Comets have tails due to the vaporization of ice and dust as they pass into the inner solar system. The tail can be longer than one astronomical unit and the coma can be larger than the Sun. The brightness of the tail comes from condensed ions and not just dust reflecting sunlight. The tail can be blown off if the magnetic field is disrupted.
Every active comet has a tail—it’s part of what makes a comet a comet. Where does a comet’s tail come from? For most of their lives, comets are icy objects at the edge of the solar system and lack tails. They are space rocks made up mainly of ice and dust: when a comet is active, this rock is called the nucleus. As the comet orbits the Sun, it eventually passes into the inner solar system, where the solar wind and sunlight become intense enough to begin vaporizing some of the comet’s ice and dust, which subsequently ionize and become the comet’s long tail. elongated comet.
A comet’s tail can become extremely long – one astronomical unit (Earth-Sun distance, 150 million km or 93 million mi) – the coma, or immediate atmosphere around the comet, can be larger than the Sun. All this from a nucleus between 100 meters (328 ft) and 50 km (31 mi) in diameter, with a diameter of 10 km (6 mi) typical of comets visible to the naked eye. Because comets spend only a small minority of their time in the inner solar system, they have plenty of ice and other volatiles to release when heated by the sun.
The reasons a comet tail is so bright despite its low density are complex. When ultraviolet rays from the sun strike the dust around the nucleus, the photoelectric effect causes these particles to release electrons for each ultraviolet ray that excites them. These electrons produce an “induced magnetosphere” around the nucleus, which blocks outward flowing solar wind particles. Since comets move at supersonic speeds relative to the solar wind, a bow shock forms at the front of the comet, similar to the bow shock formed around a ship moving at sea. Cometary ions aggregate in this area, “charging” the solar magnetic field with plasma. The luminous plasma flows backwards from the direction of the Sun, merging between the extracted magnetic field lines generated by the comet’s induced magnetosphere.
Thus, the brightness in a comet tail comes mainly from the brightness of the condensed ions, not from the dust reflecting the sunlight, although the latter does contribute some. Since the comet’s tail is not generated solely by the material presence of dust particles behind the comet, it can be blown off if the magnetic field is disrupted, which can occur during magnetic reconnection. This event, which may be triggered by a coronal mass ejection from the Sun, has been observed on several occasions, most recently in Comet Encke in 2007, and is called a tail disconnect event.
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