19P/Borrelly

19P/Borrelly matters because it was only the second comet nucleus humans ever saw in detail, after Halley. In 2001 NASA's Deep Space 1 — a probe built mainly to test new technologies like an ion engine — made a bonus flyby and returned the sharpest comet images of its time. The result was a clear look at an elongated, bowling-pin-shaped body about eight kilometers long.

What makes Borrelly interesting is how dark and varied its surface turned out to be. The nucleus was one of the blackest objects in the Solar System, reflecting only a few percent of the light hitting it, yet bright jets of gas and dust streamed from a smoother central region rather than uniformly across the whole body. That confirmed a key idea: comet activity is concentrated in specific active zones, not spread evenly over the surface.

That is why Borrelly belongs on this scale. It bridges the gap between Halley's pioneering 1986 flyby and the detailed comet studies that followed, and it did so almost as an afterthought on a technology-demonstration mission. Borrelly is proof that even a small, dark comet can reshape our picture of how these bodies come alive.