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【太阳系小常识】一个浪漫的约定
Astronomers classify Halley’s Comet as a short-period comet, meaning that its orbital period is less than 200 years. Halley’s Comet belongs to the group of short-period comets with periods 20 years or longer, a group called Halley-type comets or intermediate-period comets. According to current theories, Halley’s Comet likely originated in part of the Oort Cloud, a vast sphere of icy bodies theorized to exist at a distance from the Sun of 20,000 to 200,000 astronomical units (AU). (An AU is the mean distance between Earth and the Sun, or about 150 million km [93 million mi].) Some calculations suggest that Halley’s Comet may have come from an inner, disklike region of the Oort Cloud.
When it first became a comet and entered the inner solar system thousands of years ago, Halley’s Comet may have been a long-period comet. Over time the gravitational pull of planets in the solar system may have reduced its orbital period, turning it into a short-period comet.
1986年，欧洲飞船乔托（Giotto）成为第一艘靠近并且对哈雷彗星的彗核进行拍摄的飞船。据测量，彗核直径大约有15公里。哈雷彗星每一次进入太阳系，彗核将会向太空抛出6米深的冰和岩石物质，形成一条长长的彗星尾巴。这些碎片留在轨道上，每当地球经过哈雷彗星轨道，这些碎片就会划破夜空，形成每年十月的猎户座流星雨以及五月的宝瓶座 η流星雨。
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