The four astronauts of NASA's Artemis 2 moon mission went into quarantine on Jan. 23, keeping everything on track for a possible launch in early February.
Comets inhabit the cold reaches of the Solar System: the Kuiper Belt and the Oort Cloud. Occasionally, one passes through the inner Solar System, but mostly they keep to themselves out there. These dirty snowballs are agglomerations of rock and dust, and frozen volatiles like water, carbon dioxide, methane, and ammonia. They also contain organic materials.

