[f-word]

Suppose you put a largeish amount of polyurethane foam in orbit, possibly around some scaffold to take up even more space, had it inflate when up there and had it absorb any wandering garbage for a while. How long would it take for the giant garbage mound to deorbit and burn?

[MauranKilom]

I'm not exactly sure how you imagine the collisions to play out. If you think anything gets "stuck" in your foam, you're underestimating the speeds at which objects in orbit move. It's more like shooting a railgun at your impactor. I also recommend Netwon's impact depth approximation: https://en.wikipedia.org/wiki/Impact_depth

For the purposes of deorbiting junk it may be sufficient to just impart some delta-v, but you neither want that to be uncontrolled collisions sending stuff into even more erratic orbits, nor would you want to risk generating even more tiny pieces.

[BurningFrog]

I don't have a good feel for the numbers either, but surely if you make the Space Sponge big enough, things will get caught in it.

There is almost certainly a better material to use as well, but I don't think either of these practical concerns invalidates the idea.

Of course, big enough Space Sponges to make a practical difference would be very visible to the naked eye.

[MauranKilom]

Well, the reason why I brought up Newton's impact depth formula is that it quite clearly shows you have to have enough "braking" material up there. Distributing it over a greater area reduces the effectiveness proportionally.

The problem in the end is launch costs. Yes, we're getting better, but that doesn't mean we can shoot up a "visible to the naked eye"-sized impactor. Not to mention what difficulties an object of that size would bring for non-garbage that also occupies those orbits.

Lastly, I have no idea how much research there has been in containing the fragmentation of hypervelocity impacts. But presumably it would be an important part of the mission to not generate more garbage.

[BurningFrog]

OK, you've convinced me.

Even if launching costs ever makes this feasible, any Space Sponge big enough to be efficient would be a big collision problem in itself.

[jl6]

There’s a lot of junk but it’s spread out over a lot of space. You probably won’t catch much by accident. Also, it may be going dramatically faster than your catcher, in which case it would be less like a mitten catching a ball and more like a mitten catching a bullet.

[f-word]

Sure, but there's also a lot of uh mitten in this case, getting progressively denser as more stuff gets trapped in it. I don't think there's that many ways to trap all the tons of hyperfast shrapnel we've up up there, unless we can get starlink to just sporadically throw small nets around in hopes of catching enough junk on any given throw?

[jerf]

No, your "mitten" is getting blown apart, not getting progressively stronger. Your intuition is not tuned for space. Imagine trying to strengthen a structure by firing extremely high-powered bullets at it. "I have a tree near my house that is about to fall on us... how should I fire my high-powered rifle at it to reinforce it so it stays up?"

[f-word]

Right, so what's your solution then?

[jerf]

Who says I have a solution? Or that there even is one?

[londons_explore]

I suspect if you did the analysis, you'd find rather than it getting denser over time, you'd find most bits of space debris (even tiny ones) would have enough energy to explode your catcher...

[drfrank]

A spherical Whipple shield might be able to capture debris. A 1 mm thick aluminum shell with a radius of 100 m would mass 810,000 kg. If SpaceX's Starship can achieve launch costs of 10/kg, that's $8 M per sphere.

But passive objects are only as likely to provide value as the ratio of the number of the total volume of the sacrificial objects to the total volume of assets, so you'd want a bunch of them.

Since the debris of interest is really just the material traveling in orbit, a cylinder might be more efficient, if some primitive attitude control could be included efficiently.

[JulianMorrison]

It might help to put it in a suborbital trajectory, so it would basically go up, squirt, expand to a huge size, and drop back down, being hit by the intended object as it falls. You'd need a lot of foam to knock significant delta-v off the junk, and it would be catastrophically smashed by the thing impacting, but most of it would drop straight back down, and the bits blasted into orbit would be small and light enough to deorbit themselves.

[mdotshell]

Because of orbital mechanics, anything that would be traveling faster than the scaffold would be at a higher orbit and wouldn't be able to be caught.

The exception to this would be if the object were in an elliptical orbit, which would allow for contact to be made with the scaffold at its perigee, which be impossibly rare.

[usrusr]

If you'd ever build an artificial "sponge moon" with the goal of assimilating as much spacejunk as possible you'd probably not want to circulate its orbit. But I doubt you'd ever face that decision, given the absurd materials requirements imposed by orbital collisions.

[JshWright]

Faster orbits are lower, not higher.

[steerablesafe]

This is in fact correct, orbital velocity is sqrt(G*M/r) for circular orbits.

[f-word]

Wouldn't this be a matter of simply getting it to an useful starting position?

[Chris2048]

what if that thing had an elliptical orbit?

[gbrown]

In addition to the other objections, the polyurethane would likely disintegrate.