23
$\begingroup$

I was watching this video of a magnet aboard the ISS floating freely. When the person lets go of the magnet, it behaves like a compass needle and immediately orients itself to the magnetic field of the earth.

Why doesn't the entire ISS (or any other space vehicle for that matter) do the same? I would expect that there must be some net magnetic field to the ISS (regardless of how small its magnitude), or that some objects aboard or fixed to the ISS would have some net magnetic field, causing the station to reorient itself to align with the Earth's magnetic field.

Is there some care given to ensure that the station does not have a net magnetic field or to actively neutralize it? Or is the station constantly fighting this magnetic force with its control moment gyroscopes? And if so, how significant is the effect (seems significant, judging by the magnet in the aforementioned video), and furthermore does that mean that all spacecraft or satellites would need the same feature, to be able to combat the tendency to align its orientation with the Earth's magnetic field?

$\endgroup$
5
  • 25
    $\begingroup$ It's made of aluminum. $\endgroup$ Jun 28 at 21:43
  • 25
    $\begingroup$ Why don't ships do that in the ocean? $\endgroup$
    – DrSheldon
    Jun 28 at 23:13
  • 2
    $\begingroup$ Related: Could Magnetorquers be used on the ISS? The answers to the linked questions say "this is a bad idea". $\endgroup$ Jun 29 at 10:32
  • 6
    $\begingroup$ @DrSheldon Most ships are in forward motion, and keep their orientation because of hydrodynamic forces, subject to waves, and current. The ones that aren't are overwhelmingly at the mercy of the wind. It seems you are suggesting that atmospheric drag forces exceed the magnitude of magnetic forces acting on the ISS. Actually, I'm seeking to understand the relative magnitude of the magnetic force compared to all the other forces involved, including forces I haven't explicitly mentioned like gravity gradient torque. $\endgroup$
    – Wyck
    Jun 30 at 13:42
  • $\begingroup$ Even pieces of aluminum can have an eddy-current induced magnetic field. $\endgroup$
    – spuck
    Jun 30 at 21:29
14
$\begingroup$

Because the people that run it don't want that:

The magnetic field of the ISS is quite weak. This would be a combination of deliberate and inadvertent , as it is built out of mostly nonmagnetic aluminum, plus any really significant magnetic field would be a bother to the many instruments, science packages and electronic equipment on board.

The ISS has a much stronger force acting upon it than the Earth's magnetic field. The gravity gradient would really like to orient the ISS so that its mass axis is pointing through the Earth's gravity vector. (see "tides")

But the ISS has yet another force acting upon it, that is overwhelmingly larger than either of these. It is called "Stationkeeping". The rules dictate that the ISS shall maintain a certain orientation relative to the surface of the earth, and the solar angle, and no mere tiny physical forces will overwhelm the rules as stated by the Administrators!

The actual implementation of this attitude control is via a number (4?) of Control Moment Gyroscopes, occasionally supplemented by attitude thrusters if they need desaturation.

$\endgroup$
4
  • $\begingroup$ While we call it a "space station" it is actually a "spacecraft". The word "station" comes from the same root as "stationary", both something we consider as something that does not move. In a way the station does not move, it is built to have a consistent track around Earth so we always know where it is at any given time. As we all know, and often apparently forget, is that it is moving all the time and is flying through space like any other spacecraft will. Unlike other spacecraft it doesn't exactly have a pilot seat but it does have engines and control systems to control it's flight. $\endgroup$
    – MacGuffin
    Jun 29 at 11:37
  • 2
    $\begingroup$ @MacGuffin It's "stationary" because it just sits there (in the LVLH frame) in the same inclined LEO for two decades, doesn't make weekend trips to the Moon like a proper spacecraft would, doesn't even come down occasionally to meet the Shuttle anymore. $\endgroup$
    – TooTea
    Jun 29 at 13:07
  • 3
    $\begingroup$ @MacGuffin, TooTea: I'm quite certain that "space station" derives immediately from "naval station" and "air station". These are places where ships and aircraft come to rest. So it's not the space station itself that is stationary, but rather the visiting spacecraft (relative to the station). $\endgroup$
    – DrSheldon
    Jun 29 at 18:18
  • $\begingroup$ @DrSheldon That's actually a question for SciFi Stack Exchange. "Space Station" was probably used in Pulp Magazines even before Werner was paperclipped out of Germany. $\endgroup$ Jun 30 at 8:21
26
$\begingroup$

The ISS is more of an aluminum can than a bar magnet

Sources: bar magnet (annotated with a red "X"), aluminum can (annotated with a green "OK" check mark)


Beautiful video and demonstration, thank you!

However...

That magnet in microgravity doesn't actually orient itself at all

I watched the video carefully and at 00:44 the first time the magnet is released it is pretty far away from the aligned direction and it does not "immediately orients itself to the magnetic field of the earth." Instead it oscillates dramatically about that point as energy is exchanged between kinetic and potential.

Eventually eddy current losses and viscous drag losses in the air would damp it down (loosing it's energy by heating the magnet and the air) and it would then look like a 3D magnetic compass.

Only two seconds later at 00:46 game developer and private astronaut Richard Garriott(?) recognizes the oscillations are to large, grabs it, and tries twice more until at 00:54 when they re-release it close to the median point, where it still oscillates at the same frequency but at a lower amplitude.

The ISS is not a bar magnet, it's an aluminum can!

The actual bar magnet that wealthy game developer and amateur science video maker Garriott uses is no model for a mostly-aluminum structure in space.

While as @OrganicMarble points out the station's structure is primarily aluminum, there will be enough ferromagnetic material with some residual magnetization (unless everything sent up is methodically degaussed, which is always a possibility) some permanent magnets inside motors with imperfect return yokes, and of course some loops of current that all together are likely to result in at least a tiny residual magnetic moment for the ISS.

Even a good captain of a wooden ship will (presumably) do both "soft iron" and "hard iron" corrections for the compass every time a substantial change has been made to the metal content of the ship.

But mostly you should think of the ISS as more of a non-magnetic aluminum can than a bar magnet.

The ISS is already spinning quite nicely

It orbits the Earth every 90+ minutes and in order to keep its down-side-down it also rotates about one of its own axes every 90 minutes as well. There's a lot of angular momentum stored there and that probably dominates any interaction between any residual magnetic moment and Earth's field.

It also works very hard to maintain that state.

And if it lost that ability to control its attitude and rotation, atmospheric drag (a very lossy and damping influence) would either forcefully align the ISS to a new equilibrium rotation state with a minimum drag orientation, or cause it to spin and tumble wildly.


Further reading:

$\endgroup$
2
  • 4
    $\begingroup$ FWIW the can in the photo is very likely steel, not aluminum (or tin). $\endgroup$
    – Reid
    Jun 29 at 21:18
  • $\begingroup$ @Reid Good point! I have a similar-looking pull-top food can here at home and gave it a magnet test and in fact it is steel. Okay I'll look for a proper aluminum can, maybe soda or beer... $\endgroup$
    – uhoh
    Jun 29 at 23:31
11
$\begingroup$

This isn't an answer, just a minor addition to the previous answers. There have actually been cases where satellites have accidentally become magnetized and caused an undesirable magnetotorque; it is a legitimate consideration!

From https://llis.nasa.gov/lesson/642 :

An unanticipated roll torque was imparted to a commercial communications satellite launched in 1989. This torque mandated excessive thruster corrections by the attitude control system until the propellant was prematurely exhausted, resulting in mission failure.

Subsequent magnetic characterization tests performed on an engineering model of the large stainless steel propellant tanks revealed that the tanks could be easily magnetized during or after fabrication. The dipole moment of a magnetized tank would interact with the earth's magnetic field. This force would attempt to align the spacecraft with magnetic north, like a compass needle, producing the observed roll torque and requiring excessive attitude corrections by the thrusters. Testing showed that once magnetized, the tank would be capable of retaining a residual field that produces a dipole moment much greater than the 580 A-m2 required to produce the observed torque.

The satellite's two stainless steel propellant tanks had been substituted for titanium tanks due to schedule deadlines. Since stainless steel material is generally assumed to be non-magnetic, no magnetic testing was performed. However, it is possible for the stainless steel sheet metal to become residually magnetized, with an associated moment, either by working it into the tanks' hemispheric shape or by exposure to an external magnetic field.

The AMS-2 uses a large solenoid magnet. The paper

Blau, Bertrand, et al. "The superconducting magnet system of the alpha magnetic spectrometer AMS-02." Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 518.1-2 (2004): 139-142.

contains the following text:

  • to minimize the magnitude of the stray field outside the magnet (maximum stray field is 4 mT at a radius of 3:0 m);
  • to minimize the magnetic dipole moment of the magnet system to avoid an undesirable torque on the ISS resulting from the interaction with the Earth magnetic field 0.27 Nm
$\endgroup$
1
  • 1
    $\begingroup$ Great point about the AMS. $\endgroup$ Jun 29 at 17:17
7
$\begingroup$

As other answers have noted, the bodies of most spacecraft are made of nonmagnetic aluminum. However, some smaller spacecraft do use the Earth's magnetic field for attitude control. These smaller spacecraft use current that the spacecraft runs through magnetic torque rods to adjust their attitude.

There are two key issue with magnetic attitude control. The torque is small, which is why magnetic torquers are only used for smaller satellites. Another issue they do not and cannot provide control over all the three degrees of rotational freedom. There is always at least one degree of freedom that is not controllable via magnetic control, which means that spacecraft that do use magnetic torquers need to have some other attitude control mechanism.

$\endgroup$
3
  • $\begingroup$ Hypothetically speaking, If you have a lot of time to do it (~weeks or more) and have an orbital plane far from the magnetic equator, and have an ephemeris, clock and/or knowledge of your orbit, can't you eventually zero out any rotation with 3 axes of magnetotorquers since the direction of the field wrt spacecraft local coordinates will cycle through different directions over one orbit? $\endgroup$
    – uhoh
    Jun 29 at 0:31
  • $\begingroup$ Just for example, the magnetic field for an $i=0$ equatorial orbit i.stack.imgur.com/YUBaD.png (from here) $\endgroup$
    – uhoh
    Jun 29 at 0:45
  • $\begingroup$ @uhoh It depends on what kinds of disturbances the vehicle is subjected to. There is always one instantaneous axis over which a vehicle using only magnetorquers has zero control authority. $\endgroup$ Jun 29 at 10:40
2
$\begingroup$

The compass needle you saw is made from a magnetic material like steel. The ISS is made from a nonmagnetic material aluminum.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.