounderBruce (translated version), Ariel Provost (French version), Tahc (original version), Licence: CC BY-SA 3.0
An Euler diagram showing the relationship between objects in the Solar System.
The central star has been excluded. Also excluded are artificial satellites.

Cosmic dust found within the Solar System falls under two categories depicted here: Small Solar System bodies and natural satellites. If the dust is orbiting the Sun, as with zodiacal dust, then it fits with small solar system bodies, which have no lower limit defined. And if the dust is orbiting a body other than the Sun, as with the countless particles of Saturn's rings, then these fit with natural satellites, which also have no lower limit defined. This diagram could be further refined to show how moonlets and dust fall within the category of natural satellites, with a separate set of cosmic dust orbiting the Sun falling within the category of small Solar System bodies.
W3C grn.svg The SVG code is valid.
Methone PIA14633.jpg
It's difficult not to think of an egg when looking at Saturn's moon Methone, seen here during a Cassini flyby of the small moon. The relatively smooth surface adds to the effect created by the oblong shape.

Small moons like Methone are not generally spherical in shape like the larger moons. The closer to Saturn they are, the greater their tidal bulge. Their small sizes means that they lack sufficient gravity to pull themselves into round shape. Scientists think that the elongated shapes of these moons may be a clue to how they formed.

Lit terrain seen here is on the leading side of Methone (2 miles, 3 kilometers across). North on Methone is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 20, 2012.

The view was obtained at a distance of approximately 3,000 miles (4,000 kilometers) from Methone and at a Sun-Methone-spacecraft, or phase, angle of 63 degrees. Scale in the original image was 88 feet (27 meters) per pixel. The image has been magnified by a factor of 2.

The Cassini Solstice Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.

The original NASA image has been modified by cropping and a further doubling of the linear pixel density.
Nix best view.jpg
Processed image of Nix
Jupiter and the Galilean Satellites.jpg
This "family portrait," a composite of the Jovian system, includes the edge of Jupiter with its Great Red Spot, and Jupiter's four largest moons, known as the Galilean satellites. From top to bottom, the moons shown are Io, Europa, Ganymede and Callisto. The Great Red Spot, a storm in Jupiter's atmosphere, is at least 300 years old. Winds blow counterclockwise around the Great Red Spot at about 400 kilometers per hour (250 miles per hour). The storm is larger than one Earth diameter from north to south, and more than two Earth diameters from east to west. In this oblique view, the Great Red Spot appears longer in the north-south direction. Europa, the smallest of the four moons, is about the size of Earth's moon, while Ganymede is the largest moon in the solar system. North is at the top of this composite picture in which the massive planet and its largest satellites have all been scaled to a common factor of 15 kilometers (9 miles) per picture element. The Solid State Imaging (CCD) system aboard NASA's Galileo spacecraft obtained the Jupiter, Io and Ganymede images in June 1996, while the Europa images were obtained in September 1996. Because Galileo focuses on high resolution imaging of regional areas on Callisto rather than global coverage, the portrait of Callisto is from the 1979 flyby of NASA's Voyager spacecraft.
PIA21055 - Pandora Up Close.jpg
This image from NASA's Cassini spacecraft is one of the highest-resolution views ever taken of Saturn's moon Pandora. Pandora (52 miles, 84 kilometers) across orbits Saturn just outside the narrow F ring.

The spacecraft captured the image during its closest-ever flyby of Pandora on Dec. 18, 2016, during the third of its grazing passes by the outer edges of Saturn's main rings. (For Cassini's closest view prior to this flyby, see PIA07632, which is also in color.)

The image was taken in green light with the Cassini spacecraft narrow-angle camera at a distance of approximately 25,200 miles (40,500 kilometers) from Pandora. Image scale is 787 feet (240 meters) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter. The Cassini imaging operations center is based at the Space Science Institute in Boulder, Colorado.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini. The Cassini imaging team homepage is at http://ciclops.org.
Daphnis (Saturn's Moon).jpg
Saturn's moon Daphnis as seen from the Cassini spacecraft.
Proteus (Voyager 2).jpg
Proteus is the second largest moon of Neptune behind the mysterious Triton. Proteus was discovered only in 1989 by the Voyager 2 spacecraft. This is unusual since Neptune has a smaller moon - Nereid - which was discovered 33 years earlier from Earth. The reason Proteus was not discovered sooner is that its surface is very dark and it orbits much closer to Neptune. Proteus has an odd box-like shape and were it even slightly more massive, its own gravity would cause it to reform itself into a sphere.

Original NASA caption: This image of Neptune's satellite 1989N1 was obtained on Aug. 25, 1989 from a range of 146,000 kilometers (91,000 miles). The resolution is about 2.7 kilometers (1.7 miles) per line pair.

The satellite, seen here about half illuminated, has an average radius of some 200 kilometers (120 miles). It is dark (albedo 6 percent) and spectrally grey. Hints of crater-like forms and groove-like lineations can be discerned. The apparent graininess of the image is caused by the short exposure necessary to avoid significant smear.
Titania (moon) color, cropped.jpg
Original caption: This high-resolution color composite of Titania was made from Voyager 2 images taken Jan. 24, 1986, as the spacecraft neared its closest approach to Uranus. Voyager's narrow-angle camera acquired this image of Titania, one of the large moons of Uranus, through the violet and clear filters. The spacecraft was about 500,000 kilometers (300,000 miles) away; the picture shows details about 9 km (6 mi) in size. Titania has a diameter of about 1,600 km (1,000 mi). In addition to many scars due to impacts, Titania displays evidence of other geologic activity at some point in its history. The large, trenchlike feature near the terminator (day-night boundary) at middle right suggests at least one episode of tectonic activity. Another, basinlike structure near the upper right is evidence of an ancient period of heavy impact activity. The neutral gray color of Titania is characteristic of the Uranian satellites as a whole. The Voyager project is managed for NASA by the Jet Propulsion Laboratory.
N00151485 Calypso crop.jpg
Cassini narrow-angle camera raw image N00151485.jpg was taken on February 13, 2010 and received on Earth February 14, 2010. The camera was pointing toward CALYPSO, and the image was taken using the CL1 and GRN filters. This image has not been validated or calibrated. A validated/calibrated image will be archived with the NASA Planetary Data System in 2011.

The original NASA image has been modified by cropping, doubling the linear pixel density and sharpening.
Larissa 1.jpg

Image:Larissa.jpg cropped and cut down to show only one image.

The photojournal caption for the original image:

These Voyager 2 images of satellite 1989N2 at a resolution of 4.2 kilometers (2.6 miles) per pixel reveal it to be and irregularly shaped, dark object. The satellite appears to have several craters 30 to 50 kilometers (18.5 to 31 miles) across. The irregular outline suggests that this moon has remained cold and rigid throughout much of its history. It is about 210 by 190 kilometers (130 by 118 miles), about half the size of 1989N1. It has a low albedo surface reflecting about 5 percent of the incident light. The Voyager Mission is conducted by JPL for NASA's Office of Space Science and Applications.
Ganymede - Perijove 34 Composite.png
Author/Creator: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill, Licence: CC BY 2.0
Ganymede photographed by Juno in 2021, Projected from the perspective of '3.
FullMoon2010.jpg
(c) Gregory H. Revera, CC BY-SA 3.0
Full Moon photograph taken 10-22-2010 from Madison, Alabama, USA. Photographed with a Celestron 9.25 Schmidt-Cassegrain telescope. Acquired with a Canon EOS Rebel T1i (EOS 500D), 20 images stacked to reduce noise. 200 ISO 1/640 sec.
Moons of solar system v7.jpg
Moons of solar system scaled to Earth's Moon
Rhean rings PIA10246 Full res.jpg
Artist's conception of Rhean rings
Hyperion true.jpg
Approximately true-color mosaic of Saturn's moon Hyperion. Composed of several narrow-angle frames and processed to match Hyperion's natural color. Taken during Cassini's flyby of this lumpy moon on 26th September 2005.
25 solar system objects smaller than Earth.jpg
Author/Creator: User:primefac, Licence: CC BY 3.0
Relative sizes of 25 solary system objects smaller than Earth. This version was created and uploaded to fix flaws in the original created by User:tony_g100. All images taken from the source image or NASA images
Ariel (moon).jpg
This mosaic of the four highest-resolution images of Ariel represents the most detailed Voyager 2 picture of this satellite of Uranus. The images were taken through the clear filter of Voyager's narrow-angle camera on Jan. 24, 1986, at a distance of about 130,000 kilometers (80,000 miles). Ariel is about 1,200 km (750 mi) in diameter; the resolution here is 2.4 km (1.5 mi). Much of Ariel's surface is densely pitted with craters 5 to 10 km (3 to 6 mi) across. These craters are close to the threshold of detection in this picture. Numerous valleys and fault scarps crisscross the highly pitted terrain. Voyager scientists believe the valleys have formed over down-dropped fault blocks (graben); apparently, extensive faulting has occurred as a result of expansion and stretching of Ariel's crust. The largest fault valleys, near the terminator at right, as well as a smooth region near the center of this image, have been partly filled with deposits that are younger and less heavily cratered than the pitted terrain. Narrow, somewhat sinuous scarps and valleys have been formed, in turn, in these young deposits. It is not yet clear whether these sinuous features have been formed by faulting or by the flow of fluids.
Dione in natural light.jpg
This southerly view of Dione shows enormous canyons extending from mid-latitudes on the trailing hemisphere, at right, to the moon's south polar region.

This view looks toward the Saturn-facing side of Dione (1,126 kilometers, or 700 miles across) and is centered on 22 degrees south latitude, 359 degrees west longitude. North on Dione is up; the moon's south pole is seen at bottom.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Feb. 8, 2008. The view was obtained at a distance of approximately 211,000 kilometers (131,000 miles) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 20 degrees. Image scale is 1 kilometer (0.6 mile) per pixel.
Dactyl-HiRes.jpg
Galileo highest-res image asteroid Ida's moon (See below for details).
PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg
PIA18317: Tethys the Target

http://photojournal.jpl.nasa.gov/catalog/PIA18317

Like most moons in the Solar System, Tethys is covered by impact craters. Some craters bear witness to incredibly violent events, such as the crater Odysseus (seen here at the right of the image).

While Tethys is 1,062 kilometers (660 miles) across, the crater Odysseus is 450 kilometers (280 miles) across, covering about 18 percent of the moon's surface area. A comparably sized crater on Earth would be as large as Africa!

This view looks toward the anti-Saturn hemisphere of Tethys. North on Tethys is up and rotated 42 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 11, 2015.

The view was acquired at a distance of approximately 190,000 kilometers (118,000 miles) from Tethys. Image scale is 1 kilometer (3,280 feet) per pixel.

The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

For more information about the Cassini–Huygens mission visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini. The Cassini imaging team homepage is at http://ciclops.org.
PIA12714 Janus crop.jpg
Saturn's moon Janus shows the scars of impacts in this Cassini image of craters light and dark.

This view looks toward the Saturn-facing side of Janus (179 kilometers, 111 miles across). North on Janus is up and rotated 10 degrees to the right.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 7, 2010. The view was acquired at a distance of approximately 75,000 kilometers (46,000 miles) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 39 degrees. Image scale is 448 meters (1,469 feet) per pixel.

The original NASA image has been modified by cropping, doubling the linear pixel density, and sharpening.
PIA17202 - Approaching Enceladus.jpg
Original caption: NASA's Cassini spacecraft captured this view as it neared icy Enceladus for its closest-ever dive past the moon's active south polar region. The view shows heavily cratered northern latitudes at top, transitioning to fractured, wrinkled terrain in the middle and southern latitudes. The wavy boundary of the moon's active south polar region -- Cassini's destination for this flyby -- is visible at bottom, where it disappears into wintry darkness.

This view looks towards the Saturn-facing side of Enceladus. North on Enceladus is up and rotated 23 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 28, 2015.

The view was acquired at a distance of approximately 60,000 miles (96,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 45 degrees. Image scale is 1,896 feet (578 meters) per pixel.
Phobos colour 2008.jpg
Color image of Phobos, imaged by the Mars Reconnaissance Orbiter on 23 March 2008.

The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter took two images of the larger of Mars' two moons, Phobos, within 10 minutes of each other on 23 March 2008. This is the first, taken from a distance of about 6,800 kilometers (about 4,200 miles). It is presented in color by combining data from the camera's blue-green, red, and near-infrared channels.

The illuminated part of Phobos seen in the images is about 21 kilometers (13 miles) across. The most prominent feature in the images is the large crater Stickney in the lower right. With a diameter of 9 kilometers (5.6 miles), it is the largest feature on Phobos.

The color data accentuate details not apparent in black-and-white images. For example, materials near the rim of Stickney appear bluer than the rest of Phobos. Based on analogy with materials on our own moon, this could mean this surface is fresher, and therefore younger, than other parts of Phobos.

A series of troughs and crater chains is obvious on other parts of the moon. Although many appear radial to Stickney in this image, recent studies from the European Space Agency's Mars Express orbiter indicate that they are not related to Stickney. Instead, they may have formed when material ejected from impacts on Mars later collided with Phobos. The lineated textures on the walls of Stickney and other large craters are landslides formed from materials falling into the crater interiors in the weak Phobos gravity (less than one one-thousandth of the gravity on Earth).

In the full-resolution version of this image, a pixel encompasses 6.8 meters (22 feet), providing a resolution (smallest visible feature) of about 20 meters (about 65 feet). The image is in the HiRISE catalog as PSP_007769_9010.

NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, is the prime contractor for the project and built the spacecraft. The High Resolution Imaging Science Experiment is operated by the University of Arizona, Tucson, and the instrument was built by Ball Aerospace & Technologies Corp., Boulder, Colo.
PIA00040 Umbrielx2.47.jpg
Original Caption Released with Image: The southern hemisphere of Umbriel displays heavy cratering in this Voyager 2 image, taken Jan. 24, 1986, from a distance of 557,000 kilometers (346,000 miles). This frame, taken through the clear-filter of Voyager's narrow-angle camera, is the most detailed image of Umbriel, with a resolution of about 10 km (6 mi). Umbriel is the darkest of Uranus' larger moons and the one that appears to have experienced the lowest level of geological activity. It has a diameter of about 1,200 km (750 mi) and reflects only 16 percent of the light striking its surface; in the latter respect, Umbriel is similar to lunar highland areas. Umbriel is heavily cratered but lacks the numerous bright ray craters seen on the other large Uranian satellites; this results in a relatively uniform surface albedo (reflectivity). The prominent crater on the terminator (upper right) is about 110 km (70 mi) across and has a bright central peak. The strangest feature in this image (at top) is a curious bright ring, the most reflective area seen on Umbriel. The ring is about 140 km (90 miles) in diameter and lies near the satellite's equator. The nature of the ring is not known, although it might be a frost deposit, perhaps associated with an impact crater. Spots against the black background are due to 'noise' in the data. The Voyager project is managed for NASA by the Jet Propulsion Laboratory.

The original NASA image has been modified by cropping, increasing the linear pixel density by a factor of 2.47, and converting from .TIF to .JPG format.
Puck.png
translated from French: Image taken from Voyager 2 on 24 January 1986 of the Uranian moon Puck (at a distance of 493 Mm (thousand kilometers))
Phoebe cassini.jpg
Phoebe, as imaged by the Cassini probe.
Small bodies of the Solar System.jpg
Author/Creator: Antonio Ciccolella, Licence: CC BY-SA 4.0
Small bodies of the Solar System, including natural satellites, dwarf planets, asteroids, and minor-planet moons. Some of the Solar System's planets are also given for scaling orbital distances. Photomontage created from several images taken by space probes and telescopes.
Occulting Enceladus PIA10500.jpg
Original Caption Released with Image: