What is Orbit?
Flying is learning how to throw yourself at the ground and miss
— Douglas Adams
Even if you’ve never followed the space program, or know anything about rockets, you probably know what “orbit” is. I mean, all the time, the news reports when a spacecraft “reaches orbit”, and the Space Shuttle was often known as the “orbiter”. So you know, orbit is “up there”, and you circle a planet or moon or something. But have you ever thought about what it takes to get into orbit?
Lets imagine you are me — oh, I don’t know, 10 years ago, before I ever started looking into the specifics of how space stuff works. “Self”, I might ask you, “How do you get to space?”
The answer is clearly to go up. I didn’t know it 10 years ago, but “space” is basically anywhere with an altitude greater than 100km. So if you go up 100km, you will be in space. Will you then be in orbit? No. As soon as you stop going up, you will come right back down. This is what’s called a “ballistic trajectory”, and that’s not how you get to orbit. It doesn’t actually matter how far up you go, you’ll either come back down, or you’ll go away forever*. That’s not orbit.
Instead of going up, what happens if we go over? If you throw a baseball, what happens? It falls to the ground. But the Earth is round, and if you could throw the ball a thousand miles, when it eventually hit the ground, you can check on a globe and see that it would have followed the curve of the Earth. And if you threw it 4,000 miles? It would curve even more.
Because the Earth’s gravity is always trying to pull the baseball down, the baseball always curves toward the center of the Earth (well, specifically the center of the Earth’s gravity well). But because the baseball has a sideways motion, the ground also constantly curves away as the ball moves. If you can throw it fast enough that the ground curves away just as quickly as the ball goes toward the center, then the ball will never hit the ground. Instead, it will go all the way around and around, thereby completing an orbit.
Fortunately, we live on a planet with an atmosphere, so there’s really no way that you could throw a baseball and get it into orbit — the air would slow it down too much. But we have rockets, and we can use rockets to deliver baseballs. So lets use a rocket to make a baseball achieve orbit.
We have two choices. We could go straight up like we tried before, except this time, because we have a powered rocket, we can turn sideways once we’re out of the atmosphere and apply thrust in the direction we want to go, or we can turn sideways much sooner, and fly diagonally through the atmosphere, picking up speed vertically and horizontally at the same time.
The first one will work if you’ve got a rocket with enough fuel on it, but you end up wasting an awful lot. Whenever you’re just thrusting straight up, you are making no progress going sideways, and in order to get an orbital trajectory, where you can make a full circle without hitting the ground (or, importantly, without coming back into the thicker parts of the atmosphere), you have to be going very fast unless you’re going very, very far away from the planet.
Instead, it almost always ends up being a good idea to go diagonally, though the flight profile (that is, how much thrust you use, when, and what direction you’re going) depends on the orbit you’re trying to reach, the mass of your rocket, and its performance capabilities. This technique is known as a gravity turn, because it uses gravity to help turn the vehicle into the orbit you want it to achieve.
Most of the things we launch are launched “prograde”, that is, they are going in the same direction that the planet spins. On a map, that means they go from West to East. The reason for this is that the planet is turning, and you get a free boost if you go in the same direction as the planet.
There are also times where you might want a “retrograde” orbit, where the orbit will end up going East to West. This is useful if you want your space craft to cover a lot of ground in a little bit of time (because the spacecraft is orbiting against the rotation, it sees the entire ground path in less time than if it were prograde and traveling with the ground). This orbit is harder to achieve because you’re working against the rotational boost of the planet.
A third option is a polar orbit, where the spacecraft orbits north to south, or south to north. You don’t get a rotational boost with this orbit, though your inherent rotation does need to be corrected so that it doesn’t skew your orbit away from the pole. This orbit has the benefit of being able to observe 100% of the body’s surface. Because the vehicle is orbiting from pole to pole as the planetary body turns underneath it, all of the surface winds up under the ground path.
As an aside, a ground path is, literally, the part of the ground under the spacecraft. If you’ve ever seen a picture or movie of Mission Control, there’s usually a map with curved lines tracing the path that the craft will follow. Those curves don’t mean that the craft is in orbit bobbing and weaving back and forth. It means that the craft is orbiting at an angle, called the inclination.
Inclination is almost always measured from the equator of the planetary body, so if a ship is orbiting at a 0% inclination, it means it’s basically tracing the equator. If it has an inclination of 51.65 degrees like the International Space Station, then the orbit the ship is taking is slightly steeper than halfway between a flat orbit on the equator and a flat orbit on the poles.
On a globe, it looks something like this:
and if you trace the path that orbit follows over the ground, it looks like this:
Now, whenever you see a ground track like that in mission control, you’ll usually see a couple of them together. That’s because the planet is constantly turning, and by the time one orbit is done, what was underneath the ship has moved, and now something else is.
The last thing we should talk about is orbital altitudes — that is, the distance from the ground.
It’s important to realize that almost no (and, in fact, probably no) orbits are perfectly round, and a lot of them are very elliptical. Simplistically, an orbit looks like a conic section. That is, if you take a cone, and cut it, the edge of the cone you cut resembles a potential orbit. In reality, irregularities in the mass distribution of the body a ship is orbiting (and in the ship itself) means that the orbit won’t perfectly match the conic section, and that you won’t be able to describe an orbit using a perfect ellipse. But you can get a pretty good idea of how orbits work by thinking about them in terms of ellipses.
The two things that determine a ship’s orbit around any given body are its altitude and its orbital velocity. With a circular orbit, the ship maintains a constant orbital speed (that is, how fast it travels through space) and a constant altitude (that is, distance from the ground). Remember that since this is an orbit, you don’t need to thrust constantly to maintain the orbit. You’re falling down and sideways at the same speed — you just keep going. That’s what orbit is.
- — There are very small possibilities where you could launch straight up, get a gravity assist from something else, then come back and orbit Earth via free return trajectory. But that’s cheating.
