 Artificial satellites, the kind we strapped to rockets and launched into orbit around the Earth, are responsible for huge strides in human development and standards of living. In 2014, global satellite industry revenues were $203 billion, encompassing telecommunications, agricultural observation, disaster mitigation, meteorology, navigation, scientific observation, and national security. In this video, we'll analyze a couple of different orbits to see why satellites are positioned where they are. The first ever artificial satellite, named Sputnik-1, was launched by the Soviet Union in 1957. Today there are thousands of satellites in orbit around Earth. The first kind of orbit we'll investigate is known as a geostationary orbit. This is an orbit where the satellite stays at the same point in the sky from the perspective of people on Earth. To keep a geostationary satellite at the same point in the sky, it needs to orbit the Earth with the same period as the Earth's rotation, or else it would advance eastwards or westwards in the sky. Furthermore, it needs to be travelling in a uniform circle, or else it would sometimes be going faster than the Earth's rotation, and sometimes slower. So we can use the satellite equation derived in a previous video, and we can rearrange this to solve for r, the radius of the orbit. Rearranging, we find that r is equal to the cube root of the universal constant of gravitation times mass, times the period squared, divided by 4 pi squared. If we plug this into our equation, we find that the radius of the orbit will be 42,000 kilometers. Remember that this number is from the centre of the Earth. As we know that the Earth has a radius of 6,400 kilometers, then we know that geostationary satellites are located at about 36,000 kilometers above the surface of the Earth. Finally, we can figure out that the geostationary orbit needs to be above the equator, anywhere else and it would appear to move northwards and southwards. So, overall, we have found that for a satellite's orbit to be geostationary, it must sit on a ring around the Earth's equator, at a distance of about 36,000 kilometers above the surface. Well, that's all well and good, but why do people want geostationary satellites? Well, it turns out they're very useful for telecommunications in particular. By keeping your satellite at one point in the sky, you can build a huge satellite dish that doesn't have to move around and follow the satellite. You can just build it pointing up, put the satellite in the right direction, and broadcast away. However, geostationary satellites have one major disadvantage. They're very far from the surface of the Earth. If you want to take pictures of the Earth for weather or surveillance, you'll want to be as close as possible. In addition to this, a satellite in a stationary location would also not be very useful for weather and surveillance where you want to survey a wide range of land. For these purposes, satellites are put into an orbit known as a polar orbit. Polar orbits are low Earth orbits that orbit from the North Pole to the South Pole. Every time they complete a circuit, they can observe a strip of the Earth's surface underneath them, and as the Earth rotates, that strip of land changes. A typical polar orbit has a period of a few hours, and they can build up a picture of the entire Earth over the course of a day, strip by strip. So for example, the GRACE mission uses satellites in a polar orbit.