What causes the velocity of a comet to change?

The velocity of a comet changes primarily due to its varying distance from the sun. As a comet approaches the sun, it experiences an increase in gravitational pull, which accelerates its speed as it travels along its elliptical orbit. Conversely, as the comet moves away from the sun, the gravitational influence decreases, leading to a reduction in its velocity.

This relationship between distance and speed is a fundamental aspect of celestial mechanics, as described by Kepler's laws of planetary motion. Specifically, the law of areas states that a line segment joining a planet (or in this case, a comet) and the sun sweeps out equal areas during equal intervals of time. Therefore, a comet speeds up as it nears the sun and slows down as it recedes from it.

While factors such as solar wind and planetary alignment can influence a comet's trajectory or behavior, they do not have a direct impact on the basic principle of velocity changes due to distance from the sun. Changes in the comet's composition may affect other characteristics, such as its activity level or brightness, but they do not directly cause changes in velocity.