Artificial satellites remain in orbit around the Earth because of the combination of their horizontal velocity counteracting the force of gravity, keeping them in dynamic equilibrium.
It is thanks to Earth's gravity that satellites orbit around it instead of flying straight off into space. This force acts like a kind of invisible rope, constantly pulling the object toward our planet. The satellite is constantly falling toward Earth, but since it moves very fast sideways, it always misses the surface. The result: it gets stuck on a curved path called an orbit. The higher the altitude, the weaker the influence of this force becomes, which changes the speed needed to maintain a stable orbit. Without this gravitational pull, satellites would simply continue on their straight path instead of orbiting around us.
For a satellite to orbit the Earth without crashing or drifting into space, it must reach a certain orbital speed. Essentially, it needs to move fast enough for its trajectory to match the curvature of the planet. More simply put: the Earth pulls the satellite towards it due to gravity, but as it moves at the right lateral speed, it constantly misses the planet while falling towards it. At an altitude close to that of the International Space Station (about 400 kilometers), this required speed is around 28,000 km/h. If the satellite slows down too much, gravity takes over and it descends towards the Earth; if it speeds up too much, it risks escaping. The key is to find the right speed to achieve a kind of perpetual fall around our planet.
Satellites remain in orbit thanks to the clever combination of two essential laws stated by Isaac Newton, particularly the famous law of universal gravitation: every body attracts another body, and the heavier or closer it is, the stronger the attraction. In addition to this are the clear rules of motion described by his three laws of motion. The coolest of the three here is surely the one known as the law of inertia, which states that an object in motion will continue to move in a straight line unless acted upon by a force. These two sets of laws explain why a satellite, launched at the right speed, keeps "falling" around the Earth without ever crashing into it.
The altitude of a satellite changes a lot about its orbit. The higher it is, the slower its orbit becomes, because gravity is weaker up there. Conversely, low-altitude satellites, like those in low Earth orbit (LEO), zoom around the Earth quickly. A geostationary satellite, placed very far away at about 36,000 km, rotates at exactly the same speed as the Earth on its axis: it always points to the same spot on the planet. This is super convenient for communications or weather. At low altitude (just a few hundred kilometers), the satellite travels around the globe several times a day, which is useful, for example, for detailed earth observation or accurate photography. But if it is too low, it will encounter the residual atmosphere, gradually slow down, lose energy, and eventually fall gently back to Earth: hence the necessity to be at the appropriate altitude depending on the missions.
Even though satellites gently orbit around the Earth, many small things interfere with their orbit. For example, atmospheric drag gradually slows down some satellites, especially those in low orbits, which pushes them down over time until they eventually re-enter and burn up in the atmosphere. Other factors, like the gravitational pull of the Sun and the Moon, slightly affect their trajectory. Even the Earth poses a problem: it is not perfectly spherical, and its mass is not evenly distributed, making its gravity a bit irregular. As a result, orbits shift slowly, and it is necessary to regularly adjust the trajectory to keep a satellite on the right path.
A geostationary orbit corresponds to a circular orbit approximately 35,786 km above the Earth's equator. Satellites in this orbit have an orbital period equal to that of the Earth's rotation (about 24 hours), which makes them appear stationary in the sky to an observer on the ground.
Satellites can gradually lose altitude due to disruptive factors, including friction with residual atmospheric particles. If the speed and altitude decrease sufficiently, Earth's gravity eventually pulls them entirely towards the Earth, and the satellite ultimately disintegrates upon re-entering the atmosphere.
The orbital duration of a satellite mainly depends on its altitude and structure. Some low Earth orbit satellites remain in orbit for a few years before gradually falling back to Earth. In contrast, high or geostationary satellites can stay in orbit for several decades or even much longer.
Although in theory a satellite can remain in high orbit for a very long time where atmospheric friction is practically nonexistent, various disturbing factors (the effects of the Moon and the Sun, solar pressure, collisions with space debris) will prevent any satellite from remaining perfectly stable forever without periodic corrections or maneuvers.
The choice of orbit depends on the specific mission of the satellite. Engineers take into account the desired geographical coverage, the type of data being collected or transmitted, communication constraints, the expected lifespan of the satellite, and the costs of the space launches required to reach the chosen orbit.
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