9P/Tempel 1

9P/Tempel 1 matters because it is the comet humanity chose to hit on purpose. In 2005 NASA's Deep Impact mission released a heavy copper-laden impactor into the comet's path, and the spacecraft watched the collision from a safe distance. The goal was not destruction but excavation: by punching into the surface, the mission exposed material that had been shielded from sunlight since the comet formed.

What makes Tempel 1 valuable is what that impact revealed. The plume of debris was finer and more powdery than expected, and it carried water ice, organics, and dust that had stayed largely unaltered for billions of years. A comet's outer crust is processed by repeated passes near the Sun, so reaching beneath it offered a rare look at genuinely primitive Solar System material rather than a weathered shell.

That is why Tempel 1 belongs on this scale. It is a small icy body only a few kilometers across, yet it became the site of an experiment that turned a comet from something we observe into something we can probe. Tempel 1 is the proof that even a distant comet's interior is within reach of a well-aimed spacecraft.