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We usually divide the planets into two groups, but we have two ways of doing this. One way is by location in the Solar System, and the other is by what they are like. The planets are located in the inner and outer parts of the Solar System, so two groups are then formed. The planets are also further classified in several ways. Those ways are by composition, size, position relative to the sun, position relative to Earth and history.

What determines what group each planet will be placed in, when discussing its composition, is whether it is terrestrial and rocky or gas. Terrestrial planets are composed primarily of rock and metal and relatively high densities, slow rotation, solid surfaces, no rings and few satellites. Gas planets are composed primarily of hydrogen and helium, and generally have low densities, rapid rotation, deep atmospheres, rings and a lot of satellites. The terrestrial planets are Mercury, Venus, Earth and Mars. The gas planets are Jupiter, Saturn, Uranus and Neptune. Then there is Pluto which is in a group all its own.

The next method of classification is by size. The small planets are Mercury, Venus, Earth, Mars and Pluto. In order to be classified in the small planets group, the planet must have a diameter less than 13,000 km.. The giant planets are Jupiter, Saturn, Uranus, and Neptune. The giant planets must have the diameter greater than 48,000 km.. The

giant planets are sometimes referred to as gas giants, while Mercury and Pluto are sometimes called lesser planets.

When discussing the position relative to the sun, the planets are categorized in groups of inner and outer planets. The planets in the inner Solar System are the Sun, Mercury, Venus, Earth and Mars, and in the outer Solar System are Jupiter, Saturn, Uranus, Neptune and Pluto. There is a belt called the asteroid belt, which is between Mars and Jupiter. This belts purpose is to form a boundary line between the inner and outer Solar System.

The planets are also classified by its position to the Earth. The planets are placed in three groups when using this method, inferior, Earth and superior. The inferior planets are Mercury and Venus. They are located closer to the Sun than Earth, and they show phases like the moon s when viewed from the Earth. The superior planets are Mars through Pluto. They are further away from the Sun than the earth is, and they always appear full or nearly so.

The last method used to classify the nine planets is by history. This method is also

classified into three groups. The groups are classical, modern and Earth. The classical planets are Mercury, Venus, Jupiter and Saturn, which have been known since prehistoric times. The modern planets are Uranus, Neptune and Pluto. The modern planets were discovered in modern times, and are visible with only a telescope. Then there is Earth, once again in a category all its own.

The inner planets, which are Earth, Mars, Mercury and Venus, are similar in several ways. Each of these planets are terrestrial. Terrestrial meaning, planets that are made

mostly of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites. They are also the smallest planets in size, each under 13,000 km. in diameter. All of the inner planets are, by position, relative to the Sun by being closer than the outer planets will ever be.

The outer planets, which are Jupiter, Saturn, Uranus, Neptune and Pluto, are similar in many ways. Each of these planets are made of gas. Gas meaning, planets that are composed primarily of hydrogen and helium and generally have less densities, rapid rotation, deep atmospheres, rings and a lot of satellites. They are also the largest planets in size, each over 48,000 km. in diameter. All of the outer planets are, by position, relative to the Sun by being farther away than the inner planets will ever be. The only exception is Pluto. Pluto is not classified as a large planet, for it is under 13,000 km. in diameter which is the requirements for a small planet.

Each of the nine planets have a characteristic that makes it stand out from the rest, to make it unique.

Mercury: Temperature variations on Mercury are the most extreme in the Solar System ranging from 90k to 700k.

Venus: Venus orbit is the most nearly circular of that of any planet, with an eccentricity of less than 1%. It is also the brightest object in the sky except for the Sun and the Moon.

Earth: Earth is the only planet whose name does not derive from the Greek/Roman mythology. The name derives from Old English and Germanic.

Mars: Olympus Mons is the largest mountain in the Solar System, rising 24 km. above the surrounding plain. The base is more than 500 km. in diameter and is rimmed by a cliff 6 km. high.

Jupiter: Jupiter is more than twice as massive as the other planets combined.

Saturn: Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.

Uranus: Uranus magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with the respect to the axis of rotation. It is probably generated by the motion at the relatively shallow depths within Uranus.

Neptune: Because Pluto s orbit is so eccentric, it sometimes crosses the orbit of Neptune, making Neptune the most distant planet from the Sun for a few years.

Pluto: Pluto is about 2,320 km. (1,440 miles) in diameter. Charon is about 1,270 km. (790 miles) in diameter, making Pluto and Charon the planet-satellite pair closest in size in the Solar System.

An asteroid is one of many small or minor planets that are members of the solar system and move in oval-shaped orbits, for the most part, between the orbits of Mars and Jupiter. The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common those with diameters of less than 160 km. Most asteroids may be binary, or have satellites of their own. Three-quarters of the asteroids visible from earth belong to the C type, which appears to be related to a class of stony meteorites known as carbonaceous chondrites. These are considered to be the oldest materials in the solar system, with a

composition reflecting that of solar nebula. They are extremely dark in color because of their hydrocarbon content. They have never been melted or reheated since they first formed.

Asteroids of the S type, related to the stony meteorites, make up 15 percent of the total population. Consisting of iron-nickel, they may represent the cores of melted, diverse planetary bodies whose outer layers were removed by impact cratering. A very few asteroids are probably related to the rarest meteorite class of all, the achondrites. These asteroids appear to have an igneous surface composition like that of many lunar and terrestrial lave flows. Scientists are puzzled that some of the asteroids have been melted, but others have not.

A satellite, or moon, is a secondary object that evolves in a close orbit about a planet or star. This referred to the primary of the satellite. The motion of most of the solar system s satellites about their planets is direct, from west to east. That is the same direction of the rotation of their planets. Only a few satellites of the large outer planets revolve in the retrograde direction, from east to west. That is opposite the direction of the

rotation of their planets; they were probably captured by the planets gravitational fields some time after the formation of the solar system.

A comet is a nebulous celestial body revolving around the sun. A comet is characterized by a long, luminous tail, but only in the segment of the comet s orbit when it passes closest to the sun. A comet is generally considered to consist of a small, sharp nucleus embedded in a nebulous disk called the coma. American astronomer Fred L. Whipple proposed in 1949 that the nucleus, containing practically all the mass of the comet, is a “dirty snowball” combined of ices and dust.

Major proofs of the snowball theory rest on various data. For one, of the observed gases and meteoric particles that are ejected to provide the coma and tails of comets, most of the gases are fragmented molecules, or radicals, of the most common elements in space: hydrogen, carbon, nitrogen, and oxygen. The radicals, for example, of CH, NH, and OH may be broken away from the stable molecules CH 4 (methane), NH3 (ammonia), and H3O (water), which may exist as ices or more complex, very cold compounds in the nucleus. Another fact in support of the snowball theory is that the best-observed comets move in orbits that vary significantly from Newtonian gravitational motion. This provides clear evidence that the escaping gases produce a jet action, propelling the nucleus of a comet slightly away from its otherwise predictable path. In addition, short-period comets, observed over many revolutions, tend to fade very slowly with time, as would be expected of the kind of structure. Finally, the existence of comet groups shows that cometary nuclei are fairly solid units. The head of a comet, including the hazy coma, may exceed the planet Jupiter in size. The solid portion of most comets, however, is equal to only a few cubic kilometers.

A comet is classified by its period, the length of time it takes to travel around the sun. A short-period comet has an orbit approximately as large as Jupiter s; such a comet has a period of 3.3 to 9 years. A long-period comet follows a path about the size of Neptune s orbit. A very long-period comet may take thousands of years to orbit the sun, or it may pass the sun once and never return.

If some great force moved the Earth away from the Sun the oceans would freeze all the way to the bottom. They would be like solid rock. The air will liquefy then crystallize, leaving a layer of snow or ice to cover the whole planet. The sky gives us the seven days of the week because it is a period of seven days now in universal use as a division of time. It is of Hebrew or Chaldean origin and is mentioned as a unit of time in the Bible. The division of the lunar month into seven-day periods probably began as a celebration of the creation of the world in six days with the seventh day for rest. The Roman week was an eight-day period until 303 AD, when official recognition of the Christian religion made it necessary to celebrate the Sabbath every seventh day. The English names for the days of the week Sunday (Sol), Monday (Moon), Tuesday (Tui, the Saxon Mars), Wednesday (Woden, or Mercury), Thursday (Thor, or Jupiter), Friday (Frygga, or Venus), and Saturday (Saturn) come from Roman or Norse names for the planets.

Some facts about the rocket engine system that launches the Shuttle into space are, the two SRBs, with their combined thrust about 5.8 million lb., provide most of the power for the first two minutes of flight. The SRBs take the space shuttle to an altitude of 28 mi. and a speed of 3094 mph before they separate and fall back into the ocean to be retrieved, refurbished, and prepared for another flight.

After the boosters fall away, the three main engines continue to provide thrust. These engines are clustered at the rear end of the orbiter and have a combined thrust of almost 1.2 million lb. The space shuttle’s liquid-propellant engines are the world’s first

reusable rocket engines. They fire for only eight minutes for each flight, just until the shuttle reaches orbit, and are designed to operate for 55 flights. The engines are very large, 14 ft long and 8 ft in diameter at the wide end of the cone-shaped nozzle at the rear of the orbiter.

Another propulsion system takes over once the space shuttle’s main engines shut down as the ship approaches the altitude at which it will begin orbiting around the earth, known as the orbital insertion point. Two orbital maneuvering system (OMS) engines, mounted on either side of the aft fuselage, provide thrust for major orbital changes. For more exacting maneuvers in orbit, 44 small rocket engines, known as the reaction control system, clustered on the shuttle’s nose and on either side of the tail, are used. They have proven indispensable in performing the shuttle’s important work of retrieving, launching, and repairing satellites in orbit.

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