On this day in 1993, the Galileo spacecraft slipped past a lumpy grey rock called 243 Ida, some fifty-odd kilometres from end to end, and photographed it on the way through to Jupiter. Nothing unusual there. Here's the clever bit: Galileo's big umbrella of a main antenna had refused to open properly two years earlier, so everything it saw had to come home down the little low-gain antenna instead — a trickle measured in bits per second rather than anything you'd boast about. Images were stashed on the tape recorder, squeezed with new compression code uploaded across half the solar system, and dribbled back to Earth over months while engineers reassembled them line by line.

And sitting inside that trickle, unannounced, was a moon. When the Ida pictures finally arrived and were examined, someone on the imaging team noticed a small round speck off to one side: Dactyl, a bit over a kilometre across, quietly orbiting an asteroid. The first asteroid moon anyone had ever seen, and it travelled home in a data stream so thin you could almost count the bits.

You can see it in the photograph below, the tiny bright dot to the right of all those craters. Lyra will tell you about the orbit; I'll just admire the plumbing, and the patient people who wrote the software patch that made a broken antenna good enough.