# Which force keeps satellites circling Earth?

A satellite is any object that circles the Earth, Sun, or any other massive body. It can be classified into two: natural satellites or artificial satellites. The moon, comets, and the planets are examples of natural satellites. Artificial satellites, on the other hand, are those launched from Earth for various purposes, such as scientific research, intelligence, communication, and weather forecasting, etc.

The first satellite launched from our planet is Sputnik-1 by the former U.S.S.R. on October 4, 1957. It became an insignia of modernity and roused the United States to create its own plans of exploring the space. Just a few months after Sputnik, the U.S. launched Explorer-1 on January 31, 1958. In the succeeding decades, more and more satellites have been put in space.

But, regardless if it’s a natural or artificial satellite, the science behind its circling motion falls defined by the same mathematical equations and fall under the same physics principles of velocity and the universal force of gravity.

To fully understand how satellites orbit the Earth, it is imperative to know first what orbits means. It was Johann Kepler, a German astronomer who first precisely described the shape of the planet’s orbits in a mathematical sense. While it was thought that the orbits of the planets around the Sun are circular, Kepler found the concept of these satellites having elliptical orbits.

With that, for an object to remain in orbit around the Earth, it requires sufficient speed to follow its path. This principle applies to natural satellites or man-made ones. Kepler’s learning then allowed scientists to conclude that the nearer the satellite to an object, the higher force of attraction. Thus, it needs to travel faster to stay in a circular motion.

Now, let’s talk about the force that actually keeps satellites circling the Earth – gravity. Each object, regardless of size, has its own gravitational field. However, it can only be felt particularly on massive objects, like the planets. In the case of the Earth, its gravitational pull is at 9.807 m/s². But that only applies if the object is at the surface of our planet.

When referring to objects that revolve around the Earth, a different formula applies. It is calculated as v=(GM/R)1/2, where v pertains to the velocity of the object, G is the gravitational constant, M refers to the planet’s mass, and R is the object’s distance from the Earth’s core.

Through the given formula, scientists are able to infer the amount of velocity required for a satellite to orbit around the Earth and counterbalance the planet’s inward pull or its gravitational grip.

With that, man-made satellites are launched into space by using rockets. The rockets need to fly around 100 to 200 kilometers on top of Earth to surpass the atmosphere. Once set on its desired orbit elevation, it will begin to move sideways at speeds, which can reach up to 18,000 miles per hour. The higher the altitude the satellite has been placed, the less velocity is required to keep its orbit. As long as it maintains the speed it needs to stay balanced, the satellite will remain for long.

Most satellites are placed around 2,000 kilometers on top of the Earth. Satellites situated nearer the Earth will only last for a few weeks or months as they run into friction and will melt. At 600 kilometers, the altitude where the Internation Space Station revolves, objects can remain for decades. They need to travel fast at around 5 miles a second to stay in orbit. An incredible thought though, is that leaves a footprint that can run hundreds of miles long, making the massive space appears not as empty as you think.

To sum it all up, what keeps satellites circling the Earth falls down to the balance between two factors: the satellite’s velocity, which refers to the speed it needs to stay on its path, and the pull of gravity between the object and the planet where it revolves. The higher the altitude, the less velocity is needed. The closer the orbit, the faster speed it needs to ensure that it won’t fall back to Earth.