Probably a dumb question

[VitalApparatuz]

If an object in a similar orbit distance as Hubble had no power would it eventually fall to earth or be flung out into space?
Hubble seems so close to Earth,I know they say its 0 gravity but I don't understand that since the moon is further than Hubble but is obviously under Earth's gravity....my history and English lit. degree offer me little answers. :roll:

 

[Master_Ghost_Knight]

It would eventualy fall to earth but due to other type of phenomena.
Gravity is never actually 0 even if you were at the other end of the universe, but it is rediculously small. And the fat that hs is orbiting arround whatever gravity is left is counter balanced by the centrifuge force (in a semi-stable balance).
It would eventually fall of because the space isn't really empty and the satelite would hit sveral particles that are also caught in that orbit.

 

[VitalApparatuz]

Hey thanks MGK,
I figured my facts were too simplistic.You science guys rule

 

[Netheralian]

The propulsion systems on spacecraft like Hubble are there ultimately to counteract the aerodynamic forces (as Master Ghost Knight said - small particles - but they aren't actually in orbit) due to the earth's atmosphere (exosphere, ionosphere etc: see http://en.wikipedia.org/wiki/Earths_atmosphere#Temperature_and_layers).

To reach escape velocity (flung out into space) you would have to inject quite a lot of extra delta V (velocity increase) otherwise you would just put your self into a higher orbit from which you would also eventually degrade.

The aerodynamic forces are a bit of a drag (pun intended). They are generally the reason why you need a propulsion system in the first place. Pointing your satellite is easily taken care of using momentum wheels (sort of like flinging your arms around when your on a slippery surface) because they only require energy and not propellant mass. Without a propulsion system your orbit slowly degrades until you burn up in the atmosphere or crash into the earth. Which is why satellite disposal is a big thing these days. Often, to put a new satellite up, you also need a disposal plan that causes a controlled crash into the atmosphere so you don't end up with very large law suits or dead people...

Hubble is actually a fair way out at 560 kms or so. ISS on the other hand is at 347 kms with an exponential increase in atmospheric density so they require a considerably larger amount of fuel to keep it there.

Off the top of my head, a spacecraft I was working on a few years ago also had an operational orbit of around 500kms. Its plan was to purposely degrade its orbit due to aerodynamic drag so it can do science on the way down. From memory it had about a 4 year life before its orbit had decayed so much that it was earth fodder...

Was that more than you needed. Sorry - I'm just little bored at the moment. My Work PC is chugging away on a thermal problem and i can't do much else in the meantime...

 

[VitalApparatuz]

Awesome thanks Netheralian

 

[Ozymandyus]

It would eventualy fall to earth but due to other type of phenomena.
Gravity is never actually 0 even if you were at the other end of the universe, but it is rediculously small. And the fat that hs is orbiting arround whatever gravity is left is counter balanced by the centrifuge force (in a semi-stable balance).
It would eventually fall of because the space isn't really empty and the satelite would hit sveral particles that are also caught in that orbit.

I just wanted to clarify this 'Balanced by centrifuge force' thing... Centrifugal force isn't even a real force, and it isn't acting here. The gravity is acting as the centripetal force, and it's not really balanced by any opposite force - The orbiting object is moving forward fast enough that the acceleration towards earth caused by gravity is curving its path around the earth rather than pulling it into the earth.

 

[Master_Ghost_Knight]

I just wanted to clarify this 'Balanced by centrifuge force' thing... Centrifugal force isn't even a real force, and it isn't acting here. The gravity is acting as the centripetal force, and it's not really balanced by any opposite force - The orbiting object is moving forward fast enough that the acceleration towards earth caused by gravity is curving its path around the earth rather than pulling it into the earth.

In fact centrifugal force is a virtual force (a real force if the coordinate system was for instance polar) it is a linearized model to describe what the object naturaly does (i.e. that keeps going forward escaping away from the geometric path of the earth if nothing was done) in a sense we can say that the moon is allways falling to the earth (but because the earth is round the floor escapes from him), but that is a more complicated way to see things for those who are not to knowlageable of the mechanics. The virtual centrifugal force is a simpler way to put it, in fact it is the general way we do most calculations.

 

[Ozymandyus]

And as I said, centrifugal force as you used it is not a real force, and then you corrected me by saying it is, in fact, a virtual force. Which is precisely to say that its Not Real.

It seems to me to confuse the issue in practical terms. But you can disagree if you want... either way saying

whatever gravity is left is counter balanced by the centrifuge force (in a semi-stable balance).

seems to me to be seriously confusing the issues at hand. Saying centrifugal force 'balances' a real force(gravity) implies that there is a real force being exerted, which there isn't, and directly confounds the issue. It may be a convenient way to do calculations but it is not a convenient way to explain a concept in my opinion. The only real force at work is gravity which is constantly changing the direction of the object's velocity. And of course the small forces of atmospheric drag solar winds, and tidal friction which result in a very slow orbital decay.

 

[Master_Ghost_Knight]

And as I said, centrifugal force as you used it is not a real force, and then you corrected me by saying it is, in fact, a virtual force. Which is precisely to say that its Not Real.

I never said it was real or ficticiouse before my last post, and because it is ficticiouse it doesn't mean that it isn't real.

seems to me to be seriously confusing the issues at hand. Saying centrifugal force 'balances' a real force(gravity) implies that there is a real force being exerted, which there isn't, and directly confounds the issue.

No. People tend to have the preception of falling to be getting closer to the earth, that generaly mathematicaly translate into a polar coordinate system (altough he doesn't realise that that's what it is) and in that prespective you must necessarly use the centrifugal force or you have no balance, removing it would implicate that it must fall straight down which isn't true. In cartesian system you can not even refer to centrifugal or centript forces, it makes no sense.

 

[VitalApparatuz]

Well I feel I know more than I did when I asked so thanks to all of you

 

[sgrunterundt]

I just thought I would link to this classic

I agree with MGK, fictious forces are very real in rotating coordinate systems. In general relativity they can even be explained by gravitomagnetic effects from the rest of the universe rotating around the system. It simply depends on perspective.

 

[bemanos]

the gravity affects it normaly but its in constant fall so its falling all the time but becuase earth's curve it never reaches the surface(due to hight speeds and no friction of air) it will burn in atmosphere eventualy( becuase of the friction it will slow down and fall )

also if a object leave earths gravity that mean that it have reached an escape velocity around7-9 km/s for earth if i remember right
if the object travels below escape velocity speed then it stays in orbit

*note that there is friction in leo it is just very small to affect short time orbit periods(there are plenty of molecules hitting the object orbiting so they affect it eventualy)


*note 2 . if there is a mistake in my explanation please correct me