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As I try to comprehend how spacecraft are able to stay in orbit around the Earth I began to think about why the Earth's own orbit around the Sun does not influence spacecraft in Earth's orbit.

I came to the conclusion that the Earth's gravity holds spacecraft in place and everything in orbit around the Earth also orbits together with the Earth around the Sun, orbiting spacecraft are therefore not influenced by the 30km/s orbital speed of the Earth.

In my naivety I imagined this to be similar to a train journey, the passengers inside the train are not aware of the trains speed. If I go for a walk along the train my velocity is measured relative to other stationary objects in the train, i.e. a few km/h whilst I am in fact travelling at 100km/h+. I am held in place inside the train by the physical boundaries of the coach I am in. If I was to step out of the train whilst it was moving I would then certainly realize it's true velocity and it would speed off into the distance away from me.

In terms of my train analogy this means that the ISS for example is orbiting at 7km/s around the Earth and also orbiting at 30km/s with the Earth around the Sun and the ISS crew and spacecraft (similar to us on Earth) are not aware of the extra velocity?

My thoughts lead to another (possible very naive) question - with the Earth and everything around it moving so fast in its orbit is it possible to travel far enough away from the Earth or the Solar system - to step out of the train so to speak - such that one would observe the Earth speeding away on its 30kms/s orbit?

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  • $\begingroup$ This is an interesting question! I removed "speed" from the title because there may be answers about orbital acceleration that you also find interesting. $\endgroup$
    – uhoh
    Sep 3, 2020 at 14:00
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    $\begingroup$ If you were to step off the train, you would at first travel at the same speed, but the air would start to slow you, and gravity would pull you down, and then the ground will do a much more thorough job of slowing you than the air did $\endgroup$
    – user20636
    Sep 3, 2020 at 14:31
  • $\begingroup$ This sounds like it may receive the best answers on the physics stack exchange. $\endgroup$ Sep 3, 2020 at 14:31

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Your thoughts are not really naive, they are quite good for own ideas. In deed there is just a small adjustment you have to make:

The ground around the train is the sun.

What happens when you jump out of a train? First you are "in the air", falling down. That is exactly what happens in interplanetary space. You are falling around sun.

What happens when you are on the ground? That would be exactly as if (theoretically) standing on suns "surface". In both cases you see the train/Earth passing your position.

The main difference between your model and reality are physics in interplanetary space/"in the air": In your model two main forces will affect you: -drag, slowing you down and gravity, pulling you down. In reality you would not have drag, but gravity. But now gravity will not lasch you down but will keep you in in orbit around sun. So leaving earths sphere of influence you would end up in in orbit next to earth. You would not see earth passing, because you would still move in the same direction with the same speed.

Technically it is not so easy to leave earths sphere of influence: you need to speed up in one direction till you are far away to sun influences you more than earth (very simplified). The speed you needed for this, would end up making you faster than earth or slower than earth (speeding up in reverse direction). So you would see earth passing, but just because you have a different speed than earth.

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  • $\begingroup$ your answer was very useful, thanks! $\endgroup$
    – paj
    Sep 4, 2020 at 9:13

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