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In the movie gravity fast moving space junk destroyed a space shuttle - in reality this could happen, since the Russians lunched SputnikI human race went on a launching spree that left a lot of junk in space. Its space right? So it wouldn't be a problem - it will just fly away to a distant galaxy? Sadly no, space junk in low orbit are still experiencing gravity like our satellites and like satellites they will stay in orbit which will possibly result into a collision with our future space flights.
Harmlessly? If those 300,000 things burns out wouldn't they cause degradation in the atmosphere? If they have a laser powerful enough to reach low orbit and slow down space junk - what if a something accidentally passed by and boom! What will you guys say? Sorry we zapped your plane to pieces :p Ide have object tracking and identification set up first before having the laser zap anything. Better yet - lets launch the laser up there, to be charged using solar power and will zap the debris while its in orbit.