Dr. Carlos Calle, lead scientist in Kennedy Space Center’s Electrostatics and Surface Physics Lab, is developing instrumentation that addresses the problem of electrostatic dust. The technology will be used for future exploration missions on Mars and the Moon.
NASA Tech Briefs: Where is the electrostatic dust coming from, and what kinds of problems does it create?
Dr. Carlos Calle: The electrodynamic dust shield (EDS) originated in 1967, in a white paper that a NASA scientist wrote on possible technologies to remove dust — something that had been an issue during the Apollo missions. A professor at the University of Tokyo put that technology into practice. He developed the basic concepts, and later on abandoned it and moved on to develop other methods to remove dust for terrestrial applications. The professor was not interested in space applications and ended up working on what is called electrostatic precipitators, which is a very mature technology used in large, dusty industrial settings on Earth.
In 2003, we joined forces with the University of Arkansas at Little Rock and wrote a proposal together to the NASA Science Mission Directorate. We won a NASA Research Announcement (NRA) award to work on using the idea of the “electric curtain,” as the professor at the University of Tokyo had named it, to maintain solar panels on Mars and free them of dust. We developed that technology for about 4 years and applied it to the glass covers that are used to protect the photovoltaic arrays.
At the end of that grant, NASA had started working on the possible missions to the Moon. We changed gears a little, and we began to apply it to the lunar problem, which was how the technology was introduced in the 1960s, to solve the problem of dust accumulating on surfaces, space suits, and visors during the Apollo missions.
NASA had an agency-wide project, funded by the ESMD (Exploration Systems Mission Directorate).We worked on that for three years and developed that technology further for the lunar environment, which was a much more challenging environment. The moon has essentially no atmosphere, and the dust is highly charged due to the electrostatic environment of the moon, the solar wind, and the cosmic rays. We were able to overcome all of those difficulties and develop the technology for different applications.
NTB: How does the technology work? What does today's electrodynamic dust shield technology look like?
Calle: It involves a very thin coating of electrodes that are embedded in a substrate. We activate the electrodes with a very low-power electric signal. The signal is applied to the electrodes, and we generate an electric field wave that propagates through the surface. It’s pretty much like when you throw a pebble on a pond, and you see the ripples propagating away. In this case, it’s an invisible electric field that is propagating across the surface, and that propagating electric field carries along the dust particles that are electrostatically charged.
NTB: How is this version different from previous EDS technologies?
Calle: The original technology production that took place at the University of Tokyo used metallic conducting copper wires that were attached to different insulating surfaces. In principle, it is the same technology. What is different now is we don’t use actual wires. In the case of an application for solar panels, astronaut visors, or viewports, in which the application needs to be transparent, we start with a transparent conducting film. We use indium tin oxide, a film that is used on touchscreens for computers and tablets. To generate the wave, we end up etching the film off of the glass, for example, leaving only the electrode traces that we then apply the electric signal to. These electrodes are no longer actual copper wires but conducting films.
In other cases, in which the application does not need to be transparent, like thermal radiators, we place a copper film electrode that we attach to the metallic aluminum surface. The surface is painted over with reflective paint. From the outside, you don’t see any difference from a regular painted reflective surface, but underneath we have these thin electrodes that we activate to produce the wave that carries the dust off of the surfaces. That is what’s different.
NTB: What kinds of challenges did you face when building the technology for the Mars missions specifically?
Calle: The other difference is that in applications to Mars, for example, because of the very low atmospheric pressure on Mars, the actual electric fields that are allowed have very low strength; the breakdown limit between the electrodes is very low — much, much lower than Earth. It becomes very challenging to generate these fields that are strong enough to move the dust off of surfaces and still maintain the integrity of the films and the electrodes. We were able to overcome that difficulty when we worked on that project to maintain solar panels on Mars. There are challenges that are very different when you think about lunar or Martian environments, as compared to the terrestrial environments.
NTB: How will the dust shields be used in Mars missions? Are there other applications aside from solar panels?
Calle: We developed applications of that shield to not only solar panels, but also to protect optical systems, camera lenses, and spectrometers. To that effect, we apply the transparent electrodes to a filter-like, optical-quality glass that goes over the camera lens or the spectrometer, and it will keep the device free of dust. The same transparent application would work to maintain helmets and visors for future manned missions, or to maintain windows in a habitat free of dust. You won’t have to use any contact device, like a brush, which would scratch the surfaces with repeated usage.
Also, [the shields are used on] thermal radiators, to maintain the surfaces of instruments that need to be kept at a certain temperature. Those are painted with a reflective paint. Also, [the technology can be used with] second-surface mirrors, which are the reflective silver- or aluminum-coated films that reflect heat on structures. If you have a reflective surface or a metallic reflective surface that is covered with dust, the efficiency of that radiator, the reflective surfaces, is compromised.
NTB: You mentioned manned missions. Will this technology be embedded in the fabric of future space suit?
Calle: That is correct, and that is another application. It is very important, and we’ve been able to apply it using carbon nanotube solutions/inks to fabric. We’re working on applying it to actual spacesuit fabric, to protect and maintain spacesuits free of dust.
That was a major problem on the moon during the Apollo missions. Even though they were short-duration missions, the astronauts ended up covered with dust from the EVA activities on the moon. During the early missions, including Apollo 11, the astronauts had been instructed to remove the dust before coming back into the lunar module; they actually tried to do that, and it ended up impossible. They ended up tracking a lot of dust into the lunar module, which later on became a little bit of a problem. The missions were short-duration and the return flight was not that long. It was more of a nuisance at that time. For long-duration missions, however, they become a hazard. We’re working on that, to be able to successfully keep the dust off of spacesuits.
NTB: What is the MISSE-X mission?
Calle: The MISSE-X (Materials International Space Station Experiment) mission is going to be the tenth mission to the International Space Station. They have flown eight [MISSE] missions. The eighth is up on the station right now. The purpose of these experiments is to expose materials to the space environment. The experiment consists of a couple suitcase-sized panels that are populated with different sample materials that different researchers around the country want to expose to the space environment for a length of time, a year or two.
Panels are deployed outside the space station in different orientations. There’s a panel that faces in the wake direction, there’s a panel facing the ram direction, and there’s one side of one panel facing the zenith. The other one faces a nadir direction. The reason for that is to provide experimenters with the ability to expose materials to not only the space environment, but also the plasma of the earth and the atomic oxygen in the upper atmosphere.
In our case, we are going to be in the wake direction, so that we only experience the space environment and are shielded from the earth plasma. This is to more closely simulate the lunar environment. We are planning on flying on the 10th mission, which is scheduled to fly on the SpaceX rocket, on a Dragon capsule in 2015. We are currently developing 4 of those applications that I mentioned before: one transparent dust shield for optical systems, camera lenses, and visors; two thermal radiators, one for the white reflective painted metallic surfaces and the other one for the second-surface mirror; the reflective metallic coated films; and the fourth one on fabric for spacesuits. And the idea is to expose them to the space environment for at least a year, and to activate the shields daily to see how they perform day-after-day while exposed to that environment.
NTB: Is that what you’re working on currently? The testing process for the EDS technology?
Calle: We’re developing the actual panels that we’re going to fly, and the electronics for that. We are teaming up with the Johnson Space Center, and they are developing the electronics package that will be compact and will be part of the flight package. They’ll have to have space-rated coatings, and we’ll test them in our vacuum chambers in our lab.
NTB: What is your favorite part of the job?
Calle: Definitely the lab work. Interacting with the researchers, planning and running the tests, trying to figure out the issues the external difficulties that we encounter, and being able to solve the problems that we find.
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