So I’ve just heard they’ve set a year of 2032 to decommission the International Space Station. Since if they just left it, its orbit would eventually decay and it would crash. Rather than have a million tons of metal crash somewhere random, they’ll control the reentry and crash it into the spacecraft graveyard in the pacific.
But why not push it out of orbit into space? Given that they’ll not be able to retrieve the station in the pacific for research, why not send it out into space where you don’t need to do calculations to get it to the right place.
In: Planetary Science
Everyone is explaining how leaving orbit requires a lot of energy but nobody seems to really touch upon exactly WHY, because orbital mechanics and motion without air resistance is highly counterintuitive.
The first mental model people often have about space and orbit is that “there’s no gravity up there”, so the spacecraft is just “floating”. There may be some awareness that it’s not staying still, that it’s rotating around the earth (every 90 minutes it turns out) but it’s hard to connect that to something intuitive of what that REALLY means. After all, maybe the rotation around the earth is just an artifact of the fact that we had to get up there in the first place, and didn’t want to slow down? Or maybe we want to see many parts of the earth? All of these would be plausible.
But the reality is just a bit more weird. Let’s start with the facts essentially nothing in the universe moves unless there is a force applied to it. And once applied, once motion is occurring, you need another force to slow it down or stop it, otherwise it keeps going indefinitely. This is [Newton’s first law of motion](https://en.wikipedia.org/wiki/Newton’s_laws_of_motion) Right away this is already not intuitive to our experience on earth because nothing behaves like that here, but that’s simply because of friction. Objects in land/water/air slow down by themselves because of friction or air resistance. In space they don’t do that.
So when you have a spacecraft in space near a planet there are two forces to think of (we can ignore the rest of the universe).
* There is the engine of the spacecraft – and keep in mind the speed here is not just relevant when the engine is firing, but also whatever the engine fired at the start to get it moving to a certain speed (like a rocket taking off into space)
* And there is the force of gravity of the earth pulling the spacecraft towards it.
So if these are the only facts that you had – the first law of motion, the engine of a spacecraft pushing it away from the earth, and the gravity of the earth pulling it back – you might think there’s only 2 options – get away from the earth, or crash back onto it. This is where the frankly magic (aka insanely complicated science) of orbital mechanics come in.
Being in orbit is like balancing a coin on it’s edge. It turns out there is just the right distance, just the right speed, and just the right direction to put the two in balance. You set a rocket on it’s path, and it’s trying to get away from the earth. But at just the right time the earth pulls it back. Only there’s still enough speed in the rocket that it can’t bring it back to it, so instead it just rotates around.
But what makes orbital mechanics hard is that this isn’t just a middle ground where if you overshoot too much in one direction, and you’ll head directly for the earth, and if you overshoot too much in the other, you escape it. There is a pretty wide stable range where you just well, stay in orbit. You can visualize this by taking a ball and spinning it in a large bowl, and noting how it keeps going around, and eventually circling towards the middle, but it stays pretty reliably spinning. This is a decent model for gravity “curving” spacetime, but that’s a discussion for another day.
That’s basically it other than a few other factors (there is SOME friction in orbit, it’s not a perfect vacuum, which is why the orbit naturally decays over time). This is also why rockets on spaceships need such powerful engines. It’s not that it takes that much energy to “lift” something above the ground to overcome gravity. We can lift a ton of mass way up high just with a hot air balloon or a drone. The powerful rockets aren’t there to overcome gravity, they’re there to **speed up** the payload fast enough to be able to spin around the earth without falling back down.
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