8.11 Satellites. Figure 1 Artifi cial satellites provide valuable monitoring and communication services for humankind.

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1 8.11 Satellites As you know, Earth has one natural satellite orbiting it the Moon. Earth also has thousands of other satellites circling it at different altitudes and orbits, but these are all made by humans (Figure 1). Artificial satellites help forecast weather, monitor agriculture, aid in telecommunication or navigation, assist military activities, and explore the Universe (Figure 2). Figure 1 Artifi cial satellites provide valuable monitoring and communication services for humankind. DID YOU KNOW? Space Debris In February 2009, two communications satellites (one American and one Russian) collided over Siberia. The resulting debris added to the pieces of space junk already orbiting Earth. What safety hazards does this space debris pose? Will more satellites in orbit contribute to an increased incidence of these collisions? Figure 2 Today, more than 40 countries have put over 3000 functioning satellites into orbit around Earth. To learn more about the satellites in Figure 2, GO TO SON SCIENCE READING TIP Finding the Main Idea The main idea gives the author s thoughts about a topic or key concept. Start by identifying the topic or key concept of the text (satellites) and whether the text breaks it down into subtopics (natural and artifi cial satellites). Then check if the author gives a perpective on the topic (Canada is a world leader in developing artifi cial satellites). Human-occupied spacecraft, such as the Space Shuttle, and space facilities, such as the International Space Station, also function as artificial satellites. In 1957, the first artificial satellite (Sputnik 1) was sent into space by the Soviet Union. Its mission was to orbit Earth. Five years later, Canada s first satellite (Alouette 1) was launched. Canada has been a world leader in developing satellite technology over the last 50 years. We have become renowned for building some of the most powerful telecommunications and Earth observation satellites. Canadian engineers and scientists employ this expertise to design and build satellites used in astronomy and space exploration for everything from measuring characteristics of planets around distant stars to searching for potentially dangerous asteroids. 346 Chapter 8 Our Place in Space

2 Staying in Orbit How do all of these satellites keep orbiting Earth without plunging back to the ground? The force of Earth s gravity continuously pulls the satellite toward Earth. However, the forward motion of the satellite and the curvature of Earth prevent the satellite from getting any closer to the surface. Imagine firing a powerful cannon as pictured in Figure 3(a). The cannonball would fly through the air until it curved toward the ground, pulled down by gravity. If the cannonball was fired with more velocity, it would travel farther before coming to rest on the ground. It would still curve toward Earth, but Earth also starts to curve beneath it because Earth is shaped like a sphere (Figure 3(b)). If we could fire the cannonball with enough velocity, it would fall toward Earth but never actually hit it because its flight would extend around the curve of Earth (Figure 3(c)). When engineers launch a satellite into Earth orbit, they launch it much like the powerful cannon. The satellite needs to attain sufficient velocity to make sure that when Earth s gravity pulls it down, it continuously falls around Earth s curvature (Figure 3(d)). It is called a continuous Earth orbit. LEARNING TIP Diagrams Diagrams often help clarify what is explained in the text. How did the diagrams in Figure 3 help you understand about satellites staying in orbit? (a) (b) satellite gravitational pull velocity (c) (d) Figure 3 The satellite stays in a stable orbit because of the balance between its forward velocity and Earth s gravitational pull. cience 9 SB Ontario Science 9 SB C08-F23a-UCOS9SB FN C08-F23b-UCOS9SB 8.11 Satellites 347

3 T RY THIS ORBITING SATELLITES SKILLS: Controlling Variables, Observing, Communicating In this activity, you will model and observe how satellites stay in Earth orbit. Equipment and Materials: computer with Internet access 1. Research some websites where you can simulate the launch of a satellite from Earth. SKILLS HANDBOOK 3.B Try and find other stable orbits at different altitudes. A. How do you know if the launch speed of the satellite is too high? T/I B. How do you know of the launch speed of the satellite is too low? T/I C. In which direction does gravity pull the satellite? GO TO SON SCIENCE T/I D. In which direction does the satellite s velocity point? 2. Select a website applet (short for application) that has various settings to adjust, such as launch speed, launch height, or gravity. There might be other variables to adjust. Play with the variables and see what happens when you launch satellites at different speeds. T/I E. To send a satellite into a high-altitude orbit, how would you change the location or speed of your launch? T/I F. If you wanted to send a spacecraft to the Moon or to another planet, how would you change the location or speed of your launch? 3. Adjust the variables until the satellite is successfully in orbit around Earth. Types of Orbits Artificial satellites orbit outside Earth s atmosphere at altitudes of 200 km to more than km. The higher the satellite is, the longer the orbital period the time it takes to circle Earth. At an altitude of about 350 km, the International Space Station (ISS) takes 90 minutes to orbit Earth, whereas Canada s MOST space telescope satellite completes one orbit 820 km above Earth s surface in 101 minutes (Figure 4). Figure 4 Canada s MOST space telescope was designed to probe stars outside the Solar System by measuring changes in the light they emit. Low Earth Orbit Satellites Satellites are placed into different orbits around Earth, depending on their function. For instance, most human-occupied spacecrafts and those conducting Earth observations are set at low-altitude orbits. These low Earth orbit satellites revolve around our planet at altitudes up to 2000 km. 348 Chapter 8 Our Place in Space 6646_Sci9_Ch08_pp indd 348 8/7/09 1:34:31 PM

4 Satellites that are required to see every part of the planet as they orbit it are placed in a special type of low Earth orbit called a polar orbit (Figure 5). A satellite in polar orbit generally travels at an altitude of 200 to 900 km in a path that takes it over both the North Pole and the South Pole. This type of orbit allows the satellite to view all parts of Earth. As the planet rotates beneath the satellite, a new pole-to-pole slice of Earth is monitored with each orbit of the satellite. These satellites have many uses, including military and Earth observation. Canada s RADARSAT satellites are in polar orbits to keep an eye on a variety of natural and human-made events. They chart icebergs in Canada s far Arctic oceans, monitor shifting patterns in agriculture in Africa, and play an important role in natural disaster response in Asia. satellite 879 kilometres Earth Figure 5 A polar orbit provides the best global coverage. Medium Earth Orbit Satellites Medium Earth orbit satellites travel at altitudes up to km. Two dozen of these satellites are part of the global positioning system (GPS). GPS satellites travel in medium Earth orbits at about km (Figure 6). They aid in navigation by transmitting signals down to GPS receivers on the ground, providing them with precise geographical coordinates of their location. The orbital height of GPS satellites is governed by the fact that at least three satellites must be visible at any one time by a GPS receiver anywhere on the ground. global positioning system (GPS) a group of satellites that work together to determine the positions of given objects on the surface of Earth Ontario Science 9 SB FN C08-F25-UCOS9SB CO Creative Freelancers Figure 6 Six orbital planes host 24 GPS satellites. Sam Laterza 8.11 Satellites Pass 349 Second Pass Approved 6646_Sci9_Ch08_pp indd 349 8/4/09 3:42:42 PM

5 geostationary orbit an orbital path directly over Earth s equator with a period equal to the period of Earth s rotation DID YOU KNOW? Satellite Television The advantage of satellite TV is that it provides a wide coverage area, with the signal reaching millions of homes across large regions of the world. Some of the latest satellites can transmit more than 200 high-defi nition channels simultaneously. Launched in September 2008, Canada s communications satellite Nimiq 4 sends TV broadcast signals across most of North America. Geostationary Orbit Satellites One type of satellite orbits Earth at a distance of km, which is about a tenth of the way to the Moon. This altitude is significant because it produces an orbital period equal to the period of the rotation of Earth. When a satellite is orbiting at this height directly above the equator, it is said to be in geostationary orbit. Satellites in geostationary orbit appear motionless in the sky, which makes them useful for communications and other commercial industries because they can be linked to antennas on Earth. Weather satellites, for example, track weather in this manner (Figure 7). Communication industries use geostationary satellites for satellite broadcast television and radio. Television satellite dishes attached to the sides of homes are able to receive the satellite signal by aiming at a fixed spot in the sky (Figure 8). Figure 7 This satellite image shows Hurricane Katrina off the coast of Louisiana in August Figure 8 The satellite dishes on these apartments are aimed at a satellite in geostationary orbit. RESEARCH THIS SKILLS: Researching, Evaluating The many different satellites orbiting Earth perform various functions. In this activity, you will research a satellite and describe its function and what effect it has on the lives of people. 1. Research a satellite that interests you and conduct research to answer the following questions. A. What function does your satellite perform? T/I B. In your opinion, does your satellite perform a valuable function? A DIFFERENT KINDS OF SATELLITES SKILLS HANDBOOK 4.A.5., 4.A.6. C. What kind of orbit is your satellite in: low, medium, or high (geostationary)? T/I D. RADARSAT-1 is Canada s fi rst commercial Earth observation satellite. Research its function, orbit, and scientifi c and commercial applications. Summarize your fi ndings and present them to the class in a pamphlet, short story, or visual presentation. T/I C A GO TO SON SCIENCE 350 Chapter 8 Our Place in Space

6 TRY THIS SATELLITE IMAGES SKILLS: Performing, Observing, Analyzing The altitude of a satellite determines the speed at which it travels and is also an important factor in determining how much of Earth s surface it can see at any given time. Low orbit satellites, such as GPS satellites and weather satellites, are much closer to Earth s surface and cannot see as much of its surface as higher altitude satellites in geostationary orbit. In this activity you will use a globe to understand how much of Earth s surface different satellites can see. Then you will classify pictures of Earth taken by various satellites. Equipment and materials: standard-sized (30 cm diameter) globe; metre stick; pictures of Earth 1. To get a sense of the height of the different orbits, use the metre stick to fi nd the following distances above the surface of the globe. They represent the altitudes of the different orbits at the same scale of the globe. low Earth orbit: 4 cm medium Earth orbit: 20 cm geostationary orbit: 84 cm SKILLS HANDBOOK 3.B. 2. Place your eyes at the given distances and look at the surface of the globe. Move your eyes around at each altitude to determine how much of Earth s surface you can see at each orbital height. 3. Look at the pictures your teacher has provided. A. Classify the pictures in one of the three categories: low Earth orbit, medium Earth orbit, or geostationary orbit. T/I B. If you were developing a satellite to observe the entire continent of South America, which orbit would you pick? Why? A C. If you were developing a satellite to observe weather patterns over the city of Hamilton, which orbit would you pick? Why? A UNIT TASK Bookmark How can you apply what you learned about travelling in Earth orbit in this section to the Unit Task described on page 446? IN SUMMARY Many countries, including Canada, have launched artificial satellites into Earth orbit to study Earth and objects beyond Earth. Objects need to be sent into orbit with enough velocity to avoid being pulled back to Earth s surface by gravity. Different types of orbit (low, medium, and geostationary), are categorized based on the satellite s altitude. Satellites have many different applications for technologies on Earth, such as satellite television and global positioning systems. CHECK YOUR LEARNING 1. What is the difference between an artifi cial satellite and a natural satellite? Give one example of each. K/U 2. Describe three ways in which artifi cial satellites benefi t society. A 3. (a) When was the fi rst artifi cial satellite launched into Earth orbit? What country was responsible for this? (b) When was Canada s first artificial satellite launched into Earth orbit? What was its name? K/U 4. Identify three artifi cial satellites that Canada has launched into Earth orbit, and describe their function. K/U 5. How does Earth s shape enable engineers to put satellites into stable orbit around our planet? 6. Identify four different reasons for launching a satellite into orbit around Earth. K/U 7. Compare and contrast the three types of Earth orbit regarding their altitude and what they are used for. K/U 8. Sketch Earth with the three different types of Earth orbit. Label the altitudes and a typical satellite found at each level. K/U C 9. What does the acronym GPS stand for, and what is it used for? K/U 8.11 Satellites 351