> blowing an engine nearly horizontal to the surface

I doubt that objects put in "orbit" with a periapsis of almost zero (launched from the ground) would stay in flight for too long.

Well lunar gravity varies a huge amount by location, it may be possible for some of it to somehow end up in actual orbits, especially when it's intentionally done so.

But by the same token, those orbits will be unstable and the particles will quickly lithobrake.

Well probably, but they could also stay there perpetually.

No, as light pressure will push them out of frozen orbits.

How long would that take?!

Not long.

My understanding was that those variations in gravity ("mascons") rendered stable lunar orbits next to impossible. On Apollo missions, orbits degraded by many kilometers over the course of just a few days.

Interestingly, a researcher looking for probable impact sites found Apollo 11's ascent module could potentially still be in orbit. (Though, it's probably not.)

https://phys.org/news/2021-07-apollo-ascent-stage-orbiting-m...

> 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?

Yes. You don’t have to aim as you inundate an orbital plane with debris.

An orbit that start on the surface ends on the surface, in a single turn.

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 mechanism would loft a cloud of particles with a wide range of trajectories through a notoriously uneven gravitational field.

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.

Good ol’ kessler syndrome.

Actually, Moon is rather immune to Kessler syndrone due to its bumpy gravity caused by sub-surface mass concentrations.

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.

I wonder if "would be cleaned up rather quickly" means hours, months, or decades?

IIRC single digit years probably - Apollo missions released a couple sub satellites and missions control was then very surprised when those satellites lost altitude and crashed in a matter of months.

In comparison, there is likely still stuff from the 60s in orbit around Mars, and that's for a body with (thin) atmosphere.

> Good ol’ kessler syndrome

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.

It is also not permanent. The orbits of the debris would intersect the point where they depart, which is pretty close to the engine. Basically the engine would be hitting itself with everything it fired.

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.)

The expansion of the gas after it leaves the nozzle in vacuum would give the particles an additional kick. I'm not sure if their orbit would still intersect the engine or effectively boost higher.

I left myself some wiggle room in the phrase "where they depart" for that reason. It won't all be a straight line out of the rocket motor or whatever is pushing, because in the first fractions of a second the gasses and the particles can interact and bash each other into slightly different orbits.

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.

Or add an atmosphere to the moon. Even a tenuous one would prevent this.

A discussion about that: https://space.stackexchange.com/questions/12576/could-the-mo...

As mentioned in the link below, the Moon has a tiny atmosphere.

No, it has an exosphere. The difference being an exosphere gas particles are more likely to collide with the ground at the end of a parabolic trajectory than to collide with other gas molecules, so you don't really get any of the properties characteristic of atmospheres.

I hope all of these failures to try to land on the moon do not damage existing satellite infrastructure.