Rocket science is hard, everybody knows that. But what is it about putting stuff into space that’s so hard? The answer is complicated, but the short answer is escape velocity.
Think about throwing a ball straight into the air. As it gets higher in the air it moves slower and slower as the force of Earth’s gravity pulls it back down until it succumbs and starts to fall back to Earth again. Gravity does start to get weaker as you get further away from Earth, but that drop in gravitational pull is not nearly as much as the drop in speed of the ball, so a ball thrown by a normal human can’t escape the Earth’s gravity well.
To get to space you need to be moving at a high enough speed that the decrease in your speed is less than the decrease in the Earth’s gravity, and the speed necessary to do this is very easy to figure out with a simple equation. Just multiply the mass of the Earth by 2, multiply again by the gravitational constant, divide by the radius of the Earth (i.e. you get a head start the further away you are from the center), then take the square root of that. So let’s do that.
And the answer is 23,000 miles per hour, or almost 7 miles per second. That’s really really fast. How on Earth do you get something to move that fast? To put it simply, a very large, controlled explosion. In other words a rocket engine.
As you can imagine this is difficult to do. You need to not only design the engine so that it can withstand the tremendous forces being applied to it, but you need to carefully control the chemical reaction so that the entire fuel supply doesn’t explode all at once. On top of that you begin to run into a weight problem. The faster you need to go the more fuel you’ll need, but adding more fuel makes the rocket heavier, requiring more fuel to make it go, with more fuel making the rocket heavier, requiring more fuel to make it go, with more fuel making the rocket heavier, you get the idea.
Of course all of that just gets a thing into space. If you want to put that thing into orbit (like if it’s a satellite) or put a person on the Moon and get them back alive then getting up to escape velocity is just the beginning of your problems.