Grounding yourself... in space!

[cljohnston]

So I'm watching the astronauts installing the racks in Kibo, and I notice they're all wearing grounding straps, like good electronics technicians should.
But then I thought, "Hey wait a second... How the heck do you ground yourself in space — where there's no actual GROUND?"

The Brits call it "earthed", largely 'cause you're connected to a pipe that runs into the Earth!

I'm sure there's a nifty method they use.

 

[Coolhand]

I'd assume static electricity just gets dispersed into the metal structure of the station, since it's so huge compared to the astronaut. didn't know they 'earthed' themselves in space or if it was possible either. In fact is static buildup a big concern on an orbital structure like the station, as it rubs through the outer atmosphere or is exposed to solar wind?

 

[simonpro]

Think how a plane grounds itself

 

[Woo482]

Lands on a runway?

 

[simonpro]

Electrical equipment aboard aircraft is grounded, but having stuff grounded doesn't mean it has to be grounded to the same as the earth

 

[Ursus]

Even if there's no ground to which to "ground", you still need to equalize electrical potential. I'm sure there's many ways in which the technician or the vehicle can lose or gain a few (billion) electrons. The grounding strap keeps the electrical potential even between the two, so that it isn't suddenly equalized via a discharge that goes directly through a piece of expensive and delicate electronic equipment. Even if it's a piece of fairly inexpensive and delicate equipment, the shipping charges for replacement parts are awfully steep out there.

Most vehicles (and electronic assemblies) have what's called a "ground", or common return, even if they have several inches of rubber, several thousand feet of air, or several miles of air and empty space between the vehicle and the earth. One pole of the battery (or other electrical source) is connected via a direct electrical path to the chassis, and/or a grounding bus, while the other pole is routed through a fuse or breaker bus to supply power to the various electrically operated systems.

Of course, in the case of aircraft, which can build up a high difference in potential between itself and the earth, the chassis ground needs to be connected to an earth ground while it's being refueled, so that it finds common ground with the refueling system before the fuel nozzle is inserted, so... Well, I guess we weren't talking about refueling and making planes go boom, but we were talking about ground.

 

[to be]

Right, grounded refers to having everything at the same voltage, you could be charged to a million volts compared to earth and be "grounded", in that there is no difference in voltage between you and everything else in the space station.

 

[Coolhand]

So what happens when a vehicle docks with the station? is there a vast difference in potential between the 2 objects that needs to be 'grounded'.

 

[simonpro]

There can be, yes. Not sure how it's done with the ISS but typically docking bodies will have an extended boom that acts as a conductor to equalise the P.D between the vehicles. Of course, if both have been travelling in formation for a while this is less of a problem, as the spacecraft are still grounded to the ionosphere.

 

[Artlav]

I remember reading a scene from one of Lem books - a starship about to accelerate back to Sol system meets a small dusty asteroid in the interstellar space. One of the crafts aboard are sent to investigate.
As it approaches the asteroid and is about to land, an almighty spark goes between the two, "like an engine pulse to zero out relative velocity", effectively creating a good setting for some rescue in the dust cloud scenes.

I thought the electric charge was not a relative thing - if the station will keep to accumulate either of the charges, it may not create sparks inside, but should the things not start repel each other?

 

[gawinnard]

Little experiment for all of you who are 'static' magnets, like myself:

I always get static shocks when I'm riding on escalators in shops. The first think I do is touch a metal component on the escalator, usually just below the moving rubber 'grip', stay in contact with it and you should not be shocked. I think the 'theory' is that you're not getting chance to build up enough static electricity as you're grounded/earthed all the time, returning any charge to the ground/earth.

 

[Zatnikitelman]

Technically, they should, but because there is nowhere for things to go and because the charge is equalized between all parts of the structure, the charges don't cause things to fly apart from each other.

 

[Andy44]

I imagine one way to equalize the charge between two docking vessels would be to place a resitive material between the two where they first contact each other. The charge would drive current through the resistor at a controlled rate and be dissipated as heat instead of frying electronics.

 

[James.Denholm]

The Brits call it "earthed", largely 'cause you're connected to a pipe that runs into the Earth!

Ah, but there is a pipe running from the ISS into the Earth. A guy called Frank runs around holding it up, and periodically sticks it into some dirt so he dosen't get fried.

 

[Scrooge McDuck]

There can be, yes. Not sure how it's done with the ISS but typically docking bodies will have an extended boom that acts as a conductor to equalise the P.D between the vehicles. Of course, if both have been travelling in formation for a while this is less of a problem, as the spacecraft are still grounded to the ionosphere.

Yes I was also wondered about how this worked at docking between spacecraft, but now I see.
About grounding to the ionosphere: isn't that what the space shuttle experiment with the very long cable unrolling was about? I rembember that this went wrong at some point, but I'm not sure of the cause.

I also rembember something from an AC Clarke book about the geostationairy towers, they somehow extracted the energy from the huge potential difference. Would this be possible for the ISS too, if it had a few tens of kilometers cable pointed to the ground?

 

[Urwumpe]

The main problem for this is the aspect, that "THE" Ionosphere is a pretty bad term. It is a very dynamic environment, which is unpredictable. The next problem is, that each spacecraft creates it's own ionosphere. RCS exhaust, paint, or glue used for creating solar panels all contributes to a relatively thicker cloud of ions around a spacecraft.

Thats what caused a shuttle experiment TSS-1R to fail. The cloud was a far better conductor as expected, making the thin tether the weakest point in the circuit. They measured a stronger current in the circuit as expected. I think the potential difference at 19 km way also stronger as expected, but not critical.

 

[simonpro]

Well the ionosphere doesn't include all the other things near the spacecraft: It's generally assumed they'll have the same p.d as the spacecraft.

 

[tblaxland]

Thats what caused a shuttle experiment TSS-1R to fail. The cloud was a far better conductor as expected, making the thin tether the weakest point in the circuit. They measured a stronger current in the circuit as expected. I think the potential difference at 19 km way also stronger as expected, but not critical.

The power in the TSS-1R tether was not due to any potential differences in the ionosphere, it was due to electromagnetic induction as a result of moving through the Earth's magnetic field. The free electrons in the ionosphere effectively provide a diffuse return path for the tether current so that the return current does not cause a magnetic field that would otherwise cancel out the Earth's magnetic field local to the tether.

Stronger than expected power was measured in the tether (3 times stronger, actually) but this was not determined to be the cause of the failure. From the report:

"The tether failed as a result of arcing and burning of
the tether, leading to a tensile failure after a significant
portion of the tether had burned away," the report concludes.
The arcing occurred because either external foreign object
penetration (but not orbital debris or micrometeoroids) or a
defect in the tether caused a breach in the layer of
insulation surrounding the tether conductor. The insulation
breach provided a path for the current to jump, or arc, from
the copper wire in the tether to a nearby electrical ground.

The higher than expected power was because of the orbital velocity of the tether relative to the thermal motion of the free electrons increasing the efficacy of the cathode. This effect was previously thought to be negligible because the orbital velocity is much slower than the thermal motion.

I am not aware of any current related effect due to the ions around the orbiter or satellite. Some measurements of the satellites shock wave and plasma trail were taken though.

 

[joiz]

In fact is static buildup a big concern on an orbital structure like the station, as it rubs through the outer atmosphere or is exposed to solar wind?

i think it is, there was some plan some time somewhere, that like all good plans at NASA got "grounded" (pun) to use electromagnets to make an energy shield to repel little meteors that could damage a ship by pushing against their electric charge...

 

[dougkeenan]

They shipped up a bucket of dirt and stuck a wire into it.