This image taken by NASA's Spitzer Space Telescope shows the comet Encke riding along its pebbly trail of debris. Every October, Earth passes through Encke's wake, resulting in the well-known ...

Caption Comet 67P/Churyumov-Gerasimenko as seen by the European Space Agency’s Rosetta spacecraft in March 2015. Comet 67P was the first comet ever known to emit molecular oxygen, a molecule ...