Final answer:
The length of a comet's tail depends primarily on the interaction between the comet and the Sun. The solar wind pushes gas and dust particles away from the Sun, forming the tail. The tail always points away from the Sun due to the recoil of dust particles when photons scatter from them.
Step-by-step explanation:
The length of the tail of a comet depends primarily on the interaction between the comet and the Sun. When a comet approaches the Sun, the heat causes the icy nucleus of the comet to evaporate, releasing gas and dust particles. The solar wind, which is a stream of charged particles emitted by the Sun, pushes these particles away from the Sun, forming the comet's tail. The dust particles in the tail recoil away from the Sun when photons scatter from them, carrying momentum in the direction away from the Sun. This phenomenon is the primary reason why the tail always points away from the Sun.