Sabrina: Hi there. Sabrina here, just hanging out with Mr. Moon. If you're wondering why he looks a little... green, it's because he's a little jealous. We've spent an awful lot of time talking about the Sun and what a star is and very little time on this guy. So cheer up, Mr. Moon, today's episode is all about you.

(Intro)

To be more specific, today's episode is all about orbits. Lots of things in space are in orbit, but let's just start by talking about things that orbit the Earth, and those are two things in particular: artificial satellites, which are made by people, and the Moon.

You should remember that an orbit is the path that an object follows around another object, like how the Earth travels around the Sun. The Moon orbits the Earth in a similar way. You should also remember a little thing called gravity. Gravity is a force or the natural pull of objects towards each other. It's the thing that keeps you on Earth.

But doesn't that make you wonder why gravity doesn't just pull things out of their orbit? I mean, if gravity pulls things toward each other, then why don't things that orbit the Earth crash into the Earth?

(Big Question)

Before we answer that question, we should learn a little bit more about the things that orbit our planet and the paths they take around it. Earth has only one Moon, I'm sure you've seen it. It's covered by huge craters that were formed by asteroids hitting it, and great plains that were likely shaped by the lava that once flowed on its surface.

We think the Moon formed billions of years ago when chunks of the Earth were blasted into space after a planet-sized object hit it. These pieces of Earth then came together to form our moon. Neat, right?

Now it takes the moon about 27 days to fully orbit the Earth. The Moon's not the only thing to orbit our planet, but it is Earth's only natural satellite. A satellite is just a body that orbits a planet, planet-like object, or an asteroid. Plenty of human-mad, or artificial satellites orbit our planet, too.

Probably the most famous artificial satellite is the International Space Station, or ISS. The space station is the biggest object ever flown in space. Lots of countries, including the United States, Russia, Japan, and Canada, built this satellite where astronauts live and perform scientific experiments in near-weightless conditions. The ISS even has a gym on it.

The ISS moves pretty fast, completing 16 orbits around our planet every day, and it's not the only artificial satellite up there. There are thousands of different satellites traveling around Earth. Some of them help us forecast weather, some monitor areas of drought, and some send signals to our phones and TVs. So, thanks for the SpongeBob, satellites.

Anyway, if all of these things are traveling around our planet, and Earth's gravity is constantly pulling things toward it, then why isn't the Earth's surface full of fallen satellites, or chinks of moon for that matter? I think a visual would really help here. Let's have our moon buddy show us exactly how he stays in space.

(Investigation)

Welcome back, Mr. Moon! Two things determine the orbit of a satellite, whether it's an artificial satellite like the ISS or if it's natural like the Moon. The first factor is the gravitational pull of the Earth, which pulls things towards the center of the Earth. The second factor is the speed and direction, or the velocity at which the object is moving.

If these forces are balanced, then the satellite remains in stable orbit around the Earth. This delicate balance is super important. Why? Well, the balance between an object's velocity and the Earth's gravitational pull is the cause that produces the effect of the object remaining in orbit.

If the Moon's speed was much faster, it would escape the Earth's gravity and just fly off into space, but if the Moon's speed was much slower, the Earth's gravity would be able to pull it down to Earth.

And the orbits of artificial satellites work the same way, even though they're located closer to the Earth than the Moon. As long as they move fast enough, Earth's gravity won't be strong enough to pull those satellites down to Earth.

(Conclusion)

So, technically, the Moon and other satellites are always falling towards the Earth, but they're also moving so fast that they don't hit it. Now you know why things that orbit the Earth don't crash into the Earth. If the Moon and all those artificial satellites maintain a certain velocity - not too slow and not too fast - they'll avoid losing their battle against our planet's gravitational pull.

Very impressive, Mr. Moon! Take a bow.