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January 14th, 2010 - Space Power

I came across an interesting article the other day that I had saved from years past.
Having been schooled in the Black Arts of Electricity myself (it's still just a theory kids, never forget that) in what now seems like a former life, (and yes, my fellow Hogwarts at school - there were three distinct groups of us: the "Wire-pullers" = "Electricians", the "Sparkys" = "Industrial Electronic Technicians", and the "Propeller-Heads" = "Computer / Consumer Electronic Technicians" - I was one of these) did learn more than how to stuff a garbage bag into a fooseball table and play all day for a quarter, and what happens after you put a giant ball of Silly Putty into a microwave oven and turn it on, but that is another story.
So with that 'knowledge' and from recently spending considerable time inside commercial airliners buzzing through the Earth's stratosphere, I again found the piece particularly interesting.
The article concerns the running of electrical systems aboard  spacecraft.
It's something I hadn't considered very thoroughly, thinking that space is nothing more than a dead vacuum and what would there be out there to interfere with anything...?

Duh, wrong. At least in the case of the ISS, (International Space Station) which cruises around the Earth's ionosphere in Low Earth Orbit (LEO) at around 17,500 mph (it orbits Earth about once every 90 minutes) there is plenty to bump into.
And they don't call it the ionosphere for nothing. They call it that because it is ionized by solar radiation, and is filled with "charged plasma," which is something of a "particle Minestrone" filled with all kinds of tasty space stuff.
It seems that before the dawn of the ISS, electrical power on spacecraft was operated at a fairly nominal +28 VDC, which was a standard inherited down from the aircraft industry. Most cars these days use a battery at +12 VDC, which keeps bringing to mind an image of the craft used in the movie "Spaceballs" when I think of the +28 volt standard.

Because of the relatively low voltage back then, interactions with charged plasma in LEO were not really an issue. However, with the ISS, the standard needed to be changed to a higher voltage to reduce power losses and to reduce the overall system weight. This came with a trade-off of some electrical "issues."

It's like this; electrically conducting surfaces of the ISS that are highly negative in respect to the ionic soup in LEO tend to undergo electrical "arcing." This is not unlike what happens when you put a giant ball of Silly Putty into the microwave or hold the coil wire of a car close to the distributor and turn the key.
Because the electrical potential is great enough, an electrical current in the form of a spark or "lightning bolt" jumps the air gap between the two conductors without them actually physically touching. This can look cool, but it is also the principal behind 'arc-welding', which if continued will cause pits and burn marks on the metal's surfaces, and could eventually generate enough heat to be able to weld metals together.

So not only does this arcing damage the outside surfaces of the spacecraft, but because there is a current flow though the  plasma, there are intermittent power drains both on the local area that is doing the arcing and over the entire 'power array' of the ship. This causes 'sputtering' (electrical farts) and significant electrical 'noise', and even more worrisome it causes differences in electrical potentials between the different areas of the ship's power array. This is generally considered a bad thing. Notably because you are pretty much a dead duck in space with no power. It's cold, there's no breathable air, not much light; you get the picture.
Conversely, the solar arrays and parts of the ship that are at a highly positive electrical potential compared to the atmospheric plasma begin to collect ions from it, which causes a parasitic power drain to the ship's systems. Also a bad thing.
Due to the physical differences between the tiny particles involved, when the potentials are high, weird things begin to happen, such as materials that are normally electrical insulators (tend to stop current flow) become electrical conductors (allow current flow). Again, bad thing.
Because the spacecraft is plowing through the ionosphere like a boat, the "ram & wake" effects in the plasma near the leading and trailing edges of the craft begin to occur causing large differentials in voltage from one end of the ship to the other.
If that weren't enough, the electrical characteristics of the plasma outside of the ship are constantly changing, and can vary even more with 'solar storms' and 'sunspot events'.
In some cases the differences can be as large as what the ship's solar arrays can put out! This little problem can lead to loss of contact with the geosynchronous satellites guiding the craft (Are you with me, Major Tom...?)
When the conditions are bad enough, the insulating surfaces of the ship actually begin to conduct electricity and all of the parts of the ship can be enveloped in a "conducting plasma", also known as the "sheath effect."
It's kind of like putting your spaceship into a wool sock and sending it through the cosmic dryer without a fabric softener sheet. You can imagine how well your onboard computers and electronics appreciate this.
To reduce the problem, engineers have learned to play a 'reverse psychology' game, saying "if you can't beat the plasma, join it" and coating the outside of the ship with conductive material to eliminate electrical differences around it's outer surface. The next step is to use a clever device with a cool name, called a "plasma contactor" (not related to the flux capacitor of 'Back to The Future' fame, sorry).

A bunch of these objects resembling little spark plugs attached to the outside of the ship act as grounding rods, connecting it to the "local environment" by discharging their own stream of ions and electrons into space. They use a small amount of gas to create this ion stream, which the excess surface charges of the ship follow like a hoard of lemmings.
This all sounds groovy, except for one small issue. When the ISS has a problem with say the gimbals that support the mounts of it's solar panels as they face the sun, some lucky astronaut gets to go out there and work on them with the plasma contactors turned off, effectively creating an electrical 'float' or difference between parts of the ship of about 260 VDC. (Bad thing.)
At the time of this writing, it is not known whether straws are drawn, coins are flipped, or if it's always 'the new guy' that gets to go.

More from your intrepid space reporter after we come out of radio silence around from the dark side, and more data flows in...

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