of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons (within above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Comets already visited are: Halley's Comet 26P/Grigg-Skjellerup Tempel 1 (also hit with impactor) 19P/Borrelly 81P/Wild 103P/Hartley C/2013 A1 (Siding Spring) -unplanned encounter with Mars spacecraft 67P/Churyumov–Gerasimenko (also landed on) See also Coma (cometary) Hypatia (stone) List of comets visited by spacecraft References External links Nucleus of Halley's Comet (15×8×8 km) Nucleus of Comet Wild 2 (5.5×4.0×3.3 km) International Comet Quarterly: Split Comets 67/P by Rosetta2 (ESA) Comets