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View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
If things in space are falling, where are they falling towards and why are they falling in different directions?
Comment by bluesbrother21 at 13/09/2023 at 21:26 UTC
1 upvotes, 0 direct replies
"Falling" might not be the best term to use for building intuition. It's technically accurate (if we interpret "falling" to mean under constant unopposed gravitational acceleration), but may lead to some incorrect interpretations of what's really happening.
Everything in space is being accelerated by the gravity associated with other massive objects. In practice, this generally means something very large (e.g., a star or a black hole) has other things orbiting around it (e.g., planets or stars). Let's use Earth orbit as a useful example; Something like the ISS is "falling" because it's constantly being pulled towards the Earth with nothing opposing it. The reason the ISS stays in orbit is because it's moving so fast sideways that the Earth curves out under it to match the curvature of the ISS trajectory. Different orbits are all being accelerated by Earth's gravity, but had different initial states that let them travel in different ways.
The "different directions" thing is also addressed by the existence of other gravitating bodies. Something near the Moon, for example, is still being accelerated by the Earth but is being accelerated more so by the Moon, enough so that it travels in a captured Lunar orbit. Gravity scales linearly with mass, but scales inversely with the relative distance squared. This means that bigger objects exert more gravity, but closer objects exert **much** more gravity. This is why things can orbit the Earth, as opposed to exclusively being affected by the Sun or the black hole in the center of the Milky Way.
Comment by Rayleigh_The_Fox at 13/09/2023 at 21:45 UTC
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"Free falling" means an object is not being affected by any other forces besides gravity. When an object is in motion, it will move with constant speed in a straight line unless it is acted on by a force. In space, objects "want" to go straight, but the gravity of nearby objects can pull them away from a straight path.
That exact path depends on the strength of the gravity and the velocity of the object. In space, that path can go a very long distance before an object crashes into something. Sometimes the path loops around a planet or star many times, and we call that an orbit. On a bigger scale, objects like stars and galaxies can move in any direction, and when objects get close together their paths will be curved by their mutual gravitational attraction, then they will keep flying away on their new paths.
tl:dr - In space, things fall in different directions because there are many sources of gravity pulling in different directions.