The Space Station Grounding Problem

Cecil's Columns/Staff Reports
1

Fine and good, in a well-made circuit, there’s no need for a ground, as there is little danger of too much energy being suddenly delivered.
But can every single one of the millions of circuits on that giant mass of tubes be perfect?
More importantly, perhaps, is the issue of the solar wind. Depending on whether the ISS is near the north or the south poles of the Earth, it will be bombarded by positively or negatively charged particles most of the time.

Unless it travels quickly from over one hemisphere to over another, it will build up an overall charge from all those collisions.

No convenient place to discharge.
I wonder… if it’s near the north pole in the morning of Houston, and near the south in the evening, the charge picked up in the first few hours will be canceled out by its return to the same spot the next day. Or returned to its earlier state, rather. Granted that nothing abnormal happens.
So, the whole shebang is gaining a slight amount of mass at all times. I wonder if this is noticeable, or comparable to the losses of evaporation, discharge of wastes, leaks, etc… probably not.

Anywhoo, say there’s an electromagnetic storm. You can’t do sensitive experiments, because even if you have only a ratio of 1 billion protons to 1 billion and 1 electrons over the whole station, there will be a measureable difference in how things behave.
Because they will be charged the same, objects will tend to repel one another.
I’m lazy and don’t want to do the math.

I dunno the mass of the ISS either.
Lessee… to figure out how much experiments are whacked up… remember, this is IMPORTANT! Crystals need to grow in a known fashion, they need to know what forces are affecting everything. And electric discharges in the electronics… and the BRAIN! Hmm…

This has probably been figured out by NASA.
P.S. I bet there’s a fat old spark (well, maybe not visible - just a tranmsfer of charge) whenever something docks at the ISS.

2

The link to the Staff Report under discussion is: How can electrical outlets in space be grounded?

It helps to provide a link to the Staff Report… helps keep everyone on the same page, so to speak, and avoids repetition.

3

Actually, the International Space Station IS grounded!

Sounds weird, I know, but the ISS is grounded. Not to the Earth, obviously, but to the ionosphere. Well, sort of, anyway. There’s a device for disposing of excess electrons. It’s actually not so much to protect onboard electronics inside the station (lightbulbs, laptops, electric razors, etc) but to protect the delicate equipment on the outside and to protect the astronauts during a spacewalk from having a few thousand volts go right through them (which, though it might not kill them outright, could do terrible things to the life support systems on their suits).

The system that does it is called the “ISS Plasma Contactor Unit” (PCU). There’s also an experiment mounted on the top of the P6 Integrated Truss Structure (the thing holding up the really big solar panels) to measure voltage and see if the plasma contactor really is working.

Read about it here:
http://space-power.grc.nasa.gov/ppo/projects/iss/plasmacontactor.html

The device is needed. On craft like the Space Shuttle and smaller things like the Soyuz, static charge isn’t as much of a problem. You can provide some common internal “ground” and keep the internal equipment working fine, and things will be great. But the ISS is much bigger. And most importantly, it has huge solar panels. And there will be a lot more panels on it when it’s done. These are the biggest solar panels ever deployed in space. And they are conductive. And they are plowing through the ionosphere at 17,000 MPH. The air is so thin up there that as far as our respiratory systems are concerned, it’s nonexistent. But in fact there actually is some air – it’s just very very thin. It’s also very energetic because the higher up you go, the more exposed you are to solar radiation. The ionoshphere is called that because it’s made up of ions – charged particles. As the solar panels plow through them, they accumulate a charge which is swiftly distrubted around the station’s sealed exterior. When an astronaut floats out and touches it, they risk getting an almighty shock.

As another user pointed out above, it also poses a problem for docking if that additional charge is allowed to accumulate, it could result in a large shock between the ISS and the arriving spacecraft. I would bet that there is some grounding at this time, but it’s not much. I’ve watched videos of dockings and I’ve never seen a visible spark, at any rate.

4

This URL from Science@NASA goes into more detail on the subject of why the ISS needs a ground and why older spacecraft could get away without one.

http://science.nasa.gov/headlines/y2001/ast13nov_1sidebar.htm

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It would appear that I was partly wrong. From those two NASA pages, though, it would appear that the primary problem is not the overall voltage of the station (relative to the ionosphere), but differences in the voltage at different points, due primarily to the operation of the solar panels. I’m not clear on why one of the voltages on the panels had to be at ground, though: It seems to me that they could have avoided the problem by having the panel leads straddling ground.

In any event, the problem is not the solar wind. The solar wind is fairly uniform, and electrically neutral on net: The protons are more massive, but there are still just as many electrons. There is not a north-south bias for the different charges: The Sun (and the Earth) has a north-south magnetic field, not an electric field. The problem is the ionosphere, and craft leaving the Earth would not have to worry about that.

6

On behalf of engineers everywhere… thbbbbbbbbbbtttttt :stuck_out_tongue:

Anyway, “ground” is a very much abused term. Voltage is a measurement of potential between two points, so for the most part it’s pretty meaningless to talk about voltage at a single point (one of those pesky scientist types will probably jump in right about now and start going off about point charges, but let’s just ignore that for the time being). If you go down to ye ol local radio shack and buy yourself a voltmeter, you’ll notice it has 2 leads. You measure voltages between two points. Simple enough?

In any system, if you want to keep your sanity, you choose a common reference point, then make all voltage measurements relative to that reference. Let’s take a simple flashlight, and to make things really simple let’s assume it is a single battery flashlight (one of those little pen lights). You can call your reference anything you want, but if you want to save yourself a lot of headaches it’s best to call it zero. So, in our example, let’s call the negative terminal of the battery our refernce and we will arbitrarily call that zero volts. The positive terminal of the battery is now 1.5 volts (because batteries are basically constant voltage sources). We could just as easily have called the positive terminal our zero reference. In that case, the negative terminal would have been -1.5 volts.

We could just as easily have called our positive terminal 1000 volts (or any other arbitrary number). If we did that, our negative terminal woud have been 998.5 volts. No matter what arbitrary number you assign, the light bulb always has 1.5 volts difference in potential across it.

Electricity always flows in circuits. You have electricity going out (this is often called the “hot” wire) and you have it coming back in (this is often called the “return” wire). Engineers very often tie all of the returns together, and we (arbitrarily) call this our zero volt reference point.

For noise and other reasons like safety, we often connect all of these returns physically to earth ground (quite literally, sometimes by driving a copper rod right into the mud). Because of this, zero volts and returns are often call “ground” regardless of whether a true connection to earth ground exists. This is what is happening in the space station. They are not making a “ground” connection, they are making an equipotential reference point throughout the station.

This isn’t really anything new. Your automobile does the same thing. There’s a 12 volt battery, and the negative terminal is called “ground” and arbitrarily assigned to be the zero volt reference. All measurements in a car are made relative to this, so the 5 volts in your radio’s computer is 5 volts between the logic circuits and the negative terminal of the battery. The negative terminal of the battery is connected to the car frame, and all of the electronics use this as a reference. It keeps the car at a constant potential, but since there is no physical connection to earth ground, the voltage at any point in the car with respect to earth is pretty much meaningless.

Grounds that aren’t grounded (I told you “ground” was an abused term) are often called “floating grounds.”

There are two types of power systems, grounded and isolated (not grounded). Isolated systems, because they have a floating ground, are actually safer. You can touch either conductor and not get shocked. In a grounded system, because your body makes a connection to earth ground (a fairly poor connection, but an electrical connection nonetheless), if you touch the ground wire you are safe, but if you touch the other wire you get shocked. If you are ever in a hospital, look for the red outlets. These are fed from isolation transformers and are not grounded. Residential wiring is always grounded, because in a large system if you don’t ground it, mother nature will usually randomly insert ground connections for you, and rather than constantly fight to make sure your system stays isolated (which hospitals do) it’s easier to just ground one conductor and be done with it.

Since residential systems are grounded for safety reasons, this leads to the misconception that ground = safety, therefore the space station, if not grounded, is unsafe! Good heavens, it needs a ground! Actually, the space station doesn’t need a ground any more than your car needs a ground, but to keep the equipment operating they do need to get rid of voltage differences throughout the station, hence the “ungrounded ground” that they use.

7

Craft leaving the Earth might have to worry about it eventually. It depends on where they are going, what their power systems are like, and how long they are going to take getting to their destination. Other planets with strong magnetic fields (such as the gas giants) also have ionospheres. (Planets like Mars don’t, as such, though there is still enough atmosphere in a low Martian orbit to generate a small amount of friction. Not anywhere near as much as what the ISS experiences, however.) And there is a very diffuse plasma not only in the solar wind but in the interstellar medium. It’s much sparser, though, so it won’t generate static charge as quickly.

The solar wind is a contributing factor, however. What gives the station its charge is, indeed, friction with the ionosphere and not the solar wind. But the solar wind (and other aspects of solar activity) is responsible for the shape of the Earth’s ionosphere and affects the strength of the Earth’s magnetosphere (magnetic field). During solar maximum (as now), the ionosphere is generally denser and bigger. This has a direct affect on how quickly charge is accumulated. It also has a direct affect on the station’s altitude; greater density means greater drag, which means the station slows down. Orbital mechanics tells us that slowing down means dropping to a lower altitude, and then the problem gets worse because the atmosphere is even thicker there. So when the station is picking up the most charge, it also needs the most engine firings to maintain its safe altitude. (Skylab fell out of orbit in the late 70s partly because of scientists underestimating how much denser the ionosphere can get. They were planning to reboost it, but never got the chance.)

8

engineer: Thanks. Between you and calliacale and Chronos’ original reply, I think I understand all this a bit better. I still worry about all those loose electrons looking for route home from space, but at least I know the engineers are on top of it.

Okay, related but different question: How will all these issue affect a space elevator? One of those big cables that dangle from a space station and are anchored to the earth, allowing freight and passengers to go up and down. Is it sufficiently grounded? Will the large distance between top and bottom cause greater problems?

9

engineer_comp_geek has it right, and is pretty much agreeing with what I said. You’d be amazed, though, at how many engineers and engineers-to-be I’ve met who didn’t understand that the potential reference is arbitrary. And physicists generally use the convention that “zero volts” is the potential an infinite distance away from any charges, but that only matters if you have a wire going out to infinity. It’s still only the potential difference that has any effect on experiments.

DRomm, don’t worry about the electrons trying to get “home”. Home for those electrons isn’t the Earth, it’s just the other end of the solar panel or power cell they came from. There’s a perfectly good path for them to take to get there, though.

With a space elevator, electrical grounding can be a very big issue, though. It’s partially resolved by the fact that the cable itself would almost certainly be conductive: The only plausible candidate material so far is carbon nanotubes, which are excellent conductors. Even so, that’s an awfully long wire, and even nanotubes might not be a good enough conductor. I understand that the folks working on space elevator ideas have the problem under control, but I don’t know all the details.

10

================================================
I used to work at JPL, where making sure that a deep space probe has the same potential as it’s surroundings and/or its exhaust stream can be very important!

The problem was that particles from space or from the exhaust would be attracted to the craft (and specifically its instruments or optics) if the charge balance weren’t maintained. Evidently most thrusters spall out positive charge (tribocharge, technically - like when you rub something and get static electricity), because the solution is to build a tiny electron gun into the probe to eject negative charge into space. If you do it right the spacecraft doesn’t attract nearly as much junk and the instruments work a lot better.

I heard that something similar was used on most big sattelites but that it was somehow different (ie. we couldn’t just use one of those standard modules for our deep space probe).

I was working on an IR imager at the time so it wasn’t my specialty but there was definitely a whole particle gun group who specialized in puttig=ng the spacecraft ground at the correct level relative to its surroundings.

DG

11

Hello - don’t mind me … I’d just like to throw in my two centidollars: (my apologies for the length of the post - I’m a teacher - it is my nature to ramble)

The discussion seems to be going well and I’m learning a lot, but I feel obliged to clarify one thing: earlier posts have refered to how potential in general and zero volts is arbitrary (engineer_comp_geek in particular seems to understand the physics, but has brushed it off - please don’t take that as an insult).

While it is true that in most “ordinary” situations it is common to arbitrarily assign a zero (or “ground”) potential, and make potential measurements relative to it, this cannot always be done - especially in extreme situations. - I suppose that this makes me the “pesky scientist type” referred to above.

Physisits define zero potential as being infinitely far from all charges (one can see why an engineer would dislike this). Consider this though: Suppose I make a ball (of macroscopic size - say a kilogram) which is made up of only protons (impossible, but oh well, let’s try it anyway). A physicist would have to assign the object a potential that is very high … assigning this a potential of zero would be a mistake, because if a neutral object ever came near it, the atoms of the object would become ionized, which would change the characteristics of that object. (actually with one kilogram of pure protons it would be much much worse)

Consider the ISS (shuttle etc). If it developed a very large charge (+ or -) this charge could (if high enough) actually alter the electrical properties of the devices near it or on it. Granted, it is also possible to shield sensitive devices from these effects, and I assume this is done.

By the way - I agree with the above posts in how it is not necessary to ground all circuits anyway - provided the charges are not extreme, circuits will function just fine. Airplanes are good examples: dangling a ground wire down from them would probably not be a good thing.

By the way, when an airplane lands, it can have a fairly significant charge on it - the ground crew actually places a ground cable to the fuselage to bring it to the same potential as the earth, and will also use a line to connect the fuselage to the fuel truck, to ensure that they are at the same potential. Without knowing it, I would suspect that a ground crew that touched a just landed plane (or shuttle) would receive a good sized shock.

Hope this is of some help - if anything I said is not clear feel free to drop me an email.

12

Yes, but that’s because the potential of that ball would be very different from its surroundings, not because the ball is at a high potential. And if we can suppose one such ball, why not two? Put an object midway between the balls, and it won’t get any net polarization.

13

Those pesky engineers that get things messed up!

:rolleyes:

14

Wouldn’t the cable run the risk of shorting out whatever insulated natural potentials there may be around the earth? Would it affect thunderstorms? Wasn’t there an experiment with the Shuttle trailing a long cable, and the cable building up voltage due to relative motion with the ionosphere?

15

Relative motion to the ionosphere would, indeed, produce very significant currents, but a space elevator would be essentially at rest with respect to the ionosphere. Last I heard, the possibility of shorting out the ionosphere was considered, and found to not be a problem. And such a cable would, indeed, tend to prevent thunderstorms, but you would want to put it somewhere which gets very few thunderstorms in the first place: I believe that the current propoasal is off the coast of Ecuador, which gets very few storms. This is a good thing, because in addition to the cabel affecting lightning, lightning would also damage the cable.

16

I agree with the above, but will use it to add validity to my earlier post.

The ball is very different than its surroundings - true, which is exactly why it should not be given an “absolute zero” potential. It is very positively charged compared to the “normal” (ie neutral), which is why it is at very high potential.

As for two positive balls - again, I agree that being in the midpoint will result in no polarization … but how would you get an object there? Begin with an object that is far from either of the two. If that object is like- charged, work will need to be done to get it to the midpoint (this requires the addition of energy - which is the true meaning of potential). If the object was neutral, it will become polarized on the way to the midpoint, which can result in the same problems discussed above.

17

Hey, you’re the guy who introduced pure proton spheres. If you can imagine those, then I can imagine putting something between them. Yes, it takes energy to assemble the balls, but that’s just because we live in a universe that doesn’t already have such things assembled in it. We could, in fact, call the surface of those balls “zero potential”, and no harm would be done by it. Oh, it might make calculations a little uglier, and it’d be silly, but it’d be no less valid.

18

Another factor that the article did not mention is that some circuits, it’s useful to be able to "dump"current without changing the voltage of the destination. Take an xor gate, for instance. If a single input is “1”, then it just flows through. But if both are “1”, then you have to get rid of the current, and the simplest way is to send it to “ground”. In this case, “ground” need not be at any particular voltage, but it does need to have to have a capacitance much greater than the rest of the circuit.

19

To get back to the original question:
It’s not much different than an automobile. A car (in dry weather) does not have an electrical contact with the earth because there is no need for one. The parts of the vehicle are electrically bonded to each other, to reduce the possibility of differing potentials. It would be helpful to use the term “ground” (“earth” in British terminology) only when referring to contacts with the earth. Even bureacrats at OSHA understand this distinction.

As a scientist, I’m inclined to agree with the sentiments about engineers posted above. But the scientists here would do well to remember the caveat of one of the scientist characters in Kurt Vonnegut’s Cat’s Cradle (paraphrased, of course): “Any scientist who cannot explain his field to a 9-year old is a charlatan.”

20

Doh! Beaten to the punch! By an engineer! But there’s much more detail there than needed to make the point. And not all cars have 12 volt systems or negative grounds. Older British sports cars are notorious for having positive ground systems that confound Amnericam mechanics. Many old tractors have 6 volt electrical sytems. Needless detail tends to confuse folks.