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Why is a comet’s dust tail curved rather than straight?

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Comet Comet McNaught with filamentary tail and the Moon over the Pacific Ocean, photographed from Paranal Observatory, Chile, January 2007.

A comet’s dust tail, also known as a Type II tail, appears curved because of how solar forces interact with the dust particles as the comet moves along its orbit.

Unlike ion tails, which are pushed away in a straight line by the high-velocity solar wind, the fine dust in a Type II tail is pushed by radiation pressure. As these dust grains are blown away from the Sun, they are forced into a larger orbit than the comet nucleus. According to the laws of planetary motion, this increase in orbital size causes the particles to slow down, resulting in them lagging behind the nucleus as they follow the comet along its path. This combination of the comet's orbital motion and the gradual displacement of the dust creates the characteristic broad, curved appearance of the tail.1