1. WHAT ARE SATELLITES AND SPACE PROBES? Objective: Explain how artificial satellites and space probes are used to explore space. Key Terms: Satellite orbit

2. ARTIFICIAL SATELLITES A satellite is any object, natural or artificial, that follows a curved path around another object in space. The curved path of the object is its orbit. For thousands of years, astronomers were only able to study the skies from the surface of Earth. Then, on October 4, 1957, the Space Age began. On that day, the Soviet Union launched the first artificial satellite. It was called Sputnik. Sputnik circled Earth every 96 minutes.

3. ARTIFICIAL SATELLITES Most satellites are launched into one of four main orbits. Nearly circular orbits are usually about 250 km above Earth’s surface. Polar orbits are usually about 800 km high.

4. ARTIFICIAL SATELLITES The height of a highly elliptical, or eccentric, orbit depends on where the satellite is in its orbit. A geostationary orbit is 36,000 km above Earth’s surface. A satellite in geostationary orbit stays above the same spot on Earth. This is because it travels at the same speed as the planet rotates.

5. USES OF SATELLITES Since Sputnik, thousands of artificial satellites have been placed in orbit around Earth. These collect information about Earth, the Sun, the stars, other planets, comets, and other bodies in the solar system.

6. USES OF SATELLITES For example, the Global Positioning System (GPS) is a radio navigation system of about 24 satellites operating in circular orbits. GPS is a guidance service provided by the U.S. Air Force Space Command Unit.

7. USES OF SATELLITES The system relays positions on Earth’s surface in degrees of latitude and longitude. GPS receivers with computers have improved surveying techniques. Surveyors are now able to monitor changes of Earth’s crust caused by tectonic motion. Meteorologists are able to use GPS signals to measure the temperature and water content of the atmosphere. Geologists can accurately map changes in the Greenland ice sheet to help them understand climate changes.

8. USES OF SATELLITES Computers are essential in turning satellite data of measurements into meaningful information. For example, computers help meteorologists translate weather satellite data into current temperatures, pressures, humidity readings, and wind speeds. This information is used to create a weather report.

9. SPACE PROBES Astronauts have been sent to the Moon to explore it and have returned safely to Earth. Space can also be explored by space probes, which do not carry people. In fact, many kinds of space explorations are best done with space probes. Space probes can go places that would be too far or too dangerous for astronauts to travel to.

10. SPACE PROBES For example, part of the Galileo space probe sent to Jupiter entered its atmosphere in 1995. It radioed important information to Earth before being destroyed by high temperatures and pressure. Cassini, launched in 1997, reached Saturn in 2004. It was scheduled to orbit the planet and its moons until June 2008.

11. SPACE PROBES Space probes are usually sent on one-way missions. They do not return to Earth. Some space probes to the outer planets, such as Voyagers 1 and 2, have even been sent out of the solar system.

12. SPACE PROBES The Voyager space probes were launched in 1977. In 1989, Voyager 2 became the first spacecraft from Earth to reach Neptune. Both space probes sent back stunning images to all of the giant planets. Scientists believe Voyager 2 will continue traveling for thousands of years. They hope to continue receiving signals from both space probes until at least the year 2020.