Dear Deewakar Sharma

I probably should defer to James Garry and Daniel Baldomir but I'm curious and so will venture an answer and await a better response from them. I believe that there is quite a bit of "space debris" tumbling in orbit around the Earth. I also would guess that much of it is metallic. I also would imagine that most of it has high reflectivity that would reflect away much of the sunlight that it receives. If it is tmbling then all sides would be illuminated by the Sun part of the time. Also the objects would pass in and out of the Earth's shadow. The net result, I think, would be that the objects likely experience a fluctuating temperature regime whose average would be very close to the absolute cold of space. In other words they might warm up a bit under direct sunlight but would cool rather quickly when in shadow to remain cold in the long run over time.

Deewakar,

Your intuition is correct. But the majority of metals are not ferromagnetic - hold an aluminium pan next to a magnet, and you will feel no force between them.

And the magnetic field strength falls away with altitude rapidly (mumble: inverse cube between dipoles).

But, one can use the faint magnetic field of the Earth. If the satellite can generate its own magnetic field, then a torque can be created, which *very* slowly may be used to reorient a satellite.

This is how magnetorquers work - a multi-turn coil on a spacecraft, through which a current is passed, which creates a local magnetic field, that interacts with the Earth's field to generate a torque: turning the satellite around.

It would have to be made of a paramagnetic material like iron, cobalt, or nickel. Even then the effect would probably be very weak since the Earth's magnetic field is weak.

James Garry

Thanks for your kind reply. Do you have idea in terms of quantification how much field needs to be generated locally to cause some change in motion.

I read on web that earths magnetic field is very weak ( around 0.25 to 0.65 gauss) and a normal fridge magnet has around 100 gauss ( just for comparison)

So it may seem that in order to be affected by earths magnetic field, a very strong local field needs to be created but keeping in mind the fact that in space there is hardly any drag to cause resistance to some motion, was keen to know how much in terms of some numbers it may be required.

Apologies if question is too naive.

Thanks

Deewakar Sharma Strictly, any magnetic field will create a torque. So the answer could be "any".

But it's better to think of how large a field you need in order to create a given rate of change of pointing angle?

(how fast do you want the craft to slew from out attitude to another)

Here's a little more detail:

https://space.stackexchange.com/questions/2239/what-is-the-math-behind-magnetorquers

It all boils down to the mission requirements for pointing. And note that one can only pivot around the local field - it's not a substitute for full three-axis control via reaction wheels or thrusters.

James Garry

Thanks for your nice replies.It helped to some extent.

Magnetic materials like iron are heavy and I should imagine that most spacecraft parts are made of lightweight but strong materials that are nonmagnetic. Also I would suppose that magnetic materials would be avoided precisely because they are magnetic and might interact with the Earth's magnetic field in unpredictable ways.

Dwight Hoxie The materials do not need to be magnetic to be problematic.

Any conductive material (aluminium, etc.) will have eddy currents induced in it from the motion through Earth's field.

But the effect is small. Here's a nicely worked example.

https://opentextbc.ca/physicstestbook2/chapter/motional-emf/

You may recall STS-75 - which deployed an electrodynamic tether as a means to *boost* orbital altitude - reversing the effect that a passive conductor would show.

Deewakar Sharma Feel free to ask if my replies are obscure or unsatisfying.

Dear James,

No, No I understand you completely and you are completely correct in what you say. But are not the effects of which you speak small and thus manageable? If they weren't how did we get into space in the first place? We drive steel cars through the Earth's magnetic field with nary a tug but if the Earth were an industrial strength electromagnet I dare say it wouldn't be so easy. Compass needles, of course, are little magnets unto themselves and are light of weight and balanced on pivots so as to be free to align with the Earth's magnetic field. A spacecraft in Earth orbit is falling freely in space and like the free-floating compass needle is susceptable to (gentle?) buffeting by the Earth's magnetic field. On the other hand the Earth's magnetic field is buffeted by the solar wind and a major solar storm can most certainly wreak havoc on planet Earth so the Earth's field is not without signficance. The Northern Lights are a joy to behold!

James Garry

Thanks for your kind help.I have another random and naive question. Like in any river or flowing water body ( just for analogy ), we see sometime there are vortices/wakes which may entangle an object flowing on the surface ( say a paper or leaf).

Similarly, in space is there something following which similar behavior can be observed ( or can be artificially created) ( not touching the space time web aspects :D ). Was searching on web but no hits till now.

Thanks again

Although there are many different situations ( angle of the magnetic field respect to the metal, kind of the metal, i.e. ferromagnetic, antiferromagnetic, ....). Let me try to help to understand some ideas which are not well explained here

1. A metal without magnetic order cannot interact with the magnetic field of the Earth.

2. The magnetic field of the Earth is stationary and therefore cannot give any energy to a moving electric charge.

3. Hall effect uses the magnetic field in metal only for changing the trajectory of the electrons under an electric field but never give energy to them.

4. The Eddy currents on a metal only are produced by an oscillating electromagnetic fied and never by a stationioray magnetic field. It is true that the magnetic field is strong enough for curving the trajectories of the electrons then it is possible to create magnetic vortices which in some aspects are quite similar to the Eddy currents. Another example are the Abrikosov-Nielsen magnetic vortices in a II-type superconductor. But in any case the constant magnetic field can give energy to a material as it happens with the magnetic field of the Earth.

Daniel Baldomir

Hello,

Thanks for your kind answer. If I understood well, then despite the fact that earth magnetic field is present in outer space ( whose magnitude may not be that strong to cause some tangible change), a more important aspect is that it is stationary. Thus apart from the requirements of the material ( as you have mentioned), it is essential that the field must be dynamic ( I hope I am using the correct terminology) to affect the motion of the material.

Towards the end you mentioned that static magnetic field can give energy. Do you refer here to the energy which is just energizing the material ( just like potential energy) but it doesn't affect the motion as such.

I hope I have not created some mess in understanding.

Thanks

Dear Deewakar,

Vortices are common in fluid media in both gases and liquids. On Earth in our gaseous atmosphere we have a wide variety of eddies nd currents that come and go on short time scales and in addition we hve longer-liived well-organized dust-devil vorticies in desert terrains; rotaing low-pressure weather systems on a megascale, including the north Polar Vortex; and, of course, tornados, hurricanes and typhoons on macro- to megascales. Dust devils are common on Mars. Vortices arise in Jupiter's atmoshere and likely in those soalar system moons with atmospheres.. Because of low densities I wouldn't expect there to be self-organized vortices in the interstellar medium but possibly vortices form in the presence of magnetic fields. And of course there are the spiral galaxies.

Dear Deewakar Sharma,

You are asking on the effect of the Earth magnetic field on the metals, materials which are characterized for having almost electric charge. And I think that I didn't explained well in my last sentence which is not coherent with my previous reasoning. Sorry.

I wrote

"But in any case the constant magnetic field can give energy to a material as it happens with the magnetic field of the Earth".

which should be

But in any case the constant magnetic field cannot give energy to a metallic material as it happens with the magnetic field of the Earth.

Daniel Baldomir And most satellites (all?) orbit the Earth such that both the magnitude and direction of the magnetic field they experience changes.

The Earth's field is broadly a dipole (almost) but it would be a strange craft that had its RAAN, eccentricity, and argument of perigee such that it experienced a constant field magnitude.

In short, all satellites experience a time-varying field.

Deewakar Sharma Your observation of flows in media is interesting - and Dwight offers a few more.

In all of those cases the medium itself is in bulk motion. And as far as I understand it, spacetime has a geometry, can expand, contract (and do so in particular directions) but cannot have a bulk flow. So one cannot stand in a frame of reference and 'measure' spacetime (one measures events embedded in their inertial frame) moving past.

However, one could engineer a 'vortex' with no bulk motion. A rotating mass creates a 'grain' in the spacetime around it - and this is the well-known Lense-Thirring effect.

An extreme example of this would be seen in the vicinity around a well-starved rotating black hole (don't feed it if you want to perform physics: accretion discs are violent places!). A string, dangled from your hovering rocket, would not point directly down to the black hole - but would be drawn to one side, showing that 'in' is no longer purely a radial direction (from your perspective).

Vortices are very intriguing phenomena that Nature gives us but perhaps we would not have predicted a priori even given the "curl" of vector calculus. Amazingly vortices are ubiquitous. Nature seemingly wants to enwrap herself in the form of dust devils, tornados, hurricanes and spiral galaxies when circumstances permit and those circumstances seemingly occur on many occasions and in many environments. What benefit to the Universe attends the center of a vortex? Were there nothing to be gained then why the vortex?

The mission of the Universe is to produce entropy, the more the better! To produce entropy the Universe allows entropy to decrease locally with a commensurate increase elsewhere because advancing time ultimately will erase the low-entropy fluctuation and in so doing produce more entropy than was given up in creating the fluctuation. It is a game with vortices, and we humans as well, participating as entropy deficits. Vortices and humans originate by, if you will, "shedding" entropy to create low-entropy states of enhanced internal organization. In the case of humans the internal organization is fairly obvious as is its dynamic maintenence by ingesting low-entropy asparagus and irreversibly converting it into high-entropy excrement.

The internal organization of a vortex may not be so obvious but the tightly circulating air at the central core of a dust devil or of a tornado is more highly organized than the air that it pulls in. The irreversible havoc that tornados wreak is testimony to the dynamic.production of entropy that maintains them.

These low-entropy states are unstable deficits in the entropy scheme of things and are maintained by producing entropy. They are like holes dug in the ground that can only be maintained by continually shoveling out sand that otherwise would fill them. Ultimately the entropy deficits decay having produced far more entropy in the process of their maintainence than that originally given up to create them. Five shovel-fulls of earth were removed to dig the the hole and thousannds more were required to keep it open. So vortices and humans survive as long as they are able to utilize their envrironments irreversibly to produce entropy to maintain their low-entropy states.

@ Dwight Hoxie: Nature seemingly wants to enwrap herself in the form of dust devils, tornados, hurricanes and spiral galaxies when circumstances permit and those circumstances occur on many occasions.

For another source of vortices, consider light reflected off curved surfaces. For example, consider sunlight reflected of the curved surface on the inner wall of a coffee cup and resultant light caustic. A light caustic may or may not be the beginning of an optical vortex. For more about this, see the wonderful 1999 book on light by Nye.

I follow answers

best regards

Dear James,

My posts were for answering a sentence which is in the question:

In case of satellites or any part of rocket that goes into space, to my understanding it surely has some metallic component and thus can be attracted by a magnetic field.

The magnetic field cannot do any work on the moving "free" electrons of a metal and only a change of trajectory is obtained. The reason is very simple, take the Lorentz force and multiply it by the distance (energy) and as the velocity and magnetic field and as distance goes in the same direction as velocity (scalar product of a vector proportional to other which is in the vector product) it means that you obtain a zero energy. Thus the metallic parts of the satellite cannot take electric eynergy of the magnetic field of the Earth in this form.

But your reasoning of the magnetic field depedent of time respect to the satellite is right and good. You apply Faraday's law and you obtain always an electromotriz force. No! The things are not so obvious because the geometry is involved and you need to have a circuit where the electric field can arise. For instance, if after using Stokes you obtain a closed circulation of the electric field (integral of line) within the metal (for free charge) its value is zero.

Let me to finish saying that cars are metallic and move under the magnetic field of the Earth and you don't get any kind of electricity for them, if you obtain some is mainly due to the ionization (electrostatic) due to the scattering with the air molecules. This could be the main electric effect on the metallic structure of a satellite wihile was moving within the atmosphere.

Dwight Hoxie

Thanks for your nice explanation. Somewhere between reading the answer, I was wondering from where I started and where I am. :) :D

Dear James Garry and Daniel Baldomir ,

Thanks for your kind contribution to the topic. If I have got it right, then, as per James Garry as the metallic parts ( could be any) are moving , then even though the field is stationary , their motion cause the field to behave dynamic relatively and thus there would be some effect. But as Daniel Baldomir has described, it will depend on path ( by giving an example of car, the effect is circumvented).

If I have understood well so far, then I have another point which comes to my mind. ( sorry if it is somewhat naive and stupid).

As we know, in the space, we have large temperature variations, like the sunny side may be around 2000+ while dark side below -100 deg. I understand that satellites may be designed to take care of these aspects, let us consider some space debris which has metallic component (I am not sure if space debris comprises significant metallic part).

Owing to this vast variation, there will be significant variation of the magnetic behavior of the material. In that case, even though the path may be closed, wouldn't this aspect also account for the effect? Do you think, just for discussion, that tumbling of such materials are also attributed to this?

Thanks

Dear Deewakar Sharma

I probably should defer to James Garry and Daniel Baldomir but I'm curious and so will venture an answer and await a better response from them. I believe that there is quite a bit of "space debris" tumbling in orbit around the Earth. I also would guess that much of it is metallic. I also would imagine that most of it has high reflectivity that would reflect away much of the sunlight that it receives. If it is tmbling then all sides would be illuminated by the Sun part of the time. Also the objects would pass in and out of the Earth's shadow. The net result, I think, would be that the objects likely experience a fluctuating temperature regime whose average would be very close to the absolute cold of space. In other words they might warm up a bit under direct sunlight but would cool rather quickly when in shadow to remain cold in the long run over time.

Deewakar,

Lest we forget, space is an excellent insulator - and is a blackbody at 3K. So an object exposed to raw sunlight (1.5 kW/m^2) can indeed heat rapidly (if it's small and dark) and cool equally rapidly.

There is a radiation pressure associated with sunlight - so all objects are 'pushed' slightly by sunlight, and are warmed. They radiate that heat isotropically. So the nett effect is a tiny secular increase in the eccentricity of the orbit - no change to the orbital energy as such.

The mean temperature can be trivially calculated - and is dictated by the balance of emissivity to albedo. The type of orbit also matters.

Polar? Illuminated continuosly.

Equatorial? In darkness for half their orbit.

Hi Deewakar

The vast majority of the metal in man made space debris, such as satellites is non-ferrous and so is not attracted by magnetic fields. Even those metallic components which contain iron tend to be high end alloys, many of which are not directly magnetic.

The most likely effect you will see is the opposite action, repulsion of the metal by the magnetic field, and this is only in specific situations where the magnetic field is varying by getting stronger or weaker. In this situation the metallic object will have electric currents induced by the time varying magnetic field and these currents will then induce a force which is opposite to the magnetic field.

You can observe this by dropping a strong neodymium magnet onto a copper or aluminium plate. As the magnet approaches the plate (IE the magnetic field increases) then it induces these electric currents and the magnet dramatically slows down and lands on the plate very gently.

You can see this on many Youtube videos. Yet when the magnet is stationary with respect to the copper plate there is no interaction with the non-magnetic metal

In practice the Earth's magnetic field is too weak to generate these electric currents in metallic parts so the impact is negligible.

It would be perfectly logical to put a satellite in space to use this effect to send much of the space debris whizzing around the earth into a gravity trap or towards the sun.

Dear James,

I think we agree, don't we?

Dwight,

Well, we agree in the sense that objects will cool and warm during most orbits.

There's still a secular action - and perhaps a periodic force (depends on the orbit).

http://adsabs.harvard.edu/full/1961SAOSR..60.....W

The final equilibrium temperature is entirely set by the ratio of emissivity to albedo - for a homogenous object (mirrors forbidden). One can have an arbitrarily hot object if you engineer low emissivity values with second surface mirrors (for eg).

If an aluminum body was in a circular polar orbit, then there would be some small eddy currents induced in the material than could cause a bit of drag on the object. A body on circular equatorial orbit would not be going through a significantly changing magnetic field so the drag cause by eddy currents would probable be less than the gas molecules in that zone. I'm just thinking out loud here.

Adding to answers of the colleagues above. It is so that the space vehicles are constructed form nonmagnetic light weight metals such as aluminum. Only the magnetic materials that can be magnetized by an external magnetic field like the magnetic field of the earth. In addition the earth magnetic field is not strong and its intensity decreases with altitude.

In very small satellites as pico and nano satellite, the satellite is preserved in its path by orienting it with earth magnetic field. To realize this, strong permanent magnets are used onboard of the satellites. For using permanent magnets in cubsat please refer to the link:https://www.hou.usra.edu/meetings/lpsc2015/pdf/1969.pdf

Best wishes