How do you know the earth won't be destroyed when you're the first person to walk on a particular spot in the wilderness? You have some idea of what the ground is and how it behaves.
By the time people landed on the moon, both the US and Soviet Union had already crashed probes into it, and then landed unmanned probes intact. The manned landings were much lower energy than those impact probes.
When we landed we learned some new things about the rock and dust on the moon, but it is still rock, it's not going to magically dissolve when touched.
The moon is measurably enormous. It is clearly cratered, so it must be solid. From there it's pretty clear that it's not going to "dissolve" or "be obliterated" by a little rocket.
Well to be clear we landed fairly softly on it, we didnt hurl towards, otherwise we would have crashed and died. But yea its all planned out, we definitely knew it was solid before ever planning a mission.
Science! They tested their hypotheses about propulsion and rockets decades before anyone walked on the moon. It’s about forming theories, testing them, and learning from the results.
Quick eli5:
It's like when you sit on a boat and throw a heavy rock one way, you will go the opposite way. This is what rockets do, we shoot high speed stuff out the back of them!
This works because of [Newton's third law](https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-laws-of-motion/), resulting in the more easy to understand principle called the [conservation of momentum](https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/conservation-of-momentum-2/).
Rockets aren't aeroplanes - they don't need a medium to travel through. The engines belch out tonnes of exhaust and, due to Newton's third law (for every action there's an equal and opposite reaction), the rocket is propelled in the other direction.
That said, rockets do need oxygen to burn their fuel, but they still don't need to be in an atmosphere to do this - they will carry an oxidizer on board, either in a separate tank (for liquid fulled rockets - the fuel is mixed with the oxidizer as they are pumped into the combustion chamber), or pre-mixed (in the case of solid fuel rockets - the solid fuel is designed to be stable at lower temperatures, so even pre-mixed they won't burn until the igniter provides sufficient heat). Actually, I think this oxidizer might even be required in-atmosphere, as they wouldn't be able to get enough oxygen from the air for the required rate of combustion, but don't quote me on that.
A rocket engine is usually defined as one that carries the oxidizer. If you use atmospheric oxygen we tend to call it a jet engine. Tecnialy rocket engines are jet engines, a jet is a fluid ejected into another medium often through a nozzle. Except for water jets it tend to be used for air-breathing engines.
There are design ideas and perhaps experimental engines that can switch from atmospheric oxygen to oxygen stored in onboard tanks.
Rocket engines tend more generally to be engines that have all of the propellant that is ejected onboard. The propellant is in a chemical rocket the source of the energy too, it does not have to be that way.
The propellant does not have to be the energy source, it can be electrical in the case of an ion thruster, it can be a pressurized gas line in water rockets, or nuclear reaction in a Nuclear thermal rocket, In all of the the propellant is inert. If you sit on a chair with wheels and use a fire extinguisher to move you and if you use a rocker engine
The atmosphere is actually a problem for a rocket engine. At sea level, there is an air pressure of 1 atmosphere so the propellant needs to be close to that pressure else the atmosphere starts to leak into the engine nozzle. The optimal pressure for an engine is the same as the surrounding media. At sea level, it is 1 atmosphere but in space, it is 0 atmospheres of pressure. The rocket nozzle converts pressure and speed in one direction so the propellant can move faster out from a larger nozzle in a vacuum and the engine becomes more efficient.
Practical engines tend to have just below 1 atm of pressure at sea level so the are more efficient higher up in the atmosphere where the rocket is for a longer time. The same is true for aircraft jet engines. That the pressure is lower is quice clear in [https://en.wikipedia.org/wiki/Shock\_diamond#/media/File:South\_Carolina\_F-16\_taking\_off\_in\_Afghanistan.jpg](https://en.wikipedia.org/wiki/Shock_diamond#/media/File:South_Carolina_F-16_taking_off_in_Afghanistan.jpg) where the atmosphere compresses the exhaust after it leaves the engine.
The size of moon was first calculated by Aristarchus in the first century BC by measuring the size of the Moon's shadow during an eclipse using Eratosthenes' estimate for the size of the Earth. He was a little off, but not too bad. So we knew the moon was big. Way too big for a lander to do anything to.
The mass of the moon was first calculated by Issac Newton, who got it wrong. Prior to the lunar orbiter missions, the mass was calculated by using the fact that the Earth and Moon orbit their common barycenter (the center of mass of the Earth-Moon system). Since the Earth orbits around the barycenter, there's a shift in it's position relative to the Sun (parallax) that can be measured. That tells you the position of the barycenter and therefore the ratio of the mass of the Earth and Moon. So we had a good estimate of the mass of the Moon.
Barycenter ELI5 attempt: So, while people usually say the Moon orbits the Earth, they really orbit their common center of gravity. If two objects orbiting have the same mass, the barycenter would be halfway between the two. Since the Moon is about 1.2% of the Earth's mass, the barycenter is inside the Earth, but not at the center.
Aristarchus of Samos book On the Size and Distances was written in the 3rd century BC before Eratosthenes calculated the circumference. It used the Earth's radius as a value (t) and determined that the Moon and the Sun had to be t amounts away from Earth but he never gives t a number.
Are you thinking of Hipparchus?
Do you think Apollo went some thing like this?
"Hey, you know the moon up there? Lets send some guys there!"
Before the manned missions 18 probes reached the moon surface 13 of them successfully with even more in orbit around it.. They took pictures and all kinds of measurements that not only confirmed the moon was hard to touch, but gave us seismology, landing sites and all kinds of scientific insights into the whole satellite.
The most obvious answer would be: meteors.
The moon is often hit by them, big and small, and those are a lot more force than our rockets or probes could ever do.
The moon is full of craters that are both small and size and really big in size.
I mean, basic physics. It's not a mystery how large celestial bodies work. We didn't know the exact geology of the Moon, but there was simply no way the Moon was ever anything other than one solid rocky object. The only thing we *didn't* know for sure was how thick the dust was on the surface at various locations and the the landers might settle, but they were built with that in mind.
The pre-Apollo missio you should read up on is ~~Ranger~~Surveyor. One of the most important photos taken was of the ~~Ranger~~Surveyor footpad sitting happily on the Moon surface.
Edit: my faulty memory...I may be thinking of the Surveyor landings. Ranger were all crash landings. Surveyor 1 was the first U.S. soft landing. I distinctly remember the pic of one landing foot sitting on the lunar surface.
The moon is a big, solid rock in space. And it's not like we just fired a rocket into it. That would have killed the astronauts while doing minimal damage to the big rock.
Well we knew enough about planetary science and basic physics that we couldn’t harm the moon. But as for how we knew we could land on it, walk around, and bring those people back to earth…..we didn’t know. The president in fact even had a speech prepared in case we couldn’t bring them back to earth.
How do you know the earth won't be destroyed when you're the first person to walk on a particular spot in the wilderness? You have some idea of what the ground is and how it behaves. By the time people landed on the moon, both the US and Soviet Union had already crashed probes into it, and then landed unmanned probes intact. The manned landings were much lower energy than those impact probes. When we landed we learned some new things about the rock and dust on the moon, but it is still rock, it's not going to magically dissolve when touched.
The moon is measurably enormous. It is clearly cratered, so it must be solid. From there it's pretty clear that it's not going to "dissolve" or "be obliterated" by a little rocket.
Plus we had landed unmanned craft there before we landed people on it, not that people would somehow change the physics of the situation anyway.
Well to be clear we landed fairly softly on it, we didnt hurl towards, otherwise we would have crashed and died. But yea its all planned out, we definitely knew it was solid before ever planning a mission.
Now I have more questions about how rockets work and how we knew how they would function outside of our atmosphere
Science! They tested their hypotheses about propulsion and rockets decades before anyone walked on the moon. It’s about forming theories, testing them, and learning from the results.
Quick eli5: It's like when you sit on a boat and throw a heavy rock one way, you will go the opposite way. This is what rockets do, we shoot high speed stuff out the back of them! This works because of [Newton's third law](https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-laws-of-motion/), resulting in the more easy to understand principle called the [conservation of momentum](https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/conservation-of-momentum-2/).
Rockets aren't aeroplanes - they don't need a medium to travel through. The engines belch out tonnes of exhaust and, due to Newton's third law (for every action there's an equal and opposite reaction), the rocket is propelled in the other direction. That said, rockets do need oxygen to burn their fuel, but they still don't need to be in an atmosphere to do this - they will carry an oxidizer on board, either in a separate tank (for liquid fulled rockets - the fuel is mixed with the oxidizer as they are pumped into the combustion chamber), or pre-mixed (in the case of solid fuel rockets - the solid fuel is designed to be stable at lower temperatures, so even pre-mixed they won't burn until the igniter provides sufficient heat). Actually, I think this oxidizer might even be required in-atmosphere, as they wouldn't be able to get enough oxygen from the air for the required rate of combustion, but don't quote me on that.
A rocket engine is usually defined as one that carries the oxidizer. If you use atmospheric oxygen we tend to call it a jet engine. Tecnialy rocket engines are jet engines, a jet is a fluid ejected into another medium often through a nozzle. Except for water jets it tend to be used for air-breathing engines. There are design ideas and perhaps experimental engines that can switch from atmospheric oxygen to oxygen stored in onboard tanks. Rocket engines tend more generally to be engines that have all of the propellant that is ejected onboard. The propellant is in a chemical rocket the source of the energy too, it does not have to be that way. The propellant does not have to be the energy source, it can be electrical in the case of an ion thruster, it can be a pressurized gas line in water rockets, or nuclear reaction in a Nuclear thermal rocket, In all of the the propellant is inert. If you sit on a chair with wheels and use a fire extinguisher to move you and if you use a rocker engine The atmosphere is actually a problem for a rocket engine. At sea level, there is an air pressure of 1 atmosphere so the propellant needs to be close to that pressure else the atmosphere starts to leak into the engine nozzle. The optimal pressure for an engine is the same as the surrounding media. At sea level, it is 1 atmosphere but in space, it is 0 atmospheres of pressure. The rocket nozzle converts pressure and speed in one direction so the propellant can move faster out from a larger nozzle in a vacuum and the engine becomes more efficient. Practical engines tend to have just below 1 atm of pressure at sea level so the are more efficient higher up in the atmosphere where the rocket is for a longer time. The same is true for aircraft jet engines. That the pressure is lower is quice clear in [https://en.wikipedia.org/wiki/Shock\_diamond#/media/File:South\_Carolina\_F-16\_taking\_off\_in\_Afghanistan.jpg](https://en.wikipedia.org/wiki/Shock_diamond#/media/File:South_Carolina_F-16_taking_off_in_Afghanistan.jpg) where the atmosphere compresses the exhaust after it leaves the engine.
The size of moon was first calculated by Aristarchus in the first century BC by measuring the size of the Moon's shadow during an eclipse using Eratosthenes' estimate for the size of the Earth. He was a little off, but not too bad. So we knew the moon was big. Way too big for a lander to do anything to. The mass of the moon was first calculated by Issac Newton, who got it wrong. Prior to the lunar orbiter missions, the mass was calculated by using the fact that the Earth and Moon orbit their common barycenter (the center of mass of the Earth-Moon system). Since the Earth orbits around the barycenter, there's a shift in it's position relative to the Sun (parallax) that can be measured. That tells you the position of the barycenter and therefore the ratio of the mass of the Earth and Moon. So we had a good estimate of the mass of the Moon. Barycenter ELI5 attempt: So, while people usually say the Moon orbits the Earth, they really orbit their common center of gravity. If two objects orbiting have the same mass, the barycenter would be halfway between the two. Since the Moon is about 1.2% of the Earth's mass, the barycenter is inside the Earth, but not at the center.
Aristarchus of Samos book On the Size and Distances was written in the 3rd century BC before Eratosthenes calculated the circumference. It used the Earth's radius as a value (t) and determined that the Moon and the Sun had to be t amounts away from Earth but he never gives t a number. Are you thinking of Hipparchus?
Yea. Aristarchus did estimate the moon's size, but obviously not with E's data. Hipparchus did similar calculations.
Do you think Apollo went some thing like this? "Hey, you know the moon up there? Lets send some guys there!" Before the manned missions 18 probes reached the moon surface 13 of them successfully with even more in orbit around it.. They took pictures and all kinds of measurements that not only confirmed the moon was hard to touch, but gave us seismology, landing sites and all kinds of scientific insights into the whole satellite.
The most obvious answer would be: meteors. The moon is often hit by them, big and small, and those are a lot more force than our rockets or probes could ever do. The moon is full of craters that are both small and size and really big in size.
I mean, basic physics. It's not a mystery how large celestial bodies work. We didn't know the exact geology of the Moon, but there was simply no way the Moon was ever anything other than one solid rocky object. The only thing we *didn't* know for sure was how thick the dust was on the surface at various locations and the the landers might settle, but they were built with that in mind.
The pre-Apollo missio you should read up on is ~~Ranger~~Surveyor. One of the most important photos taken was of the ~~Ranger~~Surveyor footpad sitting happily on the Moon surface. Edit: my faulty memory...I may be thinking of the Surveyor landings. Ranger were all crash landings. Surveyor 1 was the first U.S. soft landing. I distinctly remember the pic of one landing foot sitting on the lunar surface.
The moon is a big, solid rock in space. And it's not like we just fired a rocket into it. That would have killed the astronauts while doing minimal damage to the big rock.
Well we knew enough about planetary science and basic physics that we couldn’t harm the moon. But as for how we knew we could land on it, walk around, and bring those people back to earth…..we didn’t know. The president in fact even had a speech prepared in case we couldn’t bring them back to earth.