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Comment by dcow | Hacker News Reader

Real question… how do you “ground” electrical in a space craft? There’s no earth. Large capacitors or something? Or do you just have to live with a floating ground (literally)?

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The latter, much the same as the "ground" in an automobile or airplane (or even just any old battery-operated device).

"Ground" is an arbitrarily-chosen voltage convention. In space the floating ground won't cause a fault because there's never an electrical circuit with both the floating ground and the 'real' ground.

EDIT: turns out the ISS is a bit more complicated than that, and it also has a "grounding strap" that connects to the local plasma. https://news.ycombinator.com/item?id=41417136

You do need to take some care when docking, however.

Indeed cars, airplanes, boats, all have “grounds” that are not “earthed” in any way.

Fun tidbit, old English cars (and the original Ford Model A!) used to tie positive DC to the frame rail as a “ground” or “common” instead of the negative DC. They are referred to as positive ground cars (and tractors!)

https://www.restore-an-old-car.com/positive-ground-cars.html

There are modern cars that do this still. One needs to be careful when connecting jumper cables for this reason.

Which modern cars, for example? As far as I know the era of positive ground vehicles ended roughly during the fifties, and very much by the sixties at the latest.

Hmm, I stand corrected… and maybe aged a little. I have a memory of having to care about this many years ago :-/

Airplanes do have static wicks [0] which could be seen as a form of local ground. I love these as an example of the adjacent engineering challenges that arise.

Busses also (used to?) have ground straps that you may see dragging. Apparently this was somewhat common on cars as well. "bus ground strap" is unsurprisingly difficult to search for, but I did find a discussion on Quora [1] that claims rubber formulations were responsible for vehicles picking up a static charge.

All of this is interesting once seen as an attempt to remove the potential difference between local "ground" and ambient environment.

[0] https://en.wikipedia.org/wiki/Static_wick

[1] https://www.quora.com/Cars-used-to-have-grounding-straps-han...

In one office I had to ground a desk chair that would build a static charge. Just a wire down the back to drag on the floor between the wheels.

NASA has a handbook on it:

https://s3vi.ndc.nasa.gov/ssri-kb/static/resources/NASA-HDBK...

Apparently you attach all grounds to some common component, like the chassis and then you have a zero volt reference. As long as it is stable and the relative voltages to it are consistent, that should suffice for electrical systems. Handling overall charge of the craft might be a bigger challenge, but no idea how that would be handled.

How are docking spacecraft handled, which likely have a different base charge?

Venting xenon plasma. (Also important for not frying astronauts on spacewalks.) https://en.wikipedia.org/wiki/Plasma_contactor

Protective actions are required by the docking standard so it’s taken care of at docking but I find it very hard to find detailed information about implementations. It’s either very niche or considered sensitive enough to not be published widely.

I guess they use the same solution than during EVA which is running the Plasma Contactor Units to dissipate any buildup.

The docking spacecraft must follow a NASA standard[1] to avoid building up electrostatic charge.

From the Commercial Crew requirements document[2]:

   3.9.3.13 Integrated Space Vehicle Electrostatic Charge Control
   
    3.9.3.13.1 LEO Charging Design Standard
      The spacecraft shall meet the intent of the requirements contained in NASA-STD-4005, Low
      Earth Orbit Spacecraft Charging Design Standard. [R.CTS.285]

How the contractors achieve that might be proprietary. It looks like (at one time) SpaceX used electrically conductive paint[3] as part of their mitigations.

[1] https://standards.nasa.gov/standard/NASA/NASA-STD-4005

[2] https://ntrs.nasa.gov/api/citations/20150010757/downloads/20...

[3] https://phys.org/news/2013-03-white-coating-spacex-dragon-tr...

You missed this one: "International Docking System Standard (IDSS)" [0] which I actually read that before answering hence my "required by the docking standard".

You will see that it says: "IDSS compliant mechanisms protect against electrostatic discharge through the soft capture system" which is to say not much.

That’s still more than the previous standard for which the paragraph on ESD just says "RESERVED".

[0] https://ntrs.nasa.gov/api/citations/20170001546/downloads/20...

This deck talks about how NASA handles overall charge on the station:

https://ntrs.nasa.gov/api/citations/20110014828/downloads/20...

TL;DR they dissipate charge using three Plasma Contactor Units (PCUs) on the Z1 truss, and measure it using the Floating Potential Measurement Unit (FPMU) on the S1 truss. This clamps the ISS to within 20 volts of the local plasma.

https://en.wikipedia.org/wiki/Plasma_contactor

https://web.archive.org/web/20060929171819/http://space-powe...

https://www.nasa.gov/image-article/floating-potential-measur...

Fun fact: it's very difficult to get rid of built-up charge in space. And guess what incident EM radiation does to your electronics: that's right, it can build up charge! To my understanding, this is especially a problem when charge is built up in specific dielectrics/insulators or the very delicate structures of modern transistors and other semiconductor devices. This so-called "total ionizing dose" [1] can lead to "leakage currents, degrade the gain of a device, affect timing characteristics, and, in some cases, result in complete functional failure" [2].

Any RF or EMC engineer will tell you that ground is a dirty word, especially in extreme conditions. There's no one-size-fits-all approach, since so much depends on the physical context.

[1] https://radhome.gsfc.nasa.gov/radhome/tid.htm

[2] https://www.analog.com/en/signals/thought-leadership/challen...

Same as an aircraft or car. The chassis is the ground. (Edit: as in, the "ground" is the chassis)

At least it's not there's no Earth. I think we've seen that movie where Earth disappears from the space station.

I would assume everything on ISS and capsules are running DC. Does DC need a ground? Every electronic circuit I've built just uses ground as the common leg to battery or power.

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