11
$\begingroup$

What is the distance two spacecraft will spark/arc at in SSO? Eg if one has just launched and one has been there a long time (eg a 10-20kV worst case voltage difference). I think Paschen's Law is the right equation, but I'm not sure how to deal with this in the space environment...

$\endgroup$
  • $\begingroup$ As an aside I suspect that charging does not take "a long time" though that opinion is more based on GEO experience of the time between the onset of enhanced fluence and an ESD event on the satellite - 1 - 3 days. I have heard of more recent research that supports the long-term view too. I will see what I can find. Differential charging on a single satellite can be driven by surface properties and sunlit/shadow conditions. It seems an obvious thing to check before a docking manoeuvre so there must be literature out there. I'd start with Paschen's law too. Good question. $\endgroup$ – Puffin Feb 22 '16 at 13:16
  • 2
    $\begingroup$ How would an arc come to be? There are hardly any gas molecules in a typical SSO and spacecraft shouldn't normally emit any either, afaik. Neither should they emit electrons. Maybe if they fired their thrusters. Still, I can't imagine arcing at such moderate voltages at more than the sub-millimeter level. $\endgroup$ – Rikki-Tikki-Tavi Feb 22 '16 at 13:31
  • 1
    $\begingroup$ There is plasma. Plasmas are believed to support satellite arcing between exposed current carrying conductors - though whether that includes the natural plasma rather than a locally created one from the satellite is another matter and whether the plasma, natural or otherwise, can support inter-vehicle discharges is the point of the question. Either way you've made a good point - perhaps Paschen's law is not the right starting point if its limited to neutral atmosphere. That said, newly launched satellites will out-gas for a little while. $\endgroup$ – Puffin Feb 22 '16 at 15:30
  • $\begingroup$ A couple of background links for charging: holbert.faculty.asu.edu/eee560/spc-chrg.html and dev.spis.org/projects/spine/home/tools/sctc/VIIth/… $\endgroup$ – Puffin Feb 22 '16 at 15:31
  • 2
    $\begingroup$ @Rikki-Tikki-Tavi - Self-arcing is a big issue with spacecraft, especially during times when a spacecraft is in the shadow of the Earth (i.e., no photoelectrons being emitted) and can lead to severe damage to the body and instruments on the bus. Generally designs incorporate relatively high conductivity materials on the exposed parts of the spacecraft in order to minimize this effect. The competing interests then become between how much conductivity vs. how much money one wants to spend... $\endgroup$ – honeste_vivere Feb 22 '16 at 16:22
6
$\begingroup$

I'm unable to give you precise numbers, but for orders of 10kV this will be a sub-millimeter distance.

The mechanism of arcing in vacuum is significantly different than in air. While in air the air particles, excited, become the plasma and start conducting electricity, there is no such medium in vacuum. There's Field Electron Emission, especially from any sharp points; either into empty space, or towards a positively charged target (the other spaceship). Now this emission must achieve such intensity as to heat up the electrode until its own material evaporating is emitted into space and carried by the electrons reaches the target - creates a plasma bridge. Only then arcing can occur, as it excites more of the electrode material producing more arc-sustaining plasma.

This largely depends on ability to emit enough electrons in an area small enough so that the current vaporizes the electrode surface. And here the actual calculation problems begin: what is the evaporation temperature of the material? What is its resistance - corresponding to wattage of the current being emitted. What is its shape? Sharp blades produce much higher field emission than flat surfaces. How fast and how far will the emitted material fly before it cools down enough to become resistive? It ceases to be an electrical problem, and becomes a problem of calculating behavior of mechanics of production and travel of evaporated metal plasma in vacuum.

edit: Wikipedia gives:

High vacuum (field emission limited) 20 - 40MV/m (depends on electrode shape)

so, 20-40kV/mm.

$\endgroup$
  • $\begingroup$ Thank you I hadn't really grasped field emission before. This seems to be consistent with the notion that arcing due to field emission could be exacerbated a) if parts of the satellite(s) were already charged to a few thousand volts from incident particle flux or photo-electric emission or b) if there was a plasma rather than vacuum surrounding the satellite. Would the latter in particular not enable larger gaps to be bridged in an arc? $\endgroup$ – Puffin Mar 31 '16 at 22:03
  • $\begingroup$ @Puffin: no sources behind this, but from my "engineer's hunch": a) static electricity can carry massive voltages but rarely has any significant amperage in it. If it can overheat some sharp tip so much it starts not just glowing but evaporating, it's losing the charge really fast. b) how would that plasma get there? Skimming upper atmosphere? Coronal mass ejection? This is space, any condensation of plasma should evaporate really fast. $\endgroup$ – SF. Mar 31 '16 at 22:51
  • $\begingroup$ @Puffin ...although re: a) : if one craft has pretty much neutral charge and the other has a positive one, the discharge might happen. Field Electron Emission is the easy mechanism to get rid of excess negative charge. I'm not exactly sure how to get rid of positive charge in vacuum though, so it might be much longer-lived than negative. $\endgroup$ – SF. Mar 31 '16 at 22:56
  • $\begingroup$ For my point (b) there is a permanent plasma in some Earth orbit regions. This comprises the ionosphere at low altitudes such as used by the ISS, and which I understand is relatively low density, and the Van Allen belts which are applicable to higher LEO missions such as Globalstar, the various navigation fleets in MEO and all GEO satellites. $\endgroup$ – Puffin Apr 2 '16 at 10:14
  • $\begingroup$ At GEO the belts can be agitated by solar particle events which change the energy spectrum of the trapped (belt) particles. It is new to me that field electron emission could be a significant means of loss of negative charge, to date I had understood that the dominant process at all altitudes was the photo-electric effect, except of course in eclipse. The transition from eclipse to sunlight has been known to trigger ESD anomalies. $\endgroup$ – Puffin Apr 2 '16 at 10:19
1
$\begingroup$

It is going to depend upon the orbital region and so this is a partial answer relating specifically to the ISS.

The space-flight environment: the International Space Station and beyond provides this description:

Ionospheric plasma

Spacecraft in low orbit around the earth have a complex interaction with ionospheric plasma. The solar arrays on the International Space Station operate at 160V, and the distribution system is at 120V DC. The negative side of the power system is grounded to the structure of the space station, resulting in a large amount of energy stored in the structure at –140V. High voltage solar arrays, coupled with the design and material properties of the International Space Station, can lead to detrimental interactions with the ionospheric plasma.

Two plasma contactor units have been placed on the International Space Station to provide a “ground wire” to prevent arc discharging. These devices emit a low-energy stream of electrons during spacewalks that reduces the buildup of electrical charge.[13] As long as the plasma contactor units are functional, an astronaut floating freely during a spacewalk has no risk of exposure to arcing. However, the steel tethers used by astronauts to attach themselves to the structure of the space station and the exposed metallic surfaces of the spacesuit or tools used during the spacewalk are potential sources for arcing if both of the plasma contactor units were to fail during a spacewalk.

For full credit please note that the reference "[13]" in the quote refers to this book:

Tribble AC. The space environment and its impact on spacecraft design. 31st American Institute of Aeronautics and Astronautics (AIAA), Aerospace Sciences Meeting and Exhibit; 1993 Jan 11–14; Reno (NV). Webster (TX): American Institute of Aeronautics and Astronautics; 1993. p. 491.

To address the question: its not clear what distance can be inferred here. The risk of metal hand tools appears to be that if left "floating" in electrical terms then at some point in their own charge build-up a discharge event can bridge insulating materials such as in a space suit. It seems reasonable that an arc could also occur when such a tool is returned to close proximity with the space station.

Its not clear what the role of the steel tether would be. Superficially it would appear to be a good opportunity to reduce the risks if each loose item could be connected back to the space station 0 volt reference, but it appears that a different design philosophy applies from this context.

To get more on the distance answer I'd suggest, as a next stop, reading more into the plasma contactor unit purpose and design.

$\endgroup$

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.