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View Full Version : can it orbit in any direction? or not at all?



Chunky
2007-Sep-26, 11:38 PM
can a space craft orbit in any direct artound the earth. n to e e to n s to w w to s ect ect?

can if also not orbit the earth...and stay in space without crashing down to earth.....??

you know...like...say the space craft is directly over NY city....and it stays there...not orbiting around the earth...but in sync with it..??

would it take tremendous fuel?

Neverfly
2007-Sep-26, 11:44 PM
can a space craft orbit in any direct artound the earth. n to e e to n s to w w to s ect ect?

I could be wrong but I believe it can.
The space shuttle is not designed for flying far out into outer-space. In fact the orbiter is sometimes low enough that it's still in the highest part of the atmoshpere (albeit it is extremely thin...)


can if also not orbit the earth...and stay in space without crashing down to earth.....??
I'm not certain what you are asking here... Above I said it is not designed for leaving Earth orbit. For example the Space Shuttle is not designed to fly off to the moon. That is not its purpose.
That is also why it is referred to as an Orbiter.


you know...like...say the space craft is directly over NY city....and it stays there...not orbiting around the earth...but in sync with it..??

would it take tremendous fuel?

That is called "geosynchronous" orbit.

Nowhere Man
2007-Sep-27, 01:00 AM
Strictly speaking, that last case is still orbiting the Earth. But the only place where geosynchronous is possible is on the celestial equator. If the ship is off the equator, it will appear to move north and south in the sky. So, without expenditure of fuel, a ship cannot orbit directly above NYC.

If an Earth satellite were to stop its orbital motion, it would fall straight down (or as near as).

Fred

grant hutchison
2007-Sep-27, 01:09 AM
It can orbit in any direction, provided the plane of the orbit passes through the centre of the Earth. So you can't have a satellite moving constantly above a particular line of latitude, unless that line is the equator.
For any given altitude, there's a speed that will result in a circular orbit: the higher the orbit, the slower the necessary speed. In low orbits, the satellites always goes around faster than the Earth rotates, but if they're placed high enough, they go around once a day: that's the "geosynchronous" orbit Neverfly mentions.

But: you can't position a geosynchronous orbit so that a satellite stays above New York, because New York isn't on the equator. If you placed a satellite above New York in a geosynchronous orbit, it would move away to the south, reaching a south latitude corresponding to New York's north latitude after 12 hours, and then looping back up to pass over New York again 12 hours after that.
To stay above New York you'd need to supply a constant force, which would indeed consume fuel. (Another trick that can be done at high latitudes is to use light pressure from the sun to "levitate" a very light solar sail in a stable position above one or other of the Earth's poles.)

Grant Hutchison

grant hutchison
2007-Sep-27, 01:16 AM
But the only place where geosynchronous is possible is on the celestial equator.I think there's a contrast to be made between a geosynchronous orbit, which has the same period as the Earth's rotation, but which can have any old inclination and eccentricity, and a geostationary orbit, which has to be a circular geosynchronous orbit above the equator. A satellite in the latter type of orbit will stay always in the same position relative to the Earth's surface (barring the effects of perturbations from local mass concentrations and from other solar system bodies).
So you can have a geosynchronous orbit that passes over New York; you can't have a geostationary orbit that stays over New York.

Grant Hutchison

Nowhere Man
2007-Sep-27, 01:30 AM
Nit well picked. :clap:

Fred

schlaugh
2007-Sep-27, 02:28 AM
can a space craft orbit in any direct around the earth. n to e e to n s to w w to s ect ect?

Yes, but there's a reason that NASA decided on Florida for launching most of its missions - you get a "free ride" of some 800 MPH (IIRC) from the Earth's spin because Cape Canaveral is located nearer to the equator than say, Atlanta or New York. And that launch location comes equipped with a handy ocean in which to dump expended missile bits.

At the equator that free ride increases to 1,000 MPH. So if you wanted to launch from East to West (i.e. against the turning of the Earth) then you'd need to add that extra amount of boost to compensate for the Earth's motion. And that would indeed cost more in the way of fuel.

I don't think you can orbit from West to South - although NW to SE would still obey the laws of celestial mechanics. ;)

hhEb09'1
2007-Sep-27, 04:22 AM
Nit well picked. :clap:Worse, there are "official" websites that disagree. Here's a post from an early BABB discussion about it, although it looks like I may have said NASA when I meant NOAA (http://www.bautforum.com/small-media-large/27-bad-newspaper.html#post374).

Note: last post in that thread points out that it is possible to maintain geostationary orbit over New York City if you take advantage of atmospheric and lithospheric friction.

grant hutchison
2007-Sep-27, 12:37 PM
Worse, there are "official" websites that disagree. Tut. Why do people insist on muddling useful differences in terminology? I bet they also think that a lectern and a podium are the same thing.

Grant Hutchison

hhEb09'1
2007-Sep-27, 01:11 PM
Tut. Why do people insist on muddling useful differences in terminology? I bet they also think that a lectern and a podium are the same thing.Not only useful, but perfectly logical! :)

I can't find any source now, but I'm sure I found a reference a while back that claimed geosynchronous also applied to satellites that just happened to be over the same place at the same time each day, regardless of the orbit period. This wiki article (http://en.wikipedia.org/wiki/Geosynchronous_orbit) now calls those semisynchronous, at least.

John Mendenhall
2007-Sep-27, 05:16 PM
No one has mentioned Trojan points up to now.

Useless thought: If we identify the Trojan points as L(N), then we can have L(N) of Troys.

dtilque
2007-Sep-29, 06:04 AM
No one has mentioned Trojan points up to now.
Objects in Earth-Moon Trojan orbits do not stay over one point of the Earth. In fact, they would not even stay over one point even if the Earth were tidally locked to the Moon.

The reason is that Trojan orbits actually oscillate about the Trojan point. Now you could put a space craft at the exact Trojan point and it wouldn't oscillate, at least for a while. But perturbations by the Sun and planets would cause it to oscillate after a while.

Robert Carnegie
2007-Sep-29, 12:49 PM
Was I then mistaken to believe that the term "geostationary" was rejected as misleading because indeed the object is not stationary? Oh well.

If the earth were not rotating, and if God picked up let's say an automobile and placed it stationary in the sky and let it go, 100 km or 1000 km or 100,000,000 km - it'd fall straight down.

If it's like in [Independence Day] then it takes the same energy to hold up as any other flying machine - like a helicopter. the further out it is, the less energy is required to accelerate in the opposite direction to gravity. However, acceleration takes more than energy, it takes force, and usually mass, like the material ejected from a rocket... hey, could you hang a truly stationary "satellite" over Earth's dark side on a solar-wind sail? (But it would drift off station. Also it'd be in the shadow... and other than spectacle, what's the point?)

Since the earth -is- rotating, a satellite can be "stationary" in relation to the ground. This is how satellite television works (the cheap version; you can, or could, use a motorised dish that turns to follow your satellite.)

Arthur C. Clarke predicted it, but he assumed it'd be crewed.

grant hutchison
2007-Sep-29, 01:50 PM
Was I then mistaken to believe that the term "geostationary" was rejected as misleading because indeed the object is not stationary? Oh well.
...
Since the earth -is- rotating, a satellite can be "stationary" in relation to the ground.So, in exactly the same way that you are stationary, sitting in your chair? :)

Grant Hutchison

hhEb09'1
2007-Sep-29, 02:21 PM
Objects in Earth-Moon Trojan orbits do not stay over one point of the Earth. In fact, they would not even stay over one point even if the Earth were tidally locked to the Moon.

The reason is that Trojan orbits actually oscillate about the Trojan point. Now you could put a space craft at the exact Trojan point and it wouldn't oscillate, at least for a while. But perturbations by the Sun and planets would cause it to oscillate after a while.That's a bit of a nit, since even satellites in geostationary orbits oscillate too. A great deal, in fact, unless they are at two specific points in the celestial equator (the points about which they'd otherwise oscillate if other stationkeeping wasn't performed).

dtilque
2007-Oct-01, 06:41 AM
That's a bit of a nit, since even satellites in geostationary orbits oscillate too. A great deal, in fact, unless they are at two specific points in the celestial equator (the points about which they'd otherwise oscillate if other stationkeeping wasn't performed).

Not really. Trojan asteroids can get a long ways away from the L4/L5 points of Jupiter. The Lagrange points are 60degrees away from the planet. Trojan asteroids are found from 30 to 90 degrees away. See the image at

http://en.wikipedia.org/wiki/Trojan_asteroids

grant hutchison
2007-Oct-01, 07:22 AM
Not really. Trojan asteroids can get a long ways away from the L4/L5 points of Jupiter. The Lagrange points are 60degrees away from the planet. Trojan asteroids are found from 30 to 90 degrees away.Likewise for geostationary orbit. The stable points are at 75E and 105W longitude, so a geostationary satellite that didn't expend station-keeping fuel could resonate through 90 either side of one of the stable points.

Grant Hutchison

hhEb09'1
2007-Oct-01, 02:24 PM
Not really. Trojan asteroids can get a long ways away from the L4/L5 points of Jupiter. The Lagrange points are 60degrees away from the planet. Trojan asteroids are found from 30 to 90 degrees away. See the image at

http://en.wikipedia.org/wiki/Trojan_asteroidsBut satellites in geostationary orbit will oscillate much farther than that.

PS: oops, didn't see your post there grant, thanks

mugaliens
2007-Oct-02, 05:55 PM
can a space craft orbit in any direct artound the earth. n to e e to n s to w w to s ect ect?

can if also not orbit the earth...and stay in space without crashing down to earth.....??

you know...like...say the space craft is directly over NY city....and it stays there...not orbiting around the earth...but in sync with it..??

would it take tremendous fuel?

There are two types of orbits about which you speak. The first is geosynchronous (http://en.wikipedia.org/wiki/Geosynchronous), which allows the spacecraft to orbit the Earth once a day, but need not be directly over the equator. Instead, it wanders back and forth across the equator.

The second is geostationary (http://en.wikipedia.org/wiki/Geostationary_orbit), which allows it to remain over one spot on the Earth at all times. By definition, this must be a spot over the equator.

So, no, a spacecraft would not be able to remain stationary over NY city as it's not on the equator.

To answer your other question, yes, a spacecraft could orbit in any direction, but because the Earth is already spinning 1,000 mph towards the East, most spacecraft orbit West to East (counterclockwise if you're looking down at the North Pole). It's just easier to send them that way than the other way.

However, there are spacecraft that do polar orbits (http://en.wikipedia.org/wiki/Polar_orbit)(crossing the poles) as that provides a rapid and effective way to cover the entire globe.

Robert Carnegie
2007-Oct-03, 12:18 AM
So, in exactly the same way that you are stationary, sitting in your chair? :)

Grant Hutchison
Um, yes... if that really is accepted language used by satellite pilots, should I stop kvetching now? ;-)

Although, I am not in perceptible motion relative to the bodies around me in this room, including the floor. This wouldn't be the case if I was a satellite in that orbit; I'd see a stationary Earth but the rest of the things in space would be whizzing round and around me. Of course, the things in space are doing that now, but I closed the blinds.

neilzero
2007-Oct-03, 06:13 AM
Satelites at GEO or closer fly over great circle routes. That means the plane of the orbit needs to (approximately) intersect the mass center of the Earth. Orbits of greater radius are modified significantly by the gravity of the Moon and the Sun.
A continously running ion engine could modify an orbit at least slightly, but then it would no longer be a true orbit. Neil

Urbane Guerrilla
2007-Oct-03, 08:09 AM
can a space craft orbit in any direct artound the earth. n to e e to n s to w w to s ect ect?

can if also not orbit the earth...and stay in space without crashing down to earth.....??

you know...like...say the space craft is directly over NY city....and it stays there...not orbiting around the earth...but in sync with it..??

would it take tremendous fuel?

Yes. Earth-launched satellites traveling in any direction other than from west towards the east do take "tremendous fuel" to achieve the change of direction, and it takes more fuel the steeper the angle, or inclination, of the desired orbit relative to the Equator -- not only because of the change of velocity but also because of the power needed to carry the fuel to do it up there with everything else.

From the point of view of somebody looking at a geosynchronous satellite from New York City, he'd see that satellite, located over NYC's Earthly meridian (it's at 40 degrees 47 min N, 73 deg 58 min W) swing up towards his zenith, and back down towards his southern horizon, cycling through this once a day. It could be all the way to his zenith, or not all the way up there. Most geosynch sats are for communications and whole-earth viewing, and it's unhelpful, to say the least, for a comsat to go below the horizon where you can't send signals to it or get them from it -- so he doesn't want his comsat to dip to farther south than 40 degrees south of the Equator either. Comsats orbiting right over the Equator, on the other hand, can have a dish antenna pointed straight at them and locked. These satellites aren't going to be wiggling up and down north to south.

There are other ways to arrange comms satellites, though, if you're willing to build a system that works even if the satellite is moving about in your sky -- check the orbits of Russian Molniya comsats. They are eccentric, highly inclined, and have a big aphelion over Russia, so they spend most of their orbital time hanging high in Russia's sky, slowly moving. They spend the small remainder of their orbital cycle whipping around Earth quite close in at high speed, and getting slingshotted back up to where the Russians can use them again.