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Comment by onion2k | Hacker News Reader

What matters is the relative velocity of the projectile towards the target.

True. If you really want to mess space up forever you need to launch a rocket full of sand, slingshot around the moon, and gently sprinkle it in an retrograde orbit. That would be fast enough for something as small as a grain of sand to create pinhole-sized punctures in anything put in orbit, and after a few years the sand would be spread out enough to make any launch essentially impossible.

I might have thought about this too much.

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Space is really, really big. I think it's hard for us to comprehend just how much space is in space, especially in 3-dimensional terms. It would take a lot of sand to mine all of the useful orbital space, and that's just LEO. There's tons of room in higher orbits. It would measurably increase the risk of passing through LEO on the way to other destinations and make some orbital planes unusable for e.g. Starlink or human habitation, but when you're just passing through, the odds of getting punctured are probably still reasonable, at least for unmanned spacecraft. Keep in mind that the ISS maneuvers out of the way when there are several miles to a potential impactor, just in case. We have all our eggs in one basket right now, so everybody is really careful with that one basket.

LEO is relatively simple to destroy simply because it's already so populated with mass, and that population is growing rapidly, and because they all come pretty close to crossing each other's orbits. Collisions with even very small pieces of debris typically generate secondary debris, growing the population.

A half-hour boost phase through LEO to get to higher orbit is dramatically less risky than spending 87600 hours there over a 10-year orbital lifespan. In the event that it all goes asymptotic, what sort of timescales would be afflicted by collision risks 10^6 or 10^9 or 10^12 greater than what exist now, I am uncertain.

Then why are they making so much fuss about (tracked) small things drifting by, violating the "safety box" around a satellite or station?

Low earth orbit isn't that much space though right ? It is effectively a stack of practical orbital shells. A rogue nation state can make satellite launches financially unfeasible.

It would take a lot of sand to mine all of the useful orbital space, and that's just LEO.

Nope. You just need to make it a high enough probability of your spaceship being damaged that people won't take the risk. For humans that's very low. For anything moderately expensive it's quite low.

There's tons of room in higher orbits.

You have to go through the lower orbits to get there.

> You have to go through the lower orbits to get there

Math doesn't math.

The closest solution I've seen is putting 10^6 to 10^9 kg fragmented debris in the 700 to 1,000 km band to create a 5% failure rate at fastest transit, which is economically devastating. But that assumes away grain-grain collisions.

I think when I last ran the math, you'd need to sustain 5 to 10 Starship loads of fragmented debris to that orbit a week to create a proper barrier–near 100% hazard. At which point the solution becomes shooting down those launch vehicles. In a couple years (3 to 10, depending on the solar cycle), the skies would clear to about 1% hazard. That hazard, however, would persist for some time–how long is anybody's guess.

> you need to launch a rocket full of sand

You want an accretion disc around the planet like Saturn?

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