How a rocket gets into orbit
Height gets a spacecraft above the air. What keeps it there is sideways speed, and building that is most of what a launch is doing.
When you watch a rocket launch, the most obvious part is the climb. The rocket lifts off, rises above the launchpad and disappears into the sky. It’s natural to imagine that reaching orbit means getting high enough.
Height is part of the journey. A spacecraft needs to get above most of the atmosphere so that air resistance won’t quickly slow it down. But it also needs a great deal of sideways speed. That’s what allows it to keep falling around Earth.
Turning as it climbs
A rocket begins by rising almost vertically. Soon after launch, it starts to tilt, and its path gradually curves toward the horizon. As it turns, the thrust from its main engines builds sideways speed while it continues to climb.
You can picture the rocket’s thrust as a push along its length. When its nose points upward, that push carries it upward. As its nose tilts toward the horizon, more of the push goes into speeding it around Earth. Small changes in the direction of the engine thrust help steer the rocket.
The turn happens over the course of the ascent. By the time the spacecraft is ready to enter a roughly circular orbit, it’s moving almost parallel to the ground far below.
The model has no air in it, so nothing slows the rocket down and nothing burns up on the way back. It also flies as a single stage rather than shedding empty ones. Neither changes the shape of the argument, but both are why the fuel it uses here would not be quite enough for a real launch.
Moving fast enough to miss Earth
Imagine a spacecraft about 400 kilometres above the ground, roughly the height of the International Space Station. If it had no sideways motion and its engines were switched off, it would fall toward Earth.
Give it some sideways speed and it travels forward as it falls. Its path curves downward, taking it farther around the planet before it reaches the atmosphere. With enough speed, the curve of its fall follows the curve of Earth: the ground keeps falling away beneath it.
At this height, a circular orbit requires a speed of about 7.7 kilometres per second—roughly 28,000 kilometres per hour. Gravity keeps turning the spacecraft’s motion toward Earth, but the spacecraft keeps moving forward. It circles the planet without reaching the surface.
The rocket’s job is to establish that combination of height, speed and direction. Once it has done so, the engines can shut off. There’s no special boundary to cross or mechanism that switches on. The spacecraft is already moving along a path that takes it around Earth.
After the engines stop
A spacecraft can coast in orbit because there’s very little resistance to its motion. It needs thrust again to change its orbit, and occasional boosts can replace the speed lost to the thin traces of atmosphere at space-station height. It doesn’t need a continuous push to keep circling.
Reaching the station itself takes further manoeuvres to bring the spacecraft into the right place with the right relative speed. But the essential work of reaching orbit has already happened during the launch: the rocket has given the spacecraft enough sideways motion for its fall to carry it around the world.