The damage to lunar orbiting spacecraft caused by the ejecta of lunar landers
arxiv.org
arxiv.org
It turns out that the rate & mechanics of erosion by rocket plumes is an unsolved problem that requires a new kind of model. To quote from his thread,
In the Apollo era the thinking was that the rate of soil erosion is controlled by conservation of momentum. It turns out this is wrong.
NASA researcher Leonard Roberts, the first person to research this topic, hypothesized that the soil grains steal momentum from the gas, which slows down the gas and thus reduces the erosion rate. It was this feedback that determined the rate.
I argued some years ago this has to be wrong because the particles achieve their high velocities far downstream of where they are lifted off the surface, so momentum transfer does not provide feedback to control the rate that grains are lifted.
Conversely, blowing an engine nearly horizontal to the surface is going to shut off the Moon for a while. That's a weapon.
Is it in any way more practical weapon than just lobbing stuff?
Kessler is a chain reaction. Destruction is caused by secondary effects, i.e. bits of satellites the primary projectile broke hitting targets. This is closer to an area denial weapon: the destruction is caused directly by the debris blown off the surface.
Therefore, the logical thing to do is to put it on the correct side of a mountain, to shield the engine. But that would also collect all the debris. So it would generally only be in orbit for one orbit.
That makes it more targeted than some of the other unfriendly things you can do in an airless planetary environment: https://news.ycombinator.com/item?id=35862424
(To be honest, I think on any planetary body without an atmosphere, long term everyone is going to have to dig in to the planet, and to a non-trivial degree, too, not least of which is the complete indefensibility of surface installations.)
However, that will dissipate quickly and you'll certainly be looking at a set of orbits that all pass through something relatively close to the origin. They're not going to be interacting for the first time a quarter of the way through the orbit and bouncing around a lot there.
So Kessler syndrome could certainly develop for a whiley but would be cleaned up rather quickly as all the pulling & pushing of the "rough" gravity converts the orbital speed into heat, until all the fragments impact the surface.
Might be a bit more dangerous on the surface for a while though, with lot of stuff striking it at near orbital speed in an almost horizontal direction. That could ruin your evening stroll quite badly.
In comparison, there is likely still stuff from the 60s in orbit around Mars, and that's for a body with (thin) atmosphere.
That can be a while if you near the escape velocity, still a slim chance to hit anything.
There's a huge gap in hit probability between "dangerous enough to consider safety" and "good enough to be a weapon".
The odds that a sufficient fraction of particles would remain in orbit for long enough to make routine operations in low lunar orbit or on the surface itself seems ... plausible.
And of course, the situation could be compounded by multiple burns and/or at multiple points on the surface, at a somewhat increased cost to the attacker.
I doubt that objects put in "orbit" with a periapsis of almost zero (launched from the ground) would stay in flight for too long.
https://phys.org/news/2021-07-apollo-ascent-stage-orbiting-m...
"if LADEE did encounter any lunar soil particles thrown up by the final descent of Chang'e 3, they would have been lost in the background of Geminid-produced events." [0]
That said, the Chang'e 3 is an order of magnitude (or close to two) smaller than the lunar landers they are talking about in the study. Also my own speculation is that the more continuous thrust of a lander may get particles to higher velocities due to the additional time for acceleration in the wake of the thrust as compared to the single impact of the meteor.
I struggle to compare exactly how bad the lunar dust ejection is though. Most Micrometeoroid and Orbital Debris (MMOD) curves are specified as a Flux by particle size (velocity is sort of irrelevant, as you assume most of the velocity is from the spcecraft itself and most hits are in the direction of travel of spacecraft, the ram direction). My suspicion is that MMOD flux in a LEO orbit is still going to be far far worse.
[0] https://www.nasa.gov/ames/ladee-project-scientist-update-mil...
Edit: The paper talks about flux of particles 10 um and smaller of about 10,000 impacts/m^2 during the passes. If we assume that this is a sphere of iron (new MMOD fluxes are specified in mass, not size) its ~5e-9g. In LEO at 400 km altitude (a little above the ISS) the flux of particle this size is ~1000 impacts/m^2/year. But the paper says smaller than <10 um. And at smaller masses the flux increases exponentially to 10^7 particles/m^2/year at a particle mass of 10^-18 g. So I believe my suspicion is correct that most LEO orbits are still worse, but its hard to compare apples to apples.
(I still think this is overblowing the problem, because any lander that causes this big of an ejecta problem would also badly damage itself. All the designs will put a LOT of engineering work into minimizing debris, eg Starship putting separate landing engines high up on the vehicle.)
Not necessarily because the relative speeds will be very slow. Not so in low lunar orbit, where an orbiting spacecraft will slam into the ejecta curtain at >1 km/s.
Plus then you're dealing with damage to your hopefully-smooth landing site.
For Mars, the orbital velocity is ~3.5 km/sec, thus requiring almost 5x the energy for a given mass of detritus (E = 1/2mv^2); and while its atmosphere isn't as thick as Earth's, it'll definitely cause drag for particles going that fast.
* You don't quite need orbital velocity for a plume to get high enough to disrupt an orbiting craft, but it's a handy reference point.
Also, could regolith be bound or, well, packed down to build pads?
Masscons did perturb the Apollo missions enough that they had to switch to a doppler radio to navigate accurately.
Although it does assert that the NASA Gateway orbit passing through the ejecta sheet "will probably be several times before the sheet is dispersed."
The problem is, your orbiter is necessarily in orbit at the same time that your lander is making its landing. And the lander kicks up debris that can then threaten the orbiter.