The tail of comet Hyakutake, visible in this recent color image, is composed of dust and gas driven off the icy comet nucleus by the Sun's heat and blown away by the solar wind. Bathed in solar ultraviolet light, the gas molecules break down and are excited, producing a characteristic glow. This glow is responsible for visible light from the tail and astronomers using spectroscopes can identify the compounds involved. The close passage of Hyakutake presents an excellent chance to use this technique to explore the composition of its tail. Typical comet gas tail constituents are simple combinations of hydrogen, carbon, nitrogen, and oxygen - for example, H20 (water), CO (carbon monoxide), and CN (cyanogen) are common. In fact, the poisonous CO and CN compounds were seen in the spectrum of Halley's comet during its 1910 apparition. This caused some public concern at the time as the Earth was expected to pass through Halley's tail! However, stretching for millions of miles, comet tails are extremely thin and tenuous and don't pose a danger to the Earth's atmosphere.