In article .com,
snidely wrote:
For a collection of small pieces in reasonable proximity, could you
launch a foam dispenser at a closely matching orbit (station keeping)
slightly ahead of the debris, dispense the styrene/urethane/whatever,
let drag slow it down just enough that the debris impacts at low
relative velocity, and then let drag lower the orbit?

Feasible in principle (although some details need work), but it depends on
having a collection of small debris in essentially the same orbit.

Unfortunately, most any event that spreads small pieces gives them enough
of a shove that they end up in slightly different orbits. Not only do
they spread out along the original orbit because they have slightly
different orbital periods, but worse, the rate at which an orbit's plane
precesses around Earth's axis depends on its period. So they soon end up
so spread out that there's no way you can have them all hit at low
relative velocity.

I think there would be a real problem, though, with small isolated
pieces -- hard to track, and you don't get many of them per cleaning
trip.

Exactly. And as per above, that describes essentially all of the small
debris. You could consider sending a tug to collect big stuff, but no
way is that feasible for the small stuff.

For those you can detect on orbit as they approach, maybe you could
throw aerogel foam in their path? Urethane wouldn't slow them much if
they were fast enough to be a problem for the current-technology
shields, although I guess aerogel would get poofed pretty fast, too.

Unfortunately, there's basically nothing you can put in their path that
will help. Analogies from lower speeds fail; at relative velocities of
multiple km/s, most any impact is an explosion, regardless of the exact
nature of the materials, because there is more than enough kinetic energy
there to vaporize *any* material. This actually tends to make things
worse, because shock waves from the initial contact blow the two colliding
objects to bits, and the two debris clouds then pass through each other.

Aerogels are a partial exception -- they're thin enough to decelerate an
incoming object slowly and spread the energy out over both time and
material -- but you would need enormous volumes of aerogel to decelerate
macroscopic objects that way.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |