Henry Spencer wrote:
[...]
but that will not remove the junk that is already there...

Unfortunately true. Doing anything about the existing junk is hard.

It would vary, I think, with what you'r trying to clean up...

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?

Of course, drag effects are only useful for LEO orbits, but isn't that
where the problem is most acute?

Large skin pieces will have their own drag, and probably are self
cleaning in this region.

Large heavy pieces may require an autonomous docker that can do a retro
burn.

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.

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.

/dps