Dr. William (Bill) Farrell, Scientist, Lunar Exploration Program
- Monday, 28 April 2008
Dr. Farrell: Right. What I’ve suggested is that any kind of robotic mission would have to be radioed in; you wouldn’t want a tether going in. You want that robot to, in some sense, become acclimated. You want it to come into equilibrium – electrical equilibrium – with its environment, as opposed to having any part of it float relative to the environment.
NTB: How do you go about developing an instrument to test parameters that can’t be simulated in a laboratory environment here on Earth? Or can they?
Dr. Farrell: Well, you know, you can. Some of the chambers that are being proposed are getting pretty close, although not perfect. In particular, getting the plasma in some of these chambers to exactly match, for example, what may be in Shackleton is a little tricky.
I think probably the first thing, though, is to get something in situ just to make sure these models that we have, in some sense, work. Model validation is probably the most important thing. So any kind of precursor measurement would be tremendous. Based on that, we can then work back and talk about how other hardware would behave in these environments and try to build chambers that would properly simulate the lunar environment.
NTB: You’ve already pointed out that another interesting aspect of this problem is that the astronauts moving around on the lunar surface will carry their own electrostatic charge and, in some cases, generate additional charge. How does this factor into the whole equation?
Dr. Farrell: Well, actually it’s a pretty big factor because the big issue – and we’ve been working on this recently – is in the area of dissipation. If an astronaut walks along a surface and charges up, they have to dissipate their charge. In a solar wind and in the photoelectric environment, in particular, it should be fairly easy to leak charge back into the environment; there are enough free electrons and ions around that they’ll almost act like a ground and allow for quick dissipation.
The concern we have is if we’re starting to go into the shadowed regions of Shackleton Crater, anything going into that crater, where there’s no photoelectrons and where the plasma currents are basically flowing over the crater top and not flowing fully into the crater, is shielded from the plasma current. And the surface is cold. As it turns out, the conductivity for cold lunar surfaces is about 10-14 siemens per meter. It’s real, real low – about 1014 different than the ground outside your window. So the ground becomes very resistant as well. As an astronaut moves into these dark, cratered regions, he’s got a big electrostatic problem – in particular, how does one dissipate the charge collected by moving around or roving? So they charge up and retain it. Imagine if you charged up by scuffing across the surface, but you never dissipated that charge, and over the course of the day you just kept walking and it accumulated. Eventually, at some point, you’re going to touch something of a different potential.
NTB: You’re going to discharge in a hurry, like a big capacitor.
Dr. Farrell: Like a big capacitor – that’s exactly right. In fact, right now that’s the way we’re modeling an astronaut. I even said that to one of the astronauts when they were visiting. I said, “Imagine you’re a big capacitor,” and he said jokingly, “thanks.” I said, “Well, you might have some resistors and inductors, too.” But that’s it; they’re collecting charge and a big issue in shadowed regions is how to dissipate or leak it back into the environment.
NTB: Does the technology you’re developing to overcome the lunar dust problem have any potential commercial applications here on Earth?
Dr. Farrell: Yes and no. Some of the dust charging technology may have applications in areas where small grains are mixing, like coal mines and grain silos. Those places really want to keep the grains uncharged.
Another possible application is in thunderstorm research, which we were actually doing back in 2002. So there are some applications, although not as direct at this point, primarily because we’re dealing in a low vacuum environment. The Mars package that we were developing back in 1999 probably had more of an application, in some sense, than the lunar package. The Moon is such a different animal from anything going on here on Earth or Mars – bodies with substantial atmospheres.