Last night, your brother helped you look at stars through your new telescope. While looking at the night sky, you saw a bright dot moving in a straight line over your head. You asked your brother, "How can a star move like that?" He replied that the moving bright dot was probably a satellite orbiting the Earth.

Satellites are objects that travel around other objects. Our Moon is a natural satellite because it orbits Earth. Artificial Satellites carry instruments that can make measurements for scientists on Earth. Scientists are able to use these measurements to help people.

There are thousands of satellites orbiting Earth. A part of NASA, called the Earth Science Enterprise (ESE), uses about 20 of these satellites. NASA's ESE uses these satellites to study how the Earth changes today and predicts how it might change in the future. These satellites can also study how people affect Earth (such as by polluting the air or water).

NASA places satellites into orbit for many private groups and companies. These satellites help people make telephone calls to people across the ocean. They also make it possible for us to watch live television programs from another country. We know how important satellites are when we see pictures of hurricanes from weather satellites.

Satellites must orbit in the upper parts of Earth's atmosphere. The atmosphere can push against a satellite and slow it down so much that gravity can pull it down, which is why many need to give themselves a boost every so often. Satellites orbit at least 180 miles (300 kilometers) above Earth.

Some satellites are in orbit 22,300 miles (35,888 kilometers) above the equator. This high orbit is called geosynchronous. The satellite's speed in orbit is the same speed the Earth turns. These matching speeds mean the satellite is always over the same spot on Earth. Most weather satellites, such as the GOES satellites, have geosynchronous orbits.

NASA's ESE will continue to use satellites to study Earth?s atmosphere, water, and land. Information collected from these satellites will help scientists observe and measure changes as time passes.

Don't be surprised if the next bright dot you see moving across the night sky is one of NASA's Earth Science Enterprise satellites. Someday, you may even become a scientist who uses these satellites. You can spend the rest of your evening studying more about NASA's Earth Science Enterprise on the Internet. You can do the same by visiting a special web site designed just for kids at FOR KIDS ONLY .