第1个回答 2024-08-05
Comets, captivating celestial bodies in our solar system, are primarily composed of frozen gases like water vapor, ammonia, methane, carbon dioxide, and carbon monoxide, mixed with rocky and metallic materials. Their unpredictable nature stems from their elongated orbits, some of which take hundreds of thousands of years to complete a single orbit around the Sun, with short-period comets, like Halley's, frequently entering the inner solar system within 200 years.
The glow of a comet's coma, which forms when solar energy vaporizes these gases, greatly varies in size, ranging from Jupiter-sized to even larger than the Sun for rare cases. Initially, when comets are far from the Sun, they appear small due to their distance and lack of developed coma or tail. As they approach, tails can stretch millions of kilometers, despite their overall small size compared to other solar system members.
Early astronomers' observations led to the understanding that comets' tails form due to the Sun's repulsive forces, including radiation pressure and solar wind, which push particles away. The tail's shape and composition are influenced by these forces, with some comets exhibiting a dual tail of dust and ionized gases.
As comets move away from the Sun, their coma recondenses and tails disappear, revealing that most comets cannot survive more than a few hundred orbits near the Sun. Short-period comets, like Halley's, originate beyond Neptune in the Kuiper belt, while long-period comets form a spherical shell called the Oort cloud. Long-period orbits are more eccentric and more susceptible to external gravitational influences, like passing stars, that can send them towards the inner solar system.
Halley's Comet, a famous short-period comet, has a 76-year orbital period and has been observed numerous times, with its longest tail reaching over a million kilometers in length during its 1910 appearance. The comet's history and periodic reappearance highlight the importance of studying short- and long-period comets for understanding their origins and behavior.