In July 1988, the automatic station Phobos 1 lifted off from Earth. Its target was Mars and its mysterious moon, Phobos. Two months after launch, an incorrect command was sent, communication was lost, and the probe fell silent forever. Since then, it was considered lost.
Decades later, astronomers spotted a strange object—1998 KY26. It doesn't fly like a typical asteroid: its trajectory changes, as if something is nudging it. Usually, such behavior (non-gravitational acceleration) is a signature of comets: they emit gas jets from their nucleus, creating thrust. But this object has neither a gas cloud (coma) nor a tail. The mystery was dubbed a dark comet.
The authors of the study tested a bold idea: what if 1998 KY26 is not a space rock at all, but a remnant of Phobos 1? They calculated that two engine burns would have been enough to put the probe on its current orbit. The first nudge (right after the loss of contact in 1988, the second) in May 1996. The total impulse of 1.9 km/s fits well within the capabilities of the station's main engine, which ran on nitric acid and amines. Interestingly, in 2031, Japan's Hayabusa2 probe is scheduled to arrive at this object. If the hypothesis holds, it will meet not an asteroid, but a piece of Earth technology.
The word 'dark' here doesn't refer to color but to stealth: the object behaves like a comet but shows none of the usual signs, as if hiding its activity.
The situation echoes Arthur C. Clarke's story 'The Sentinel', where a lunar artifact turns out to be a trace of an alien civilization — only here, the trace was left by us.