We have known about comets as long as we have been human. When we see comets in the sky from Earth, the first thing we notice is their tails: streamers of gas and dust pointing away from the Sun. And those tails can be very impressive, stretching across a significant fraction of the sky. The most famous, Halley’s comet, is a regular visitor to Earth (though its last pass-by was less than astounding to those of us without decent telescopes), and I personally remember Comets Hyakutake and Hale-Bopp very fondly.

However, a lot of things about comets are still mysterious, which is why the Rosetta spacecraft is exciting. After ten years of maneuvering to get into position, the probe is now in “orbit” (actually a series of corrective maneuvers) around Comet 67P/Churyumov–Gerasimenko. While other craft have flown by comets, Rosetta is the first to visit one on a long-term basis, and will even send a lander to the surface. My latest column in The Daily Beast explains why this is a big deal:

What we primarily see is the clothing of the actual comet: its tail is a flowing robe of melted ice (consisting of water, carbon dioxide, and other familiar chemicals) and dust knocked off by particles streaming out from the Sun. While it may stretch a long way, that robe contains relatively little material. The body beneath is a chunk of ice and dust and interesting complex chemical compounds, such as amino acids; studying that body is the goal of Rosetta. [Read more…]