1. Newton's thought experiment: what happens when a cannonball is fired very hard from a high mountain?
In the 17th century, the scholar Isaac Newton imagined a thought experiment. He pictured a very high mountain. From the top, he fired a cannonball horizontally, straight out into the air. If he fired it with weak force, the ball would fall to the ground quickly. If he fired it with more force, the ball would fly farther before hitting the ground [1].
But what if he fired it with even more force? Newton realized that at a certain speed, the curve of the falling ball would match the curve of the round Earth. The ball would never hit the ground. Instead, it would go all the way around the world and return to where it started [1]. This idea helps us understand how satellites work.
2. Satellites are not "not falling" but are falling continuously around the Earth
Using Newton's idea, we can say that satellites are not "not falling." They are actually "falling toward the Earth." However, because they have a very fast sideways speed, they keep moving forward without hitting the ground [2].
Imagine throwing a ball very fast in a place with no air resistance. It would fly in a circle around the Earth. In reality, this applies to satellites at heights where there is almost no air. They are falling, but they miss the ground because they are moving sideways so fast [2].
3. How fast do real satellites have to move to stay in orbit?
To circle the Earth just above the ground, a satellite needs to move at about 7.9 kilometers per second. This speed is thought to be necessary for orbit [3]. That is incredibly fast.
The International Space Station (ISS) flies at a height of about 400 kilometers (about 250 miles) above the ground. It moves at about 7.7 kilometers per second [3]. If we change this to kilometers per hour, it is about 28,000 kilometers per hour. At this speed, the ISS goes around the Earth in only about 90 minutes [3].
4. Why satellites slowly fall because of thin air high above the ground
There is more to this story. Even at the height where rockets fly, there is very thin air. This thin air slowly slows down the satellite [4]. As the speed drops, the satellite slowly sinks lower.
Sputnik 1 was the first artificial satellite launched in 1957. Because of this thin air, it eventually burned up. It fell back into the atmosphere 92 days after it was launched [4]. So, satellites do fall, but it takes time.
5. Why the ISS is lifted up to a higher height from time to time
The ISS is also affected by this thin air. If left alone, its height would drop over time, just like Sputnik 1 did [5]. To prevent this, engineers perform a "reboost." They use engines on spacecraft attached to the ISS to push it back up to a higher orbit [5].
Because people live on the ISS, it must be kept safe. Constant effort is needed to keep it from falling.
6. Geostationary satellites for weather and TV fly much higher than the ISS
Other satellites, like those used for weather forecasts and TV broadcasts, are called "geostationary satellites." They fly much higher than the ISS, at about 36,000 kilometers (about 22,000 miles) above the ground [6]. At this height, there is almost no air resistance.
However, they are pulled by the gravity of the Earth's uneven field, as well as the Sun and Moon, which can shift their position. So, even these high satellites need to use rocket engines sometimes to correct their position [6].
7. An experiment: how a ball falls when you throw it weakly and strongly
You can see this principle in action. In a wide open space, throw a ball weakly. Then throw it strongly. You will see that the harder you throw, the farther it goes before it hits the ground. This helps you imagine Newton's idea: if you threw it hard enough, it might go all the way around the Earth.