Static Surface Charging During Apollo

[kryton]

Scientists reviewing Lunar Prospector data from 1998/1999 found surface charging on the moon of up to 4,500 volts of electricity. They are concerned that future missions to the moon could be hampered by this phenomenon by burning out electrical equipment and posing a threat to the astronauts. How concerned were they about static discharge during Apollo or were they not aware of it? Hammering rods into the ground - flags, cameras, etc. Did the lunar modules suffer from any static buildup on the moon?

[Mister Earl]

Hrm. Interesting... solar wind, unhampered by an atmosphere, striking lunar dirt, and creating a small charge. Like sandblasting a large PVC pipe? Hehe.

[PhantomWolf]

I don't recall any major problems, though Apollo 12 did have trouble with one of the "skirts" on an ALSEP experiment when it got charged and refused to sit on the soil.

[Thomas(believer)]

I'm sure the moon is no place for me.

[gwiz]

I seem to remember reading somewhere that this phenomenon occurs at local sunrise, and briefly lifts clouds of charged dust particles. It was initially picked up by one of the instruments left behind on one of the Apollo missions.

[JonClarke]

Charging on the Moon (and Mars) is a great favourite amongst the "let's panic about everything remotely possible" crowd. Before the Mars rovers arrived there was considerable FUD generated (pun intended) by such people about the electostatic risk posed by dust stroms, dust devils, or simply moving round on the surface. There would be arcing, short circutining, damagae ot instruments, motors, solar cells etc. Of course, simple engineering design meant that so such problems occurred.

Jon

[Paul Beardsley]

Interesting topic, but does it belong here? Surely it's to do with space exploration as it's not a conspiracy theory. Although I rather like the idea of some HB thinking the flag was meant to serve as a lightning conductor...

Somewhat OT, but this has bugged me for a long time... If you are in the car during a thunderstorm, I understand that the car behaves as a Faraday cage and so the lightning cannot get you. However, the car itself can build up quite a charge.

So, what do you do when you want to get out?
a) Wait for the car to discharge itself, then step out. (Do the tyres conduct?)
b) Force the car to discharge itself by means of a dragging cable or something, which presumably has to be deployed by remote control.
c) Leap out of the car, so that you are well clear of the highly charged outer skin of the vehicle.
d) Don't worry about it. You never hear stories about people being electrocuted after a thunderstorm, so presumably the car does not charge up to dangerous levels.

(I might have asked this before but I don't recall getting an answer.)

[Nicolas]

d) if it would, we would have a nice way of storing energy. You can get a static shock though, but you can have those on any (dry) day.

[kryton]

With the moon being constantly bombarded by the solar wind and activity, I was assuming that the Apollo astronauts would have encountered some of these high voltages with their tools and equipment. Weren't they concerned with the rover and traveling many kilometers away from the lander that this may be a problem. I haven't found too much on the subject but wouldn't they have noticed something if this were true. The larger rocks may have been supercharged. DId they have a procedure for discharging the dust and rocks before bringing them into the LEM cabin? Thank you

[JayUtah]

4,000 volts is not very much at all for an electrostatic charge.

[Donnie B.]

For comparison, the electronics industry's most commonly-used human discharge model for electrostatic testing uses 15,000V. It's intended to simulate someone shuffling across the carpet on a dry day before touching the equipment under test.

[kryton]

I had read that some devices can be damaged with as little as 20 volt static discharge. And also weren't they concerned about this static charge being carried into the LM ascent stage? Wasn't the trans-lunar coast and the LM a pure oxygen atmosphere?

[JayUtah]

I had read that some devices can be damaged with as little as 20 volt static discharge.

No doubt some can, but not the kind put into human-rated spacecraft. A 4,000 volt electrostatic discharge is negligible. It's interesting from a purely scientific standpoint, but it won't even foul up your PDA.

And also weren't they concerned about this static charge being carried into the LM ascent stage?

No, because it's not a significant charge.

Wasn't the trans-lunar coast and the LM a pure oxygen atmosphere?

Yes, but at the same partial pressure at which it occurs in Earth air (ca. 5 psia). The Apollo 1 fire was especially problematic because it was pure oxygen at 17 psia -- a very dangerous concentration.

[PhantomWolf]

A 4,000 volt electrostatic discharge is negligible

According to this, you can build up a 12,000V charge just walking over a nylon carpet!!!!!

[kryton]

So a 4000V or even a 12000V discharge in the pure Oxygen atmosphere of the LEM wouldn't be a concern because of the atmospheric pressure in the cabins.

[JayUtah]

So a 4000V or even a 12000V discharge in the pure Oxygen atmosphere of the LEM wouldn't be a concern because of the atmospheric pressure in the cabins.

Essentially correct. It would be no more worrisome than ordinary static sparks from walking across your living room and touching a doorknob, in terms of ignition and combustion.

Do electrostatic discharges on the order of 10,000 V harm avionics? Not in the least. Airplanes routinely build up and discharge electrostatic discharges an order of magnitude greater by slipping through the air. This is why they have to be electrically grounded before you can start pumping fuel into them. The LM and CSM were built to human-rated aerospace standards. Why do static charges damage other electronics? Because those devices are not engineered for reliability; they're typically optimized for small size, low cost, low power consumption, or high performance. Those compromises can create a static-sensitive situation.

What about Apollo 1? Apollo 1's cabin at the time of the launch-pad fire was filled with pure oxygen at something like 17 psia, or almost four times the concentration that was normal for the CM in flight and almost four times the concentration of oxygen in normal air. This condition accelerated combustion dramatically.

Earth air is a mix of gases, chiefly nitrogen and oxygen. In a mix of gases, each gas is responsible for a certain amount of overall gas pressure, called its partial pressure. For many chemical reactions such as combustion, the partial pressure of a gas reactant dictates how fast the reaction takes place. For the special case of respiration, the partial pressure dictates how readily the oxygen penetrates the alveola walls in the human lungs and diffuses into the bloodstream. The total gas pressure of Earth air at sea level is 14.7 psia, of which about 5 of that is provided by oxygen. Humans don't need nitrogen, nor do they need 14.7 psi of external pressure. Technically you only need the oxygen at the right partial pressure, and if it's the only gas present it's also the total pressure. So by supplying only oxygen at 5 psia (or 3.5 psia for the space suits) you can make the spacecraft walls thinner and lighter without increasing the ignition hazard or hampering breathing.

But because of how the oxygen plumbing was designed in the SM, the CM at liftoff had to be pressurized to at least 3 psi greater than the outside (i.e., sea level + 400 feet at Launch Complex 39), and the only gas available in the system to do that with was oxygen.

The spark that ignited the Apollo 1 fire was from the ship's electrical system and as such carried far more energy than an electrostatic discharge. It is not likely that even in the oxygen-rich environment of the CM on the launch pad an electrostatic spark would have ignited anything.

As manned space flight progressed, we realized the important role of the diluent gas. Nitrogen doesn't participate chemically in conduction, but where present it absorbs energy that would otherwise go toward combustion. So for that and other reasons, manned spacecraft today are pressurized to a mix very close to sea level atmosphere on Earth, and at the same pressure.

So what about static discharges and methane or gasoline explosions? Those occur not because there is a richness of oxygen, but because the highly-combustible fuel vapor/gas is mixed ideally with the oxidizer at the discharge site. If there's no fuel nearby, the spark has nothing to ignite. Oxygen alone does not obviously burn.

The electrostatic spark carries enough energy to ignite ideally mixed fuel and oxidizer, but it also can damage photographic film. In the 70mm still cameras the film was dragged across the glass reseau plate. Designers feared that would build up a static charge. So they actually coated the reseau plate with a very thin layer of metal and connected it electrically to the film body and magazine. This is likely the reason why the lunar surface Hasselblads were especially sensitive to halation in the reseau plate and interreflection from the rear lens element.

But that design feature helps us understand the LM condition further. A static discharge occurs only when two objects hold a substantially different electrical charge, and then are given a conduction path between them. As you shuffle across the nylon floor, you build up a charge that has nowhere to go. It doesn't leak out through your feet very fast. When you touch a door, the charge equalizes because there is a conduction path.

As the astronauts climbed the ladder at the end of the EVA and groped their way through the forward hatch, they and anything they were carrying would equalize the charge with the LM body. This would occur long before the cabin was pressurized.

[sts60]

Another thing about Apollo 1 is that materials which will not burn all at all, or at least burn readily, in ordinary atmosphere may do so rather violently in an oxygen-rich atmosphere. Velcro, IIRC, is one such material.

[JayUtah]

And yet another thing about Apollo 1 is that because the countdown demonstration test was characterized as a test, there were many things in the cockpit that ordinarily wouldn't have been there during the flight, and which in retrospect shouldn't have been there for the test either -- things like the Remove Before Flight tags and protective covers on instruments and equipment.

NASA was not entirely blind to the danger of a high-pressure pure oxygen environment, but they believed they had mitigated the risk by isolating and eliminating ignition sources, and specifying non-flammable materials. The relative carelessness with which the wiring harnesses were handled eroded the former, and overlooking the danger of the test environment eroded the latter. You fix wiring harness problems by tightening the constraints on the processes by which they are designed, installed, and tested. You fix the other problem by changing the way people think about and do their jobs.

[Extravoice]

Jay, thank you for the excellent post explaining partial pressure and relating it to the Apollo I fire situation. I sometimes think you should write science textbooks for a living.

I am a little confused by one statement, though...

As manned space flight progressed, we realized the important role of the diluent gas. Nitrogen doesn't participate chemically in conduction, but where present it absorbs energy that would otherwise go toward combustion. So for that and other reasons, manned spacecraft today are pressurized to a mix very close to sea level atmosphere on Earth, and at the same pressure.

(my emphasis)

Did you mean combustion? If not, would you elaborate a little?

Since spacecraft currently use a nitrogen/oxygen mix, am I correct assuming that fires are worse than expected in a low pressure oxygen-only atmosphere (albiet not nearly as bad as a high pressure oxygen-only atmosphere)?

[JayUtah]

Yes, I meant combustion, and I remember seeing that in Preview and believing I had corrected it.

I may have misled you slightly by discussing nitrogen-oxygen mixes in the context of fire safety. The mixed atmosphere indeed poses a measurably lesser risk of ignition and fire, but its primary goal is the shirt-sleeve work environment. When using a 5 psia pure oxygen environment, you have to "blow down" slowly with oxygen prior to entering it, the same way surfacing divers have to decompress. Nitrogen does indeed penetrate the alveolar lining and enter your bloodstream, and it remains in equilibrium. When you remove the nitrogen from your breathing air, the nitrogen diffused in your blood wants to leave it rapidly and painfully. That's why the Apollo astronauts suited up and then walked to the pad under full airtight integrity carrying their portable air supplies.

[Nicolas]

What, those weren't space rated ghettoblasters??

In the shuttle they don't have pure oxygen. The astronauts still cary a bag when going to the elevator though. Can't be oxygen, as they haven't got their helmets on . What is that bag/box?

[HelluvaNASAEngineer]

Correct me if I'm wrong, but I would suspect the reason why Apollo missions never worried about ESD on the moon is because back in those days their electronic systems didn't consist of sensitive microprocessors, but rather analog components with fewer, larger transitors that were much less sensitive.

[Stuart van Onselen]

In the shuttle they don't have pure oxygen. The astronauts still cary a bag when going to the elevator though. Can't be oxygen, as they haven't got their helmets on . What is that bag/box?

I've always assumed that those boxes are for cooling. After all, they're weariing very heavy clothing in a hot part of the world.

[Nicolas]

I think you're right, I remember them saying something like that was their suit "airco" unit.

[kryton]

Thanks for the responses and to Jays very thorough explanation. Just one more question on this subject. Is this also how they kept their hair from catching fire? Since hair can easily catch fire even in a 22%-24% oxygen enriched environment.

[PhantomWolf]

I'd suggest the best way to keep your hair from catching fire is to avoid having ignition sources near it. Hence why the CSM and LM were non-smoking flights.

[JayUtah]

Since hair can easily catch fire even in a 22%-24% oxygen enriched environment.

No, it's not the percentage of oxygen; it's the partial pressure of oxygen that largely determines flammability.

In air at sea level, only 20% or so is oxygen. But its partial pressure is about 5 psia, part of the total pressure of 14.7 psia. In the command module during flight, the atmosphere is 100% oxygen, but it's still at only 5 psia. The partial pressure of oxygen is also the total gas pressure of the CM contents.

When you speak about enriching with oxygen, that means to raise the partial pressure of the oxygen in air. Removing the diluent gases does not change oxygen's partial pressure.

[Nicolas]

With 5 psia, the pressure feels like in the mountains, though without the lack of oxygen in the air in this 100% oxygen environment. That must feel strange .