Dr. William Farrell, a scientist with the Lunar Exploration Program at Goddard Space Flight Center, is an expert on the problem of lunar dust and its effects on astronauts and equipment.
NASA Tech Briefs: What is lunar dust, and how does NASA currently deal with it?
Dr. Farrell: Lunar dust is usually particles that are less than 100 microns in size. However, the particles that the Exploration Program is really interested in are the grains that can possibly get into the lungs. Particles at about a few microns and below are of particular interest for the lunar crew in terms of human health since they can stick to air sacs in the lungs and destroy tissue. This illness is called silicosis and in acute forms it can be pretty serious.
NTB: Won't the crew be completely encased in helmets and suits when they're on the lunar surface?
Dr. Farrell: What we learned from Apollo is that they were donning their suits in the same atmosphere that they were breathing, so they were actually exposed to lunar dust, directly, right in that atmosphere. One of the big challenges that Constellation has is how to actually isolate the breathable air environment – the "short-sleeve" environment, if you will, that the astronauts are in – from the dust. There are ideas out there about using a suit-lock, or having some kind of airlock, but that kind of thing is really critical so that you get isolation from the dust.
NTB: When did NASA first learn about lunar dust and the problems it can cause?
Dr. Farrell: Well, actually the Apollo missions were the first clue that dust really could be an issue. Sandy Wagner at Johnson SFC put together a nice white paper on the impacts of dust on the various missions, and on some missions lunar dust was just reported to be present in the lunar module, but it was not a big issue. However, on other missions, particularly Apollo 16, lunar dust was everywhere and the astronauts really had a hard time. It created eye irritations, got in the nostrils, and because of those kinds of missions – the more extreme missions – NASA really started to appreciate just how invasive this dust can be. But it wasn't every mission, so that's kind of a curiosity as to why some missions didn't experience a dust problem while others did.
NTB: One of the projects your group is working on is the development of a follow-on instrument to the Lunar Ejecta and Meteorites (LEAM) instrument developed by Otto Berg for Apollo 17. Please explain what that instrument was designed to do.
Dr. Farrell: Sure. The LEAM – Lunar Ejecta and Meteorites – was an experiment designed to detect accelerated small particles, especially those originating from micro-meteorite impacts. But as it turned out, one of the things it detected, particularly when the experiment crossed the lunar terminator, is hyper-charged dust, and the dust seemed to be moving relatively fast. The dust seemed to be moving at 100 – 300 meters per second, but the exact speeds couldn't quite be differentiated in the instrument. This dust activity was really an unexpected find! And this was found at the terminators. The reason why the terminators are interesting is because that's where electric fields have been found to be the strongest and largest on the Moon, so there's a strong suspicion that this lunar dust that's accelerated is tied to the lunar electrical environment.
Now what we want to do, of course, is to really follow through on that and not only have a dust detector – and maybe a slightly more sensitive dust collector – but also carry with that an electric field package and a plasma package, because the electric fields are actually established by the incident plasma and the nearby boundary. So in Otto Berg's case, he got the when and where, but what we're trying to do is actually get the when, where, and why by getting the full electrostatic picture. So it's a combined suite: dust detection, DC E-field sensor, and plasma spectrometers.
NTB: How far along are you on development of the new instrument?
Dr. Farrell: Well, you know, a lot of these instruments already have heritage. They've already flown, or the concepts have already been tested in other places. For example, the DC E-field sensors are commonly flown in space plasma applications in the ionosphere and the magnetosphere, and we're teaming with the University of California at Berkeley, who are the world's preeminent experts in that area. They're awesome.
The plasma spectrometers, we've been building them here at Goddard, and other places out in the community have been building these spectrometers for years and years. Really, since the dawn of the space age, since the early 1960s. So they've been really honed down. Probably the biggest challenge we have is miniaturization to a landed system, but this technology exists. A lot of it's already out there; it's how you package it. That's really the issue.
NTB: So that basically is your group's job now?
Dr. Farrell: That's right. Packaging – how would you get it into a lander, accommodations, those kinds of issues. We do have some experience with this because back in 1999 we proposed putting an atmospheric electricity package on what was the Mars 03 Surveyor mission. I don't know if your readers would be interested in the convoluted history of the Mars 03 Surveyor Mission, but basically the way it worked was prior to the MER – the two exploration rovers – the Mars Program was going to have a Mars 03 lander and sample return. This was back when there was this really aggressive push for relatively cheap missions to Mars. But this lander was a powered lander system that had heritage back to the Mars Polar Lander, which, of course, failed. After the failure, the Young commission pointed out to NASA that more money needed to be spent on Mars missions to buy down risks. Because of this new view, both the Mars 01 and Mars 03 missions developed with a leaner but riskier approach were considered suspect and replaced with the two MERs, which have been hugely successful. The MERs also avoided challenging powered landings by using the bouncing balloon system, which is probably not the technical name for it, but you know what I mean.
Our package was there to measure dust and electricity in a Martian dust storm. As it turns out, in dust storms the grains mix with themselves and the surface of Mars, and when they do that it also creates an electric field. Actually, the electric fields are very strong on Mars as opposed to the Moon. We went on for two years in Phase A design looking at how to miniaturize some of these sensors, combining processing units, etc. So we have experience doing that. We're kind of going through that right now.
NTB: You've mentioned the electrical fields. Do you understand yet how the dust develops its electrostatic charge?
Dr. Farrell: That is an interesting question. The short answer is "yes;" the long answer is "not really." The short answer is, on the Moon, the dust can get charged a number of different ways. Since the dust is sitting in a plasma, it actually charges just by being in equilibrium with the plasma. The plasma is an ionized gas. On the dust surface, the dust will try to do everything it can to maintain perfect current balance, so it will charge up, basically, to fend off electron currents – plasma electron currents – which tend to be stronger than ion currents. This is true in the night-side region.
In the day-side region they'll charge up positive, in some sense to draw back the photoelectrons that have been emitted from its surface. So in some sense the grains are pre-charged. But any kind of rubbing, like, for example, an astronaut walking along or a rover driving over it, will add extra charge – actually a lot of extra charge – to what's known as "triboelectric effect." When we talk about it more formally and in the field of solid state, it's called "contact electrification." And we're familiar with this. In the wintertime, when you scrape over a carpet, you charge up. You've got the same kind of thing going on as the astronauts walking across the lunar surface with this loose dust. It will charge up because it's come in contact with an object.
So you actually have a couple of different sources of charging on the Moon. The question is, which one is dominant? The other question is, all of this is in an electrical environment that itself is variable because the surface electric fields will vary, depending on solar conditions, particularly during a solar storm that gives off a lot of energetic electrons. The surface can get strongly negatively charged, particularly in the night-side region up near the terminators. If that's the case, now your dust is going to feel more charged because it becomes more charged during solar storms.
So there are a lot of different inputs. There's a natural input, which is actually a function of what is known as space weather. Then there's also the anthropogenic input – which is the way we refer to it – where humans and rovers drive over it, or excavate it in the case of digging for something, which people might want to do in a polar base.
NTB: You've noted that the electrostatic charge forms differently on the day side of the Moon than it does on the dark side of the Moon. Can you explain why, or how, that works?
Dr. Farrell: The key element there is photoelectrons, or what's known as the photoelectric effect. Basically, when the sun shines on the day side, the incoming photons release electrons, so the surface – and grains on the surface – emits an electron current. As a consequence, bodies on that side will tend to charge a few volts positive. But on the night side you've gotten rid of these photoelectrons, which are really the dominant source on the day side, and now you're left with very tenuous plasma, both ions and electrons. On the night side, as it turns out, the electron currents dominate by about a factor of 10 over the ion currents, so your surface will tend to charge negative to repel those incoming electrons.
It's really an issue of how does the surface balance itself so it gets zero current at the surface? These models that we use are very much like spacecraft charging models. You set the boundary conditions – you want zero current on the surface, so what kind of potential do you need to dial in to directly balance the electrons and ions on the surface? If you have too many electrons coming in, which you get in a plasma, then the surface will charge negative. You can dial in a negative voltage, the electrons will be repelled, and then you'll draw on ions and the current will be balanced at the surface. So it's almost like this self-regulating property, if you will.
On the night side though, what's interesting is the plasma is very, very tenuous because the solar wind, in particular, has been blocked out in the lunar day side region. You're essentially in this plasma void behind the Moon, so there's a tenuous, kind of warmer plasma. The night side surfaces can charge up very strongly negative – like 100 or 200 volts. During a solar storm, Jasper Halekas at the University of California, Berkeley, reported that Lunar Prospector detected electron beams from the surface emitted at 4 kilovolts, suggesting that the surface was charging up to 4 kilovolts during solar storms. When we talk about this with Exploration folks, even though they're interested in the dust, they are totally floored that the surface potential varies as much as it does. In fact, from my perspective, the surface potential is really the driver of all things bad, including the dust. It's almost like, if you put any human electrical system in this environment on the surface and you start cranking around the voltages, you're going to affect power systems, you're going to affect dust, you're going to affect how easy it is for an astronaut or system to dissipate their own charge, particularly on the night side.
NTB: What about the area of the Moon where the two sides meet, that moving line between lunar day and night known as the terminator?
Dr. Farrell: That's kind of a real important area, particularly if we have a lunar base going to Shackleton Crater very near the South Pole, which is what the NASA lunar architecture people are thinking. We'll probably be crossing very close – if not crossing directly into – that region. There's going to be a potential difference, and in particular there's a lot of debate whether people will actually go into Shackleton or not. It depends on who you talk to. But there are definitely resources believed to be at the bottom of Shackleton that human explorers may want. For example, there's going to be, maybe, water, or hydrogen, some kind of hydrogen-based molecules. People want to bring that out. So whether they use robotics to excavate, or send in humans to do that, there's going to be a big charge differential, a voltage differential between the sunlit region up at the top of the crater and the dark region down below, which will not get direct solar wind flow. Within that crater, the surface potential should get strongly negative. If you send in a human system that is powered from sunlit regions, with a power ground near zero volts, the surface all around might be at minus 300 volts, so there's a big potential difference between that object and the surrounding terrain. Hence, the issue has to really get worked out.
NTB: So the robot would have to actually compensate somehow, by adjusting its voltage?
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.