Студопедия — Satellite
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Satellite






 

Any small body orbits a larger one. Natural satellites that orbit planets are called moons. The first artificial satellite, Sputnik 1, was launched into orbit around the Earth by the USSR in 1957. Artificial satellites can transmit data from one place on Earth to another or from space to Earth. Satellite applications include science, communications, weather forecasting, and military use.

Space probes have been sent to natural satellites including the Earth's Moon, Mars's Deimos, and the moons of the giant planets Jupiter, Saturn, Uranus and Neptune.

At any time, there are several thousand artificial satellites orbiting the Earth, including active satellites, satellites that have ended their working lives, and discarded sections of rockets. The brightest artificial satellites can be seen by the naked eye. Artificial satellites eventually re-enter the Earth's atmosphere. Usually they burn up by friction, but sometimes debris falls to the Earth's surface, as with Skylab and Salyut 7.

Hundreds of millions of pieces of space junk, ranging from particles a millimeter across up to disabled satellites, are careering around the Earth. The US Space Command catalogues the larger items to make sure they are not mistaken for enemy missiles; currently about 10,000 items are listed.

Satellites in polar orbit usually complete an orbit in two hours or less and as the Earth is rotating as they orbit, they cover a large part of the Earth's surface each day. Remote-sensing satellites, such as Landsat, are in polar orbit.

 

The British satellite Prospero continues to orbit the Earth every 100 minutes and will do so until 2040. With the correct equipment, the satellite's radio transmitter can still be heard broadcasting on 137.56 MHz.

 


TEXT #18

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Computer virus

A computer virus is a computer program that can copy itself and infect a computer without the permission or knowledge of the owner. The term "virus" is also commonly but erroneously used to refer to other types of malware, adware, and spyware programs that do not have the reproductive ability. A true virus can only spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive. Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.[1][2]

The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware. Malware includes computer viruses, worms, trojan horses, most rootkits, spyware, dishonest adware, crime ware, and other malicious and unwanted software), including true viruses. Viruses are sometimes confused with computer worms and Trojan horses, which are technically different. A worm can exploit security vulnerabilities to spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a program that appears harmless but has a hidden agenda. Worms and Trojans, like viruses, may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when they are executed. Some viruses and other malware have symptoms noticeable to the computer user, but many are surreptitious.

Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, Instant Messaging, and file sharing systems to spread.


TEXT #19

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Virus

A virus (from the Latin virus meaning toxin or poison) is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. Viruses infect all types of cellular life. The first known virus, tobacco mosaic virus, was discovered by Martinus Beijerinck in 1898, and now more than 5,000 types of virus have been described in detail, although most types of virus remain undiscovered. Viruses infect all forms of life, are found in almost every ecosystem on Earth, and are the most abundant type of biological entity on the planet.

Viruses consist of two or three parts: all viruses have genes made from either DNA or RNA, long molecules that carry genetic information; all have a protein coat that protects these genes; and some have an envelope of fat that surrounds them when they are outside a cell. Viruses vary in shape from simple helical shapes to more complex structures. They are about 1/100th the size of bacteria. The origins of viruses are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer.

Viruses spread in many ways; plant viruses are often transmitted from plant to plant by insects that feed on sap, such as aphids, while animal viruses can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing, and others such as norovirus, are transmitted by the faecal-oral route, when they contaminate hands, food or water. Rotaviruses are often spread by direct contact with infected children. HIV is one of several viruses that are transmitted through sex.

Not all viruses cause disease, as many viruses reproduce without causing any obvious harm to the infected organism. Some viruses such as hepatitis B can cause life-long or chronic infections, and the viruses continue to replicate in the body despite the hosts' defense mechanisms. However, viral infections in animals usually cause an immune response, which can completely eliminate a virus. Microorganisms such as bacteria also have defenses against viral infection, such as restriction modification systems. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening and more minor infections.

TEXT #20

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