At lunch today, a question about geosynchronous orbits came up (don’t ask!). We were discussing whether or not such orbits are stable for satellites (specifically, whether satellites in geosynchronous orbits would need attitude thrusters of any type during their useful life). Googling turns up a lot of discussions of LaGrange points (familiar to any science fiction fan and/or reader of GQ :)), but I haven’t yet been able to track down anything that specifically says one way or the other.
So, does anybody know? Do satellites in geosynchronous orbits need to adjust their orbits periodically? How about geostationary orbits?
WAG - why the hell not? When you don’t know something, that shouldn’t stop you from pontificating (the teacher’s credo)…most geosynchronous satellites are very far from the earth. Some are, I believe, around 20,000 miles from the surface. This means that orbital interference from the random atmospheric molecule is pretty rare. That being the main actor in changing the orbit of one of those babies, I’d guess that the “useful life” of a satellite like that doesn’t involve repositioning it. (Now, I sit back and read what the people in the know have to say. Man, I just love the SDMB).
Firstly, geosynchronous and geostationary mean the same thing.
In order to have a geostationary satellite, the satelite must be at a a height of 22 300 miles above the earth. If it is in a stable orbit, it therefore travels at exactly the right speed to revolve around the earth in the same time that it takes the earth to revolve. It therefore stays above the same spot on the earth and appears to be ‘stationary’ from the earth, hence the name.
Now on to stability. If geostationary satellite were unstable as you hypothesise, it would very soon spiral into the earth. However, all satellites are in stable orbits, including the moon, earth (around sun) etc. So they would not need altitude thrusters. However, not even space is a perfect vaccum, so the satellite would eventually slow down until it’s orbit became unstable. However this would take a very long time, so not in its useful lifetime.
Geosynchronous satellites take 24 hours to go around the Earth. The orbits can be circular, or elliptical. They need not be above the equator. If the orbit is elliptical, or tilted, then they do not stay over one part of the Earth.
Geostationary satellites have orbits that keep them over one spot on the Earth. They orbit above the equator, in circular orbits.
Actually, satalites in ANY orbit will require adjustments every now and then to keep in orbit. There are only two points in earth space that are extremely stable, the lagrange points L-4 and L-5 (The “Trojan” points). However, those are in lunar orbit, significantly further out than GEO, and therefor of little use at the moment.
However, it requires very little correction to keep a GEO satalite in possition, so their lifespan tends to be quite long with small manuvering thrusters (Barring mistakes or accidents). I’m not sure how long their lifespan is, but my WAG would be a decade without adjustment. Typical commercial satalite, with adjustment? Maybe three to five decades? Though it would varry a lot, naturally. In either case, pretty long, but when it’s a multi-million dollar piece of hardware, you want to get all the time you can out of it
Alright… here is a link that discusses different types of satellites. Basically, any satellite that orbits at 35,788 km above the surface of the Earth is considered geosynchronous. If it also orbits above the equator, it is geostationary (i.e. it orbits over a single point, never appearing to move relative to a spot on the surface).
I believe the orbit has to be circular as well as equatorial for the satellite to be truly geostationary. If the orbit were elliptical, the satellite would trace a line in the sky back and forth.
As other posters above have mentioned, geostationary satellites have a working life that depends on the amount of fuel available for maintaining their position. Typical lifetimes tend to be around 7 to 15 years. However, there are some old satellites with orbits inclined by as much as 2 degrees that are still carrying TV signals. For example, Gorizont 32 (53 degE) (nearly 20 years old?) is inclined by 2.8 degrees, and has ORT in SECAM and Tajik TV in digital. Eutelsat IIF3 at 21.5degE is very active, carrying a lot of feeds, even though it is well past its planned 7-year lifespan (launched in 1991) and is now inclined 1.5 degrees.
I notice no one has mentioned this, and nothing turns up on the web, but when I worked in aerospace, I was told that one of the biggest reasons for a need for geostationary satellite adjustments was the quadrupole shape of the Earth’s gravity field. The equator is not a circle, it is an ellipse, so the satellites tend towards one of two positions. I also saw this in a textbook by Kaula, where the satellites would oscillate about the points. I cannot remember the size of the effect.
Nukeman, geosynchronous and geostationary do not mean the same thing, see this NASA site.
Kidding aside, I’ve also seen definitions on the web (that I can’t seem to lay my hands on right now) that say geosynchronous includes satellites that are above the same place at the same time of day. For instance, that would include satellites whose period was eight hours, since they would always over the same place at 12:00 GMT. Well, that last link that you, Cerowyn, provided, does hint at that, sorta. That link includes the Molniya orbits in the definition of geosynchronous, even though it doesn’t seem to recognize that the Molniya orbit is typically 12 hours instead of 24.
The Bad Astronomer also discusses the Molniya orbit, but he doesn’t seem to recognize that the orbit swoops down from a geostationary-like height to a low shuttle-like height every period. At its highest, the Molniya orbit approximates a nearly-geostationary comm satellite orbit, over high latitudes, and is useful for communications. The semi-major axis of the Molniya orbit is about half that of the geostationary semi-major axis, which is proportional to period.
The Russian satellites travel from pole to pole. They are not geostationary. They have to track it. As one satellite disappears from view another appears on the southern,Ithink< horizon.
Yes, I noted that above, but I don’t think that you are technically correct. Your webpage still says “So the Keyhole satellite (if that’s what it was) launched by the U.S. to watch over Afghanistan is either in a low Earth orbit, and moves rapidly, or is in a much higher orbit, and appears stationary. But it cannot be both.”
Keyhole satellites do use Molniya orbits, and at different parts of their orbit, they are low and rapid, and then high and almost stationary–that’s precisely the reason for using such orbits. They can be both–so I think that your objection to the Yahoo news item was unfounded.
Why are some satellites visible from the Earth on a clear night? Do they have a light that shines downward, or do they simply reflect light from the sun?
