What do we do about all that dust blown around when you land on the Moon?

Moon dust is evil. Plus: Capturing carbon, and finding the first stars torn apart by black holes

May 30, 2024 Issue #728

I learned a thing!

Wherein I learn a thing

I learned two things, actually.

I got a press release about a new paper looking into what happens to dust and rocky grains on the surface of the Moon as a rocket descends to land. This is a critical area of study!

The Moon is covered in this material, which is called regolith. It’s made of tiny grains of eroded rock from the surface, and is nasty stuff. Erosion on the moon comes in the form of small meteorite impacts, which can break down rocks, and thermal stress, which means big temperature swings from day to night, which, over time, also causes rocks to fracture and crumble. On Earth, where there’s air and water, eroded material tends to be smooth (think river rocks), but on the Moon there’s nothing like that, and these tiny grains are jagged and evil-looking.

If they get into machinery they can cause damage, and I don’t even like thinking of what they’d do to a telescope mirror. They’re also extremely dangerous to astronauts’ health; once breathed in they mix with mucus in the airways to form a cement-like material, and the Apollo astronauts complained a lot about how irritating regolith is. 

So studying how this material is transported by descending rockets is extraordinarily important if we plan on having more permanent bases on the Moon. For example, how far should a landing pad be from living spaces or observatories such that they aren’t inundated by lunar dust?

Long black tendrils of material extend away in all directions from a small crater on the Moon. The surface is otherwise gray.

A 12-meter crater on the Moon created by a small asteroid impact. The dust, seen dark here, gets blown a long way, and a new study shows even a rocket landing can send this material to incredible distances. Credit: NASA's Scientific Visualization Studio / LRO

Here’s the first thing I learned reading the paper: This has been studied, but it’s extremely difficult to do. The Moon is a vacuum and has lower gravity, so it’s difficult to impossible to build a chamber to physically test the situation. We have to rely on old data from Apollo and theoretical understanding of the physics. 

The second thing I learned is that this problem is far worse than I would’ve thought. The basic idea is that hot gas moving at high speed from an engine nozzle slams into the dust and accelerates it strongly, flinging it away. The way this works on the Moon is very different on Earth. For example, we have an atmosphere, so fine dust won’t get far because it immediately slows hugely as it rams into the air. On the Moon, with no air, that material is free to travel long distances.

And it does. Specifically what I learned, in what was almost an aside in the paper, is that dust from the Apollo landings were accelerated enough to travel all over the Moon. Literally any spot on the Moon could have a grain of regolith flicked there by the Lunar Module. In other words, there is no safe spot on the lunar surface from this phenomenon. So what we really need to ask is, are there ways to mitigate this problem (building some sort of windbreak, as it were, or figure out a way to design engines to minimize their impact), and how much grief from it are we willing to accept? Is there some minimally acceptable amount of dust you’re just going to have to deal with as a lunar inhabitant?

There’s lots of interesting physical details dealing with gas flow, and how its energy is transferred to the dust, and I hope to read more about that later (though admittedly my knowledge of fluid dynamics is sparse). But an important point is that this is something all space agencies need to look into, especially since international cooperation will be critical for exploring the Moon. We can’t have one country’s dust destroying another’s billion-dollar experiment. I hope there is a multi-national effort to fund this research, and even do targeted experiments to gather more data as we send more landers down to the surface.

Exploring space is hard, and one of the hardest parts is realizing just how different things are, and how can take nothing for granted. Nothing. Landing a rocket is hard, obviously, as we’ve seen recently with so many failed attempts to do so. But even if we can figure out the engineering, there are still lots of other issues to worry over. I’m glad some scientists are taking the initiative to do so now.

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