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How does the ISS maintain orbit across a range of altitudes?
I am a physics student preparing a presentation on the ISS, one of my
topics will cover orbital information. My question is that for a stable orbit to be maintained a satellite must maintain a fixed relationship between altitude and velocity, otherwise due to atmospheric drag it will decrease in velocity and be overcome by the earth's gravitational field and begin re-entry. After examining the altitude information on the ISS released by NASA, I have seen that the ISS operates over a wide range of altitudes, and as it slowly decreases in height it re-boosts back up to maintain its height, from what I have been taught so far for a satellite to maintain stable orbit it must conform to V = SquareRoot GM/R, if for example the satellite falls 10km, does it therefore increase it's speed in order to stop it losing more altitude, and when re-boosted does it decrease its speed in order to stop it exiting orbit? Or can it operate comfortably across a range of altitudes without significant effect? Anyone who can shed light on this for me your help is much appreciated, thanks. Jarrod. |
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How does the ISS maintain orbit across a range of altitudes?
"Rueffy" wrote in message oups.com... I am a physics student preparing a presentation on the ISS, one of my topics will cover orbital information. My question is that for a stable orbit to be maintained a satellite must maintain a fixed relationship between altitude and velocity, otherwise due to atmospheric drag it will decrease in velocity and be overcome by the earth's gravitational field and begin re-entry. After examining the altitude information on the ISS released by NASA, I have seen that the ISS operates over a wide range of altitudes, and as it slowly decreases in height it re-boosts back up to maintain its height, from what I have been taught so far for a satellite to maintain stable orbit it must conform to V = SquareRoot GM/R, if for example the satellite falls 10km, does it therefore increase it's speed in order to stop it losing more altitude, and when re-boosted does it decrease its speed in order to stop it exiting orbit? Or can it operate comfortably across a range of altitudes without significant effect? Anyone who can shed light on this for me your help is much appreciated, thanks. You seem to be confusing velocity and energy in your explanations. You also seem to assume that orbits must always be circular, which is absolutely not the case. If a spacecraft is in a circular orbit and is acted upon by a constant force in a constant direction opposite the velocity vector (e.g. air drag), you can think of this as a decrease in the kinetic energy of the object and can solve for velocity and altitude, assuming that the orbit will remain circular. However, if a spacecraft is in a circular orbit and fires its engines in the direction which increases the velocity of the spacecraft for a duration that's very short compared to the time it takes to complete one orbit, then we can model the burn as instantaneous (not that I'm saying you can make this assumption with ISS reboost burns). Since this is modeled as an instantaneous change in velocity (delta-V), the altitude at the point in the orbit at which the burn occurred does not change. The orbit will continue to pass through that point on subsequent orbits. What does change is the *shape* of the orbit and the altitude of the orbit at the location on the opposite side of the orbit. Since our burn was made in the direction which increased the velocity at the burn point, this increases the altitude at the point opposite the burn point. Note that at this opposite point the altitude is now higher than it was before. This is where terms like apogee and perigee come into play. It's really hard to describe these things with words, so I suggest you consult a good orbital mechanics book that has both equations and pictures. ;-) Jeff -- "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety" - B. Franklin, Bartlett's Familiar Quotations (1919) |
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How does the ISS maintain orbit across a range of altitudes?
JRS: In article , dated Wed, 14
Jun 2006 10:44:28 remote, seen in news:sci.space.station, Jeff Findley posted : If a spacecraft is in a circular orbit and is acted upon by a constant force in a constant direction opposite the velocity vector (e.g. air drag), you can think of this as a decrease in the kinetic energy of the object and can solve for velocity and altitude, assuming that the orbit will remain circular. But under those circumstances the kinetic energy actually increases. See URL:http://www.merlyn.demon.co.uk/gravity2.htm#EDOB and thereabouts. -- © John Stockton, Surrey, UK. Turnpike v4.00 MIME. © Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links; Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc. No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News. |
#4
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How does the ISS maintain orbit across a range of altitudes?
"Dr John Stockton" wrote in message ... JRS: In article , dated Wed, 14 Jun 2006 10:44:28 remote, seen in news:sci.space.station, Jeff Findley posted : If a spacecraft is in a circular orbit and is acted upon by a constant force in a constant direction opposite the velocity vector (e.g. air drag), you can think of this as a decrease in the kinetic energy of the object and can solve for velocity and altitude, assuming that the orbit will remain circular. But under those circumstances the kinetic energy actually increases. See URL:http://www.merlyn.demon.co.uk/gravity2.htm#EDOB and thereabouts. You're right. I was mixing up my terms too. It's been a few years since my orbital dynamics class. ;-) One of the interesting things to note is the relationship between the kinetic energy and potential energy, the sum of which is the total energy of an orbit. http://www.go.ednet.ns.ca/~larry/orbits/kepler.html Jeff -- "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety" - B. Franklin, Bartlett's Familiar Quotations (1919) |
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