At NASA's Jet Propulsion Laboratory, Dr. Claudia Alexander, NASA's Rosetta Project Scientist, demonstrates how comets behave as they get closer within their orbits to the Sun. The Sun increases the temperature of a comet, resulting in ice sublimation. Sublimation occurs when a solid heats up and bypasses the liquid phase and becomes a gas. As the ices sublimate, they spew out gasses and dust into space. On a comet, this process accounts for the coma or halo which we observe as it barrels towards the Sun and becomes active.