Satellites around the Earth

Not so long ago, satellites were exotic, top-secret devices. They were mainly used for military purposes, for activities such as navigation (directing the course of a vehicle) and espionage (spying). Now, satellites are an essential part of our daily lives. We see and recognise their use in weather reports, television transmission and everyday telephone calls.

In many other instances, satellites play a background role that escapes our notice: some newspapers and magazines are more timely because they transmit their text and images to multiple printing sites via satellite to speed local distribution. Emergency radio beacons from downed aircraft and distressed ships may reach search-and-rescue teams when satellites relay the signal.

What is a satellite?

A satellite is basically any object that revolves around a planet in a circular or elliptical path. The moon is Earth's original, natural satellite, and there are many man-made (artificial) satellites, usually closer to Earth.

* The path a satellite follows is an orbit. In the orbit, the farthest point from Earth is the apogee, and the nearest point is the perigee.

* Artificial satellites generally are not mass-produced. Most satellites are custom-built to perform their intended functions. Exceptions include the GPS satellites (with over 20 copies in orbit) and the Iridium satellites (with over 60 copies in orbit).

* Approximately 23,000 items of space junk - objects large enough to track with radar that were inadvertently (unintentional) placed in orbit or have outlived their usefulness - are floating above the Earth. The actual number varies depending on which agency is counting. Payloads (the total load of the craft) that go into the wrong orbit, satellites with run-down batteries, and leftover rocket boosters all contribute to this count.

Although anything that is in orbit around the Earth is technically a satellite, the term "satellite" is typically used to describe a useful object placed in orbit purposely to perform some specific mission or task. We commonly hear about weather satellites, communication satellites and scientific satellites.

The Soviet Sputnik satellite launched on October 4, 1957, was the first to orbit the Earth. Sputnik was a 23-inch (58-cm), 184-pound (83-kg) metal ball. On the outside of Sputnik, four whip antennae transmitted on short-wave frequencies above and below what is today's Citizens Band (27 MHz).

However, after 92 days, gravity took over and Sputnik burned in the Earth's atmosphere. Thirty days after the Sputnik launch, the dog Laika orbited in a half-ton Sputnik satellite, with an air supply for the dog. It burned in the atmosphere in April 1958.

Sputnik is a good example of just how simple a satellite can be. Today's satellites are generally far more complicated, but the basic idea is a straightforward one.

How a satellite is launched into orbit

All satellites today get into the orbit by riding on a rocket or by riding in the cargo bay of the space shuttle. Several countries and businesses have rocket launch capabilities, and satellites as large as several tons make it safely into the orbit on a regular basis.

For most satellite launches, the scheduled launch rocket is aimed straight up at first. This gets the rocket through the thickest part of the atmosphere most quickly, and best minimises fuel consumption. After a rocket launches straight up, the rocket control mechanism uses the inertial (motionless) guidance system to calculate the necessary adjustments to the rocket's nozzles, to tilt the rocket to the course described in the flight plan.

In most cases, the flight plan calls for the rocket to head east because the Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity speed of the Earth at the launch location. The boost is greatest at the equator, where the distance around the Earth is greatest and so rotation is fastest.

Once the rocket reaches extremely thin air, at about 120 miles (193 km) up, the rocket's navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position. The satellite is then released. At that point, rockets are fired again to ensure some separation between the launch vehicle and the satellite itself.

Read more on satellites later.

Types of satellite orbits

There are three basic kinds of orbits, depending on the satellite's position relative to the Earth's surface:

* Geostationary orbits (also called geosynchronous or synchronous) are orbits in which the satellite is always positioned over the same spot on Earth. Many geostationary satellites are above a band along the equator, with an altitude of about 22,223 miles, or about a tenth of the distance to the Moon. The "satellite parking strip" area over the equator is becoming congested with several hundred television, weather and communication satellites! This congestion means each satellite must be precisely positioned to prevent its signals from interfering with an adjacent satellite's signals. Television, communications and weather satellites all use geostationary orbits. Geostationary orbits are why a DSS satellite TV dish is typically bolted in a fixed position.

* The scheduled space shuttles use a much lower, synchronous orbit, which means they pass overhead at different times of the day. Other satellites in synchronous orbits average about 400 miles (644 km) in altitude. * In a polar orbit, the satellite generally flies at a low altitude and passes over the planet's poles on each revolution. The polar orbit remains fixed in space as the Earth rotates inside the orbit. As a result, much of the Earth passes under a satellite in a polar orbit. Because polar orbits achieve excellent coverage of the planet, they are often used for satellites that do mapping and photography.