58 The Asteroid Belt-Mid

The Asteroid Belt FOCUS Question What is the Asteroid Belt, and how did it form?

小行星带焦点问题什么是小行星带，它是如何形成的?

What Is an Asteroid? Our solar system has one star, eight planets, and many moons. But did you know there are trillions of other objects traveling around the Sun? Trillions! Some of these objects are asteroids—rocky space bodies that orbit the Sun. They are too small to be planets, and they have no atmosphere, or air. Most are irregular in shape. They are not spheres, like the Sun and planets. Their surfaces may be craggy and marked by craters. Asteroids have been around since our solar system formed 4.6 billion years ago. They are the "leftovers" from those early days. Do You Know? Asteroids may be as small as a car (a few meters across) or as large as a small moon.

什么是小行星?我们的太阳系有一颗恒星、八颗行星和许多卫星。 但你知道有数万亿其他物体围绕太阳运行吗? 数万亿!其中一些物体是小行星——围绕太阳运行的岩石天体。 它们太小了，不可能是行星，而且它们没有大气层或空气。 大多数形状不规则。 它们不是像太阳和行星那样的球体。 它们的表面可能崎岖不平，并有陨石坑。 自从46亿年前太阳系形成以来，小行星就一直存在。 他们是早期的“剩余物”。 你知道吗?小行星可能像一辆汽车(几米宽)那么小，也可能像一颗小卫星那么大。

Short Days, Long Years One day is the time it takes an object in space to make one complete rotation, or spin. Asteroids spin quickly. A day on an asteroid may last for just a few hours! A year is the time it takes an object in the solar system to make one complete orbit of the Sun. Most asteroids have longer years than Earth because they are farther from the Sun. Look at the chart below to see how long a day and a year are on some well-known asteroids. Compare them to a day and year on Earth. ASTEROID ORBITS AND ROTATIONAL PERIODS Earth Eros Gaspra Ida Lutetia Day (Rotation) 24 hours 5 hours 7 hours just over 4 hours just over 8 hours Year (Orbit) 365 days 643 days just over 3 Earth years nearly 5 Earth years nearly 4 Earth years

一天是空间中物体完成一次旋转或旋转所需的时间。 小行星旋转很快。在小行星上的一天可能只有几个小时!一年是太阳系中一个物体绕太阳一周的时间。 大多数小行星的寿命都比地球长，因为它们离太阳更远。 看看下面的图表，看看一些著名的小行星上一天和一年有多长。 将它们与地球上的一天和一年进行比较。 小行星轨道和自转周期地球爱神天(自转)24小时5小时7小时刚刚超过4小时刚刚超过8小时年(轨道)365天643天刚刚超过3个地球年近5个地球年近4个地球年

Rockin in the Ring Millions of asteroids hang out in the Asteroid Belt. This area of space lies between Mars and Jupiter. How did they get there? When the solar system was forming, dust and gas swirled around the Sun. Some of the dust and gas collided and clumped together. Once the clumps became large enough, their gravity pulled in even more of the debris. These growing clumps became the planets and their moons. Once Jupiter formed, its massive gravity tugged on the smaller, leftover clumps. This kept them from forming into larger bodies. These leftover bits and pieces are asteroids. Wowser! All the asteroids in the Asteroid Belt together would make an object far smaller than the Moon.

小行星带中有数百万颗小行星。 这片区域位于火星和木星之间。 他们是怎么做到的?当太阳系形成时，尘埃和气体围绕太阳旋转。 一些尘埃和气体碰撞并聚集在一起。 一旦团块变得足够大，它们的重力就会吸引更多的碎片。 这些不断增长的团块变成了行星和它们的卫星。 一旦木星形成，它巨大的引力就会拉住较小的剩余块状物质。 这使得它们无法形成更大的物体。 这些剩余的碎片就是小行星。 老古板!小行星带中的所有小行星加在一起会形成一个比月球小得多的物体。

Notable Asteroids The first object spotted in the Asteroid Belt was Ceres, in 1801. It is much larger than other Belt objects. Because of its size, Ceres is now classified as a dwarf planet, like Pluto. NASA's Galileo probe studied an asteroid named Ida (EYE-duh) in 1993. It found a surface peppered with craters. More surprising was a small, orbiting moon. The moon, called Dactyl (DAK-tul), may be a piece of Ida that broke away in an impact with another asteroid. Radar images show that Kleopatra is shaped like a dog bone! It has two moons. It is more like a pile of rubble stuck together than one solid rock. The moons may have broken away from the spinning asteroid.

1801年，在小行星带发现的第一个物体是谷神星。 它比其他Belt对象大得多。 因为它的大小，谷神星现在被归类为矮行星，就像冥王星一样。 美国宇航局的伽利略探测器在1993年研究了一颗名为艾达(EYE-duh)的小行星。 它发现了一个布满陨石坑的表面。 更令人惊讶的是一颗绕轨道运行的小卫星。 这颗卫星被称为达克蒂尔(达克-图尔)，可能是艾达在与另一颗小行星撞击时脱落的一块碎片。 雷达图像显示，埃及艳后的形状像狗骨头!它有两个卫星。 它更像是一堆粘在一起的碎石，而不是一块坚固的岩石。 卫星可能已经脱离了旋转的小行星。

How to Avoid Asteroids Some asteroids travel out of the Belt. If they get too close to a planet, they may crash into it. An asteroid chunk 20 meters (66 ft.) wide fell to Earth over Russia on February 15, 2013. It blew up in the air. The powerful blast broke windows and damaged buildings. More than one thousand people were hurt. Scientists use telescopes to track the orbits of nearby asteroids. We may be able to knock large ones off track before they reach Earth. Word Wise A small piece of an asteroid that could fall to Earth is called a meteoroid. If it enters Earth's atmosphere, it is called a meteor. Once it lands on Earth, we call it a meteorite.

如何避开小行星有些小行星会飞出小行星带。 如果他们太接近一颗行星，他们可能会撞上它。 2013年2月15日，一颗20米(66英尺)宽的小行星坠落在俄罗斯上空。 它在空中爆炸了。强烈的爆炸震碎了窗户，毁坏了建筑物。 超过1000人受伤。 科学家们用望远镜来跟踪附近小行星的轨道。 我们或许能在大型陨石到达地球之前将其击毁。 小行星的一小块可能掉到地球上的碎片被称为流星体。 如果它进入地球大气层，就叫做流星。 一旦它降落在地球上，我们称之为陨石。