Just a short while ago, satellites were top-secret devices, used primarily in a military capacity, for activities such as navigation and espionage. Now they are an essential part of our daily lives. We see and recognize their use in weather reports, sattelite television and everyday telephone calls.

Emergency radio beacons from downed aircraft and distressed ships may reach search-and-rescue teams when satellites relay the signal.

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 manmade satellites, usually much 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 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 placed in orbit or have outlived their usefulness, are floating above Earth. The actual number varies depending on which agency is counting. Payloads that go into the wrong orbit, satellites with run-down batteries, and leftover rocket boosters all contribute to the count.

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

The Soviet satellite, Sputnik was the first to orbit Earth, launched on October 4, 1957. Sputnik's transmissions died along with its battery after only three weeks, but its effects have been felt for decades. News reports showed that many people in the United States were embarrassed to see the Soviet Union achieving a scientific first, as well as frightened that a foreign country had placed something overhead.

Soviet rocket development seemed well ahead of the United States' efforts. The push toward getting an American satellite into space started immediately. Sputnik was a 23-inch, 184-pound metal ball. On the outside of Sputnik, four whip antennas transmitted on short-wave frequencies. After 92 days, gravity took over and Sputnik burned in Earth's atmosphere.

A rocket must be controlled very precisely to insert a satellite into the desired orbit. A rocket must accelerate to at least 25,039 mph to completely escape Earth's gravity and fly off into space.

Earth's escape velocity is much greater than what is required to place an Earth satellite in orbit. With satellites, the idea is not to escape Earth's gravity, but to balance it. Orbital velocity is the velocity needed to achieve balance between gravity's pull on the satellite and the inertia of the satellite's motion, or the satellite's tendency to keep going. This is about 17,000 mph at an altitude of 150 miles high.

Without gravity, the satellite's inertia would carry it off into space. Even with gravity, if the intended satellite goes too fast, it will eventually fly away. On the other hand, if the satellite goes too slowly, gravity will pull it back to Earth.

The orbital velocity of the satellite depends on its altitude above Earth. The nearer Earth, the faster the required orbital velocity. At an altitude of 124 miles, the required orbital velocity is just over 17,000 mph. To maintain an orbit that is 22,223 miles above Earth, the satellite must orbit at a speed of about 7,000 mph.

That orbital speed and distance permits the satellite to make one revolution in 24 hours. Since Earth also rotates once in 24 hours, a satellite at 22,223 miles altitude stays in a fixed position relative to a point on Earth's surface. Because the satellite stays right over the same spot all the time, this kind of orbit is called geostationary. Geostationary orbits are ideal for weather satellites and communications satellites.

The moon has an altitude of about 240,000 miles, a velocity of about 2,300 mph and its orbit takes 27.3 days.

In general, the higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, a satellite runs into traces of Earth's atmosphere, which creates drag. The drag causes the orbit to decay until the satellite falls back into the atmosphere and burns up. At higher altitudes, where the vacuum of space is nearly complete, there is almost no drag and a satellite can stay in orbit for centuries.

Weather satellites help meteorologists predict the weather or see what's happening at the moment. The satellites usually contain cameras that can return photos of Earth's weather, either from fixed geostationary positions or from polar orbits.

Communications satellites allow telephone and data conversations to be relayed through the satellite. Broadcast satellites broadcast signals from one point to another. Scientific satellites perform a variety of scientific missions. The Hubble Space Telescope is the most famous scientific satellite, but there are many others looking at everything from sun spots to gamma rays.

Navigational satellites help ships and planes navigate. The most famous are the GPS satellites. Rescue satellites respond to radio distress signals. Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage.

Military satellites are up there, but much of their actual activities remain top secret. Intelligence-gathering possibilities using high-tech electronic and sophisticated photographic-equipment reconnaissance are endless. The U.S. military and the forces of allied nations used more than 9,000 GPS receivers during Operation Desert Storm in 1991.

All of these types of satellites have a metal or composite frame and body, usually known as the bus. The bus holds everything together in space and provides enough strength to survive the launch. They all have a source of power such as batteries and solar cells.

All satellites have an onboard computer to control and monitor the different systems and a radio system and antenna to send and receive signals and instructions. They all have an attitude control system that keeps the satellite pointed in the right direction.

A satellite launch can cost anywhere between $50 million and $400 million. A shuttle mission pushes toward 500 million dollars. Building a satellite, getting it into orbit and then maintaining it from the ground control facility is a major financial ordeal.