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Thwap! A dart hits the elephant. The large animal falls to her knees. She is asleep. National Geographic explorer Michael Fay and his team are nearby. Carefully, they put a collar on her. They name her Annie.

Fay is at a national park in Africa. Sometimes the elephants go outside the park. Fay wants to protect them from hunters. That's why he put the collar on Annie. When Annie wakes up, the collar will send a message to a satellite. It tells where Annie is.

The satellite then sends the information to Fay's computer. So Fay will always know where to find Annie. Guards will know where to go to protect her from illegal hunters. That's how satellites help Annie. They help you, too. Let's learn how.

A satellite is an object in space. It goes around another object. The moon is one type of satellite. It orbits, or goes around, Earth.

People build satellites, too. Rockets carry them into space. Each satellite has a special job. Some help scientists study the world. They send pictures and other information back to Earth. Today, about 1,000 working satellites are in orbit.

Each one has all it needs to do its job. Solar panels collect the sun's energy. This gives it power. It has computers to receive messages from Earth. It has radios to send signals back to Earth.

Sometimes satellites stop working. They may run out of energy. Or their computers may break down. The sun can even burn them up. That's not all.

Some satellites fly close to Earth. Air slows them down. Sometimes one falls through Earth's atmosphere. There, gases rub against the satellite. This can burn it up! This doesn't happen often, though.

Most satellites are relay stations. They help people communicate. Let's say you call a friend on a cell phone. First, the phone relays, or sends, a signal to a tower. The tower then sends the signal to a satellite. The satellite sends it to a tower near your friend. Your friend's phone starts to ring!

Satellite televisions work the same way. Television stations send programs to a satellite. The satellite relays the program to your home TV. These satellites are always moving. Yet they need to be in just the right spot for TV or cell phone calls. Their secret is that they move at the same speed as Earth spins. So they are always above the same point on Earth.

Some satellites are like eyes in the sky. They take pictures of Earth. You may have seen some on the news. They show where big storms are forming. This warns people and saves lives.

Weather satellites follow lines of longitude. Those are lines on a map that go north to south. The Earth spins east to west. So the satellite "sees" almost all of Earth.

Satellite images show changes on Earth, too. They show where people have cut down forests. They also help people study global warming. As Earth heats up, ice melts at the Poles. Over time, satellite pictures show how much the ice is melting.

One day, satellites may even predict earthquakes. Here's how. Moving pieces of Earth make radio waves. What if satellites picked up these waves in space? They could warn people about an earthquake!

Satellites can be spies in the sky, too. They can see if other countries are hiding weapons. They can even pick up secret messages!

Some satellites teach us about Earth. Others teach us about space. The Hubble Space Telescope is one of these. This satellite looks far into space. It takes pictures of what it sees. It sends the pictures back to Earth.

The pictures tell scientists about the stars and planets. They help us better understand the universe. Using Hubble, scientists learned the universe is 13.7 billion years old!

Here on Earth, people use a group of 20 satellites to find their way around. The satellites are called the Global Positioning System, or GPS. It figures out where someone, or something, is. Drivers use GPS in cars. Pilots use GPS in airplanes.

Even ships at sea use GPS. A lost ship can send out a signal. A GPS picks up the signal. Then it works out the ship's location. It sends this information to a rescue team. Help is on the way!

Satellites already do many things. They explore the universe. They watch over Earth. They help Mike Fay save elephants. They make your life safer, too! What more will satellites be able to do tomorrow? Just imagine!

Article by Jonathan McDowell. Top-of-page illustration from © Shutterstock (globe); © Paul B. Moore/Shutterstock (satellite). "Eye in the Sky" appears on page 2 of the October 2009 issue.