Video Transcript
Communication satellites are often placed into geostationary orbits. Why is this? (A) Geostationary orbit is the lowest height at which satellites can orbit; thus, it is easier to place a satellite into geostationary orbit. (B) Geostationary orbit is the maximum height at which satellites can orbit without leaving the orbit of Earth. Satellites orbit at this height because it is where there is the most space; thus, the satellites will not crash into each other. (C) A geostationary orbit is the most energy-efficient orbit; thus, it is cheaper to keep a satellite in geostationary orbits than other orbits. (D) A satellite in geostationary orbit stays at the same point in the sky, making it easier to locate the satellite and send and receive signals. (E) A satellite in geostationary orbit stays at the same point in the solar system relative to the Sun; thus, the position of the satellite does not need to be tracked.
To begin, we can recall that a satellite in a geostationary orbit stays over the same position on Earth’s surface by orbiting over the equator from west to east with a period of 24 hours. Thus, as Earth rotates, the satellite moves along in a way that makes it seem stationary.
Now, answer choice (A) says that a geostationary orbit is at the lowest possible height above Earth. This is simply not true. Geostationary orbits are actually quite high, about 36,000 kilometers above Earth. For reference, the international space station orbits at about 400 kilometers above Earth. The diagram up here is not drawn to scale, but if it was, a geostationary satellite would actually be all the way out here. We should eliminate answer choice (A).
Next, (B) says just about the opposite that geostationary orbit is the highest that satellites can go. Although these orbits are quite high, they’re not the upper limit. In fact, scientists categorize satellite orbits by distance as either low, medium, or high Earth orbit. And geostationary orbit at around 36000 kilometers marks the boundary between medium and high Earth orbit. Some satellites in high Earth orbit are at a height of over 100000 kilometers. Therefore, answer choice (B) is incorrect.
Option (C) says that it’s cheapest and most energy efficient for a satellite to be in geostationary orbit. As we mentioned before, geostationary orbit is quite far from Earth, and thus a spacecraft needs more fuel to reach the required altitude. Also, at such a distance from Earth, the gravitational effects of the Moon and Sun can pull a geostationary satellite off track, unless the satellite regularly executes thruster burns, which requires extra fuel. For these reasons, answer choice (C) isn’t very convincing, but we can keep it around while we check out the other options.
Answer choice (D) says that geostationary satellites are easier to track and contact because they stay at a fixed point in the sky. This is what geostationary means, at rest with respect to Earth’s surface. Of course, the satellite and Earth are always moving, but they’re synchronized, which does make the satellite easier to track and contact. This is all true. So (D) seems like the best answer. But just to be sure, let’s check out option (E). (E) says that geostationary satellites don’t need to be tracked because they do not move with respect to the Sun. This doesn’t really make sense. After all, geo- means Earth. Also, Earth orbits the Sun at a distance of about 150000000000 meters.
So, if a satellite were to stay still with respect to the Sun, it would be hundreds of billions of meters away from Earth at times, meaning it could take up to half an hour to send and receive one signal. Sometimes, the Sun could even be between the satellite and Earth, making communication impossible. Answer choice (E) is not correct, and option (C) isn’t very convincing either. (D) is the best answer because it correctly describes geostationary orbit and why it’s useful. We know that a satellite in geostationary orbit stays at the same point in the sky, making it easier to locate the satellite and send and receive signals.
