CHAPTER II[ Universe, Galaxy and Solar System ] [ Galaxies ] [ [ Large-Scale Structure: Human's Place In The Universe ][ The Universe Today ] [ Solar System ] [ The Planets and Space Exploration ] [ Asteroids ] [ Comets ][ Planetary Fact Sheet-Metric ] [ Diameters and Distances (km) in Solar System ]UNIVERSE, GALAXY AND SOLAR SYSTEM Civil engineering is the science of coexistence of men with its environment. Thus, at the beginning, it has started with the close environment, dealing with local conditions. Men in the forests in hot countries have learned to make cable bridges, men in countries with abundent stones and lacking long fibres, have learned to make arches and used them in making temples and bridges. With the advance of the civilization and transportation, the meaning of the word “local” has changed. Men has came across with conditions which are different from those prevailing in the near vicinity, and different experiences are added to each other. It is not to be forgotten that civil engineers are still far from solving all the problems on the Earth. There are, for example, so called extreme environments like deep oceans, polar areas etc that needs to be explored more. We are still far from making structures earthquake-proof, tornado-proof. Nevertheless, we are now at a level that is “global”. This shows that humans now are getting concious about the fact that they are living on a globe, called Earth. And now we are getting concious about a further fact that we are living in the space, close to Earth we have other space objects on which we can live. We have also the “space” between these objects where we can live. So, we have to look around to see where we are. Our actual understanding of “our” universe is a relatively recent finding. A computer simulation depicting a large chunk of
our universe {S0014} [ Top ] The introduction of telescopes to the study of astronomy opened up the universe, but it took some time for astronomers to realize how vast the universe could be. Telescopes revealed that our night sky was not only populated with stars, but with other, more nebulous objects. Some of these objects were nebulae within our Galaxy, the Milky Way. As telescopes became more powerful, it was possible to see that some of the nebulae had a spiral-like structure. These were also believed to be part of our Galaxy and thus relatively nearby. In 1920, two important astronomers, Harlow Shapley and Heber D. Curtis, held a great debate about the nature of these "spiral nebulae". Were they objects within the Milky Way, or were they communities of stars distinct from our Galaxy? Edwin Hubble studied these "spiral nebulae" and found that they were composed of stars, and thus resolved the debate. These nebulae were not nebulae at all, but galaxies! Suddenly, our universe was much bigger. We realized that our Galaxy was just one of many billions of galaxies in the universe. Hubble continued to study galaxies his entire career, and we owe much
of our understanding of galaxies to him. His observations led to the
current classification of galaxies as spirals, ellipticals, or irregulars,
and to our knowledge that the appearance of these galaxies depends both
on our perspective, and on the forces which form and power galaxies.
[ Top ] Large-Scale Structure: Human’s Place in the Universe
Additionally there are much larger clusters of galaxies containing anywhere from 50 - 1000's of galaxies. The closest cluster to us is the Virgo cluster, at a distance of about 50 Million Light-years. It spans 10 Million Light-years of space. The Coma Cluster, almost 300 Million Light-Years away, contains thousands of galaxies and spreads accross 20 Million Light-years of space. The Coma Cluster of Galaxies The average cluster contains hundreds of galaxies and spans 16 Million Light-years. The average distance between clusters is some tens of millions of Light-years. Groups and Clusters cluster together to form even larger clusters known as Superclusters. The Local Group belongs to the Local Supercluster, whose center is at the Virgo Cluster. Superclusters, on average, span 100 Million Light-years of space. They often have flattened structures. In between Superclusters are enormous voids of space where there are almost no galaxies at all. These voids can be 100's of Light-years accross. The Largest structures discovered in the Universe are systems of voids and clusters. At this scale the Universe takes on a foamy look with the voids appearing as great bubbles and galaxies lying along them in great filaments connecting Superclusters. [ Top ] There is a very well organized Large Scale Structure apparent in the Universe that both excites and confuses Astrophysicists. The Universe is also not static, in fact we live in an Expanding Universe. Additionally, the entire Universe is permeated by The Cosmic Microwave
Background Radiation which gives many clues as to the origin of the
Universe.
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The solar system is made up of the Sun, the 9 planets and their 78 known moons, asteroids, comets, dust and gas. The planets, asteroids, and comets travel around the Sun, the center of our solar system. Most of the bodies in the solar system travel around the Sun in elliptical orbits that lie near the ecliptic plane. All the planets orbit the Sun in the anticlockwise direction, and all but Venus, Uranus and Pluto spin on their axes in this direction as well. Solar system formation began billions of years ago, when gases and dust began to come together to form the Sun, planets, and other bodies of the solar system. As recently as in April 1986, astronomers found that our galaxy is smaller than they thought and the Sun is 23,000 light-years from it's center. [ Top ] The Planets and Space Exploration The four inner planets at the center of the solar system Mercury, Venus, Earth, Mars, are known as the terrestrial planets because they are solid, rocky bodies. Jupiter, Saturn, Uranus, and Neptune are all composed primarily of gas and are known as the Jovian planets. Pluto, a small solid icy body smaller than the Earth's Moon, is usually the outermost planet, but occasionally passes within the orbit of Neptune. The asteroid belt, between Mars and Jupiter, is made of thousands of chunks of rock which orbit the Sun. Comets are small icy bodies that travel past the Sun and give off gas and dust each time they pass by. Unmanned spacecraft
have travelled to all the planets in the solar system (except Pluto)
and taken pictures of the planets and many of their moons. They have
also taken measurements that tell us about the particle, electromagnetic,
and gravitational environment near the planets. This information can
be used to tell us about the planets interior, surface, atmosphere,
and magnetosphere. [ Top ]
More than 100,000 asteroids lie in a belt between Mars and Jupiter and they move in the same direction as the planets. These asteroids lie in a location in the solar system where there seems to be a jump in the spacing between the planets. Scientists think that this debris may be the remains of an early planet, which broke up early in the solar system. Several thousand of the largest asteroids in this belt have been given names. Asteroids range in size from Ceres, which has a diameter of about 1000km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. Galileo image of Gaspra Some asteroids, called Apollo asteroids, cross the orbit of Earth. It has been estimated that there are around 1000 such asteroids (Earth orbit crossing asteroids) with a diameter of a kilometer or more. The chances of an asteroid colliding with Earth are very small! But some do come close to Earth, like Hermes (closest approach of 777,000 km). [ Top ]
Now we know that comets are lumps of ice and rock that periodically come into the center of the solar system from somewhere in its outer reaches, and that some comets make repeated trips. The solid part or nucleus of comets can be oddly shaped and can be only a few miles across. When comets get close enough to the Sun, heat makes the nucleus start to evaporate. A cloud of gas and dust called the coma forms. The coma can reach a similar diameter to that of a giant planet! Jets of gas and dust form long tails that we can see from Earth. These tails can sometimes be millions of miles long. In 1985-1986, a spacecraft called Giotto visited the most famous comet, Halley, on its most recent visit to the inner solar system. In 1993, comet Shoemaker-Levy became trapped by the gravity of Jupiter and plunged into Jupiter's atmosphere! In 1996 and 1997 we saw comet Hyakutake, and comet Hale-Bopp. Hale-Bopp was one of the brightest comets ever seen from Earth. Comet Linear was discovered in 1999 and made its closest approach of the Sun in July 2000. Now scientists have identified a class of comets known as small
comets (though they originally were just called snowballs from
space!) [ Top ] http://nssdc.gsfc.nasa.gov/planetary/factsheet/index.html
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When
we look out into the Universe we observe some rather remarkable structure.
We have seen that stars cluster together to form galaxies. But galaxies
also cluster together to form much larger structures. There are clusters
of galaxies called Groups which contain 10's of galaxies. Our
own galaxy, the Milky Way, a spiral disk of 200 billion stars,
resides in such a group of about 30 galaxies, named the Local Group.
The Local Group contains 3 large spirals: The Milky Way, The Andromeda
Galaxy, and M33. The rest are all small galaxies (dwarf spheroidals
and irregulars). The Local Group spans nearly 5 Million Light-Years
of space. 
