Whaling on the moon was not too successful, so taking it out into orbit seems a reasonable next step.
While NASA finally gave up on the Opportunity Rover, a UK-built space harpoon was fired last week, much to the annoyance of one ex-Shuttle bigwig... and earlier today, the UK agreed to fling money at satellite tech. Moby Dick in Spaaaaaace: Harpooning space junk The University of Surrey was chuffed to tell us late last week …
Cursory investigation suggests that this idea may have legs, albeit very short ones; Series 4 Episode 2, "Deep Space Wombles," isn't exactly a thorough proof-of-concept but it demonstrates the stakeholders' engagement and that much of the heavy lifting has already been done.
> Wayne Hale, ... pointing out the dangers of the harpoon approach
The most comical "danger" would be if the harpoon's tether broke and it missed its target.
Then continued in it's orbit until it arrived back at the place it started from and stuck its own host satellite.
Earth's atmosphere extends out past the ISS, though tenuous in the extreme at such a distance, it is there and will slow a spacecraft accordingly. This is why satellites and the ISS must expend fuel to maintain their orbits and why satellites in near earth orbit eventually sink low enough to burn up after they have expended their fuel supplies. Depending on the orbital path, relative speed and the surface area of the spacecraft this form of atmospheric de-orbiting can take anywhere from weeks to years to accomplish, the large sail is essentially just a means to hasten the result.
Quite a bit of LEO experiences some level of aerobraking but most satellites are small enough that it doesn't impact them too badly. Exceptions include the ISS whose solar panels are big enough to catch a bit of "wind" and requires regular station-keeping boosts back up to their target orbit.
A lightweight sail that costs little to launch but which can significantly increase the cross-section of the sat is a fairly easy and passive way to deorbit sats quicker (it'll happen anyway, but this speeds the process).
I dunno, it may mean that NASA think the Tango Prince is going to throw some more histrionics and cause more shutdowns, slowing down the NASA end of SpaceX operations at KSC. Astronauts at Baikonaur waiting on a Russian launch will be less affected by such political uncertainty than astronauts sat in Florida waiting for NASA officials to sign relevant paperwork.
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