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John Dlugosz
2006-May-15, 06:23 PM
I started reading a copy of the novel about the rise of Kahn, with Gar 7 and Captain Kirk, after seeing it mentioned here.

In one chapter, the scientist Evergreen has a satelite in geosynchronous orbit over the south pole, in an attempt to close the hole in the ozone layer.

--John

antoniseb
2006-May-15, 06:26 PM
That's science fiction. On the other hand, I get the impression that in the Star Trek universe, the ships/stations had enough power that it wouldn't be impossible to simple have a powered hover over the South pole.

ngc3314
2006-May-15, 06:47 PM
That's science fiction. On the other hand, I get the impression that in the Star Trek universe, the ships/stations had enough power that it wouldn't be impossible to simple have a powered hover over the South pole.

There have been pretty serious proposals for objects with big solar sails to hang over one pole, with the stationkeeping thrust provided by tacking with light pressure. These are known as statites, an idea patented by Robert Forward in 1989.

Roy Batty
2006-May-16, 04:05 PM
Small nitpick... A polar geostationary, geosynchronous orbit would be impossible (In reality most of our polar satellites operate in Sun-synchronous orbit though).

But I know what you meant! ;) :)

ToSeek
2006-May-16, 05:39 PM
A true polar geosynchronous orbit would be one with a 90-degree inclination that passed over the same spot on the equator every 24 hours.

Dr Nigel
2006-May-18, 12:41 PM
But, ToSeek, I think the point was, how can a polar orbit in any way be geosynchronous?

Surely, a geosynchronous orbit, by definition, is one that follows Earth's own rotation. And for a polar orbit, that would mean sitting and swivelling in one spot, which even my rather inadequate grasp of orbital mechanics suggests is not a stable orbit...

SeanF
2006-May-18, 02:09 PM
But, ToSeek, I think the point was, how can a polar orbit in any way be geosynchronous?

Surely, a geosynchronous orbit, by definition, is one that follows Earth's own rotation. And for a polar orbit, that would mean sitting and swivelling in one spot, which even my rather inadequate grasp of orbital mechanics suggests is not a stable orbit...
Geosynchronous simply means that the period of the orbit is equal to the Earth's rotation - about 23 hr 56 mn. Doesn't matter whether the direction of the orbit follows the direction of rotation.

An orbit in which the satellite actually follows the Earth's rotation and thus remains "stationary" over a specific point on the Earth's surface is called Geostationary. All geostationary orbits are geosynchronous, but not all geosynchronous orbits are geostationary.

I believe the book being referenced uses the term "geosynchronous," but the context clearly implies that it's supposed to be "geostationary" over the Pole, which is, of course, impossible.

hhEb09'1
2006-May-18, 02:19 PM
Geosynchronous simply means that the period of the orbit is equal to the Earth's rotation - about 23 hr 56 mn. Doesn't matter whether the direction of the orbit follows the direction of rotation.Some sources (including NASA ones we've seen before) use the terms interchangeably, some don't. Usually, when there is a distinction, the definition is the one that ToSeek mentioned, that the geosynchronous satellite passes over the same point on the equator at the same time of day--which doesn't even imply a 24 hr period, it could just as well be 12 hr.

I'll see if I can find those old discussions.

PS: Here ya go (http://www.bautforum.com/showthread.php?p=339&highlight=geosynchronous#post339). Wow, post number 339--and I reference a previous discussion on the old BABB!

Dr Nigel
2006-May-20, 01:04 PM
Geosynchronous simply means that the period of the orbit is equal to the Earth's rotation - about 23 hr 56 mn. Doesn't matter whether the direction of the orbit follows the direction of rotation.

An orbit in which the satellite actually follows the Earth's rotation and thus remains "stationary" over a specific point on the Earth's surface is called Geostationary. All geostationary orbits are geosynchronous, but not all geosynchronous orbits are geostationary.



Oh. I guess that makes sense. You learn something every day.

Gillianren
2006-May-20, 04:41 PM
Oh. I guess that makes sense. You learn something every day.

Or at least you should, right? I despair for people who don't.

mike alexander
2006-May-20, 05:43 PM
For those interested, the Statite patent # is 5,183,225

peter eldergill
2006-May-20, 10:54 PM
Why would anyone care about a geosynchronus but not geostationary orbit? What would be the point of anorbit exactly 23 hr 56 min if it were not geostationary?

Pete

Dr Nigel
2006-May-21, 08:26 AM
Why would anyone care about a geosynchronus but not geostationary orbit? What would be the point of anorbit exactly 23 hr 56 min if it were not geostationary?


Maybe to connect two points on opposite sides of the globe at the same local time each day?

Roy Batty
2006-May-21, 04:06 PM
Why would anyone care about a geosynchronus but not geostationary orbit? What would be the point of anorbit exactly 23 hr 56 min if it were not geostationary?

Pete
Sometimes it's useful to have a satellite in an elliptical (Molniya (http://www.telescope.org/nuffield/pas/moon/moon9e.html)) orbit so that it can stay over a certain area for most of time & also can be closer (i.e. increase spatial as opposed to temporal resolution & vice-versa). Some communication satellites are just geosynchronous (though most are geostationary as well) so as to reach higher lattitudes, & also meteorological satellites take advantage of this to scan a larger area. So, some people care :)

ngc3314
2006-May-21, 04:42 PM
Why would anyone care about a geosynchronus but not geostationary orbit? What would be the point of anorbit exactly 23 hr 56 min if it were not geostationary?

Pete

There are plenty of applications where you get communications advantages from a 24-hour orbit but don't require it to hover motionless as seen from Earth. An astronomical example was the International Ultraviolet Explorer (IUE), launched in 1978 for a 3-year mission (which ended up lasting over 17 years). It used an inclined and elliptical geosynchronous orbit. for which the ground track traced a figure-8 over the Atlantic and South America. This way, astronomers could do uninterrupted command and downlink with a single tracking station near Goddard (and the ESA contingent had almost-continuous visibility from Villafranca, near Madrid).

A huge economic advantage of geostationary orbits is on the ground - you can take an antenna, align it on the satellite, and basically leave it on concrete blocks. Whole industries are based on this. The USSR (and now Russia) couldn't use this for much of their territory, as the Earth hides geostationary objects from high latitude, and went with the Molniya orbits already discussed (which still need tracking by the ground stations). Those orbits are so hgh that I remember a Russian class at the University of Arizona which watched the evenng news from Moscow by pointing a dish up over the pole.

The geosynchronous ring is getting so crowded (with the spacing usually mandated by noninterference concerns) that it's not an orbit you would use if you didn't really need it.

peter eldergill
2006-May-21, 05:15 PM
It would appear that I've much to learn about orbits around Earth!

Thanks

Pete