But robot space crafts are sent very much farther.
These have visited the distant planets.
These robot space crafts are called space probes.
What is a Space Probe?Space probes carry cameras and many kinds of instruments to study the planets they visit. They send information back to earth by radio.
Probes must be launched from the earth at a very great speed—more than 24,800 miles (40,000 kilometers) an hour.
Then they can escape from the earth's gravity.
Aiming the probe is very difficult, because the planet it must reach is always moving. The probe must be aimed so that it reaches a point in space almost at the same time as its target. It must do this after a journey of many millions of miles, lasting for years.
Voyager space probesIt is an astonishing fact that most probes sent to the planets have reached their targets. In 1989, the American probe Voyager 2 flew past the planet Neptune almost exactly on time, after a 12-year journey. The planet was nearly 3.1 billion miles (5 billion kilometers) away from earth at the time. From this distance, the radio waves from Voyager took over four hours to reach the earth.
A Voyager probe photographed Saturn on its journey into the outer solar system. It used nuclear generators to make electricity.
Solar cells couldn’t be used because sunlight is too weak near Saturn.
Venera space probeMost probes study a planet as they fly past it. But some actually land on the planet and report back from its surface.
A Russian probe, called Venera, was the first to land on a planet. It parachuted down to Venus in 1970. Since then, several Venera probes have sent back pictures of its surface.
Two American Viking probes landed on Mars in 1976. They sent back pictures and reported on the Martian weather. They also examined the soil for signs of life—but didn't find any!