(This is a follow-on to the thread on launching the space shuttle from low altitudes.)
Help me remember the differences between these kinds of orbit: geostationary orbit This one might be the kind of orbit where a satellite stays in exactly the same position above the equator. It’s an orbit with an altitude (above sea level) of something like 40,000 miles (or was that a radius of 40,000 miles) geosynchronous orbit … or maybe it’s this one. What’s the difference between the first two? parking orbit I can’t remember anything about this one, except that it was one phase of the Apollo mission. How high is this one?
What do you call the lowest orbit you can be in without, y’know, falling out of the sky and burning up? I guess this a tough call, if all orbits will eventually decay and end the satellite’s life… Maybe I should ask, how low are the lowest intentional orbits?
Do we have anything above the geo-whatever orbits? I mean, do we have anything so far out that even though it flies eastward, its position above the equator moves west?
It shows you all the satellites orbiting the earth at the moment. All the ones They’ll let you to know about, anyway. I’m guessing that big equatorial ring around the earth is the geosynchronus position for satellites.
I’d say the name for a lowest possible orbit is just “low earth orbit.” That’s what you use if you need a stable orbit but have no other requirements for the orbit - the Shuttle, space stations and many astronomical satellites use this orbit. Of course, the exact height depends on how stable you want the satellite to be (i.e. how long it needs to stay up).
There are not many satellites beyond the geosynchronous orbit. If you exclude interplanetary probes, the only one I can think of is SOHO (Solar and Heliospheric Observatory). It’s at a Lagrange point between the earth and the sun, where its view of the sun is never obscured by the earth.
By the way, am I the only one who thought ‘geosynchronous’ means any orbit with a 24 hour period, as opposed to a ‘geostationary’ orbit which is a circular, equatorial orbit with a 24-hour period? My dictionary says they are the same thing, so I must have been mistaken.
No, you’re not. Apparently a lot of the dictionaries say that. On the other hand, NASA makes a distincition: “A SC in earth orbit with a period of one sidereal day is considered geosynchronous. If its eccentricity and inclination approach zero, the satellite is further defined as geostationary.” Of course, NASA’s version makes more sense, entomologically.
I believe that Chandra is also out beyond geostationary at apogee. It has a highly eccentric orbit which allows it to spend most of its time away from the Earth, so as to decrease terrestrial interference, but the lower perigee means a significantly lower fuel cost in launching it. This is also probably the preferred method for any other satellites which need to be far from the Earth. Most satellites, however, either just need to be somewhere in space, in which case you use LEO, or they need to be geosynchronous/geostationary, in which case, you launch them higher.
Oops, yes I forgot Chandra X-ray Observatory… It has a 64 hour orbital period. Primary reason is to go above the earth’s radiation belt which can interfere with observations. It also allows a very long continuous observation - how do you take a 10-hour exposure image if the earth zips past you every hour?
Actually if you go to the link given by Erroneous and zoom out, you can see a few more satellites outside the geostationary orbit. Many are astronomical satellites, like the IUE (International Ultraviolet Explorer, I think?) and XMM (X-ray MultiMirror).
In addition to LEO (low earth orbit) and geosynchronous orbits, there is another important orbit - the polar orbit. You use it if you want to observe the whole surface of the earth. Remote sensing satellites and spy satellites use this orbit.
Whoa, I forgot all about Lagrange points. Now here’s a question: is there more than one between any pair of objects? The way you phrased your last sentence implies that there are several, but to my rusty old physics faculties, there would only be one. I mean, it’s the point where gravitational pull from each object is equal … there would be a plane through that point where gravitational pull would be equal, but it seems like anywhere but one point would be unstable…
If it’s at a Lagrange point, does that mean it’s always located right on top of the sun (from an earthling’s point of view)?
Is there anything at the Lagrange point between Earth and Moon?
Can the Lagrange point be inside one of the objects? I was thinking about where it would be located between, say, Mars and Phobos, and I decided it might be in the moon given the tremendous mass differentials. Would this mean there could be no loose rock on that face of the moon, since it all would fall up to the planet?
You’re thinking of the moon being inside the Roche limit. It’s the distance at which a “liquid” body would be torn apart by tidal stress. The rings of Saturn are within the Roche limit, I think, and that is why they don’t get together to form a moon. Here is a good description of the Lagrange points–there are five different types.
I know I posted a previous link where NASA made a “distincition” between geostationary and geosynchronous, but here is another link where they don’t! On the other hand, here is a Treasure Trove of Physics page which does make the distinction. They have a nice list of orbit types (under construction), the most interesting is the Molniya.
This page says that Chandra has a perigee of 10,157 km and an apogee of 138,672 km.
For any system of two bodies, there are five Lagrange points. The first one is in between the bodies, and this is where SOHO is. The second and third are on that same line, but they’re on the opposite sides of the respective bodies, so there’s one on the opposite side of the Earth from the Sun, and one on the opposite side of the Sun. The fourth and fifth are in the orbital plane, and form two equilateral triangles with the two primary bodies.
The significance of these points is that a body at any of those points will stay in the same position relative to the primary bodies. The first three are unstable, so there’s usually not anything there naturally, but the fourth and fifth are stable, and so tend to accumulate material. There’s a couple of clouds of dust at the Earth-Moon L4 and L5, visible to the naked eye under the right circumstances (or so I’ve heard; I’ve never seen them myself), and Jupiter has whole clusters of asteroids (the Trojan asteroids) at its points with the Sun.
Chronos I think you need to clarify for us lay people that the Lagrange points are not that point that people think about between the two bodies where their gravitational forces cancel each other out (in any case that would be one single point).
I remember a few months ago that there was a space-shuttle
mission to the ISS, and its most important objective was to
boost the station’s orbit, because it had been decaying. For that matter I remember that Skylab’s orbit eventually decayed to the point that it fell out of the sky.
With all the experience we have in launching satellites, why
does this happen? Do we really not know yet exactly how to
factor in the minute resistance of the atmosphere at 300+ miles, or do engineers actually calculate the orbits with
a planned temporary lifespan? That seems reasonable in the case of a crewed mission of short duration, but not something like the ISS, which is expected to stay up forever.
Skylab, Mir, ISS are kept low so they are easy to get to. When the orbit decays, you push it up. Skylab fell because the US chose to let it. It was never intended to be permanent. If they wanted to, they could have kept it up. It was really just practice for the ISS.
That is kind of what I was thinking of but I’d never heard of it before. I was just picturing the L1 point (the only Lagrange point I knew about, and I didn’t know it’s number) being really close to the smaller mass because of an extremely high mass ratio. Thanks for the links.