Active comets are fascinating ephemeral bodies that have both thrilled and terrified people for millennia. As a bonus, when Earth passes through a comet’s debris trail, we see a dazzling meteor shower. The phenomena of comets are spectacular, but what they can tell us about the early solar system is extraordinary. Comets are the best-preserved samples of the materials that were involved in the construction of the solar system.
The Sun, planets, and smaller bodies of the solar system all formed from a short-lived disk of material that for centuries has been called the solar nebula. The disk formed by the gravitational in-fall of gas, largely composed of hydrogen and helium, and small solid grains containing the majority of the elements in the periodic table. It lasted a few million years, only a factor of 10 longer than the free-fall time of interstellar matter onto the disk. Within those few...
