Back in 2005 (over 20 years ago!), Fraser wrote an article about the dangers of electrostatic discharge to astronauts on the Moon and Mars. Anyone that lives in the cold regions of our own planet, with its exceedingly dry interiors for half the year, knows the unpleasantness that goes along with getting shocked when you touch a metal surface. In space, that problem gets much worse, and could potentially prove fatal to astronauts or electromechanical systems if not dealt with properly. A new paper from Bill Farrell of the Space Science Institute and Mike Zimmerman of Johns Hopkins University, which was published in Advances in Space Research, goes over how that specific problem of “tribocharging” affects the operation of lunar rovers.
Rovers are expected to be the workhorses of future lunar exploration missions. Either automated or crewed, they will be the main mode of transport for both material and people around the Moon’s surface. But, as they are moving along that surface, they are building up electrical charge on their wheels. By the paper’s estimates, in some cases, that charge could reach up to 1 Megavolt. Such massive energy would essentially cause a miniaturized arc flash if an astronaut were to touch it, potentially frying their life support system. Even uncrewed rovers could have random electrical arcs between their wheels and other critical components, such as their navigation systems or electric drive trains.
Luckily, when they are driving around on the normal surface of the Moon, this effect is mitigated by a phenomenon known as photoemission, whereby the electrons gathering around the wheels are released by sunlight. That’s why you didn’t see any massive arc flashes on the Apollo rovers while they were careening around the Moon.
YouTube video from 15 years ago describing the same ESD challenges, and featuring Bill Farrell, one of the authors of the paper. Credit - NASA Goddard YouTube ChannelHowever, some of the most interesting locations on the Moon are in areas known as Permanently Shadowed Regions (PSRs). These regions literally never see the light of day, which means the photoemission effect doesn’t remove many electrons like it would on the surface, making the charge build up much faster. However, PSRs are also where the water is on the Moon, and given all the useful things that water can do (like allow humans to survive and create rocket fuel), we are definitely going to have to travel into these areas.
To mitigate the potential damage caused by the massive charge build-ups expected in these areas, the authors suggest several potential techniques. One technique is to consider the side of the crater to enter - if it’s on the “windward” side, facing the Sun, it can stay in the photoemission effect flow longer. But once that benefit is exhausted, it comes down to design choices.
One counterintuitive one is to electrically bond the wheels of the rover to the rest of the rover itself. While this might seem like you’re just inviting a 1 MV charge nearer to the sensitive electronics, it also increases the total area to spread the charge out to, and therefore the area the photoemission effect might work on. Another design choice is the type of materials the wheels themselves are made out of - if they charge “positively” (vs the negative charge of the regolith they are touching), it could minimize the initial charge they undergo. An even more unique solution is the idea of positioning UV lamps to shine on the wheels, inducing an artificial photoemission effect using the rover’s own electricity.
Video showing VIPER, one of the future lunar rovers, being tested on simulated lunar dust. Credit - NASA Glenn Research Center YouTube ChannelThere is one other technique that could limit the danger from these charges, but it’s going to make every space explorer out there very unhappy. If the rover drives extremely slowly - like 0.2 cm a second slowly - then the charges will have time to dissipate even in the dark areas of the PSRs. Such an unedifying result is a far cry from what science fiction authors and even videos of the Apollo program would have us expect. But it is the safest option, at least for now.
As Artemis gets off the ground again, and with plans to eventually start having a large presence of lunar rovers, taking this concern seriously is going to be a key consideration for their missions. Since we’ve known about it for literally decades, scientists and engineers have certainly had time to plan. But hopefully they can come up with a solution that is better than simply crawling around some of the most interesting places on the lunar surface.
Learn More:
Phys.org - How to prevent charge buildup in a lunar rover
W.M. Farrell & M.I. Zimmerman - Rover wheel tribocharging in lunar shadowed regions: deriving a speed limit for charge accumulation