# How did we land back on earth from the moon

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Wouldn’t when we land on the moon, the earth would still be in motion bc of the orbit, and so whenever we exit the orbit of the moon (Which was orbiting the earth) the earth would still be in a constant orbit moving at high speeds, how did we calculate and intercept earth in said orbit.

In: Planetary Science

The earth and the moon are taking the same orbit around the sun, so there is not as much difference in speed as you’re thinking.

The Moon orbits the Earth, so it’s not like the Earth is leaving the Moon behind. When a spacecraft lands on the Moon or orbits the Moon, it’s still orbiting the Earth because it’s “attached” to the Moon, which is still orbiting the Earth. All you have to do is burn your engines in the right direction so that your trajectory send you back to Earth.

The moon is orbiting the Earth. As the Earth moves through the solar system, the moon moves with it. Think of it like 2 runners on a track, moving at the same speed. It’s going to be relatively easy to toss a ball back and forth between each other because you’re moving the same way.

It gets harder with other planets, they still mostly are moving the same direction, but your speeds are way different and your distances are much greater. Way more chances for things to go wrong, and being off by .01% doesn’t matter much when you’re moving a foot, but if you’re moving a few hundred thousand miles, that little error will make a big difference.

The math to figure this all out has been around for a long time, the difficulty was more on the engineering and technology side of things. And of course, getting it wrong is pretty disastrous.

The moon is in freefall over the earth, and moving laterally enough that it doesn’t actually get any closer, i.e. – it’s in orbit. In order for something to get to earth from the moon, it needs to lose most of that speed, so that it can fall down to earth. So the rocket orbiting the moon needed to use fuel to thrust in the opposite direction that the moon was traveling in, to shed speed and let earth’s gravity pull it back down.

The Earth is spinning around really fast (once a day on its own axis). And it is zooming around the Sun (about 107,000 km/h). And it and the Sun (and all the other planets) are hurtling around the centre of the Milky Way (around 828,000 km/h).

So if you jump up, why don’t you flying away?

Because you are also moving with the Earth.

Motion is generally relative; it doesn’t really matter how fast you are going, what matters is how fast you are going compared with whatever you care about.

In terms of you and the Earth, even if you are both moving really fast relative to the Sun, or the centre of the galaxy, compared with each other you’re hardly moving at all.

And the same works out for getting back to the Earth from the Moon. Sure, the Earth is moving really fast through space, but the Moon moves with the Earth (rotating around it).

When dealing with space travel mostly you just have to worry about your “local” movement compared with whatever the biggest thing nearby is. So when going between the Earth and the Moon you can mostly treat the Earth as fixed in space (if spinning). At least, once you’ve got far enough away from the Moon that it isn’t affecting you much. Getting from the Moon back to the Earth is in some ways easier than going the other way, as the Earth is bigger so easier to get pulled towards and hit.

I feel like you should probably go play some Kerbal Space Program. Definitely not 100% accurate to earth/moon physics but very precise with the same laws. It sounds like you have some misunderstandings about how orbital mechanics works and rather than trying to tease out what all those are from multiple back and forth questions, you can go play around with it yourself. Lemme know if you need some help paying for it.

The movement between the Earth and Moon isn’t that big of a deal.

The Moon orbits around the Earth at the same speed a satellite the same distance from.earth would need to move.

When in orbit around the moon, you just need to move fast enough to reach the moon’s escape velocity. If you escape the moon’s gravity in the direction away from the way the Moon is currently moving, you get your remaining velocity “subtracted” from the speed of your orbit around the Earth.

The big issue with returning from the Moon is it is MUCH further away than low Earth orbit, so as you are essentially falling from the Moon back to Earth, you gain a lot of speed, so you either need to use fuel to slow down, or hit the atmosphere even harder than a normal reentry.

Imagine you are sitting on a bus traveling at high speed down the highway, holding a ball in your hand. Now toss the ball straight up in the air. Why does the ball fall back down to your lap instead of flying to the back of the bus the instant it leaves your hand? Because it has still the momentum from the moving bus and it doesn’t lose that momentum just because you aren’t holding it anymore.

The astronauts leaving the moon still have the same momentum that the moon has of “falling” around the earth (otherwise known as orbiting), and they don’t lose that momentum just because they left the surface of the moon.

The moon orbits the earth. Mercury and Mars and others orbit it sun. That’s why it takes more to reach Mars. Take off at the wrong time and angle Mars will be on the opposite side of the sun and Mars takes 687 days to get to the same position

My knowledge of this comes from KSP (Kerbal space program) which is a game where you make your own space program, and it has surprisingly realistic orbital mechanics. Search for videos on “how to get from Kerbin to the Mun and back” if you want to see a demonstration of something similar to going from the Earth to the Moon and back in real life. (Although skip the parts irrelevant to you, like rocket building).

Firstly, there is no “dead zone”. You’ll always be dominated by some nearby large mass, whether that’s the moon, the Earth, or the Sun. Most of the time, you’re going to be orbiting something, unless you’re going so fast that you’re just doing a bit of a flyby (but your path is going to be bended by the nearby big mass object anyway).

Imagine we are in orbit around the moon. We have a nice circular orbit at the moment. It is well calculated and well understood. If I start to increase my velocity in my direction of travel (we call this direction “prograde”), which is kind of parallel to the moon’s surface, using my rocket engines, then this orbit is not going to be circular anymore. At this point, it becomes an ellipse.

Due to the fun nature of how orbits work if I increase my velocity (in the prograde direction) at one point in the orbit, then the opposite side of the orbit will start to elongate. Like becoming more oval shaped. This “elliptical” orbit is also well understood. The maths for it nice and predictable. The point where we are currently in the orbit is called the periapsis – the point in the orbit closest to the moon. The opposite side of the orbit is called the apoapsis – the point in the orbit furthest from the moon. Remember these words. So far we are still orbiting the Moon, just with a more elliptical orbit.

To get from the Moon to the Earth, we need to make this orbit more and more elliptical – increase that apoapsis (point furthest from the moon) more and more until the apoapsis is now in the Earth’s sphere of influence, instead of the moon.

We also have to worry about direction here – I can’t just increase my apoapsis at some random point in the orbit and expect it to be okay. I need to make sure that my apoapsis actually meets up with Earth. It is not as simple as making the apoapsis the point that is nearest to Earth at the time I turn my rocket engines on, because as we know the moon is orbiting Earth, so by the time we get to that apoapsis the Earth will be somewhere else. Fortunately, the solution is just to do the whole maneuver later in our orbit around the moon. Like, imagine you are throwing a football to a friend, but that friend is moving in a circle around you. You throw it to where they will be (off to the side) instead of where they are right now (straight ahead). This concept works the same for orbits.

Once we have our elliptical orbit set up where the apoapsis is pretty close to where Earth will be relative to the moon, we simply turn off our engines and wait until we get near to that apoapsis. Depending on the exact specifics, we may now be caught in a new orbit – a highly Elliptical one around Earth, or we may be just doing a bit of a flyby where we bend around the Earth a bit. Regardless, when we get into Earth’s orbit there will be a new periapsis (the point in the orbit closest to Earth). Here, we can do a “retrograde” burn – that means face away from our direction of travel (which would be parallel to Earth) and fire our rockets. This will lower the apoapsis (which is also a new apoapsis – different to the one we had in our orbit around the moon), and make the orbit less elliptical, eventually even near circular if we burn those rockets long enough. From there we can do another retrograde burn until our periapsis (now at the opposite side of our orbit) intersects with Earth’s atmosphere. We can travel along that path and let the atmosphere slow us down, and activate our parachutes, and land close to where we intended.

There are definitely other ways we could do maneuvers to go from the moon to Earth, of varying efficiencies, but this is one of them!