Man-made satellites and space rockets are sent up so we can learn more about outer space. Some satellites were sent up so that their paths would look like a circle around the Earth. The instruments on such satellites tell us about the upper layers of the atmosphere.
Other satellites have very long, oval-shaped paths. They travel far from the Earth and again come close. These satellites study outer space. There is no air there. Space rockets and interplanetary stations fly even further. Scientists guide them by radio waves.
Some interplanetary stations travel close to the Moon or Venus or Mars, or land on their surfaces. Then they can study the Moon and planets with the help of automatic instruments.
The results of these observations are transmitted by radio and television back to Earth.
