Fleas are known to attack furry animals and humans alike.
No one is quite sure where these individual vehicles came from, or even how many there are in existence, but they seem to be the property of the solitary prospector Nansen, who uses them to drop in on trading posts with crates of unbelievably pure iridium, which he trades for food, money and Earth cloudberries (his favorite treat) — though, strangely, not water, leading to Multicorps speculation that he has been able to steal water from the Europa water mines, then hops away as quickly as he arrived.
Their origin is mysterious, but the components of the Fleas’ fusion nuclear/solar-powered propulsion engine seem to be common technology and relatively weak, with the nuclear reactor powered by thorium, the slowest-decaying radioactive element, which allows it to travel without ever (as far as anyone knows) refueling.
The ships’ true innovation is in a high-powered computer system that constantly recalculates the gravitational pull of a range of proximal objects in such a way that it can then slingshot itself from body to body at speeds that can far exceed what other ships are capable of.
Nansen usually keeps his Flea anchored on the back of comets like ISON, riding it at high speed to outer trading areas like Far Commissariat and then whipping back to out-Sol for more prospecting. How the sling-shotting Flea doesn’t completely jumble Nansen’s brain in his skull is another mystery.