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It is known that the Sun (and thereby all the planets in our Solar System) orbit the center of our galaxy at roughly 1 million miles per hour. From our point of view on Earth, everything looks stationary due to relative motion.

My question is: why don't planets just fly past astronauts when they go into space? In space, the Earth has a very negligible gravitational pull on astronauts, so there's no reason for them to be attracted to Earth and move along with it on its very fast journey around the galaxy.

I considered that anything inside the "field" of orbits of the planets around the Sun would be pulled along, but that doesn't seem to make much sense scientifically.

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  • $\begingroup$ I'm going to award this to best answer below, but if someone provides an even better answer within 23 hours, I may award it to them instead. $\endgroup$ – Inertial Ignorance Mar 30 '17 at 2:59
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why don't planets just fly past astronauts when they go into space?

The astronauts (because the started on the earth) are also flying around the galactic center at the same speed. The other stars in the galaxy are pulling on them in almost exactly the same way as they pull on the nearby sun and earth. So unless there's some other force that is only affecting one of the earth or the astronauts, there's no reason that they would be pulled apart.

In space, the Earth has a very negligible gravitational pull on astronauts

Not sure what you mean by negligible. Gravity pulls on the surface of the earth with an acceleration of about $9.8 \text{m/s}^2$. For astronauts on the ISS about $400\text{km}$ above the surface, the gravitational acceleration from the earth is about $8.7 \text{m/s}^2$ or a bit more than $89\%$ the pull on the ground.

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  • $\begingroup$ Wow, it's that high on the International Space Station. I had assumed Earth's gravity was at a level such as 1% there. Is the reason astronauts continue to move at the same speed around the galaxy due to their inertia of already moving on Earth? $\endgroup$ – Inertial Ignorance Mar 27 '17 at 17:07
  • $\begingroup$ Basically, yes. To move at a different speed would require a differential force (something pushing on them, but not on the nearby planets and suns). There is no such force beyond the rocket itself (which is insufficient for this purpose), so they remain together. $\endgroup$ – BowlOfRed Mar 27 '17 at 20:09
  • $\begingroup$ And I assume the person and the Earth have the same acceleration around the galaxy due to F=ma ? The galaxy exerts far less centripetal force on a person, but since that person has proportionately less mass they are accelerated at the same rate as the planets and Sun? $\endgroup$ – Inertial Ignorance Mar 27 '17 at 21:56
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"In space the Earth has a very negligible pull on astronauts" no, its pull is not negligible. The International Space Station is in orbit around the Earth, as is the Moon. No astronaut has yet been further from the Earth than this.

Any astronaut in this imaginary scenario would have to leave the gravitational influence of the Milky Way in order to see the effects of its rotation, as otherwise they will still be orbiting the centre themselves and hence also moving at a very high relative speed, along with the rest of the matter in the galaxy.

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The simplest explanation is that an astronaut and his spacecraft, being launched from the Earth, possess the same momentum as the Earth: the astronaut derives about 99 percent of his speed and his rotational motion - whilst in orbit - from the Earth, i.e. it is what he started out with: all objects on Earth share the Earth's rotational velocity and rotational direction.

This is modified only very slightly by the thrust of take-off, most of the energy of which is used up in merely transfering him from the surface to his orbital altitude.

The astronaut thus continues to share the momentum of the Earth, hence responds to external gravity in the same way as the Earth. They do not have separate responses. This is partly due to their common velocity and common rotation, but also in part due to the gravitational pull of the Earth on the astronaut. The very strong local gravity of the Earth on the astronaut, so close at hand, completely obliterates the incredibly weak gravity of distant objects: such as other planets of the solar system, but also any bodies even further away (stars, galaxies).

The much stronger local gravity of the Earth causes it and the astronaut to be so strongly bound together that they, in effect, respond to external gravity as if they were a single unit.

The Earth's gravitational effect on the astronaut is so powerful that it is impossible to treat him separately from the Earth, in calculating the incredibly tiny gravitational effects (on them both) of bodies external to the solar system, because those effects are so small that it is in practice impossible to observe any difference between the response to them of the Moon compared to the response of the Earth, never mind trying to observe any difference in the response of an object so much lower in mass than the Earth or Moon, i.e. an astronaut.

Gravitational effects decrease with distance from the mass/object generating the gravity, such that if the distance from that mass is doubled the strength of its gravitational attraction reduces to one-quarter. This means that if the gravity is measured at a distance of 1 million miles, at 2 million miles its strength is only a quarter of that value. At 8 million miles, the strength is only one percent of its strength at 1 million (distance x2 = 100%/4 = 25%) (x4 = 25%/4 = 6.25%) (x8 = 6.25%/4 = 1.56%). With each doubling in distance, the latest value for the field strength (expressed as a percentage of the value at the first measured point) is reduced to a quarter.

Applied to any object, the measured strength of its gravity reduces so quickly that its gravitational attraction at any distance reduces to only one percent of that strength when the distance is multiplied by 8. On that basis, even the closest star (4 light years away) has almost no gravitational influence within our solar system, because the strength of its gravity is only a tiny fraction of 1 percent of the gravity of our Sun, so the Earth and our theoretical astronaut are entirely governed, to better than 99.999 percent, by the gravity of the Sun -- not by that of external objects such as distant stars.

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