Have you ever looked up at the night sky and wondered how thousands of satellites stay up there, silently circling the Earth? It’s a great question. They aren’t held up by anything, yet they don’t crash down or fly away. The secret lies in a beautiful balance between two powerful forces: immense speed and the constant pull of gravity.
At its core, an orbit is a delicate dance between two fundamental principles of physics. Understanding these two concepts is the key to unlocking the mystery of how satellites stay in the sky.
An orbit is achieved when the pull of gravity is perfectly balanced by the satellite’s forward velocity. The satellite is trying to fly away in a straight line, but gravity is constantly pulling it back. The result of these competing forces is not a crash or an escape, but a continuous, curved path around the planet.
The simplest way to think about an orbit is to imagine it as a “controlled fall.” Picture yourself on a very tall mountain, throwing a baseball.
The baseball would now be in a state of continuous freefall, but it would never get any closer to the ground. It is constantly falling, but it is also moving forward so quickly that it perpetually “misses” the Earth. This is precisely what a satellite does.
To achieve this state in what is known as Low Earth Orbit (LEO), a satellite must travel at a staggering speed of about 17,500 miles per hour (roughly 28,000 kilometers per hour). At this specific velocity, the curve of its fall matches the curvature of the Earth, locking it into a stable orbit.
If going too slow means falling to Earth, what happens if a satellite goes too fast? If the launch rocket gives the satellite too much forward velocity, its inertia will be stronger than Earth’s gravitational pull.
Instead of being pulled into a stable, curved path, the satellite’s momentum would overpower gravity. Its trajectory would still bend slightly, but not enough to keep it circling the planet. It would break free from Earth’s gravitational grip and fly off into deep space.
This critical speed is known as escape velocity. For an object to leave Earth’s orbit entirely, it must reach a speed of at least 25,000 miles per hour (about 40,270 kilometers per hour). Space probes sent to other planets, like the Voyager or Mars Rover missions, are intentionally accelerated beyond escape velocity to begin their long journeys.
Not all satellites fly at the same altitude or speed. Their orbits are carefully chosen based on their mission.
Even with the perfect balance of speed and gravity, orbits are not completely stable forever. Several small forces can nudge a satellite off its intended path over time:
To counteract these forces, satellites are equipped with small engines called thrusters. Periodically, ground controllers will fire these thrusters for a few seconds to make tiny corrections, a process known as station-keeping. These adjustments push the satellite back into its precise orbital path, ensuring it can perform its mission for many years.
What happens when a satellite runs out of fuel for its thrusters? Once a satellite can no longer perform station-keeping, its fate depends on its altitude. In LEO, atmospheric drag will eventually slow it down enough that it re-enters the atmosphere and burns up. For satellites in higher orbits like GEO, they are often pushed into a final “graveyard orbit” to get them out of the way of active satellites.
How many satellites are currently orbiting Earth? As of late 2023, there are over 8,000 active satellites orbiting our planet. The number has grown rapidly in recent years due to large satellite constellations like Starlink. There are also tens of thousands of pieces of inactive satellites and other space debris being tracked.
Do satellites ever collide with each other? Collisions are very rare but have happened. Space agencies around the world track tens of thousands of objects in orbit to predict and avoid potential collisions. Satellite operators can perform avoidance maneuvers with their thrusters if a serious risk is identified.