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• Before it starts vaporizing, the comet is just a dirty snowball
• snowball part becomes nucleus
• Water vapor expanding off of nucleus that has not broken into H and O yet forms coma
• The coma is generally what we think of as the "head" of the comet
• Hard to see nucleus through coma
• Generally see jets of water vapor coming off of nucleus
• "Dirt" part of comet flys off with water, becomes dust tail
• dust is reflective, light actually pushes on it!
• radiation pressure
• Sunlight that breaks up H₂O also ionizes H₂O and other elements found in comet (e.g. CO)
• ions get swept back by solar wind
• Solar wind much more effective at pushing ions than radiation pressure is at pushing dust
• Two tails
• Ion tail (straight back)
• Dust tail (curved)
• a 3rd tail was discovered in Hale-Bopp due to a special interaction of sodium and sunlight, sticks even straighter back than ion tail
• Production of water, ions, etc. can vary from day to day, changes affect ion tail more (moving faster)
• Comet tails always point away from the Sun (at least as viewed from above

Nucleus of Comet Halley from Giotto spacecraft

Two tails of comet Mrkos (1957, two day intervals)

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