• Some newspapers and magazines are more timely because they transmit their text and images to multiple printing sites via satellite to speed local distribution.
• Before sending signals down the wire into our houses, cable television depends on satellites to distribute its transmissions.
• The most reliable taxi and limousine drivers are sometimes using the satellite-based Global Positioning System (GPS) to take us to the proper destination.

• The goods we buy often reach distributors and retailers more efficiently and safely because trucking firms track the progress of their vehicles with the same GPS. Sometimes firms will even tell their drivers that they are driving too fast.
• Emergency radio beacons from downed aircraft and distressed ships may reach search-and-rescue teams when satellites relay the signal (read this page for details).

• 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 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. This online catalog of satellites has almost 26,000 entries!

Because of Soviet government secrecy at the time, no photographs were taken of this famous launch. Sputnik was a 23-inch (58-cm), 184-pound (83-kg) metal ball. Although it was a remarkable achievement, Sputnik's contents seem meager by today's standards:

• Thermometer
• Battery
• Radio transmitter - changed the tone of its beeps to match temperature changes
• Nitrogen gas - pressurized the interior of the satellite

On the outside of Sputnik, four whip antennas transmitted on short-wave frequencies above and below what is today's Citizens Band (27 MHz). According to the Space Satellite Handbook, by Anthony R. Curtis:
Sputnik is a good example of just how simple a satellite can be. As we will see later, today's satellites are generally far more complicated, but the basic idea is a straightforward one.

How is a Satellite Launched into an Orbit?

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 minimizes fuel consumption.
After a rocket launches straight up, the rocket control mechanism uses the inertial guidance system to calculate 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 Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator, where the distance around 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.

Orbital Velocity and Altitude

Earth's escape velocity is much greater than what's required to place an Earth satellite in orbit. With satellites, the object 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 -- the satellite's tendency to keep going. This is approximately 17,000 mph (27,359 kph) at an altitude of 150 miles (242 km). 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. At the correct orbital velocity, gravity exactly balances the satellite's inertia, pulling down toward Earth's center just enough to keep the path of the satellite curving like Earth's curved surface, rather than flying off in a straight line (read this page for details on orbits).
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 (200 kilometers), the required orbital velocity is just over 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 km) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). 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.

What is a Satellite Launch Window?

With the Space Shuttle, an extremely important factor in choosing the launch window is the need to bring down the astronauts safely if something goes wrong. The astronauts must be able to reach a safe landing area where rescue personnel can be standing by. For other types of flights, including interplanetary exploration, the launch window must permit the flight to take the most efficient course to its very distant destination. If weather is bad or a malfunction occurs during a launch window, the flight must be postponed until the next launch window appropriate for the flight. If a satellite were launched at the wrong time of the day in perfect weather, the satellite could end up in an orbit that would not pass over any of its intended users. Timing is everything!

What is Inside a Typical Satellite?

• Weather satellites help meteorologists predict the weather or see what's happening at the moment. Typical weather satellites include the TIROS, COSMOS and GOES satellites. The satellites generally 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. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder -- a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.
• Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).
• 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 NAVSTAR satellites.
• Rescue satellites respond to radio distress signals (read this page for details).
• Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. The most famous are the LANDSAT series.

·         Military satellites are up there, but much of the actual application information remains secret. Intelligence-gathering possibilities using high-tech electronic and sophisticated photographic equipment reconnaissance are endless.