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Now I get what microgravity is, how spacecrafts (even moon) fall around the earth to keep up themselves in their orbits and other related concepts after reading this.

The spacecrafts at certain heights will have to travel at as much as 17,500 miles per hour to stay in their orbits. If they want to orbit at any lower altitudes, they may have to travel faster and at higher altitudes, they can afford to travel slower.

When on EVA's, the astronauts will have to have their jetpacks/MMUs/thrusters on to stay afloat, closer to the spacecraft and do what they wanted to do.

While all these things are understandable, objects outside of the spacecraft floating around is not so much. Shouldn't the objects, right after they are out of the spacecraft, decelerate and so enter the earth's atmosphere within seconds (knowing micro-gravity is still enough gravity to pull objects towards earth)?

Obviously some of these observations are based on the movie: Gravity. The dead bodies floating around, one of the astronauts (Sandra) with no thrusters on drifts off of the spacecraft due to momentum - I can't seem to get my head around these things. Do these objects travel at enough speed to fall around the earth, despite being out of the spacecraft which travels at a constant speed? Even though it's the vacuum, why does the momentum have so much effect? Surely gravitional force should be strong enough to pull objects towards earth almost as easily at those altitudes despite apparent lack of air resistance?

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    $\begingroup$ "Shouldn't the objects, right after they are out of the spacecraft, decelerate and so enter the earth's atmosphere within seconds". Why? What interaction would cause them to decelerate so aggressively? Assuming two objects; one just inside the spacecraft and one just outside. What would cause the object just outside to behave significantly differently from the one inside such that the orbit of the one outside rapidly decays? You do understand that the spacecraft isn't in powered flight when in orbit, correct? It's in free-fall, like the objects inside and outside, around the Earth. $\endgroup$ – Alfred Centauri Jul 1 '14 at 0:54
  • $\begingroup$ "knowing micro-gravity is still enough gravity to pull objects towards earth": I might be misreading, but is your understanding that microgravity pulls things out of orbit? Because "microgravity" is just a synonym for "zero-g" or "weightless," and an orbit by definition is the path you follow accounting for the force of gravity on you. $\endgroup$ – user10851 Jul 1 '14 at 0:57
  • $\begingroup$ @AlfredCentauri, You do understand that the spacecraft isn't in powered flight when in orbit, correct? It's in free-fall, like the objects inside and outside, around the Earth. Probably that's what I misunderstand I believe. My understanding was that spacecrafts are put in orbit by the rockets to travel in certain speeds that would just about counter the gravitational pull. Seems like I was wrong. Unless an object is continuously powering to keep up the speed required to keep it in its orbit, how does it even travel? Just leaving an object at altitude automatically means it will free-fall? $\endgroup$ – mystarrocks Jul 1 '14 at 3:04
  • $\begingroup$ @ChrisWhite, I see micro-gravity/zero-g as the same gravitational force that the earth (in our case) exerts on bodies, just lesser in magnitude than when objects are a lot closer to the earth's surface. At least that's what the link in my question seems to describe. $\endgroup$ – mystarrocks Jul 1 '14 at 3:06
  • $\begingroup$ @mystarrocks, once the spacecraft is effectively above the atmosphere, there is effectively no drag to slow it down. The trick is to get the velocity parallel to the Earth's surface to approximately 17,500 mph and then simply coast 'round the Earth. Of course, the gravity of the Earth causes the spacecraft to fall towards the center of the Earth but, due to the speed of the spacecraft, the result is an elliptical orbit that never intersects the atmosphere or surface of the Earth. In essence, the Earth's surface 'curves away' at the same rate as the spacecraft falls towards it. $\endgroup$ – Alfred Centauri Jul 1 '14 at 3:19
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Objects exiting the spacecraft have inertia - they will continue to move at the same speed as the spacecraft. There is nothing to slow them down enough to drop them out of orbit, so they remain orbiting the earth.

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  • $\begingroup$ So, you mean re-entering the earth after a space-flight requires special measures than just letting the speed decay by decelerating and thereby entering the earth's atmosphere? Is this what they call firing the retro rockets? $\endgroup$ – mystarrocks Jul 1 '14 at 3:09
  • $\begingroup$ @mystarrocks, by firing the rockets opposite the direction of travel, the lowest point of the new orbit intersects the atmosphere and, at that point, the atmosphere starts decelerating the spacecraft resulting in a relatively rapid deceleration and return to the Earth's surface. $\endgroup$ – Alfred Centauri Jul 1 '14 at 3:22
  • $\begingroup$ Ok, so an object in free-fall at that altitude can almost never re-enter the earth's atmosphere (it cannot just decelerate by automatic means at all, so simple speed decay / mere deceleration is out of question) without firing the rockets opposite to the direction of travel, then. Got it! Can upvote a comment yet, sorry! $\endgroup$ – mystarrocks Jul 1 '14 at 3:31

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