Satellites

Satellite

source: NASA

A satellite is an object in space that orbits or circles around a bigger object. There are two kinds of satellites: natural (such as the moon orbiting the Earth) or artificial (such as the International Space Station orbiting the Earth). There are dozens upon dozens of natural satellites in the solar system, with almost every planet having at least one moon. Saturn, for example, has at least 53 natural satellites, and one artificial one — the Cassini spacecraft, which is exploring the ringed planet and its moons. Artificial satellites, however, did not become a reality until the mid-20th century. The first artificial satellite was Sputnik, a Russian beach-ball-size space probe that lifted off on Oct. 4, 1957. That act shocked much of the western world, as it was believed the Soviets did not have the capability to send satellites into space.

The first artificial satellite, launched by Russia (then known as the Soviet Union) in the late 1950s, was about the size of a basketball. It did nothing but transmit a simple Morse code signal over and over. In contrast, modern satellites can receive and re-transmit thousands of signals simultaneously, from simple digital data to the most complex television programming.

There are three types of communications satellite systems. They are categorized according to the type of orbit they follow.

Geostationary satellite orbits the earth directly over the equator, approximately 22,000 miles up. At this altitude, one complete trip around the earth (relative to the sun) takes 24 hours. Thus, the satellite remains over the same spot on the earth's surface at all times, and stays fixed in the sky from any point on the surface from which it can be "seen." So-called weather satellites are usually of this type. You can view images from some of these satellites on the Internet via the Purdue Weather Processor. A single geostationary satellite can "see" approximately 40 percent of the earth's surface. Three such satellites, spaced at equal intervals (120 angular degrees apart), can provide coverage of the entire civilized world. A geostationary satellite can be accessed using a dish antenna aimed at the spot in the sky where the satellite hovers.

A low-earth-orbit (LEO) satellite system,it employs a large fleet of "birds," each in a circular orbit at a constant altitude of a few hundred miles. The orbits take the satellites over, or nearly over, the geographic poles. Each revolution takes approximately 90 minutes to a few hours. The fleet is arranged in such a way that, from any point on the surface at any time, at least one satellite is on a line of sight. The entire system operates in a manner similar to the way a cellular telephone functions. The main difference is that the transponders, or wireless receiver/transmitters, are moving rather than fixed, and are in space rather than on the earth. A well-designed LEO system makes it possible for anyone to access the Internet via wireless from any point on the planet, using an antenna no more sophisticated than old-fashioned television "rabbit ears."

Types of Satellite

Pupil Worksheet

Satellites are usually classified according to the type of orbit they are in. There are four types of orbit associated with satellites, and the type of orbit dictates a satellite's use.

Low Earth Orbits

Satellites in low Earth orbits are normally military reconnaissance satellites that can pick out tanks from 160 km above the Earth. They orbit the earth very quickly, one complete orbit normally taking 90 minutes. However, these orbits have very short lifetimes in the order of weeks compared with decades for geostationary satellites. Simple launch vehicles can be used to place these satellites of large masses into orbit.

Sun-Synchronous orbits

Meteorological satellites are often placed in a sun-synchronous or heliosynchronous orbit. These satellites are in polar orbits. The orbits are designed so that the satellite's orientation is fixed relative to the Sun throughout the year, allowing very accurate weather predictions to be made. Most meteorological satellites orbit the Earth 15 to 16 times per day.

Geosynchronous satellites

Earth-synchronous or geosynchronous satellites are placed into orbit so that their period of rotation exactly matches the Earth's rotation. They take 24 hours to make one rotation. However, the plane of orbit for these satellites is generally not the equatorial plane. Apart from geostationary satellites (see below), the satellites are used for communications at high latitudes, particularly in Russia and Canada. The orbits are called Molniya orbits. The satellites are placed in highly elliptical orbits which enable them to appear to hover above one point on the Earth for most of the day. In twenty four hours they move over the Earth in a figure of eight pattern centred on a fixed longitude, moving slowly where they can be useful and quickly where they are of little use.

Geostationary satellites

The majority of communications satellites are in fact geostationary satellites. Geostationary satellites like geosynchronous satellites take 24 hours to complete a rotation. However, geostationary satellites are positioned directly over the equator and their path follows the equatorial plane of the Earth. As a result geostationary satellites don't move North or South during the day and are permanently fixed above one point on the equator of the Earth.

Most video or T.V. communications systems use geostationary satellites. Geosynchronous and geostationary satellites are typically orbiting at 35,788 km (22,238 miles) above the surface of the planet (42,000 km from its centre).

Modern satellites have a mass of several thousand kilograms, compared with just 180 kilograms for Sputnik. Modern satellites are placed in space using launch vehicles like the Arianne Rocket or the Space Shuttle. Once in space, most satellites obtain their power from the Sun using solar panels. Satellites travelling deep into space often carry additional nuclear power supplies.

Satellites are launched into space to do a specific job. The type of satellite that is launched to monitor cloud patterns for a weather station will be different than a satellite launched to send television signals for Sky TV. The satellite has to be designed specifically to fulfill its function.

A telescope orbiting the Earth. An astronomy satellite's vision is not clouded by the gases that make up the Earth's atmosphere, so it gives clearer pictures than telescopes on Earth.

Astronomy satellites study stellar phenomenon like black holes, quasars, and distant galaxies. These are not to be confused with space exploration satellites, which also study these phenomena - see below .

Astronomy satellites have many different applications:

• they can be used to make star maps
• they can be used to study mysterious phenomena such as black holes and quasars
• they can be used to take pictures of the planets in the solar system
• they can be used to make maps of different planetary surfaces

A type of scientific satellite that studies the Earth's atmosphere. They were some of the very first satellites launched into space.

Before satellites, transmissions were difficult or impossible at long distances. The signals, which travel in straight lines, could not bend around the round Earth to reach a destination far away. They had to be reflected off layers in the atmosphere and mountain ranges etc. caused shadow areas.

Because communications satellites are in orbit, the signals can be sent instantaneously into space and then redirected to another satellite or directly to their destination.

A type of satellite that gives ships and aircraft their coordinate positions on the Earth.

Navigation satellites were developed in the 1950s, and they rely on the doppler effect to calculate the position of vessels emitting a radio signal. Navigation satellites are also widely used by the military.

Space exploration satellites are not really satellites at all; they are actually spaceprobes.A satellite is defined as something that is orbiting something else (usually aplanet), butspace probes don't do that - instead they travel deep into the solar system. However,they are similar to orbiting satellites in design and function.                On their journeys, space probes send back detailed pictures and other data of faraway planets and other stellar phenomena. Space exploration satellites are responsible for many of astronomy's most important achievements. Jupiter's rings, for example, were discovered by a space exploration satellite. Space exploration satellites must be built to last because it takes so long for the satellites to reach their destinations. Space exploration satellites are different from astronomy satellites (see above) because they do not operate from Earth orbit; they are actually sent out into deep space on their own.
Remote sensing is observing and measuring our environment from a distance. Remote sensing satellites are usually put into space to monitor resources that are important for humans. For example, remote sensing satellites might track animal migration, locate mineral deposits, watch agricultural crops for weather damage, or see how fast the forests are being cut down. All of these things can be done best from space because a satellite in orbit can normally take photographs of large expanses of land all over the world. Since these satellites are able to take photographs and observe areas all over the globe, the satellite is able to monitor areas in which the climate is very harsh, or which are nearly impossible tor reach by land.
Reconnaissance satellites are used to spy on other countries. They provide intelligence information on the military activities of foreign countries. These satellites can even detect missile launches or nuclear explosions in space. Reconnaissance satellites can pick up and record radio and radar transmissions while passing over a country and they can be used as an orbital weapon by placing warheads on a low orbit satellite to be launched at a ground target.