Orbit is about speed, not height
It is natural to think that reaching space is a matter of going high enough. Altitude matters, but it is only part of the story. The edge of space is conventionally placed at around 100 kilometers above the surface, and a rocket can reach that height and simply fall back down. A suborbital flight does exactly that.
To stay up, a spacecraft must also move sideways very fast. For a low orbit a few hundred kilometers up, the required speed is roughly 7.8 kilometers per second, which is about 28,000 kilometers per hour or 17,500 miles per hour. Most of a rocket’s energy goes into gaining this horizontal speed, not into climbing.
Newton’s cannonball
Isaac Newton offered a famous thought experiment. Imagine a cannon on a very tall mountain firing horizontally. A slow shot lands nearby. A faster shot lands farther away. If the ball were fast enough, Earth’s surface would curve away beneath it as quickly as it fell, and it would never land. It would be in orbit.
That is what an orbit is: continuous falling that keeps missing the ground. The idea ties into the theory of gravity, which explains why the ball keeps curving toward Earth. A satellite is not free of gravity; at the altitude of the International Space Station, gravity is still roughly 90 percent as strong as on the ground. Astronauts feel weightless because they and the station are falling together.
How thrust works
A rocket engine burns fuel and an oxidizer, and expels hot gas out of the back at very high speed. The gas is pushed backward, and the rocket is pushed forward by an equal and opposite force. This is Newton’s third law, one of the laws of motion. Unlike a jet engine, a rocket carries its own oxidizer, so it works in the vacuum of space where there is no air.
To lift off, the thrust must be larger than the rocket’s weight. A rocket on the pad is typically mostly fuel and oxidizer by mass, which is why it weighs so much. As it burns propellant, it becomes lighter and accelerates faster, even if the engine thrust stays the same.
The tyranny of fuel
There is a catch. To gain more speed, a rocket must carry more fuel, but extra fuel adds weight that needs even more fuel to lift. This relationship, described by the rocket equation, means the required amount of propellant grows rapidly as the target speed increases. Engineers have to balance engine efficiency, structure weight and payload. A launch vehicle for a satellite is mostly fuel tank, with the payload a small fraction of the liftoff mass.
Why rockets use stages
Staging is the solution to the fuel problem. Once a section’s fuel is spent, its empty tanks and engines are dead weight. Dropping them means the remaining rocket has less mass to accelerate.
- First stage. The large bottom section provides most of the thrust to get through the thick lower atmosphere and builds up speed. It burns for a few minutes and is then released.
- Upper stage(s). Smaller engines, designed to work well in near-vacuum, continue to build speed and altitude.
- Payload. The satellite or crewed capsule sits on top, often protected by a fairing that is discarded once the air is thin enough.
Some rockets also use strap-on boosters, which add thrust at the start and drop away early. In recent years, some first stages have been designed to fly back and land so they can be reused, which can reduce cost. The exact economics depend on how often and how reliably they are reused.
What a launch looks like
A rocket rises nearly straight up at first, to clear the pad and get through the densest air quickly. Soon after, it begins to pitch over and travel more and more horizontally, in what is called a gravity turn. By the time the upper stage takes over, the rocket is moving almost parallel to the ground. That is why launch trajectories curve over the ocean instead of going straight up.
Many launches head east, in the direction Earth rotates. Earth’s surface moves eastward at roughly 465 meters per second at the equator, so an eastward launch gets a small free boost. Sites closer to the equator benefit more.
The final push
When the upper stage reaches the target altitude and speed, its engine shuts off. The spacecraft is now coasting along an orbit. Often the first orbit is slightly elliptical, and a later short burn at the highest point raises the lowest point and makes the path circular. Orbits are changed by burns: speeding up at one point raises the orbit on the opposite side, and slowing down lowers it. This can seem backward at first, but it follows from the physics of falling around a planet.
At an altitude of a few hundred kilometers, one lap around Earth takes about 90 minutes. Higher orbits are slower and take longer. A satellite in geostationary orbit, about 35,800 kilometers up, circles once every 24 hours and so seems to hover over one spot, which suits communication and weather satellites.
Getting beyond Earth orbit
To leave Earth entirely, a spacecraft needs escape velocity, about 11.2 kilometers per second at the surface, ignoring air resistance. In practice, probes are usually sent into orbit first and then given another burn. Trips to other planets are timed for launch windows, when planets are well positioned, and use carefully planned paths to save fuel.
Coming back down
Returning from orbit is the opposite problem: all that speed must be shed. Most of it is lost by slamming into the atmosphere, which heats the front of the craft intensely, so reentry vehicles carry heat shields. Parachutes, wings or engines handle the final slowing. Heat from reentry comes from compression and friction of the air, not simply from friction alone.
A quick recap of the key numbers
Rough figures help build intuition: about 100 kilometers to reach the conventional edge of space, about 7.8 kilometers per second to stay in low orbit, and about 90 minutes per lap at that height. These are approximate and vary with the exact orbit. Together they explain why the hard part of spaceflight is not going up but going sideways fast, and why the rocket you see on a launch pad is mostly fuel. The same orbital mechanics also keep the satellites used for GPS navigation in place overhead.