00:03The universe taunts us by showing us all the places we can't ever visit.
00:08However, if our species wants to have a long-term future, we have to escape our prison.
00:14But what's keeping us here in the first place?
00:17Turns out we owe the universe a debt that is 4.5 billion years old.
00:24Everything with mass in the universe attracts every other thing with mass.
00:32We call this phenomenon gravity.
00:35The closer you are to a big chunk of mass, the stronger the attraction or the more you're pulled in.
00:41This effect traps us on Earth.
00:44We can imagine this as being prisoners in a gravity prison or a gravity well.
00:49It's not a literal well, but a handy concept to understand how this works.
00:53Being in a gravity prison means that you owe gravity energy.
00:58But how can you owe energy?
01:01Because in our universe, things don't want to change their speed or direction.
01:06To convince them to move, you have to expend energy.
01:10Billions of years ago, the gravitational attraction of trillions of trillions of dust particles orbiting our sun pulled them together until they formed a planet.
01:19This process used energy and created the gravity well we're now a part of.
01:24The deeper you are inside a gravity well, the more energy you owe gravity.
01:29If you don't find a way to get enough energy, you're not able to leave, no matter what you do.
01:34Because your atoms were once part of the dust that the universe expended energy on to get to this place.
01:41Okay, let's summarize all of that again.
01:45Objects in the universe don't like to move. You have to convince them to do so with energy.
01:50Gravity used energy to convince the parts that make up our planet to move together.
01:55This created a gravity prison in the process, trapping us.
01:59To escape it, we need to repay it with energy.
02:02Okay, how do we do that?
02:05To get into space, we need to go through a complicated process of exchanging energy.
02:15For this purpose, we build a negative potential energy repaying machines, known by their more boring name, rockets.
02:23Rockets work by using some of the most energetic chemical reactions humans know about to basically explode fuel in a controlled way.
02:31This converts chemical energy into kinetic energy.
02:35The exhaust of the reaction is directed outwards and pushes the rocket away from Earth.
02:41By expending a lot of energy, we are increasing our gravitational potential energy,
02:46which is a complicated way to say that we're paying back our energy debt to gravity.
02:51But it's actually a lot trickier than that.
02:54When you burn fuel to get into orbit, you lose lots of energy to heat, the exhaust, and atmospheric drag,
03:00so you actually need much more.
03:03And you can't just pile a huge amount of really explosive, dangerous fuel close to your payload and detonate it.
03:09You need a controlled burn, which is complicated and makes your rocket very heavy, which means it has more mass.
03:16The more mass something has, the more energy you need to convince it to move.
03:21So you need more fuel to lift up your rocket.
03:24But if you need more fuel, that means you need more rocket to carry that fuel.
03:28But this makes your rocket heavier, thus requiring more fuel, which requires more rocket to carry that new fuel, and so on.
03:36At the end of this madness, you need closer to a hundred times the weight of your payload to launch.
03:42Ariane 6, for example, the European rocket, will weigh 800 tons
03:46and should be able to transport 10 tons into geostationary transfer orbit or 20 tons into medium Earth orbit.
03:55But a rocket can only produce so much thrust, so there's a maximum weight after which it just won't take off.
04:02If you add too much weight, it won't lift off, so you can't just build bigger and bigger fuel tanks.
04:08This is the tyranny of the rocket equation, and it means spaceflight will never become easy.
04:14But wait, it gets worse.
04:16Getting to space is still not good enough.
04:19You're still inside the gravity prison at the edge of space and will crash back to Earth.
04:24Staying in space is much harder than getting there.
04:27To get to a stable position where it can stay for a while, a rocket has to reach low Earth orbit.
04:33To do this, you need a lot of kinetic energy, which means going extremely fast.
04:39At an altitude of about 100 kilometers, this is 8 kilometers per second, 28,000 kilometers per hour,
04:46fast enough to travel around Earth in 90 minutes.
04:50Here, we can use a trick.
04:52Instead of flying straight up, we can go sideways.
04:56Earth is a sphere, so if you're going sideways fast enough, even though you're falling towards Earth, the ground will curve away beneath you.
05:04So as long as you're above the atmosphere, about 100 kilometers up, you'll be able to stay up there in orbit.
05:11This is what the ISS does, falling around Earth, expending energy from time to time to stay fast enough.
05:18If we look at orbits in scale, we see that near-Earth orbit is laughably close to Earth.
05:24To deploy, for example, satellites or leave for other planets requires another round of energy debt repayment.
05:32Getting to orbit is the most difficult part of spaceflight for us right now.
05:36For example, if we want to send a rocket to Mars, half the energy is necessary just to get into orbit,
05:42and the other half for the 55 million kilometers to Mars.
05:46Therefore, to be as effective as possible, rockets aren't built in one giant piece.
05:51Instead, we use multistage rockets.
05:53You don't need to carry an empty fuel tank, so rockets drop it.
05:57Rockets today shed their boosters and main stage as they ascend,
06:01with each successive stage being its own fully contained rocket, complete with its own engine and fuel.
06:07Okay, so this is why getting to space is hard.
06:11If you feel all of this seems really complicated, don't worry.
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