Comets are icy bodies that formed alongside the planets in the early Solar System before being flung into its distant outer regions. When they return to the inner Solar System, the Sun’s heat and radiation activate them, causing them to release gases and dust. This material forms a glowing cloud around the comet’s solid nucleus, known as a coma.
As the comet approaches the Sun, material from the coma is pushed outward, creating two distinct tails that can stretch millions of kilometers into space. The dust tail consists of tiny particles about the size of smoke particles, reflecting sunlight with a yellowish-white glow. The plasma tail, also called the ion tail, is made of electrically charged gases, often glowing blue due to carbon monoxide ions. Both tails point away from the Sun, shaped by the force of the solar wind, with the dust tail curving slightly along the comet’s path.
Surrounding the coma and part of the tails is an invisible, irregular cloud called the hydrogen envelope. The envelope is made up of neutral hydrogen atoms which most likely come from water vapor molecules that are split up by radiation from the Sun. It is only visible in ultraviolet light, which cannot pass through Earth’s atmosphere.
The right-hand side of the image shows photographs of three real comets streaking across the dark sky.
Learn More: What Is a Comet?
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