We answer the question of the subscriber:
The tail of the comet is formed due to the impact on it of solar radiation, it warms the surface of the comet and leads to sublimation of the substance with it, in physics this term means a transition from a solid state to gas, bypassing the fluid.
Cut into a gaseous state of methane, water, nitrogen, carbon dioxide, etc. In this case, the gases are pulled from the surface a huge amount of dust particles and carry them away from the comet.
Gas and dust particles, highlighted by the Sun, form a visible tail of a comet, dust stretches under a small angle to the direction of the comet orbit, and gas stretches radially away from the sun.
Thus, the tail at the comet is not constantly present, but only during periods of convergence with the Sun, which are relatively short, and the emissions of the substance are not so significant as it may seem based on the sizes of the tail, it is actually extremely resolved.
Therefore, comets depending on their size can survive thousands and millions of cycles of rapprochement with the Sun, before they are broken, and it from hundreds of thousands to hundreds of millions of years.
It is believed that new comets regularly arrive into the inner part of the solar system from the Oort cloud, therefore, for 4.5 billion years of the existence of our system, all comets did not disappear.