The explanations of the moon rotating while always keeping one side facing earth are still perplexing to me, but I hadn’t thought about how fast it’s actually orbiting us. I know the Apollo rockets traveled 24,000 MPH, but how were the astronauts able to safely land the lunar module on a body moving so fast? The lunar module wouldn’t have been able to slowly descend to the surface, it would have to race to catch it. There’s no air resistance to make astronauts or moon dust fly off, but wouldn’t there still be an insane amount of g-force at such high speeds?
In: Physics
G-force isn’t about speed, it’s about how quickly your velocity is changing. Speed isn’t absolute, it’s just how fast the distance between two things is changing. If you move at the same speed as the Moon, in the same direction, you can land as if the Moon were standing still—because in your frame of reference, it *is* standing still.
Relativity. If two objects are moving at the same speed no matter how fast then to that object relatively speaking the other object appears to not move
Imagine two sky divers jump out of a plane 5 seconds apart from each other. The first skydiver spreads their arms out, and is falling at 120 MPH. The second skydiver points his head down and is falling at 150 MPH. So relative to the first skydiver he’s now approaching at 30 mph. He can catch up, slow down by spreading his arms out, and the two can now match their speeds and shake hands and play catch mid air. They’re both falling at 120MPH relative to the earth, but relative to each other they’re essentially not moving.
This is kind of how a spacecraft speeds up, catches up to a celestial body, and then lands on it safely. The first skydiver is the moon, the second is the space shuttle, and only the relative difference in speed between them has to be compensated for in order to land safely.
Same way you can walk to the bathroom on an airplane going 500 mph.
The orbiter is placed in an orbit around the earth that matches the moon. All objects in the same orbit travel at the same speed, so once it is there the moon is practically standing still. Then the orbiter changes its trajectory slightly so it is orbiting the moon. After that, it is “just” a matter of slowing your velocity, which causes your orbit to shrink until you reach the surface.
OP, you are (presumably) on Earth, and our planet is spinning on its axis at about 1,000mph, and the spinning globe is zooming around the sun at 67,000 mph – so how are you able to safely walk around, sit down etc? If you take a flight, how can the aircraft possibly land on airfield that’s moving so fast?
The answer is that you are gravitationally coupled to the Earth, so as it spins, you move with it, so you don’t experience that spin at all. So is the atmosphere, so is that plane you flew in.
When the astronauts visited the moon, they crossed over the tipping point between Earth gravity dominating what happened to them into the domain where lunar gravity dominates, and the same effects that keep you seated at your monitor rather than thrown sideways into your wall at 460m/s due to the spin alone kicked in.
The way orbit work is that for a specific altitude you need a specific speed. So if you wanted to orbit at the ground level on earth you would need to travel at 17,000mph. At low earth orbit around 1,200 miles up you can go at around 14,000mph. At geostationary orbit which is around 22,000 miles up you can go at around 7,000mph. And when you reach the orbit of the moon which is around 220,000 miles up you can go at 2,288mph.
So as you can see it’s a long way to go during which the astronaut are slowly decelerating. The Apollo Rocket only go at 24,000 MPH to reach a stable orbit. After that it slowly go up, while decelerating to keep their orbit stable as they slowly get closer to the moon. It took about 4 days for the astronaut to reach the moon from the launch, so the amount of G force is not really high, since the deceleration is spread across so much time. By the time they reached the orbit of the moon the astronaut are going at the same speed as the moon, so they are nearly stationary compare to it, making it relatively easy to land.
G forces have to do with acceleration. Not speed.
If you were to accelerate from 10 mph to 20 mph (so really slow to still really slow) in 0.0001 seconds you would die.
Speed is also relative. All the module would need to do is match the speed the moon is rotating and then, thanks to the lack of atmosphere causing it to slow down. It’s basically not moving all relative to surface of the moon.
I’m not entirely sure *what* they did do. But the moons speed is basically a none issue as far as being able to land on it.
Relative speed.
The rockets had to do that speed to leave Earth’s gravity. Once out of it, they could slow down. But what matter is relative speed – relative to the Earth, they would have been going as fast as the Moon. Relative to the Moon, they would hardly be moving at all.
And they slowed *lots* and orbited (I think) before they landed, they didn’t just aim at the Moon at 24000mph.
They launched up, at a point where they were behind the Moon (I believe), they then aim slightly to the side of the Moon, its gravity pulls them round and into an orbit, and they thus use the Moon itself to slow themselves down. Once the speed (actually velocity) relative to the Moon’s orbit / rotation is acceptable, then you can start to descend.
Air resistance plays absolutely not part.
G-Force is how fast you change the speed. So long as you don’t go from 24000mph to nothing instantly, you can do it well within G-forces that you experience on Earth, for example (60mph to nothing in 2.7 seconds is 1g, for example – in half that time is more g). And on launch they went from nothing to 24000mph in a matter of minutes. What makes you think it takes more the other way round?
Speed is a nonsense terms unless it’s relative to something else. Measuring space speeds relative to Earth is dumb. You measure the speed relative to where you are, where you came from, or your destination, depending on which part of the journey you are on. In that respect, the Earth is rotating at over 1000mph, going round the sun at 30 kilometers per second, etc. Those speeds mean nothing, its only the difference between *your* speed and the speed of the thing you’re aiming for that matter. And you can reduce that to virtually zero as you approach them by matching speeds and entering an orbit, so that you can touch down safely.
They flew in their rocket ship until is was going as fast as the Moon. When they were going the same speed as the Moon, they simply walked down a ladder. It’s like transferring from a moving helicopter to the deck of a moving ship. If they’re both going the same speed in the same direction, the relative motion is zero. Your foot doesn’t care how fast the water under the ship or the vacuum around the Moon is moving. (There is no absolute reference frame, the Solar System itself is moving at a high speed, but we’re all moving at the same high speed.)
> The lunar module wouldn’t have been able to slowly descend to the surface, it would have to race to catch it
The Earth is orbiting the Sun at almost 70000mph. That doesn’t make it difficult for planes to land.
Yes, the Apollo spacecraft would have to “catch up” with the moon. But the velocity it has to reach to do so is actually a lot less than what’s needed to leave Earth in the first place. As the spacecraft travels from the Earth to the moon, gravity is slowing it down. By the time it arrives at the moon, a relatively small capture burn is all that’s needed to put it in orbit.
Because there is no air on the moon, the spacecraft cannot use parachutes to slow its descent, but it can slow down using the engines in order to make a soft landing.
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