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aturn, surrounded by
the ring, is the most beautiful and attractive planet that creates the wonder of
this solar family. This impeccably beautiful planet is being orbited by
multiple satellites, large and small. Each of these satellites is unique and extraordinary
in its own characteristics. Today we will discuss about a small but mysterious
satellite Rhea.
Rhea is the second largest satellite of second largest planet of our solar system, Saturn which is also in sixth position in terms of distance from the Sun. However, in terms of size, the satellite ranks ninth in the solar family as a whole. Rhea was discovered on December 23, 1772 by Giovanni Domenico Cassini, an Italian mathematician, engineer and astronomer. It was not named until 1847. Later, the satellite was named after Rhea who is Kronos's wife and mother of other Greek gods and it was proposed by another world-famous astronomer, Sir John Herschel, the son of the world-famous astronomer Sir William Herschel. Another name for the Rhea is Saturn V because its position is fifth from Saturn in terms of distance.
With a diameter of 764 km, Rhea is situated at an average distance of 1,85,520 km from Saturn. Rhea is bound to its planet in synchronous rotation, meaning that Rhea's diurnal motion is approximately equal to its annual motion. One day on Rhea equals little more than four and half day as Earth's time. At about the same time, Rhea orbits Saturn once. Its average orbital speed is 8.48 km per second. As the mass of Rhea is very low, its escape velocity (Escape Velocity: The lowest velocity which a body must have in order to escape the gravitational attraction of a particular planet, satellite, star or other object.) is very low too. Its escape velocity is 0.64 km per second or 2,304 km per hour.
The density of Rhea is lower than calculated, indicating that 25 percent of the satellite's surface is composed of silicate rock and the remaining 75 percent is made up of ice, formed from water. Various studies have suggested that Rhea has been able to sustain its inner sea of liquid water, perhaps through the heat generated by radioactive decay. There are other features of Rhea with which Saturn's another satellite, Dione, bears many resemblances.
The difference between the light and shady areas of Rhea's surface is about 500 Celsius. The surface where sunlight reaches directly has an average temperature of -1740 Celsius and the surface where sunlight does not reach, that is the shaded area, has an average temperature below -2200 Celsius. The surface of Rhea is filled with numerous craters, created by cosmic collisions. Also some deep cracks and some fuzzy lines have been found in the equatorial region of Rhea. The area of Rhea which is situated opposite to Saturn, have two regions of the crater, created by the cosmic collision. These two areas range from 400 km to 500 km. The northern area of crater has fewer craters than the southern area of crater. The northern area of crater is known to us as Tiwara. On the surface of Rhea, a 48 km diameter ditch has also been found, called Inktomi or The Splat. It is well known for two reasons. Firstly, it is a very bright region and secondly, scientists estimate that Inktomi may be the newest ditch in this region.
The surface of Rhea has been divided into two separate geological classes in terms of the density of the craters, created by cosmic collisions. The first part usually covers those areas where the diameter of the ditch is 40 km or more and the second part covers those areas where the diameter of the ditch is less than 40 km. This second type of craters is more common in the polar and equatorial regions. Between these two classes of ditches there are a few craters that look very bright from space. Scientists speculate that these luminous regions may be covered by matters which were raised by the eruption of cryo-volcano during the formation of Rhea.
In 2010, NASA announced that Rhea had a thin atmosphere with the 5:2 ratio of oxygen and carbon dioxide. The reason for the presence of oxygen particles in the atmosphere of Rhea is the process of Radiolysis (Radiolysis: The process of separation of components that make up substance by radiation of ionized particles) by ionized particles from Saturn's magnetic field. But the cause of the presence of carbon dioxide has not yet been well established. However, it is speculated that carbon dioxide may have been formed due to oxidation of any organic matter present in the ice layer of Rhea.
Prior to this incident, NASA announced in 2008 that Rhea probably had a very thin ring. If the ring of Rhea really did exist, then Rhea is still the first and only satellite of this solar system to have a ring. Cassini Spacecraft assumed the existence of a ring by observing the change in the flow of electrons (Negatively charged particle) trapped by Saturn's magnetic field as it made its flyby (Flyby: According to space-science, the shortest distance, at the time of observation, between observation space-vehicle and the planet or satellite.) over Rhea. However, as a result of cosmic collisions, dust of Rhea can come up and may form a pseudo ring. The reason behind such thinking is that the Cassini spacecraft then observed Rhea from different planes and from different angles to be more sure about the presence of the ring but failed each time. So there is a need for more observation and extensive research in this regard in the future.
Till
date, there have been four space missions in Rhea. NASA's name is involved in
every mission. The first spacecraft which made its flyby over Rhea was called
Pioneer 11. On July 31, 1979, the spacecraft flew 3,45,303 km from Rhea.
Twenty months later, Voyager-I made its flyby over Rhea from 73,980 km
away on August 22, 1980, further, Voyager-II, the second spacecraft in the
Voyager Mission, made its flyby over Rhea from 6,45,260 km away on June
5, 1981, 10 months after the expedition of Voyager-I over Rhea and left the
solar family like Voyager-I to travel farther and farther away. (For more
information on Voyager mission, visit the article “Voyager, The Oldest Surviving
Space Mission”. The link of the article is: https://spacejagat.blogspot.com/2020/02/voyeger.html)
For
a long time after that, no space mission was made to Rhea and its planet
Saturn. Cassini, a spacecraft designed to collect information about Saturn and
its satellites, arrived at Rhea about 24 years later. The spacecraft made its
flyby over Rhea 5 times between the duration from November 26, 2005 to March 9,
2013, a period of 88 months. On August 30, 2007, the spacecraft made its
farthest distanced flyby. At that time, the distance between the spacecraft and
Rhea was 5,750 km and on January 11, 2011, the spacecraft made its
closest flyby to Rhea. At that time, the Cassini spacecraft was only 69
km from the surface of Rhea. During its orbit, Cassini, like other
planets and satellites, sent numerous images of Rhea to Earth. Cassini is still
the last spacecraft to target Rhea.
Right now, there have not been any plans to explore Rhea in recent future by any space research organization around the world. In 2026, NASA will launch the Dragonfly Mission for Titan. The trajectory (Trajectory: The specific route through which a rocket or spacecraft or meteor can complete their journey.) of that expedition has not yet been published. However, before reaching Titan in 2034 if it will make is flyby over Rhea, we may know more information about Rhea in the coming days and may recognize it anew.
Human
settlement in Rhea has never been discussed yet. But it is impossible to fully
predict the changing situation today after 500 crore years from now, when many
things will change in our solar system. Then if human settlement is successfully
established on any other planet or satellite, who can say that, this tiny Rhea
will not extend a helping hand to us.
DECLARATION: All THE IMAGES AND VIDEO HAVE BEEN SOURCED FROM GOOGLE.
