The Americom-7 satellite was launched on September 14, 2000, and currently orbits Earth at a distance of 35800 kilometers above the surface. The EOS PM-1 satellite was launched on May 4, 2002, and currently orbits Earth at a distance of 702 kilometers above the surface. Which satellite orbits Earth faster?
Let’s begin by sketching a diagram of the Earth, the two satellites, and their orbital paths, assuming their orbits are roughly circular. Notice that Americom-7 is much farther from Earth than EOS PM-1. Recall that the speed of an object in circular orbit depends on its distance from the center of orbit and the mass of the large body that it’s orbiting. Here, both satellites in question are orbiting the same thing, Earth. Therefore, the key variable that determines these satellites’ speeds is their distance from Earth or their orbital radius. We can say that there is an inverse relationship between orbital radius and orbital speed. This is because if an object has a greater orbital radius, it will have a slower speed. Likewise, an object with a smaller orbital radius moves faster.
We’ve been asked which satellite orbits faster. So we can just think about which one has a smaller orbital radius. EOS PM-1 is about 50 times closer to Earth than Americom-7 and is, therefore, the satellite that orbits Earth faster.