Or, On Alien Ground
Or, This Test Charge Earth
OK, I’m done.
Let’s say we send a probe to elsewhere. The probe is, presumably, at a potential of 0 V relative to the dirt its launchpad sits on. How do we know its first contact with the alien world won’t be a massive spark? Does it matter if this alien world is in the same solar system?
On a related note, have any chunks of ice/rock/icy rock from beyond caused thunder and lightning on their way down? IOW, did we ever get a volt from the blue?
Actually, it’s pretty likely that interplanetary probes are electrically charged, by the solar wind.
I searched, but couldn’t find any references to this regarding the Apollo landings.
That’s one small step for…pzzzzzzt…aaaaahrhhhghh. Er… Houston we have a problem
Is there any kind of bacteria that could survive on a space probe and somehow harm life forms on another planet, another possibility to consider.
Although voltage is in general used as a relative measure, there is a choice of reference for zero that’s natural and measurable, any object with no net charge.
I’m assuming you weren’t really interested in effects such as the solar wind, but a difference in potential between distant planets. “Things” in general often have some charge, for different reasons, but planets are probably close enough to each other and to zero for significant sparks to be unlikely.
Every time a helicopter lands there is usually a high potential on it to be discharged. I remember having to stamp on any cable from a helicopter to discharge it.
Even pushing a shopping trolley in the wrong footwear can generate a very uncomfortable charge. Reaching for the coffee on the top shelf can be quite painful.
Soft rocket landings will equalise the charge before they reach the surface - they have this nice big ionized plume to do this for them.
Interplanetary craft probably simply reach equilibrium with their surroundings, this may well be a dynamic thing. A charged craft may have some interesting issues with its constituent components repelling one another. Deploying large films could be amusing. I wonder if this affects the JWST?
You’ve been reading Immanuel Velikovski, haven’t you?
This thing about the planets not necessarily being neutral with respect to each other was a big deal with him, and features in his books, especially Worlds in Collision
While it might have been possible to worry about this in his day, we’ve had too many items landing on other worlds, both with and without rocket braking, and no huge sparks jumping from planet or moon to the lander. Nor has any supposed charge difference had an effect on the celestial dynamics of such planetary probes. Or of the moons and planets themselves. If there ever had been any sort of charge imbalance, I suspect that bombardment by the solar wind would’ve long ago equalized any such differences.
It’s not like electric potential can just be set arbitrarily; the only way to change the electric potential of an object is to add or subtract electric charge (electrons or ionized atoms). And, everything in the universe with no net charge is at the same potential.
On the scale of humans and electric circuits that we build, it’s not that hard to temporarily move a few electrons from one thing to another, and things small enough to pick up can get a pretty big change in potential from a relatively small number of electrons. Plus even small potential differences can screw up our circuits (especially electronics). So we worry about getting everything on exactly the same ground potential, etc.
But on the scale of planets, you’d need some really crazy mechanism to throw enough extra electrons on a whole planet to have any noticeable net charge (the mechanism would have to keep the planet from attracting neutralizing charges, too). And I don’t think we know of anything like that.
So it’s sort of possible, but it would be a very very strange planet in a very strange place, and we’d probably have some idea that crazy things were going on even before we sent a probe there.
Now, as mentioned, the spacecraft could easily pick up a static charge, and I bet NASA already makes sure that sensitive electronics are isolated from the main chassis. But there’s only so many electrons you could stuff into a landing probe, so even worst case, you’re looking at a slightly scaled up finger-to-doorknob spark, not a major lightning bolt.
But on small sections of our planet, isn’t this known as weather? Lightning, etc. And if not planets, but smallish things, would enough potential be generated by some sort of equivalent global process?
Right. This just follows from the fact that it’s axiomatic that atoms and their ions are identical and indistinguishable throughout the universe. Which means that matter must be the same everywhere, and therefore so is the property of electric charge. This makes electric charge, or its absence, a universal metric and not one relative to earth, so in order for a planet to have a net electric charge there would have to be some pretty crazy active process causing it. Which is not by any means impossible, just probably quite unlikely, even in some other stellar planetary system.
ETA: My first reading of the thread title seemed to imply it was asking whether aliens would be neutral with respect to earth in a future galactic interplanetary war, or whether they’d be for us or agin’ us. 
Required reading.
In Heinlein’s Farmer in the Sky, Bill, the viewpoint character, observes a pre-docking maneuver where a conductor is passed between the interplanetary ship and the shuttlecraft he is riding on. He sees a bluish discharge at the end of the cable, and feels a shock.
The captain informs the passengers that they had “balanced the electrical potential” between the two ships. Bill reflects that, had the captain said they had been struck by a lightning bolt, he would have been just as accurate, but doubted that the passengers would have reacted well to that statement.
It’s an interesting article but I’m not sure of its relevance here. It says that spacecraft can become charged as a result of their flight environments, and that this has the potential (!) to damage components if systems are not appropriately engineered. An example cited is the low-energy plasma surrounding the earth, distorted by the solar wind to the extent that it can be blown into the orbits of geosynchronous satellites.
I don’t see this as contradicting anything that I or Quercus have said about the absolute nature of charge (potential is relative, but charge is not), and the fact that a planet that has a non-zero net charge must have acquired it from the effect of some active causative process. Or am I missing something?
Hence the person with one of the best job titles ever:
If I had that job, I’d hand out millions of business cards just because.
Yes, if your probe is landing in the Venusian equivalent of a thunderstorm, then it could get a nasty shock. There are plenty of natural phenomenon where electrons get stripped from one part of the environment and accumulated in other parts. And eventually those charges equalize, either slowly over time or very quickly as lightning. But the larger the differences in charge become, the more likely it is that those differences will equalize.
It’s much more likely that the probe itself will accumulate some sort of charge, rather than that the planet it is visiting will be charged. As was said earlier, it’s not like all charge is relative. There’s an absolute neutral charge. And since oppositely charged particles attract each other, a lump of matter in the primordial gas cloud that was positively charged would attract electrons from the cloud, and a lump that was negatively charged would discharge electrons into the cloud. So every star and solar system is going to form neutrally charged, and every body in a solar system is going to form neutrally charged.
And any mechanisms that would tend to charge a body in the solar system would have to prevent that body from discharging somehow. We like to imagine that a vacuum is an electrical insulator, because there’s no direct conduction between two bodies separated by a vacuum. But no vacuum is perfect, especially the vacuum near a star. The sun puts out all sorts of charged particles, this is what causes auroras on Earth. So a planet that somehow became charged would quickly–geologically quickly anyway–become neutral by attracting charged particles until it had become neutral.
So the Earth as a whole doesn’t have a positive or negative charge, beyond some rounding error, and neither does any large body in the solar system. What could much more easily happen is that the probe accumulates a charge relative to both Earth and the target body, since the probe is very small, it is existing for a short time, and has some active processes that might make it accumulate charge. Descending through a planet’s atmosphere is an obvious example.