Humans clearly have a trash problem on Earth, but our track record isn't that much better in outer space, where tens of thousands of stray debris fragments whip around the planet at rip-roaring speeds, posing a very serious danger to astronauts and satellites.
Many ideas have been tossed out to deal with our orbital trash, including space-grade fishing nets and laser-telescopes. The latest so-crazy-it-might-just-work scheme? An autonomous space trash bot that sucks up junk and vaporizes it for propulsion.
It's still very theoretical, but the concept, which appeared in a recent paper on arXiv, is pretty intriguing in that the trash bot would require no external source of propulsion. In its current design, the engine would catch small pieces of debris (<10cm) that aren't easy to get a fix on with a laser but still pose a big danger to orbital infrastructure. (According to the ESA, even a <1cm nugget of junk could pack the punch of a hand grenade.) Trash would then be fed to a ball mill, a coffee grinder-esque contraption that uses abrasion-resistant pellets to pulverize everything into a fine powder.
Conceptual model of the space junk engine, via Lan et al. 2015
That powder is then fed into a charging system, which heats it into a roiling mess of plasma. From that plasma, positively charged ions are separated and accelerated to high energies to generate thrust. Each time the bot swallows some food, it gets a little kick, propelling it onward toward its next space meal.
Of course, there are still some big issues to work out: For one, while the craft wouldn't need propellant, its vacuum-cleaner-coffee-grinder trash-to-energy system still requires a power source. What's more, the thrust the spacecraft produces depends on the density of debris, and you might imagine a situation where the trash bot, having cleared the surrounding area, suddenly finds itself marooned.
Personally, I'm hoping to see a combination of these ideas make it past the concept phase in the next few years. The orbital trash problem is only getting worse, and an environmental cleanup effort involving giant lasers, space fishing, and autonomous vacuum cleaners is something I think we'd all like to see.
Top image: Artist's impression of debris swirling around the earth, via ESA -- P. Carril