I shall try to answer this, but there are probably greater scientific minds who'll do a better job.
Imagine an object travelling through space. Let's assume it's not expending any fuel, so it's speed and direction will stay the same. Now, lets imagine a large nearby planet-like object. Our original object will be pulled towards the planet-like object. Now, if it's not going fast enough, it will be pulled in by the planet's gravity and burn up in it's atmosphere. On the other hand, if the object is going too fast, it will shoot right past the planet and off into space.
But if the object is travelling at just the right speed, the planet's gravity will alter it's course at the same rate that it is moving in a perpendicular direction. This has the effect that the object will circle the planet at a fixed distance form the planet. It is especially interesting because it requires very little energy to maintain the orbet. (In a perfect universe, it would be possible to maintain a stable orbit without expending any energy. However, there are many factors such as the upper layers of the Earth's atmosphere, the gravitational effects of the moon and other bodies that affect the orbit and make course corrections necessary.)
However, the speed an object must be travelling at to orbit the Earth at a given distance is fixed. The further an object wishes to orbit from the Earth, the faster it must be travelling. However, as the circumfrance of the circle it traverses increases by a factor of Pi, the time it takes to traverse the orbit increases. At a distance of 36Mm the orbit takes 24 hours, which means that the satellite appears to stay at a fixed point in the sky.
While it would be possible for an object maintain a fixed point in the sky at any distance form the Earth, it could only do so by consuming large amounts of fuel. This could not be considered an "orbit", and any satellite we could produce with current technology would expend its fuel supply within hours.
Also, the faster the object is travelling, the more centripetal force is required to keep the object going round in a circle instead of shooting off in a straight line - but at the same time, the further the object is from the Earth, the less effect the Earth's gravity has on the object. So there's actually only a fairly thin band where gravity is sufficient to keep the object going round the Earth, but not crashing into it.
do geosynchronous satellites have to keep generating bits of thrust to keep their speed up & stay in the 36000Km orbit? Or once plonked there with the right initial nudge, do they stay there forever?
They stay put for a fair while - their orbit ends up decaying sometimes as has been previously stated elsewhere, because of the action of the moon's gravity, micro-meteorites, solar wind, and other such teeny-tiny forces that can have an effect over time. Hence they send up the Space Shuttle every so often to straighten out the orbits of the more expensive satellites. :-)
Not for refulling geosynchronous satellites though as the shuttle only goes up 300Km or so (Low Earth Orbit?). unless the satellite thought "ohh, need fuel", dropped to a lower orbit, picked up some gas & then nipped bak to higher orbit? EssoSpace division :)
must find the rather cute java app showing satellites orbits, can drag it around, rotate the earth, zoom in & out, highlist specific satellites. GPS satellites on that looked like a crowd of angry flies buzzing round the earth, with LEO satellites making a close to the surface sphere, the bunch of geostationary ones making a Saturnesque ring, and things like Chandra nipping well away from Earth then zooming back :) Way to waste hours wondering : http://science.nasa.gov/RealTime/jtrack/3d/JTrack3D.html
I say 'that's the best thing I've ever seen on the Internet' quite often, but honest to God I mean it in regards to this applet. I had no idea that anything so fucking amazing had been made.
I... I might have to enable Active Desktop and set this as my wallpaper. Oh God.
it *is* rather cool! Wish it did a lot more as well - imagine if it linked in a planetarium & the voyager & mariner & other probes and everything we've ever done in space ;)
A geosynchronous will generally be more stable than a low-Earth orbit because there isn't nearly as much of Earth's stray atmosphere floating around at that altitude. However, even at this distance there is matter floating around which has a slight effect on the orbit. There is also the gravitational pull of the Moon whill will gradually pull the satellite about in it's orbit, so geosynchronous do need to expend fuel to make course corrections too. I think ground stations sometimes need to make adjustments for sattelites that have drifted out of their original orbits too.
ta for the explanation and yes, whilst simple to point at the nice nasa link, 'tis good to be able to explain it oneself :)
I did think that any orbit could be "geosynchronous" in terms of staying over a fixed spot on the Earths surface, at the expense of fuel to make it stay there...
Need a satellite refulling station for all those geoSynchronous satellites using up (albeit) small bits of fuel (hydrazene?) for station keeping. Opportunity for Esso? - may not be as cheep as 1.30euro/litre though!
Interesting that you should bring it up - the US military is currently looking at satellite "tug-boats". The idea is that there are lots of perfectly good satellites up there that have come to the end of their lives because they've run out of fuel. Instead of spending a billioin dollers on a new satellite, send up a low-cost tug that would hitch onto the old one and take over the guidance/course correction functions.
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Date: 2003-10-21 06:10 am (UTC)no subject
Date: 2003-10-21 06:20 am (UTC)ta for the link (been there before, forgot to follow the orbital equations link, doh!)
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Date: 2003-10-21 06:48 am (UTC)no subject
Date: 2003-10-21 06:47 am (UTC)Imagine an object travelling through space. Let's assume it's not expending any fuel, so it's speed and direction will stay the same. Now, lets imagine a large nearby planet-like object. Our original object will be pulled towards the planet-like object. Now, if it's not going fast enough, it will be pulled in by the planet's gravity and burn up in it's atmosphere. On the other hand, if the object is going too fast, it will shoot right past the planet and off into space.
But if the object is travelling at just the right speed, the planet's gravity will alter it's course at the same rate that it is moving in a perpendicular direction. This has the effect that the object will circle the planet at a fixed distance form the planet. It is especially interesting because it requires very little energy to maintain the orbet. (In a perfect universe, it would be possible to maintain a stable orbit without expending any energy. However, there are many factors such as the upper layers of the Earth's atmosphere, the gravitational effects of the moon and other bodies that affect the orbit and make course corrections necessary.)
However, the speed an object must be travelling at to orbit the Earth at a given distance is fixed. The further an object wishes to orbit from the Earth, the faster it must be travelling. However, as the circumfrance of the circle it traverses increases by a factor of Pi, the time it takes to traverse the orbit increases. At a distance of 36Mm the orbit takes 24 hours, which means that the satellite appears to stay at a fixed point in the sky.
While it would be possible for an object maintain a fixed point in the sky at any distance form the Earth, it could only do so by consuming large amounts of fuel. This could not be considered an "orbit", and any satellite we could produce with current technology would expend its fuel supply within hours.
no subject
Date: 2003-10-21 07:02 am (UTC)no subject
Date: 2003-10-21 07:21 am (UTC)no subject
Date: 2003-10-21 07:33 am (UTC)no subject
Date: 2003-10-21 07:43 am (UTC)must find the rather cute java app showing satellites orbits, can drag it around, rotate the earth, zoom in & out, highlist specific satellites. GPS satellites on that looked like a crowd of angry flies buzzing round the earth, with LEO satellites making a close to the surface sphere, the bunch of geostationary ones making a Saturnesque ring, and things like Chandra nipping well away from Earth then zooming back :)
Way to waste hours wondering : http://science.nasa.gov/RealTime/jtrack/3d/JTrack3D.html
no subject
Date: 2003-10-21 10:47 am (UTC)I say 'that's the best thing I've ever seen on the Internet' quite often, but honest to God I mean it in regards to this applet. I had no idea that anything so fucking amazing had been made.
I... I might have to enable Active Desktop and set this as my wallpaper. Oh God.
no subject
Date: 2003-10-21 11:52 am (UTC)Wish it did a lot more as well - imagine if it linked in a planetarium & the voyager & mariner & other probes and everything we've ever done in space ;)
no subject
Date: 2003-10-21 11:59 am (UTC)would be fun to set it to show spacecraft & satellites popping up (and disappearing :( as years go by
Sputnik, all alone...
no subject
Date: 2003-10-21 07:34 am (UTC)no subject
Date: 2003-10-21 07:20 am (UTC)I did think that any orbit could be "geosynchronous" in terms of staying over a fixed spot on the Earths surface, at the expense of fuel to make it stay there...
Need a satellite refulling station for all those geoSynchronous satellites using up (albeit) small bits of fuel (hydrazene?) for station keeping. Opportunity for Esso? - may not be as cheep as 1.30euro/litre though!
no subject
Date: 2003-10-21 07:40 am (UTC)