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Why doesnt the moon just fall to the earth?

  1. Jan 4, 2005 #1
    I know its in orbit but things like the hubble telescope need fuel dont they?
     
  2. jcsd
  3. Jan 4, 2005 #2
    Nope, no fuel. They have exactly the right velocity so that they are always falling down to earth, but the earth curves away at the right amount to keep it always falling. Think about throwing a rock. You throw it lightly, it will not go very far before gravity brings it down. Then throw it a little bit harder, and it goes further. Keep throwing harder and harder. Eventually it will go very far, but as it falls, the earth will curve beneath it, and so it will always "fall" forever, and thats what an orbit is.
     
  4. Jan 5, 2005 #3
    good explanation. :)
     
  5. Jan 5, 2005 #4

    SGT

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    Actually, the space telescope as well as other artificial satellites loose velocity gradually, mainly because of friction with particles in their orbits.
    So they must from time to time use fuel to correct their orbits. The amount of fuel the satellite carries initially gives its lifetime. Once the fuel is spent the satellite will loose velocity and gradually fall to Earth, as has happened with several ones.
    Of course, a very valuable satellite, as the space telescope, can be refuelled in order to increase its lifetime.
     
  6. Jan 5, 2005 #5

    HallsofIvy

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    The moon DOES fall! It is falling toward the earth constantly. Fortunately, it keeps missing!
     
  7. Jan 5, 2005 #6

    DaveC426913

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    "The secret to flying is to throw yourself at the ground ... and miss."
    DA-THGTTU
     
  8. Jan 5, 2005 #7

    BobG

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    Low orbiting satellites need fuel to stay in orbit because the atmosphere doesn't suddenly stop. Instead, it gets thinner and thinner until, eventually, there's so little left you can just disregard it. Around 300 miles high, the atmosphere is so thin that it would only affect satellites with an extremely high area to mass ratio. By around 600 some miles, you could disregard the atmosphere for virtually every orbiting object.

    The International Space Station and the Hubble telescope orbit at an altitude a little under 200 miles high, so the atmosphere does slow them down, causing them to lose altitude.
     
  9. Jan 5, 2005 #8

    DaveC426913

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    "Low orbiting satellites need fuel to stay in orbit because the atmosphere doesn't suddenly stop. "

    But just to clarify - they don't burn continuously like a rocket, they merely make occasional orbital adjustments, when they get too low.
     
  10. Jan 5, 2005 #9

    russ_watters

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    Kinda irrelevant, but I'm curious: Does Hubble have fuel or does the Shuttle just give it a boost when it stops by for servicing? I know it doesn't use fuel for pointing...
     
  11. Jan 6, 2005 #10
  12. Jan 6, 2005 #11
    Ok, i have another question.
    Do we see the same side of the moon and why? I seem to always see the face. although this could be an illusion.
     
  13. Jan 6, 2005 #12

    BobG

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    Oops. My bad. The Hubble orbits around 350 miles high. The atmosphere does have a very small effect on the orbit, but it degrades slowly enough that the shuttle raises it back again whenever other maintenance is performed on the Hubble. It would take around 10 years for the Hubble to re-enter the atmosphere if were left unattended.
     
  14. Jan 6, 2005 #13

    BobG

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    Yes, we see the same side of the moon. The time for one rotation matches the time for one orbit. All objects orbiting another tend to wind up with the rotation rate matching the orbit rate, including the Earth and the Sun (even though we're not even close to getting to that point yet). The closer two objects are, the faster they reach that state.

    What is really happening is that the long-axis of any orbiting object winds up pointing along the radius. Quite a few low precision low cost satellites use this principle as their method for attitude control. It's called gravity gradient stabilization and is caused by the fact that the gravity field around a planet or sun is really spherical and decreases in proportion to the square of distance - in other words, the force of gravity at the ends of a see-saw really aren't quite the same as the force of gravity at the fulcrum of a see-saw, since the ends of a perfectly balanced see-saw are further away from the center of the Earth.
     
  15. Jan 6, 2005 #14

    russ_watters

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    That's still a shorter timeframe than I would have expected. Thanks for the info.
     
  16. Jan 6, 2005 #15

    From Earth, we don't always have the EXACT same view of the moon. It wobbles a little; its called libration. Here is a link explaining it...

    http://www.stargazing.net/david/moon/moonlibration.html

    -Glenn
     
  17. Jan 6, 2005 #16

    Andrew Mason

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    I believe that this is an incorrect interpretation of the physical data. What is the evidence that the moon actually moves around relative to the em barycentre? It seems more likely to me that it is our position on the earth relative to the em barycentre that changes.

    Satellites whose attitude is automatically stabilised via gravity gradient do not move around like that - if they did we would be constantly having to adjust our antennae. So why would the moon move?

    AM
     
    Last edited: Jan 6, 2005
  18. Jan 6, 2005 #17
    Don't both momenta of the Earth and moon act as an action-reaction pair, keeping them apart from each other in orbit?
     
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