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Saturn

Saturn is a gas giant planet. Saturn is the second largest planet in the solar system, the sixth from the Sun, it belongs to the four giant planets that are made of gasses. It is the lighter planet as it mean density is only 0.687 g/cm³, quite bright with magnitude from +1.47 to ?0.24, it orbits the Sun in  29.4571 years and has a synodic period of 378.09 days with respect to the Earth. Its equatorial radius is 9.45  Earth radii with a smaller polar radius of 8.6 Earth radii. Its mass is 95 times the one of the Earth. Its heliocentric distance is almost double than the one of Jupiter, with aphelion of 10.1 04 AU and perihelion at 9.0 AU, semi-major axis 9.58 AU and eccentricity 0.0557. Saturn has very many satellites as ~ 200 have been observed, 61 of them with well known orbits and 53 fifty-three of them officially named. The equatorial surface gravity is almost equal to the one of the Earth (1.065 g) and with its radius leads to an escape velocity of 35.5 km/s. It rotates around its axis with respect to the stars in 10h 34m. The chemical composition of the outer atmosphere is 96.3% molecular hydrogen and 3.25% helium and smaller amounts of ammonia, acetylene, ethane, phosphine, methane, while the upper clouds contain ammonia crystals and the lower ammonium hydrosulfide (NH₄SH) and or water. It does not contain but very little helium, much less than the Sun and Jupiter.

 The axial tilt is similar to the Earth’s 26.73°. The reflectivity (albedo) is 0.342 (Bond) or 0.47 (geometric). As expected for a far from the Sun object the temperature is very low as it varies between 84 K and 134 K. It has been observed for the first time with a telescope by Galileo Galilei in 1610 who observed the rings as two smaller spherical bodies. In 1695, Christian Huygens was the astronomer who suggested the existence of a thin, flat ring, while Jean-Dominique Cassini observed a division between the -now named- A and B rings.

The magnetic field of Saturn has the axisymmetric shape of a magnetic dipole aligned with the rotational axis but the dipole is positioned 0.037 R_saturn towards the North Pole. It is somewhat weaker than Earth's magnetic field, as at the equator is 20 µT while the magnetic moment is 4.6 X 10¹⁸ T m³, 580 times larger than Earth’s and 30 times smaller than the one of Jupiter.

Saturn has a huge magnetosphere with dimension is about one-fifth that of the Jovian magnetosphere, as a result of the very low density of the solar wind at the distance of Saturn, which is 100 times more rarefied than at the Earth’s distance. The magnetosphere of Saturn has a diameter that is usually the same as the one of the Sun. The magnetosphere has radiation belts that are affected by the presence of the rings and the satellites.

The observation and exploration of Saturn consisted from three phases: The era of the ancient observations (naked eye), the epoch of the advanced telescopic observations (starting in the 17th century) and finally in the 21st century the phase of the spacecrafts orbiting the giant planet begun.

During the beginning of the era of observations with telescopes Titan, a very important moon of Saturn, was discovered by Huygens and few years later Giovanni Domenico Cassini gazed four other moons: Iapetus, Rhea, Tethys and Dione. The same astronomer also viewed the largest gap in the rings, which is now known as the Cassini Division. In 1789 William Herschel discovered two other important and equally interesting moons: Mimas and Enceladus.

Enceladus has geiser like cryovolcanoes and creates Saturn's E Ring,

Credit: CICLOPS, JPL, ESA, NASA

During the 20th and the 21st century ESA and NASA collaborated for studying Saturn in-situ. The first visitor was PIoneer 11 in 1979. After those low-resolution images in 1980 Voyager 1 probe sent us the first high-resolution images of the system (planet, rings and satellites).  After Voyager 1, Voyager 2 (1981) continued with a detailed study of Voyager 1, as they had cameras onboard. Thanks to this mission we were able to get more close-up images of the moons, and more detailed studies of the Saturn's atmosphere and rings. With Voyager 1 and 2 we achieved the confirmation (or even the discovery) of several more moons and the discovery of important features of the rings like the small Maxwell Gap and the Keeler gap.

The last mission to Saturn is the Cassini-Huygens probe, which performed its Solar Orbit Insertion (SOI) in 2004, and since then it keeps sending back high-resolution images and data. This mission can be split into phases based upon the inclination of the spacecraft's orbit. These varied orbits will allow views onto the rings and the studying of all parts of the magnetic field and energetic particle environment.

LINKS:

http://en.wikipedia.org/wiki/Saturn#Magnetosphere

http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1973-019A

http://www.nasa.gov/mission_pages/voyager/index.html

http://saturn.jpl.nasa.gov/index.cfm

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn

http://www.planetary.org/explore/topics/space_missions/cassini_huygens/tour.html

PICTURES:

http://pds.nasa.gov/planets/special/saturn.htm 

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