why cant a flying object just leave the atmosphere at a slower speed? why does it need to achieve ‘escape velocity’? if a rocket goes straight up at 100kmph without stopping, it should escape the atmosphere eventually right?
In: Physics
Yes. The key part of your question is “without stopping.” That implies an absolute immense amount
of fuel, more than a rocket can possibly hold.
Things can’t just go at a set speed in atmosphere without constantly pumping in energy to fight drag and gravity.
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You’re right, escape velocity is just a term for the initial speed something would need to escape earth’s gravitational pull to such an extend that it reaches an orbit. But we don’t just toss things into space, as that would result in a ridiculous force on the spacecraft and the people in it. Keep in mind that the escape velocity gets lower as you go higher.
If I throw a ball into the air, the gravity of planet Earth will pull it back down and the ball will fall. If I throw it a little stronger, the ball will be in the air longer, but still will eventually fall back to Earth. I would have to throw the ball really really really strongly to make it go so high that Earth’s gravity doesn’t pull it back down.
That’s sort of the same idea here. A rocket that isn’t going very fast will also be pulled back down, and won’t be able to escape Earth’s gravitational pull. It has to go really fast or else it will fall back down, even if it doesn’t happen immediately.
Escape velocity is what is needed for an object that goes up to keep going up.
The problem with a rocket going straight up at 100 kph and just maintaining that is that you will end up running out of fuel before the earth runs out of gravity. If you were to create a better, more efficient propulsion system then you would eventually get far enough away at just 100 kph you would no longer fall back down and continue outwards.
So its not that it can’t work, its just that we don’t have the technology or capability at this point to make it work.
Remember, gravity is supplying a constant acceleration down towards earth. The farther you get from earth the lower that acceleration is.
Escape velocity is a concept that basically says “If I start at this point going at speed x straight up, I will move so far away from the planet that gravity will never reduce my outward velocity to 0.”
Most of our rocket launches aren’t concerned with reaching an escape velocity, they want orbital velocity. We don’t send many things away compared to what we want to stay nearby.
What gravity is is you constantly falling towards the centre of mass of, well technically everything in the universe simultaneously, but mostly Earth. I assume you’re sitting still right now. That means then that you are just falling towards the earth in a straight line and being rudely interrupted by a chair. That chair is falling towards Earth too, and being prevented from falling further by the ground. Now, stand up and run 100 meters in a straight line, or if you don’t want to do that, imagine what it would be like if you had. If gravity didn’t exist and earth wasn’t moving, and you could run on air, you would end that run floating an imperceptible distance above the ground, because the earth has curved away from your straight line. However, you are not floating right now. Why? Because of gravity. Or because you didn’t bother doing it. You’re propelling yourself forwards, but gravity is also adding some downwards movement to you, so whenever you get the opportunity to fall back towards earth, you will.
The next thing I want you to do is get up again and go to space. Or, if you can’t be bothered, imagine what it would be like if you had, but know that I’m judging you for half-arsing it. Imagine that you have magic shoes that let you run on the top of the atmosphere, but don’t prevent you falling through it. If you stand still on the atmosphere, you’ll fall directly towards earth in a straight line. But what if you run? You still have that downwards movement from gravity, but you also have sideways movement. If you can run 100 meters in one second and gravity can pull you down 100 meters in one second, then after on second, you haven’t moved a total of 100 meters directly towards Earth, you’ve instead effectively moved down a diagonal line, because you’ve both run 100 meters across and fallen 100 meters down. If you keep running and keep falling, the diagonal line you’re moving on will eventually collide with the Earth, but it’ll do so a long way away from the spot directly below where you started, and because the earth has been curving away from you, the total distance you fell will be slightly higher than the distance you’d have fallen if you stood still.
In fact, here, have a [diagram](https://imgur.com/Xm48MWn). The big circle marked Earth is Earth. The red dot is where you started. The horizontal grey line indicates you moving in a flat line. If there was no gravity, the end of that line is where you’d end up. The line stretching from there towards the earth shows the downward movement of gravity. If you stood still there, you’d fall down that straight line and hit Earth. However, when you do both at the same time, you follow the path of the green line. It’s curved because gravity is actually an acceleration, not just a movement – you fall faster and faster as you go down, so the curve starts shallow and gets steeper towards the end of the fall. And notice that when you hit the ground as the orange dot, you’re further “down” than the ground directly beneath your starting point, and the angle gravity is pulling you at has changed.
Taking that then, imagine what would happen if you could run so far in the time it takes for gravity to pull you down that the curve gravity pulls you down actually misses Earth completely. You wouldn’t keep falling towards the bottom of the image forever, because the angle gravity is pulling you down at is constantly changing. Instead, you’ll circle around the earth, and if you keep running fast enough to always miss the earth, you’ll eventually end up back where you started. Congratulations, now instead of gravity making you hit the earth, you’re in an orbit! The moment you stop running though, your speed will fall, and the curve gravity pulls you down will make you hit the ground again.
This should illustrate why speed is important then. To escape the pull of gravity, you need to travel sideways fast enough that by the time gravity has pulled you down to the ground level of where you started, the earth has curved away from you enough that you’re not only still in the air, but higher up in the air compared to the new ground than you were when you started. You’re always falling when you’re in gravity, but sometimes you can run so fast that you miss the ground and just keep falling and missing and changing direction and falling again and missing again forever.
The atmosphere is very thin compared to the size of Earth. Low Earth orbit is just a few hundred kilometers up, meaning the strength of gravity remains pretty much unchanged.
If you go straight up you have to keep your rocket engines running all the time, even beyond the atmosphere. Sure, you can do it slowly. If you go sideways once you reach orbit you no longer need any propulsion to stay there.
Think of gravity like a rubber band. Not taking in frivction or resistance of any kind, the rubber band will continue pulling it back correct? So the idea is to go as fast as possible t break free of the rubber band which is a constant force (gravity) pulling back on the object. Less energy/fuel is used to get away from earth by going faster. Once it’s in orbit it’s actually the inertia of the object fighting the force of gravity trying to pull it back in. Once the object slows down it will fall back to earth. If it keeps going out of gravity’s pull (massive amounts of energy) it will still be acted on in some way by other large masses
Yea, but getting into orbit isn’t about just getting out of the atmosphere, it’s about going sideways really fast. If you went straight up at 100km/h you’d be in space in about an hour but you’d fall right back down as soon as you turned off your engines.
Suppose you’re in a spaceship near to a planet. Let’s assume for a moment that the planet is very very tiny so we’re never at risk of crashing into it, and that it has no atmosphere. Let’s also assume that our rocket has run out of fuel. If our rocket’s initial speed is low enough, we will remain in orbit around the planet – specifically we will trace out an ellipse around it. If the speed is high enough, we will keep getting further and further from the planet – specifically we will trace out the shape of a hyperbola. Somewhere in the middle there is a speed, depending only on our initial distance from the planet (EDIT: and the mass of the planet), which marks the boundary between those two outcomes. This is the escape velocity.
Now in real life there are a few complications. Depending on the direction we’re travelling, we might collide with the planet. This is possible regardless of how fast we’re going. If we’re close enough to the planet that its atmosphere causes substantial drag, then things get a lot more complicated and whether or not we can escape its gravity depends on the density and viscosity of the atmosphere, the path we take through it, and how streamlined our rocket is. And if we still have fuel left and are using the engine to accelerate, this also complicates things. So “escape velocity” is not really a very relevant concept when we’re talking about a rocket that is on the ground on a planet with a thick atmosphere.
But if we’re already far out in space with no atmosphere around us, we’re orbiting a body and we want to leave it and travel to a different body, it’s a very useful concept. The “cost” of maneuvers out in space is often measured in terms of “delta v”, i.e. the amount by which our engine needs to change our speed. The difference between our current speed and the escape velocity is the delta v we need to escape the orbit of the body.
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