How a Satellite is Placed in Orbit

            A satellite is an object that revolves around a larger object such as a planet. The moon, for example, revolves around the earth. The moon, therefore, is a natural satellite of the earth.
            Scientists are now able to place man-made satellites in orbit around the earth. Indeed several of these man-made satellites are right now traveling at great speeds around the earth.

How is a Satellite Launched?

            Launching a satellite is done with rockets. Everything on the earth is held to it by the force of gravity. When a rocket is launched, it has to overcome the force of gravity and the resistance of the air through which it travels. Gravity and air resistance put a limit on the height that a single rocket launched from the ground can reach.
            A multi-stage rocket is used to launch a satellite. A multi-stage rocket is really two, three, or more rockets or stages built on top of one another.
            The force of gravity is strongest near the earth. Near the earth, too, a rocket meets its greatest resistance from the earth's atmosphere. Therefore, the first stage is large. It needs more fuel and more thrust to push the other rockets out through the dense air near the earth. When the first stage has used all its fuel, it drops off, and the second stage begins to burn.
            The second stage, like the first one, burns until it has used up all its fuel. Then it, too, drops away, and the third stage starts to burn. The final stage is very light compared to the original weight of all the stages that took off from the earth.
            The multi-stage rocket accomplished three things: (1) it gets the satellite to the right altitude, (2) it points it at the correct angle and (3) it gives it the proper speed. The first stages of the rocket provide the thrust to get the rocket 300 miles or more above the earth's surface. The last stage turns the rocket to a horizontal position and launches the satellite at the proper speed. Horizontal here, means parallel to the horizon. At a height of 300 miles the horizontal velocity or speed is about 18,000 miles per hour. The speed is also called its orbital velocity.

The Orbit of a Satellite

            The path a satellite takes around the earth is called its orbit. The orbit of a man-made satellite is not a circle but an ellipse, or oval. Therefore, the satellite is not always the same distance from the earth. Its speed, too, varies at different places in its orbit. As it approaches perigee (the closest point to the earth in its orbit), the satellite is pulled to its highest speed by the gravity of the earth. As it approaches apogee (the farthest point from the earth in its orbit), the earth's gravity slows it down.
            To stay in orbit permanently, a satellite must, at all times, be outside of the earth's atmosphere. When perigee is close to the earth, the friction of the earth's atmosphere affects the speed of the satellite. The satellite is slowed down each time it passes this point. Its orbit becomes smaller and more nearly circular. Eventually, it loses so much speed that the earth's gravity pulls it toward the earth. Then friction caused by the resistance of air makes the satellite very hot. It is soon burned up and destroyed very much like a meteor.

What Keeps a Satellite in its Orbit?

            Obtain a rubber ball that is fastened to a long rubber band. Swing the rubber ball around your head. Notice as you swing the ball it moves in a circular orbit. The rubber band holding the ball is like the earth's gravity holding a satellite. The forward movement of the ball provides a centrifugal force that balances the pull of the rubber band. The forward movement of a satellite (its orbital velocity) also provides the centrifugal force that balances the pull of gravity of the earth.
            Actually an earth satellite "falls around" the earth. If a gun were mounted on the top of a high mountain and a bullet was fired horizontally, a slow-moving bullet would move forward a short distance before it fell to the earth from the pull of gravity. A faster-moving bullet would go farther before it fell. Keep in mind that the earth is round and curves away from the gun. An extremely fast bullet would also fall, but because of the bullet's high speed, its path would always remain about the same distance from the center of the earth. The bullet would travel around the earth.
            A satellite must also have enough orbital velocity to provide a centrifugal force to balance the earth's gravitational pull (centripetal force). A satellite close to the earth must have a speed of at least 18,000 miles per hour.
            The higher the satellite orbits, the less speed it needs. This is because the earth's force of gravity decreases with distance. The moon is about 240,000 miles from the earth. It travels only about 2,100 miles an hour to stay in its orbit around the earth.

Satellites Are Used to Get Information

            Man-made satellites are, in a sense, science laboratories in space. They carry many different kinds of scientific instruments to help us get accurate information. Artificial satellites have radio transmitters to send this information to the earth. Some also have radio receivers to pick up signals from the earth which operate special equipment in the satellite. The process of sending information by radio from instruments in a satellite is called telemetering.

A Satellite May Collect Information about Meteors

            Billions of meteors shower into our atmosphere every day. Friction with the air causes most of them to become white hot and burn up. The dust that remains settles to the earth.
            Most meteors are very small, even smaller than grains of sand. They are called micrometeorites. Man wants to know more about these meteors. To collect information about them, some satellites are equipped with erosion gauges. These gauges are made of a metallic alloy deposited on glass. Electricity passes through it. The alloy is worn away when meteors hit it and the resistance to the flow of electricity increases. The change in resistance tells scientists the amount of wear that has taken place.
            Other satellites have sensitive crystal microphones connected to their inner shall. Whenever a meteor hits the shell, vibrations are produced which cause the crystals to vibrate and produce electricity. The electrical impulses are stored in a memory device and telemetered back to earth whenever the satellite passes over a receiving station. The frequency of the pulses of electricity give a clue to the number of meteors in outer space.

Satellites Are Equipped to Measure Temperature

            Some satellites are equipped with thermistors, temperature-measuring devices. They measure the exact temperature of the satellite at all times.
            A weak electric current passes through the thermistor. Whenever the temperature changes, the amount of electricity flowing through the instrument changes. The change in current affects the radio signal sent out by the satellite. The altered radio signal is received at a receiving station on the earth and is interpreted as a change of temperature.

Satellites Contain Instruments to Detect Radiations

            The earth is continually being bombarded by radiant energy from the sun. Some of this energy is heat and light. the sun also gives off ultraviolet rays, X rays, and cosmic rays. These are strong enough to kill living things. Fortunately, some of these radiations are filtered out by our atmosphere and so they do us little or no harm. But scientists want to know more about the strength of these rays in outer space.
            Some satellites contain instruments called Geiger counters, which are sensitive to cosmic rays and the other types of radiations. When a radiation passes through a Geiger counter, an electric impulse passes through the instrument. These electric impulses are amplified, stored, and then radioed to the earth.

Satellites Can be Equipped to Study the Earth's Magnetism

            The earth is a huge magnet. It has one magnetic pole in the Northern Hemisphere and an opposite magnetic pole in the Southern Hemisphere. Magnetic force is concentrated at these two places.
            Like any magnet the earth is surrounded by a magnetic field. But it seems to be affected by electric currents flowing in the ionosphere. Changes in these currents seem to change the strength of the magnetic field at the ground.
            Some satellites are equipped with instruments called magnetometers. The information obtained from these instruments will help scientists map magnetic lines of force at all altitudes, continuing into outer space.

Some Satellites Carry Live Animals

            Man has already entered outer space. But before he could make this daring venture, he had to know whether living things could survive in outer space. To find out about this, animals were sent into outer space.
            To carry an animal into space, a satellite must have room for air, food, water, and temperature controls. The animal is strapped down. Automatic feeders dispense food and water. Russian scientists launched a satellite with a dog in it. Instruments to record its heartbeat, temperature, rate of breathing and moisture were attached to it.
            On May 28, 1959, two small monkeys were sent about 1,500 miles in the nose cone of a Jupiter missile, fired at Cape Canaveral, Florida.
            The monkeys were in the air to an altitude of 300 miles for about 15 minutes. And they rode at speeds up to 10,000 miles per hour. The nose cone was recovered and the monkeys were reported to be in excellent condition. This was a tremendous breakthrough in man's progress toward space travel.
            On April 12, 1961 the Soviet Union reported that their first astronaut, Major Gagarin, had been put into orbit around the earth and had returned safely. On August 6, 1961 another Soviet Astronaut, Major Titov, was put in orbit about the earth and remained there for about one day. He returned to earth safely.
            On May 5, 1961 Navy Commander Alan B. Shepard became the first U.S. astronaut ever sent into space. He reached an altitude of 115 miles and traveled a distance of 300 miles. A second American astronaut, Captain Virgil I. Grissom, later made a successful flight. On February 20, 1962 John H. Glenn became the first American to make an orbital flight about the earth.