Tickets to usa for The Sun’s radiation pressure then pushes the debris back to form a coma and a dust tail fig. FIG The nucleus of Comet Hartley as it degasses. The hyperactive comet is km in diameter. This image was taken by NASA’s EPOXI mission spacecraft during a lyby on November , Image credit NASA/JPL-Caltech/UMD/Wikimedia Commons. FIG How a comet develops a coma and tail. Image credit Sirscha Nicholl. How close a freezing cold comet nucleus must come to the Sun before it begins to degas varies greatly, based largely on the concoction of chemicals that make up the comet’s ices. Tickets to usa 2016.