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Author Topic: Why don't satellites gravitate towards the Earth but stay in orbit?  (Read 22298 times)

Ellingson, Kenneth L

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Ellingson, Kenneth L  asked the Naked Scientists:

Why don't satellites gravitate towards the Earth but stay in orbit?

Knute Ellingson, Chicago

What do you think?


 

Offline DoctorBeaver

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The satellite is actually trying to travel in a straight line to conform to Newton's 1st law of motion (an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force). However, the Earth's gravity is, at the same time, pulling the satellite down (the external force in Newton's law).

OK, imagine a firing a gun horizontally. The bullet will eventually fall to Earth. But what if the bullet were travelling fast enough that it passed the horizon before starting to drop? As the bullet fell to Earth, the curvature of the Earth would mean the ground was curving away underneath it.

Now, apply that to a satellite. If the satellite is travelling fast enough, the curvature of the Earth will cause the ground to fall away from beneath the satellite as gravity is pulling it down. This process continues, causing the satellite to orbit the Earth.
 

lyner

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As Douglas Adams said " flying is falling but missing the ground".
 

Offline DoctorBeaver

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As Douglas Adams said " flying is falling but missing the ground".

Very well put.
 

lyner

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It should, of course, be mentioned that, in order to be in orbit, an object must me moving. Otherwise it would fall straight down.
For a circular orbit of a given radius, it has to have just the right speed. If it starts in a circular orbit and you give it an extra kick forward, the orbit will get more and more elliptical - being furthest away from Earth when it's is the opposite side to where you 'boosted' it..
 

Offline graham.d

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I was wondering whether you guys had misread Kenneth's meaning when he says "gravitate towards the Earth but stay in orbit". It may mean "why don't satellites gradually get closer to the earth on a lower and lower orbit", which in fact they do (at least the ones in a low orbit to start with do).

The reason is of course that the earth's atmosphere does not just stop at some point but just gets thinner and thinner. There is sufficient atmosphere at close-in orbits to create some friction with the satallite and slow it down gradually. This has the effect of gradually lowering the orbit of the satellite which, in turn, increases the friction. Eventually the satellite falls to earth, mostly breaking up and burning up, though usually not sufficient to stop some big chunks reaching the ground. Fortunately the chances are still quite small of these hitting anyone, but it is a finite risk nonetheless.
 

lyner

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I was wondering whether you guys had misread Kenneth's meaning when he says "gravitate towards the Earth but stay in orbit".
Quite possibly. We all batted on about our favourite thing, as per usual.
 

Offline chris

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One satellite that isn't gravitating towards the Earth and is in fact moving further away (by about 3.8 cm per year) is the moon, which is being sling-shotted out into space by the Earth. Consequently our last ever eclipse will probably be in about 700 million years...

Chris
 

Offline Flyberius

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One satellite that isn't gravitating towards the Earth and is in fact moving further away (by about 3.8 cm per year) is the moon, which is being sling-shotted out into space by the Earth. Consequently our last ever eclipse will probably be in about 700 million years...

Chris

Something to do with the tides I heard. 

The moon pulls at the earth but slips a little due to the water sloshing around.

One solution they proposed was giant tide stopping dams.

Mind you this was all 3am Discovery channel stuff so they may have pinned it down to just the one thing.

Also, whilst playing the space sim Orbiter, I heard of gravitational torque degrading an orbit. 

What is that exactly?
 

Offline DoctorBeaver

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One satellite that isn't gravitating towards the Earth and is in fact moving further away (by about 3.8 cm per year) is the moon, which is being sling-shotted out into space by the Earth. Consequently our last ever eclipse will probably be in about 700 million years...

Chris

Something to do with the tides I heard. 

The moon pulls at the earth but slips a little due to the water sloshing around.


I've never heard that before. Maybe 1 of our physics whizzkids has come across it.
 

Offline sHiMmY

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Well this is indeed the result of tidal forces. The gravitational force of the Moon "pulls" on Earth's oceans and crust, creating the tides. Earth's gravity also pulls on the Moon, distorting it so that it is slightly egg-shaped. These tidal effects produce bulges on the Moon's surface, and Earth pulls on these bulges a little more than on the surrounding regions because of the concentration of mass there. Earth's constant pull on the Moon's tidal bulge caused its rotation to slow down from its much faster initial spin rate to its current rate of 27.3 days.

Hope this has helped ~sHiMmY
 

lyner

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Whilst slowing Earth up it is transferring energy into the Moon's orbit by dragging it along a bit as Earth rotates. This extra energy hoiks the Moon up into an increasingly high orbit. (barely detectable - you realize)
 

Offline shinzumonk

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The moon and any other satellite of Earth that are either drifting away from, or falling toward the planet are doing so mainly because they are either exceeding or falling slightly short of the required orbital speed based on the size of the satellite, and it's distance relative to Earth.
 

lyner

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The moon and any other satellite of Earth that are either drifting away from, or falling toward the planet are doing so mainly because they are either exceeding or falling slightly short of the required orbital speed based on the size of the satellite, and it's distance relative to Earth.
Actually, that's wrong in two important respects. The size of the satellite doesn't have much bearing on things - unless it is a sizeable proportion of the  planet.
Also, there is no ''required orbital speed". Most orbits (all, in practice) are not circular but elliptical, with various eccentricities. The speed varies from faster, when it is close, to slower when it is further away on its path. Whatever the speed of the satellite and wherever it is, it will follow an orbit, based on its Kinetic and Potential energy (per unit mass of the satellite). The only two things that can disturb the orbit are
1. Energy Loss, due to atmospheric friction (causing the orbit to decay). or
2. Energy transfer effects due to the planet not consisting of spherical shells of constant density (which may cause energy transfer from the planet's rotation to the satellite's orbit - causing it to move away) - the planet needs rotational symmetry to be treated as a true point mass..
 

Offline 112inky

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it's happening The moon and any other satellite of Earth that are either drifting away from, or falling toward the planet are doing so mainly because they are either exceeding or falling slightly short of the required orbital speed based on the size of the satellite, and it's distance relative to Earth
 

lyner

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If you took them to a place with no atmosphere and with no tidal effects, there would be no such thing as a "required velocity" for a particular orbit. Their energy would be constant and nothing would change, whatever the speed was and you would have a stable elliptical orbit.
Where did you get the idea that the size of the satellite came into it? It is only relevant for a very big satellite - and even then, the orbit is just around their mutual centre of mass.
As you say "it's happening" but there is an explanation for it which is outside the simple theory of lossless orbits around point masses.

Just read round about orbits - Keppler had it more or less sorted out hundreds of years ago.
 

Offline Ravish

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If the satellite has to travel at a precise speed in order to fall to earth at the same rate at which the earth's curvature falls away, is it purely coincidental that this speed matches the speed at which earth rotates, thereby keeping satellites in the same position in the sky? Also, does the weight of the satellite and its height above the earth not have an impact on the speed it needs to travel to stay in orbit?
 

lyner

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If the satellite has to travel at a precise speed in order to fall to earth at the same rate at which the earth's curvature falls away, is it purely coincidental that this speed matches the speed at which earth rotates, thereby keeping satellites in the same position in the sky?
You seem to be describing Geostationary Satellites here. Yes, their speed does have to be controlled precisely, in order to keep on station, with an orbit time of 24 hours. But they are in a minority. Left to their own devices, they would gradually assume a different orbit - but they would no more "gravitate to Earth" than anything else going around up there.
 Also, does the weight of the satellite and its height above the earth not have an impact on the speed it needs to travel to stay in orbit?
The word is Mass (not weight). The force of attraction is proportional to the mass of the object and the acceleration it experiences due to that force is Inversely proportional to its mass. The value of mass cancels out, when you do the sums, so any object, at a given speed and traveling in a given direction will follow the same orbit. It experiences the same acceleration towards the centre of the Earth .  As I said before, if the satellite is not going at an appropriate speed for a circular orbit, when it is at a certain place in its orbit, then the orbit will be elliptical - not circular.
 

Offline Ravish

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That makes mass(ive) sense, thanks!  :)
 

lyner

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No prob.
 

Offline muzammalbaig

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½mV² is the kinetic energy in the satellite orbiting a body(Planet) This is in perpendicular direction to the gravitational pull being experienced by it. Thus the gravitational pull only causes angular acceleration and the satellite turns towards the planet taking a curved path.
now if V exceeds a certain limit (escape velocity) the satellite will fled away for ever otherwise it will start orbiting the planet.

every flying object(satellite) e.g an aeroplane will orbit the earth unless its kinetic energy depletes or it gains more kinetic energy. the aeroplane always falls because the air resistance very rapidly takes away its energy so we have to add more energy constantly into it.
i.e. if there was no air then after having an aircraft flown, we would not need running engines to travel. this is true for all satellites (stationary or not) and once up and running we leave them up there on their own.

But practically atmosphere never ends and eventually we have to control the satellite to keep it in its orbit.

space offers extremely low resistance to flying objects hence the object once up will not fall down very soon, because in the procedure of going up it has gained both tangential and perpendicular components of kinetic energy hence it orbits the earth for a very long time.
  [O8)]
 
 

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