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Question about sending satellite circular orbit

  1. Oct 27, 2008 #1

    fluidistic

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    Is it possible to put in circular orbit an object from Earth's ground? My friend said yes and told me we were going to see how exactly in 2 years when we'll learn about a more formal mechanics.
    But I've thought about it and I believe it's impossible. My thought is that the satellite will describe a spiral and not a circle. My friend says it will describe a spiral and then a circle... but how could that be? If it describes a spiral then it will always have some velocity in a direction that is not parallel to Earth's ground, so the circular motion is not possible. At best it could be very very close to it, but still it will never be a perfect circular path. (Unless the satellite has a little reactor, but we suppose it hasn't).
    So who is wrong and why?
    Thanks.
     
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  3. Oct 27, 2008 #2

    D H

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    If your satellite doesn't have thrusters, no. It won't spiral, however. It's orbit will be in the shape of an ellipse. Since you don't know this, I'll assume you are fairly young. Have you been introduced to Kepler's Laws?

    An elliptical orbit that starts from the surface of the Earth will hit the Earth sometime later. The only way to escape this is to escape the Earth's gravity field entirely -- or have the satellite perform an orbit insertion maneuver once it clears the atmosphere.
     
  4. Oct 27, 2008 #3

    Mapes

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    The engines are used again after launch to circularize the orbit. You might be interested in reading about the Hohmann transfer.
     
  5. Oct 27, 2008 #4

    fluidistic

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    I think it's relative, but I'm not that young (21 years old), university freshman.
    I've been introduced to the laws of Kepler in about 5 minutes, so my professor didn't emphatize at all on them... and I have no exercise that is related to them. I'm surprised in a way that the path will be elliptic. This is quite complex to guess : Earth's rotation on itself (neglecting the influence of the Moon) makes things hard for me to guess.

    And Mapes
    this is exactly what I thought... Of course I'm interested in reading the "Hohmann transfer". I'll check it out, thanks.

    Thanks to both!
     
  6. Oct 27, 2008 #5

    D H

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    I'm more than a bit surprised this never came up in your high school physics class, if you took one. Kepler's Laws are pretty standard fare for high school (or even lower level) physics.

    Since you haven't even covered the rudimentary basics, jumping in to the concept of orbit transfers is not a particularly good idea. They will just look like a bunch of equations that have no rhyme or reason. You need to understand not only that orbits around a body with a spherical mass distribution are elliptical, but how and why they are elliptical before you can make sense out of transfer mechanisms. You have to learn how to walk before you learn how to run.
     
  7. Oct 27, 2008 #6

    fluidistic

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    In fact they teach that in high school but I missed a whole year. I had no chance with that... I turned 17 and due to family problems I had to move from Canada to France. In Canada I'd have to take 2 years before entering into University while in France only one year. So when I came to France the students already saw many concepts such as precalculus and a lot of physics/chemistry while I had no idea about what was going on. I had to pass the "bac" which is a big exam at the end of the year (I had homeschooled at that time because I couldn't understand ANYTHING in math, so I took particular classes with a tutor... but still, I couldn't do 2 years in 6 months) and failed badly. But at least I learned some basic stuff in math, so the year that followed I went to high school and was an average student overall. (but particularly weak at sciences courses.... which was my speciality). And finally I could get my diploma this time.
    From there I moved to Argentina and study at University. Basically it restarted all precalculus and also restarted from 0 for physics, which helps me very very much.
    I remember in high school when they were doing exercises about RLC circuits with plenty of differential equations... I had a vague idea about the meaning of everything... but now even if I didn't get RLC circuits (second year course), I can understand much more than before.
    Just to say that Kepler's Laws were teached 2 years before entering University in France, so I totally missed that year.
    So I more than agree when you say I need to understand well why orbits are elliptical before getting involved into something that already assume that!
    Last word : from what I've learned till yet in Argentina in physics, I am just excited about what's coming up!
     
  8. Oct 27, 2008 #7

    D H

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    The good news: you didn't miss much. Algebra-based physics is in a sense harder than calculus-based physics. Algebra-based physics involves a lot of memorization of seemingly disparate equations that the book and your teacher pull out of thin air. Those equations seem to work, but why, and how are they related, and where do they come from? The answer: A lot of them come from calculus. In calculus-based physics there is a lot less memorization, things are more interrelated, and you can see why at least some of those stupid equations are the way they are (i.e., you or the instructor will derive them).
     
  9. Oct 28, 2008 #8

    fluidistic

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    I agree. At university we tend to "demonstrate" all the formulas we are using while in high school the book says : the solutions of this differential equations are under the form... Maybe some books detailed everything but it wasn't necessary to know the details to pass the final exam.
    So now my guess is that I must learn well the Laws of Kepler. (If I don't do it now, I'll still learn them later I think.)
     
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