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Saturn, in astronomy

Saturn, in astronomy, 6th planet from the sun.

Astronomical and Physical Characteristics of Saturn

Saturn's orbit lies between those of Jupiter and Uranus; its mean distance from the sun is c.886 million mi (1.43 billion km), almost twice that of Jupiter, and its period of revolution is about 29 1-2 years. Saturn appears in the sky as a yellow, starlike object of the first magnitude. When viewed through a telescope, it is seen as a golden sphere, crossed by a series of lightly colored bands parallel to the equator.

Saturn, like the other Jovian planets (Jupiter, Uranus, and Neptune), is covered with a thick atmosphere composed mainly of hydrogen and helium, with some methane and ammonia; its temperature is believed to be about −270°F; (−168°C;), suggesting that the ammonia is in the form of ice crystals that constitute the clouds. Like Jupiter's interior, Saturn's consists of a rocky core, a liquid metallic hydrogen layer, and a molecular hydrogen layer. Traces of various ices have also been detected. The wind blows at high speeds—reaching velocities of 1,100 mph (1,770 kph)—across Saturn. The strongest winds are found near the equator and blow mostly in an easterly direction. At higher latitudes, the velocity decreases uniformly and the winds counterflow east and west. Because no permanent markings on the planet are visible, the planet's exact period of rotation has not been determined. However, the period of each atmospheric band varies from 10 hr 14 min at the equator to about 10 hr 38 min at higher latitudes. This rapid rotation causes the largest polar flattening among the planets (over 10%). Saturn is the second largest planet in the solar system; its equatorial diameter is c.75,000 mi (120,000 km), and its volume is more than 700 times the volume of the earth. Its mass is about 95 times that of the earth, making Saturn the only planet in the solar system with a density less than that of water. Saturn has been encountered by four space probe missions: Pioneer 11 (1979), Voyager 1 (1980), Voyager 2 (1981), and Cassini and Huygens (2004). Among the discoveries made by the Voyager probes was a magnetosphere (a region of charged particles consisting primarily of electrons, protons, and heavy ions captured partly from the atmosphere of the satellite Titan) that encloses 13 of Saturn's satellites and its ring system. Huygens landed on Saturn's moon Titan in 2005 and returned photographs of its surface.

The Ring System

Saturn's most remarkable feature is the system of thin, concentric rings lying in the plane of its equator. Although first observed by Galileo in 1610, it was not until 1656 that the rings were correctly interpreted by Christiaan Huygens, who did not reveal his findings about their phases and changes in shape until his treatise Systema Saturnium was published in 1659. Saturn's rings were believed to be unique until 1977, when very faint rings were found around Uranus; shortly thereafter faint rings were also detected around Jupiter and Neptune.

Although the ring system is almost 167,770 mi (270,000 km) in diameter, it is only some 330 ft (100 m) thick. From earth, this system appears to consist mainly of two bright outer rings, denoted A and B, separated by a dark rift—discovered by the Italian-French astronomer Gian Domenico Cassini—known as Cassini's division, plus a third, faint inner crepe ring (denoted C). The Encke Division, or Encke Gap, which splits the A ring, is named after the German astronomer Johann Franz Encke, who discovered it in 1837. Pictures from the Voyager probes show four additional rings. The exceedingly faint D ring lies closest to the planet. The faint F Ring is a narrow feature just outside the A Ring. Beyond that are two far fainter rings named G and E. In 1859 the Scottish physicist James Clerk Maxwell showed that the rings must consist of countless tiny particles each orbiting the planet in accordance with the laws of gravitation. When edgewise to the earth the rings appear as a nearly imperceptible ribbon of light across the planet; this occurs twice during the 29 1-2-year period of revolution. Twice during each orbit the rings reach a maximum inclination to the line of sight, once when they are visible from above and once when visible from below.

The Voyager 1 (1980) and 2 (1981) space probes revealed incredible new detail as they passed within 78,000 mi (126,000 km) and 63,000 mi (101,000 km) of Saturn, respectively. They recorded hundreds of tiny rings that are grouped into the seven major rings. The three brightest rings are lettered from the outermost, A, B, and C. The A, B, and C rings dissolved into more than 1,000 narrow ringlets, 100 of which are in the Cassini division. The outer F ring was found to contain braids, knots, and strands, possibly caused by nearby moons that shepherd it, that is, limit the extent of a planetary ring through gravitational forces. The origin of the rings is unknown, although it is believed that they may have been formed from larger satellites that were shattered by the impact of comets and meteoroids.

The Satellite System

Saturn has 48 confirmed natural satellites. Because the increasing number of satellites makes it difficult to continue to name them after Greek Titans, a scheme was adopted for the outer satellites. These are now named after the giants of other cultures: Inuit, Norse, and Gallic. The satellites may be divided into eight groups for convenience. In the order of their distance from Saturn, the groups are shepherd (satellites whose orbit is within or just beyond Saturn's ring system), co-orbital (two satellites that share the same orbit and trade positions within it on a regular basis), inner large (large satellites within the E ring), Trojan (satellites that are co-orbital at Lagrangian points), outer large (large satellites beyond the E ring), and Inuit, Norse, and Gallic (each a group of outer satellites that have similar orbits).

There are four named satellites, Pan, Atlas, Prometheus, and Pandora in the shepherd group. The co-orbital group comprises Epimetheus and Janus. The inner large group comprises six satellites, Mimas, Methone, Pallene, Enceladus, Tethys, and Dione. The Trojan group comprises four satellites, Telesto, Calypso, Helene, and Polydeuces. The outer large group comprises four satellites, Rhea, Titan, Hyperion, and Iapetus. The Inuit group comprises five satellites, four of which—Kiviuq, Ijiraq, Paaliaq, and Siarnaq—have been named. Of the 18 satellites comprising the Norse group, only seven are named: Phoebe, Skathi, Mundilfari, Narvi, Suttungr, Thrymr, and Ymir. The Gallic group consists of three satellites, Albiorix, Erriapo, and Tarvos.

Almost all of Saturn's inner moons form a regular system of satellites; that is, their orbits are nearly circular and lie in the equatorial plane of the planet; almost all of the outer moons' orbits are inclined. Except for Hyperion, which has a chaotic orbit, and Phoebe, all the satellites are believed to have synchronous orbits; that is, their orbital and rotational periods are the same, so that they always keep the same face turned toward Saturn. The largest satellite, Titan, is 3,200 mi (5,150 km) in diameter and has the size and cold temperatures necessary to retain an atmosphere; it is the only natural satellite in the solar system with a substantial atmosphere.

Saturn has six major icy satellites that can be easily seen through earth-based telescopes. The most prominent feature of heavily cratered Mimas, the innermost of the six, is a large impact crater about one third the diameter of the satellite. Certain broad regions of Enceladus are uncratered, indicating geological activity that has resurfaced the satellite within the last 100 million years. Tethys also has a very large impact crater, as well as an extensive series of valleys and troughs that stretches three quarters of the way around the satellite. Both Dione and Rhea have bright, heavily cratered leading hemispheres and darker trailing hemispheres with wispy streaks that are thought to be produced by deposits of ice inside surface troughs or cracks. Iapetus, the outermost of the large icy satellites, has a dark leading hemisphere and a bright trailing hemisphere.

The remaining satellites, some sharing orbits with others, are smaller. The two largest of these, the dark-surfaced Phoebe and the irregularly shaped Hyperion, orbit far from the planet; the outermost satellite, Ymir, orbits with retrograde motion, i.e., opposite to that of the planet's rotation, as do Phoebe, Mundilfari, Narvi, Suttungr, Thrymr, and many of the newly discovered, yet unnamed satellites. The smallest, ranging from c.12 to 20 mi (20 to 32 km) in diameter, are Pan and Atlas, the satellites closest to the planet, and Telesto, Calypso, and Helene. Prometheus and Pandora, c.55 mi (90 km) in diameter, share an orbit, as do Epimetheus and Janus.

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Saturn, in Roman religion and mythology

Saturn, in Roman religion and mythology, god of harvests, later identified with the Greek Kronos. Little is known of the origins of his cult. His reign was regarded as the Golden Age. He was the husband of Ops and the father of Jupiter, Juno, Ceres, Pluto, and Neptune. It was said that after the fall of the Titans, Saturn fled to Italy, where he settled on the Capitoline Hill, civilized the people, and taught them the arts of agriculture. At his festival, the Saturnalia, held at first on Dec. 17 but later extended for several days thereafter, gifts were exchanged, schools and courts were closed, war was outlawed, and slaves and masters ate at the same table.

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Saturn

Roman god of agriculture, equated with the Greek deity Cronus. His wife was Ops, the goddess of plenty, and his children included Juno, Neptune, and Ceres. His festival, Saturnalia (beginning December 17), became the most popular Roman festival; its influence is still felt in the celebration of Christmas and the Western New Year. During Saturnalia, all business transactions were suspended, presents were exchanged, and slaves were given token freedom. The remains of Saturn's temple are located in the Forum in Rome. Saturday is named for Saturn.

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Saturn

Sixth planet from the Sun, named for the Roman god of sowing and seed. The second largest nonstellar object in the solar system after Jupiter, it is about 95 times as massive as Earth and has more than 700 times its volume. Saturn's outer layers are gaseous, mainly hydrogen. Models of its interior suggest a rock-and-ice core surrounded by a shallow layer of liquid metallic hydrogen encased by an envelope of molecular hydrogen. Its mean density, about 70% that of water, is the lowest of any known object in the solar system. Saturn has at least 47 moons (including Titan, the largest) and an extensive ring system, with several main sections visible from Earth with a telescope. Saturn's rings, first observed in 1610 by Galileo, are made up of countless separate particles ranging mainly from inches to many feet in size but also including dust in some regions. Water ice probably constitutes most of the ring material. Saturn's day is about 10.6 hours; its year is 29.4 Earth years. Its rapid rotation, acting on electric currents in the core, generates a strong magnetic field and large magnetosphere. Saturn's fast spin also makes it the most flattened (oblate) of the planets; its polar diameter of 67,560 mi (108,728 km) is 10% smaller than its equatorial diameter. Its average distance from the Sun is 887 million mi (1.43 billion km).

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Saturn

Any of a series of space launch vehicles developed by the U.S. beginning in 1958 for the Apollo Moon-landing program. Saturn I, the first U.S. rocket specifically developed for spaceflight (first fired 1961), was a two-stage vehicle that placed unmanned versions of Apollo spacecraft and other satellites into Earth orbit. An upgraded version, Saturn IB, was used for unmanned and manned Apollo Earth-orbital missions and for ferrying crews to the Skylab space station. The three-stage Saturn V, the largest launch vehicle ever built by the U.S., was used for manned Apollo lunar flights and to launch Skylab.

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Saturn¹

the Roman god of agriculture and vegetation

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Saturn²

1. one of the giant planets the sixth planet from the sun, around which revolve planar concentric rings (Saturn's rings) consisting of small frozen particles. The planet has at least 30 satellites. Mean distance from sun: 1425 million km; period of revolution around sun: 29.41 years; period of axial rotation: 10.23 hours; equatorial diameter and mass: 9.26 and 95.3 times that of the earth, respectively

2. a large US rocket used for launching various objects, such as a spaceprobe or an Apollo spacecraft, into space

3. the alchemical name for lead

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Saturn [′sad·ərn]

(aerospace engineering)

One of the very large launch vehicles built primarily for the Apollo program; begun by Army Ordnance but turned over to the National Aeronautics and Space Administration for the manned space flight program to the moon.

(astronomy)

The second largest planet in the solar system (mass is 95.2 compared to earth's 1) and the sixth in the order of distance to the sun; it is visible to the naked eye as a yellowish first-magnitude star except during short periods near its conjunction with the sun; it is surrounded by a series of rings.