The explosive, green "devil comet" that recently swept close to Earth appeared to grow an impossible second tail as it passed us, new photos reveal. But this extra limb, known as an anti-tail, is actually an extremely rare optical illusion caused by the comet's proximity to our planet.
12P/Pons-Brooks (12P) is a 10.5-mile-wide (17 kilometers) comet with a green hue, which is given off by pairs of carbon atoms, known as dicarbon, in its tail and coma — the cloud of gas that surrounds its icy crust, or nucleus. 12P is cryovolcanic, meaning it occasionally erupts when solar radiation superheats the comet's icy innards, or cryomagma, causing pressure to build up in the nucleus until the core splits and shoots its guts into space. When this happens, the comet's coma expands and reflects additional light, making the comet shine brighter than normal.
