Don’t shoot yourself in the back!


Dr. Phil Metzger is a Planetary scientist at the University of Central Florida and CoFounder of NASA KSC SwampWorks. In a recent tweet thread he talked about the danger of dust being kicked up when landing on the Moon.

Dust? Really? So a spacecraft kicks up a little dust while landing. What’s the big deal? As you’ll see in this article. Something as trivial as dust can be problematic for future missions.

SpaceX Starhopper performs a successful take off and landing
Picture credit: BBC article on SpaceX

If you look at any rocket launch or the recent test of the SpaceX Starhopper there is ALWAYS dust, sand, dirt and rock kicked up, mixed with exhaust fumes and water vapor, forming impressive clouds.

Falcon Heavy launch.
Picture credit: SpaceX

Imagine standing down there, just slightly off to one side as the launch exhaust rushes at you. Madness, right? Ah, yeah… that’s precisely where Dr. Metzger’s research picks up. You see, on Earth, we have a thick, dense atmosphere, so the rush of the exhaust quickly dissipates as it runs into opposition from the atmosphere, billowing into those iconic clouds. Not so on the moon.

Dr. Metzger and his team spent considerable time analysing the footage of the Apollo lunar landings, looking at the dust and debris being kicked up. In this image, a rock the size of a fist is accelerated to 15 meters per second (about 35 miles per hour)

Rocks being propelled by the Apollo Lunar Lander
Picture credit: NASA with annotation by Dr. Metzger

As they looked closer, Dr. Metzger’s team noticed dust trails forming behind rocks embedded in the surface of the Moon. In essence, these rocks were being sandblasted, slowly stripped away by the grit kicked up by the exhaust.

Dust trails forming behind rocks as they’re eroded by the exhaust
Picture credit: NASA with annotations by Dr. Metzger

What the team realized was there was something in between the easily visible rocks and the dust trails, and that was sand. Rocks were clearly visible, tumbling in the videos. Dust formed trails that were easy to see, but the fine grains of sand were essentially invisible, although their effect was apparent, causing rapid erosion.

When they calculated the speed of these various classes of ejecta, they were surprised to see dust and sand exceeded the escape velocity of the Moon.

Unlike on Earth, where the exhaust gases and dust mix with the atmosphere, billowing into clouds as they slow down, there’s no such resistance on the Moon. Being in a vacuum, once set in motion, this ejecta keeps going until it hits something.

Ejecta trajectory
Picture credit: Dr. Metzger

The image above is a simplification, but it highlights a surprising problem when it comes to Lunar missions. You can shoot yourself in the back!

Dr. Metzger points out that there are FOUR points at which the hyper-velocity ejecta could impact a lunar orbiter. Remember, the lander is kicking out debris in all directions so (in our illustration above), some goes left, some goes right and then crosses the orbit twice (while outbound and then again when inbound).

These calculations were done for the Apollo missions. Even back then, with a lightweight lander, ejecta could be propelled off the surface into orbit. Not only did dust, grit and sand get into orbit, it went well above the altitude of the CSM (Command Service Module) waiting in orbit.

Apollo CSM
Picture credit: NASA

Now space is big. Dr. Metzger’s diagram is a generalisation to make the point. The likelihood of a direct impact is tiny as the Moon’s gravitational field is “lumpy” (for lack of a better word), so the probability of grit and dust returning to exactly the same point is low, but it is NOT zero. It could come down anywhere within a hundred miles of the original landing zone after circling the Moon.

As bigger craft land more often, the problem gets worse. (Bigger landing craft will throw off bigger debris). If we establish a surface base, the impact of micro-meteors may very well originate from our own landing craft!

What would the damage be to orbiting spacecraft or astronauts on the surface? Imagine standing in the way when the craft originally came down, as that grit and rock won’t have lost any of its momentum.

Could some of these artificial micro-meteors puncture a spacesuit? They would probably get through some layers at least. If they hit a spacecraft, the damage will depend on the size/speed, but several years ago, the shuttle was hit by a fleck of paint and the result looked like a bullet had struck.

Tiny bits of rock and sand can ruin your day in space
Picture credit: NASA

As we return to the Moon, the research of planetary scientists like Dr. Metzger will allow us to understand the risks and plan accordingly so we can avoid shooting ourselves in the back.

You can read the original thread by Dr. Metzger on Twitter.

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