SLIM: Orbit


What is an orbit?

An orbit is the curved path of an object around a point in space. While we may not see them, there are lots of things traveling in orbits that affect our daily lives. For example, our planet, Earth, orbits around the sun. The moon has an orbit around our Earth. The satellites that bring us TV and radio signals, as well as the International Space Station all travel in orbits around our planet. But what does it really MEAN to orbit?

Orbits are all about gravity. Great, you may ask, but what exactly IS gravity? Gravity is the attraction between any two things that have mass- you and the planet, the planet and the sun, the sun and the center of our galaxy… all these things have mass and, therefore, gravity. Even the objects on your desk have some tiny level of gravitational attraction between each other. However, it’s the attraction to the MUCH larger mass of the planet that you observer if you, say, knock a pencil off your desk and onto the floor that is observable.

When we talk about orbits, it’s the gravity of the mass of a planet or star that causes other, smaller masses to be attracted to it. How is it, then, that smaller things (like those satellites) don’t just crash into the larger things (like our planet)? That’s what happens when we push a pencil off our desk: it’s attracked to the floor and the huge amount of planet below it. So what’s missing?


The key to orbits is speed. We call the study of speed and orbits “orbital mechanics” which is basically a fancy term for figuring out how to keep things going around and round something in a stable path. If you’re setting up an object in a stable orbit there’s a balance of enough speed to keep you going AROUND your mass, but not so much speed that you fly off into space.

Imagine you throw a baseball as hard as you can. The baseball will travel in a straight line for a while, but eventually it starts to curve downward (there’s our friend gravity again) and ends up in the grass. Now: imagine that you shot that baseball out of a cannon. It would travel a lot farther than before, but again, it would eventually land somewhere on the plant. What if you shot that baseball away from you so fast and it was able to travel so far that when it started to curve downward for a landing the surface of the Earth curved away at the same rate. Essentially, your baseball would be eternally free-falling towards the planet, but missing. THAT is an orbit!

There are other things complicating our example (friction and resistance from air, for instance, not to mention what would happen to a baseball when you put it in a contraption capable of launching it that fast…) but this is essentially what happens when scientists put satellites in orbit in the space around our planet. The rocket provides the oomph and speed to get the satellite into orbit so that it’s in constant free fall around the Earth. The orbit is a tug-of-war between gravity pulling the satellite back to earth and speed causing the satellite to move away from the planet in a straight line.

Falling feels like Floating

Think about the pictures and videos you’ve seen of astronauts in orbit. While they’re not actually weightless, their bodies and movement make it appear that they are. Again, this is because they are travelling VERY quickly and “falling” around the Earth, not because they’re high up and away from the planet. For example, the International Space Station (ISS) (the first module of which was launched in 1998) has to travel over 7,000 meters every SECOND to maintain a stable orbit. If an orbiting satellite were to stop moving, gravity would very quickly remind everybody involved who is in charge and it would begin to fall towards the surface of the planet!