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If I make a rocket and want to reach a point in space above sea level where I don't need to burn fuel to rise, how far is that point? space or zero gravity area?

I am just trying to figure out distance where I can shut my rockets.

Put another way, how much fuel do I need, or how much distance do I need to cover so that I can orbit earth like ISS?

Nathan Tuggy
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Waleed
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    You should change your question to "Is there a point where gravity reaches zero?", or better yet; "At what point can I shut down my rockets?" Currently this question has no answer because gravity does not reach zero! – uhoh Dec 27 '17 at 21:29
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    Related, and can probably answer your question, but isn't exactly a duplicate: https://space.stackexchange.com/questions/7981/why-does-a-spacecraft-use-a-curved-orbit-to-go-to-its-destination-instead-of-tr – Nathan Tuggy Dec 27 '17 at 21:29
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    It's not a place, but a speed called escape velocity. Once you reach that speed, you won't fall back to Earth. Escape velocity is about 11km a second, so you gotta be going pretty fast. – zeta-band Dec 27 '17 at 21:39
  • @zeta-band If you would like to put your spacecraft into Earth orbit, which is precisely what ~99% of all non-suborbital launches do, then you'd better shut of your engines well before you reach escape velocity! – uhoh Dec 27 '17 at 22:47
  • Gravity won't reach zero, so your options are freefall, the sustained-miss freefall known as orbit, or to be at a Lagrange point where the gravitational attraction of the Earth is exactly balanced by that of the Sun or Moon. – Chris Stratton Dec 28 '17 at 04:31
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    In free fall, zero gravity may be just above the ground. – Uwe Dec 28 '17 at 08:18
  • @Uwe indeed, but not for very long :-) – uhoh Dec 28 '17 at 15:01
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    Possible duplicate of Couldn't I escape Earth's gravity traveling only 1 mph (0.45 m/s)? – James Jenkins Dec 28 '17 at 17:30
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    @uhoh using the Fallturm in Bremen, zero gravity is possible for 4.74 seconds during a drop over 110 m. Compared with a low orbit of 400 km, this is just above the ground, but longer than some milliseconds only.
    If the catapult is used, over 9 seconds are possible using the 110 m twice.     – Uwe Dec 28 '17 at 20:17
  • Using free fall in a very deep hole into the Earth, there may be zero gravity several kilometers below the ground. – Uwe Dec 28 '17 at 21:09
  • The question is quite good and should be kept as such (even if marked as duplicate). It shows one more mind path linked to the fact that going to space is more about speed than altitude. – Manu H Dec 28 '17 at 22:21
  • I see no problem with this question. It implies a common misconception about orbital mechanics that an good answer could address. – Erik Dec 29 '17 at 15:16
  • Infinite distance @Waleed – Starship - On Strike Mar 10 '23 at 21:11

1 Answers1

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There is gravity everywhere. It doesn't work so that enough far away from the Earth, there won't be gravity any more.

In Low Earth Orbit (i.e. things orbiting Earth), they are in nearly the same gravitational field as we are. For example, the ISS orbits roughly 400 km above the Earth, which is only 400/6378 $\sim$ 6.3 % farther from the center of the Earth than the surface. Since gravity scales as $1/r^2$ that means at that altitude, gravity is still about 88% as strong as on the surface.

The people of the ISS are in weightlessness because they are also flying 7.8km/s speed (around 28000 km/h). This results in a centripetal force which compensates the gravity of the Earth. This is why the ISS doesn't fall down.

Farther away from the Earth, the gravity of the Moon, or other planets, or the Sun would affect more. These are typically far lesser effects as we have on the Earth, but they still exist. Planning the trajectories of space probes, all of them should be calculated.

Thus, if you simply fly upward and shut down the engines, you will fall down. If you fly faster than 11.2km/s, then your spaceship will leave Earth and never come back. If you want to orbit Earth, you have to leave the atmosphere (around 100km height) and accelerate to the speed of at least 7.8km/s sideways around the Earth (because there is a little air even there, 300km is more practical).

Nathan Tuggy
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peterh
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    ISS altitude is pretty close to 400km, not 300km; the effect of gravity at ISS altitude is ~88.5% of that at sea level. – Russell Borogove Dec 27 '17 at 23:50
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    @RussellBorogove ISS fixed, 88.5% imho matches well "nearly the same". – peterh Dec 28 '17 at 01:07
  • Agreed, I just wanted to quantify it as long as I was here. – Russell Borogove Dec 28 '17 at 01:09
  • @peterh the Earth's radius is not 6640km!! I'll make a small edit. – uhoh Dec 28 '17 at 04:02
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    I suggest adding the word 'sideways' to the last paragraph to emphasize that you need to accelerate orthogonal to "up". – Roman Reiner Dec 28 '17 at 12:20
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    To put it another way, in case they don't know what "centripetal force" means: The ISS is falling towards the Earth, but it is moving sideways so fast that it keeps missing. – MJ713 Dec 28 '17 at 19:54
  • Any answer to this fundamental question that uses the notion of "centripetal force" (which is non-existant) should be downvoted. – Erik Dec 29 '17 at 14:16
  • @Erik Why? I think Newtonian Mechanics is okay in this scenario. – peterh Dec 29 '17 at 14:32
  • There is no centripetal force in Newtonian mechanics. – Erik Dec 29 '17 at 14:34
  • @Erik May I cite this: "Isaac Newton described it as "a force by which bodies are drawn or impelled, or in any way tend, towards a point as to a centre". In Newtonian mechanics, gravity provides the centripetal force responsible for astronomical orbits." – peterh Dec 29 '17 at 14:37
  • There is no physical force countering gravity for orbiting bodies. – Erik Dec 29 '17 at 14:39
  • @Erik In the GR. But we are now in Newton. What is the problem with this wiki post? – peterh Dec 29 '17 at 14:40
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    "This results in a centripetal force which compensates the gravity of the Earth." is a false statement. – Erik Dec 29 '17 at 15:09
  • @Erik I think it is a simplified abstract which matches the knowledge level of the OP (and the future readers on similar knowledge level). I can't consider it so bad. This is how it is taught in the elementary schools even today, and it is being done probably on the same reason. This is why it wasn't edited out from the Wiki long ago. It would be possible to explain it by that the ISS is in continuous free fall, but it would overcomplicate the answer. – peterh Dec 29 '17 at 16:33
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    The centripetal force is gravity It acts towards the centre. You mean "Centrifugal force", the term in a rotating frame of reference that balances the centripetal force of gravity – James K Jan 01 '18 at 19:07