Regarding space junk, can't we laser them out of existence? Or use laser to ablate their surface and put them into reentry orbit? That, I suppose, requires a orbit based laser.
No but you can use ground lasers to push things higher to avoid a collision. I guess you could do the same thing from space with a big laser. But that not really viable. If you already going for a space base based solution why use a laser in the first place? There are better solutions for that.