Laser zapping space junk

Question based on this recent news tidbit.

http://www.theguardian.com/science/2014/mar/10/australian-scientists-zap-debris-with-lasers

How does a ground based laser alter the orbit of space debris ? I don’t see how this could work.

Enormous increase in surface area does affect orbit stability…

There is some atmosphere there, so there is drag … its like you turn a rock into a pile of dust and throw it…

Two ways:

1.) Photons may not have mass, but they carry momentum. That may not make intuitive sense, but it’s true. It was first deduced thermodynamically, then demonstrated electromagnetically, and finally as a consequence of quantum theory. William Crookes invented the “Crookes Radiometer” (AKA the “Light Mill”) to demonstrate this. It turns out that it didn’t really demonstrate it (his vacuum wasn’t good enough), but the principal was eventually demonstrated once good enough vacuums had ben achieved.

Light pressure is the motive force behind solar sails and optical tweezers. The hitch is that the momentum per photon is really tiny, so you either need HUGE surface area (hence those big solar sails) or very intense light sources (hence the lasers).

The thing is that the lasers can act on the same piece of debris over long periods of time, and the effect is cumulative. Laser light pressure has long been suggested as one way to clear up the orbiting garbage to try to avoid the Kessler Syndrome.

2.) Even more immediate and dramatic, focused laser light can ablate away a surface layer of material. Since you’ve got conservation of momentum, blowing a little bit of material off one side at high velocity produces a small change in the momentum of the larger body in the other direction – but it’s generally a LOT larger than the oomph you get from laser light pressure. I know – I worked on laser propulsion for a couple of years. We could send dime-sized pieces of plastic (chosen for its high absorption of the pulsed laser light) zipping away with a pulse of light from our laser system.
If you’re really lucky and arrange things right, you can get even more of a push by sending another laser pulse in to heat that ablated material via inverse bremsstrahlung, causing it to expand and push against the larger body. This is a Laser Sustained Detonation Wave (LSD wave – and no smarmy cracks about what the experimentalists were ingesting), and was the hope for the Laser Propulsion movement. There are lots of papers and do9cuments on the topic, or you can read Jerry Pournelle’s story High Justice, or Michael Kube-MacDowell’s novel The Quiet Pools, or my own just-released novella The Flight of the Hans Pfaal for details.

Could you get enough intensity at a hundred or so miles up to heat something up enough or reflect enough light, to do much, though? When the debris is closest, you’re just pushing it up which won’t help much, and when you’re slowing debris down, it’s got to be several times farther away.

Interesting thread/Dopername combo.
My understanding is that there have been proposals for orbiting lasers and ground-based ones. The ground-based ones, despite having a much longer line-of-sight, are infinitely easier to maintain and control, and are meant to slow down debris so that they go into lower orbits, and eventually succumb to atmospheric drag.
Here are some links:

http://orbitaldebris.jsc.nasa.gov/

http://www.space.com/11157-nasa-lasers-shooting-space-junk.html

Project ORION – laser use to mitigate space debris:
http://adsabs.harvard.edu/full/1997ESASP.393..699B

I think “pushing it up” does help. That will both raise the apogee and lower the perigee of the debris. It will then experience far more drag as it drops further down into the atmosphere at perigee.

Slowing the debris down with a retrograde impulse would be a more efficient way to do that, but that would require either an orbital laser or a ground-based laser that can go through a lot more atmosphere.