The dust busters

NASA's project to return to the Moon is now in full swing and its researchers and engineers are hard at work on the technical problems that need to be solved. Edinburgh software company DEM Solutions is helping NASA to tackle one of the greatest foes for the new generation of lunar astronauts: moondust.

According to Carlos Calle, director of the Electrostatics and Surface Physics Laboratory at the Kennedy Space Centre in Cape Canaveral, moondust caused massive problems during the Apollo programme. 'The lack of atmosphere on the Moon, the lack of water, and the particle size and composition of the lunar dust are all very conducive to the development of electrostatic charges on the surface of the particles,' he said.

On the Apollo missions, astronauts reported that the dust stuck to spacesuits, degrading the zips; it got into pulleys and the joints of the lunar rovers; it coated solar panels. And once it was there, it proved very, very difficult to remove.

Calle explained that lunar dust particles often have nano-scale iron particles embedded inside glassy silicon dioxide. The grains have a jagged shape whose surface area is very large compared with their volume.

This means that when the Moon's surface is struck by the solar wind and cosmic rays it can easily pick up charges from stray electrons; other sources of ultraviolet radiation can also generate charges by knocking electrons off the surface of the dust particles.

'As soon as we have activity on the Moon, the astronauts and the vehicles moving around make the particles rub together and rub against other materials, generating triboelectric charging,' Calle said.

And once the particles are charged, they will stick to each other — and to anything else.

Dust-coated solar panels are a particular problem NASA is trying to resolve, Calle said. 'The problem is that dust covers the panel and blocks the sunlight, which reduces their efficiency quite a bit,' Calle said. 'This tended to happen with the Mars Rovers, which limited their lifespan.'

But on Mars, he explained, there is some atmosphere and dust tornadoes tend to clean the particles away from surfaces. By contrast, in the sterile, atmosphere-free Moon environment there is nothing to clean away dust.

Calle's lab is developing electrostatic screens that will be fitted to solar panels to repel dust from the screen surface or stop it settling altogether. The system was developed for use on Mars, he said, but the different conditions on the Moon and the composition of the dust means that the design systems used for earlier models are not adequate.

This is where DEM Solutions came in. The company specialises in discrete element modelling software, said founder and chief executive John Favier. 'It's the main method for modelling what we call discontinuous media, such as dusts, granules, pills and rocks, all of which are solids, but can flow,' he said.

'Trying to simulate their mechanical behaviour is very difficult using finite element analysis, which is used to model continuous solids, or with computational fluid dynamics, which models continuous liquids. Discrete element modelling simulates every particle and gives you an understanding of a material's behaviour.'

NASA first saw DEM Solutions' EDEM software suite at a conference, where one of DEM's long-standing customers, construction machinery manufacturer John Deere, was demonstrating how it used the system to model an excavation process.

'One of the features of the software is that, because it's a simulation tool, it's a virtual environment and we can set gravity to any level we want,' said Favier. 'NASA asked us if we could do the excavation model under lunar gravity, which we did, and saw different behaviour in the model.'

Another important factor is that DEM has customers in the printing industry, who use the system to model the movement of powders and magnetic particles in inks.

'In this case, the particles are charged,' Favier said. 'And if you put those two together, we have the capability to model charged particles and electric fields, under different gravitational conditions. So we had the capabilities that NASA needed.'

At Cape Canaveral, Calle's team will use EDEM to model the behaviour of lunar dust with its electrostatic screens, using actual lunar dust samples to help make the models accurate.

'We're going to advise DEM Solutions on the mathematical expressions to incorporate into their program to model the particles and the way they interact,' Calle said.

'The substrates for the screen's electrodes and the electrodes themselves have coatings on them, and we need to see how the different coatings interact with the lunar dust. We need to look at the electrostatic charge exchange between screen and particle and the motion of particles as they respond to the different fields we apply on the screens.'

Initially, Calle's team is looking at standard screens with copper electrodes and transparent screens with electrodes made from indium tin oxide, which could be used for helmet visors and viewports and windows in the proposed lunar base.

'In the long term, we could look at some kind of repelling system for all the mechanical parts of vehicles that go to the Moon,' he added.

The initial agreement with DEM is a one-year project, he said; this is part of a wider, five-year project involving several other NASA laboratories.

'At the end of the year we hope to be able to compare the simulations with actual experiments, so that we can verify the system's accuracy,' he said.

The project will also help DEM develop its EDEM software further, according to Favier. 'It's going to enhance our ability in the area of modelling printers and other areas where we need to model charged particle behaviour, such as dust extraction. It also has applications in the electronics industry,' he said.