Very basic question: How do they stay in orbit?

[BobM3]

Sorry for such a basic question but I never really thought about waht keeps a satelite at a constant height. You've got gravity trying to pull it towards the earth and centrifigal force trying to push it away from earth. It seems like that is very unstable - a little deviation either way would send it plummeting towards earth or into space. So, how do they stay in orbit?

 

[IRstuff]

Well, obviously, it's not that unstable, but it is continually falling as you indicated.

TTFN

FAQ731-376

 

[Comcokid]

Good reply IRstuff. That's exactly what Newton in his Universal Law of Gravity pointed out. A body in orbit is constantly falling. However, the earth below it is curving away at the same rate, so it's in an infinite free-fall.

Disturb an orbiting body slightly, and it remains in orbit, but maybe in a slightly different orbit, or maybe one that’s more elliptical than circular.

PBS Nova done several good programs on this.

 

[IRstuff]

Earth orbits are quite stable, unless they're too low, in which case, the wispy remnants of the atmosphere slow it down enough to cause it to continuously drop in orbit until it burns up in the atmosphere.

As for why it otherwise stays in the same orbit, it's conservation of energy. If the PE doesn't change, it can't physically have moved in altitude, so it's at a constant circular orbit. An elliptical orbit is slightly more complex, but again, conservation of energy puts it at the altitudes on a repeat orbit.

Slingshot orbits are a little bit more complicated than that, since they actually involve an energy transfer between the two bodies involved.

TTFN

FAQ731-376

 

[EricRICPDAS]

Satellites do tend to drift from exactly where you want them to be. Keeping them in place is called Station Keeping. This generally consumes propellant over time. Satellites in geosynchronous orbit that shut down at end of life tend to accumulate over India. This is because of the Himalaya mountains causing a gravity anomaly. The moon, Jupiter, and the Sun also perturb the orbits. Propellant is not always needed to keep the orientation correct, because a torque can fix this. Torque can be obtained by a current loop that interacts with the earth's magnetic field.

Eric R

http://www.icpdas-usa.com

 

[BobM3]

Thanks to all. It was Nova show that got me interested in this. Now it's time to get a book.

 

[Tophinater]

I'm not an aero engineer, Im mechanical but my take on this is energy balance. As long as a satellite has enough kinetic energy it will remain in an orbit. The mass of the object being oriented and kinetic energy contained by the orbiting object determines its radius between the two objects. If for some reason either object loses kinetic energy, from micro friction or such, the two objects fall closer to eachother accelerating and there by increasing the velocity which is required for them to maintain orbit. I believe this can be modeled using eigen pairs along with the obvious classical physics model.

Gm = V^2*r

 

[bobcvn65]

BobM3,
If you really want something to ponder, check out the Clohessy-Wiltshire equations (or Hill equations). These are by far the most interesting thing in the subject of orbital mechanics. These eqautions govern the motion of orbiting bodies and are quite non-trivial. For instance, if you were in orbit and wanted to dock with another spacecraft at the same altitude, would you speed up in the spacecrafts direction? Keep in mind that if you speed up you will ultimately put yourself in a higher orbit, and thus a slower period. Also, what if you were on the shuttle and someone on the opposite end of the ship wanted you to throw them a hammer? Would you throw it towards them or away from them? Check out the Hill equations and I'm sure you'll fall in love with the counterintuitive principles you will learn.

 

[dEARQ]

Check out The Science Teacher magazine, Jan.1984, "How To Orbit The Earth."

 

[dEARQ]

Tower of BABEL: One way to keep one in orbit might be to put up a long pole made of nanotubes and attach the satellite. This might be better than the proposed paper thin ribbon tether they are proposing. The 62000 mile tether would be attached to the earth. As the earth rotates it would swing a satellite around. Sounds hilarious, but then they plan to run an elevator up the tether! It is called the space elevator and it might be up by 2015!

 

[swearingen]

I'm quite sure it won't be 2015, dEARQ.

See Thread 25-137712

In fact I'm not so sure it will ever happen...



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