Why don’t satellites fall down from the sky?

Image via Pixabay.

Satellites are able to stay in Earth’s orbit thanks to a perfect interplay of forces between gravity and their velocity. The satellite’s tendency to escape into space is canceled out by Earth’s gravitational pull so that it is in perfect balance. This is the same principle that explains how natural satellites, such as the moon, become locked in a planet’s orbit.

But some very smart people had to run some very complicated math to design the perfect satellite launch. If the satellite moves too fast, it escapes into space. Too slow and it is destined to crash into the atmosphere.

With the right distance, speed, and trajectory, an object can defy Earth’s gravitational pull for quite a long time. In fact, gravity — the same force that is trying to drag these down to the surface — is a vital force in keeping satellites orbiting around our planet.

And to make it entertaining, we’re going to start with rollercoasters.

Perpetually falling

If you’ve ever been on a rollercoaster, you’ll know that strange sensation you get in your gut around bends or hills. Physically speaking, that sensation is produced by inertia; although the cart is changing directions, your body is resisting this shift. You’re strapped in safely to the cart, but your internal organs have a bit more leeway to move. So, for a few moments, they essentially keep moving on the old trajectory, while the rest of your body is on a new one.

This process is summarized neatly in the first law of motion: an object, either moving in a straight line or at rest, will maintain that state until acted upon by an external force. “External force” here can mean a great many things, from air resistance to gravity to you hitting a flying ball with a bat.

Artificial and natural satellites rely on this law to stay above the clouds. Since there is no air resistance in space, once a body gets moving, there’s virtually nothing to slow it down. It doesn’t lose kinetic energy (momentum), so it can keep moving forever.

The satellites we build today get their energy from the rockets that bring them to orbit. They do have internal fuel supplies and thrusters, but these aren’t used to maintain speed. They’re for maneuvers such as avoiding debris or shifting orbits. Rockets impart the satellites they carry with quite a lot of energy, as they need to travel at speeds of at least 17,600 mph (28,330 km/h) to be able to escape Earth’s gravity. After separation from the satellite, this leaves enough energy to keep that satellite in orbit around the Earth for several decades, even a few centuries.

Still, the purpose of a satellite is to stay closeby (relatively speaking) so it can beam our social media posts all over the world. But from what we’ve seen so far, shouldn’t they just travel into space forever? Yes. But there’s one other force at play here — gravity. While momentum keeps satellites moving, gravity is what keeps them in our orbit.

If you fill a bucket with water and spin it really fast, you’ll see that the water won’t pour out of it. It’s being pushed against the bottom of the bucket by inertia (in this case, centrifugal force), but that bucket and your arm work against the force. They even out in the end: the water can’t move through the bucket’s bottom, it can’t escape over the lip, either, so it kind of stays in one place.

For satellites, the Earth’s gravity acts as the arm and bucket in the above example. One really simple way to understand the process is to visualize the satellite as a rocket that’s always going forward, tied with a very long chain to the center of our planet — it will just go around in circles.

What’s important here is to get the distance right. First off, you want your satellite to be outside of the planet’s atmosphere, so as to avoid air drag and keep a constant speed. But you don’t want to be too far away, because the force of gravity is inversely proportional to the squared distance between two objects. So if you double the distance between a satellite and the Earth, gravity would only pull on it one-quarter as strongly. If you triple it, it would only be one-ninth of the force. In other words, put a satellite too close to Earth and it will fall. Put it too far away, and it escapes into space.

In essence, what engineers try to do when putting a satellite in orbit is to make it fall forever. We put it up high enough that air friction is almost zero (ideally, zero). Then we push it really fast in one direction. Finally, we rely on the Earth’s gravity to pull it down while it is moving forward, so the resulting movement is a circle. Because it’s moving forward and the planet is round, it’s essentially gaining altitude constantly. But, since it’s also falling at the same time, it’s losing altitude constantly. The sweet spot is to have it escape into space just as fast as it’s falling down to Earth, all the time.

If the math is done just right and the deployment phase goes properly, these two cancel each other out, and we get an orbiting satellite. In practice, it never goes quite perfectly, which is why these devices are fitted with fuel and thrusters so they can perform tiny adjustments to their direction of motion or altitude and keep them in orbit.

A good example of what would happen in the absence of these thrusters is the Moon. Our trusty and distinctive nighttime companion is not on a stable orbit — it’s slowly escaping Earth’s gravitational pull. Due to the specifics of how the Earth’s gravitational field interacts with the Moon, our planet is ever-so-slowly accelerating it into a higher orbit. Continuing with the above example, it’s making the ‘escape into space’ force a tad more powerful than the ‘falling down to Earth’ force. As a consequence, the Moon will probably break out of orbit with Earth in the future, but we’re talking billions of years here.

Alternatively, we have (had?) the Mir space station as an example. This Russian installation ended its mission in March 2001 and was brought to a lower orbit — it was ‘deorbited‘. Here, air friction steadily slowed it down. Because of this, gravity started gaining the upper hand and Mir eventually burned up in the atmosphere while spinning around the globe, closer and closer to the surface.

The physics of how bodies in space interact is always fascinating, at least it is to me, and generally has a weird quirk to it that spices every scenario up. The idea that something can keep falling forever without actually coming closer to the ground certainly is quirky, and it fascinated me ever since I first came upon it. Later, sci-fi would bring me to concepts such as gravitational slingshots, which are very similar to what we’ve discussed here, but they actually help you go to space faster. Cool.

Our discussion so far makes this whole process sound simple, and in theory, it is. But very many bright people had to crunch some extremely complicated math to make it possible, and many still do that, in order to keep satellites orbiting over our heads. As much as falling forever sounds like magic, it’s built on countless hours of intellectual work and, in this day and age, on some very powerful computers running calculations around the clock. 

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