Understanding how exactly lunar dust sticks to surfaces is going to be important once we start having a long-term sustainable presence on the Moon. Dust on the Moon is notoriously sticky and damaging to equipment, as well as being hazardous to astronaut’s health. While there has been plenty of studies into lunar dust and its implications, we still lack a model that can effectively describe the precise physical mechanisms the dust uses to adhere to surfaces. A paper released last year from Yue Feng of the Beijing Institute of Technology and their colleagues showcases a model that could be used to understand how lunar dust sticks to spacecraft - and what we can do about it.
One important differentiator of the type of lunar dust is whether or not it is “hypervelocity”. Hypervelocity dust particles, which travel more than 1 km/s, have a completely different physical model than “low velocity” ones that travel anywhere between .01 to 100 m/s. While the hypervelocity particles might be kicked up by retrorockets on a spacecraft or by meteoroid impacts in the lunar surface, the vast majority of the dust created by everyday activities on the Moon, such as driving or walking on the surface, will be of the low-velocity kind.
To understand how those low-velocity parties interact with a surface requires two separate physical models. First, there’s the “long distance” attraction of the surface of the spacecraft or vehicle. The other is the “clinginess” of the particles once they impact, and potentially stick, to whatever surface had attracted them.
Fraser discusses how we can use electric fields to mitigate dust accumulation.The long distance attraction is primarily caused by the charged surface of the vehicle. Every vehicle, or on the surface of a body without an atmosphere, accumulates charge by being exposed to the solar wind and radiation. This effectively creates a giant electric field, which attracts charged dust particles. The field surrounding the vehicle is actually part of what’s called a “plasma sheath” that varies the local electric potential around the vehicle. Dust, no matter what its own electric charge, can be captured in this sheath and be pushed towards the surface of the vehicle.
Once it gets within a certain radius of the vehicle, the clinginess factor takes over. This is the equivalent of van der Waals forces here on Earth that cause small particles to stick together. In the model the authors developed, the most important component of this short-range contact force is the “interface energy” between particles as the surface. They use a complex theory of deformation, known as Thornton’s adhesive-elastic-plastic model, that effectively tracks the various complex changes small particles go through as they impact a surface.
Developing the model is only a means to an end though - understanding what the model means for the design of spacecraft is really the most critical take-away from this research. There are two major findings - first, advanced, less sticky coatings can help, but only so much. That’s because the plasma sheath surrounding the spacecraft holds even many particles that bounce off the coating in the general area so it can happen again, with less bouncing energy, shortly thereafter. Therefore, the second take-away is that decreasing the charge on the spacecraft is by far the most effective way to lower (but not eliminate) long-term dust accumulation.
Fraser discusses how to deal with Moon dust with Dr. Kevin CannonTo lower the charge on a spacecraft, engineers have both passive and active options. Active options include electron/ion “guns” that shoot charge particles away into space as well as plasma contactors, like those used on the ISS, which change a neutral gas (like xenon) into a plasma and then eject them into space. Passive options for lowering the charge accumulated on the surface of the vehicle include ensuring proper grounding of the vehicle itself, and using conductive coatings - hopefully that are also relatively slick.
Ultimately, dealing with lunar dust adhesion is going to be a long-term problem that will require a variety of solutions. While models like the one described in the paper can be helpful in showcasing the factors engineers will have to consider as part of their spacecraft design, getting on the ground data from vehicles on the Moon is ultimately the best way to come up with better mitigation methods. It might still be a while before we see a permanent presence on our nearest neighbor, but at least there’s more than one space agency working on solving these problems.
Learn More:
Beijing Institute of Technology Press - Modeling of electrostatic and contact interaction between low-velocity lunar dust and spacecraft
Y. Feng et al - Modeling of Electrostatic and Contact Interaction between Low-Velocity Lunar Dust and Spacecraft
UT - Lunar Dust is Bad. But Not as Bad as Living in the City
