How would a visible spark even be possible? As pointed out, it’s not absolute vacuum, but it sure isn’t normal atmosphere. When you get an electric arc on the surface, you can see it becasue the air is getting heated to a point where it glows. Without air, you just get electrons jumping around, and I don’t think you’d be able to see that any more than you can see the electron stream in a clear CRT.
Mister.Physics said:
But isn’t that a bit like talking about gravitational potentials, i.e. gravity fields, and heights? You take your lowest point and call that zero, and define all heights off that, and that works fine unless a sinkhole suddenly appears under you and you find yourself at the top of a new potential. 
The point is, phyisicists have good reasons for relating to “absolute” voltages, whereas most people (i.e. engineers, technicians, people tinkering with their light sockets, etc) are only worried about the relative difference. It sure makes the math easier to define the common point as zero instead of +237.24.
And if you do suddenly find your ball of protons you’re calling “zero potential” approaching a neutral sphere, the engineer says it’s time to redefine terms.
Actually, the only good reason for physicists to use absolute voltages is uniformity of notation. If physicists the world over had decided to instead set the standard at infinity being at a potential of -3722.6 volts, the physics would all still be the same, and the only difference is that it would be harder to remember.
That’s not a good enough reason?
Yeah, it’s a good enough reason, especially since there’s no reason to do it any other way. Likewise, engineers have a good reason (the same good reason, in fact: Uniformity of notation) for calling Earth zero potential. Of course, the two definitions of zero potential aren’t all that far off from each other.