81P/Wild (Wild 2)

81P/Wild matters because it is the comet that sent a piece of itself back to Earth. In 2004 NASA's Stardust spacecraft flew through the comet's coma at high speed, extending a collector filled with aerogel — a foam so light it could trap delicate grains without vaporizing them. Two years later a capsule parachuted those grains down to a desert in Utah.

What makes Wild 2 especially interesting is how recently it became active. For most of its history it orbited far out in the cold outer Solar System, and only a close pass by Jupiter in 1974 swung it onto its present path near the Sun. That means its surface had been relatively undisturbed, so the dust Stardust collected was unusually fresh. To everyone's surprise, those grains contained minerals that form only at very high temperatures, evidence that material had been transported outward across the entire early Solar System before being locked into a comet.

That is why Wild 2 belongs on this scale. It is a few-kilometer ball of ice and dust, yet it delivered the first physical sample of a comet into laboratories on Earth. Wild 2 turned cometary science from remote observation into hands-on analysis.