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太阳系

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我们在总结GlanceInformation太阳系
PMS 010-A(JPL)
1991年6月

JPL 410-34-1九十一分之六
(5/93更新)

美国航空航天局
美国国家航空和航天局

喷气推进实验室
加州理工学院
加利福尼亚州帕萨迪纳

本出版物的印刷本与公共电子邮件
办公室在您所在地区的NASA中心。

简介

从我们的小世界,我们凝视着宇宙的海洋
数不清的岁月。 古代天文学家观测点
光出现在移动的星星。 他们称这些
对象的行星,这意味着娃儿,和罗马之后将它们命名为
神 - 木星,众神之王; 火星,战争之神;
水星,众神的使者; 金星,爱与美之神,
和土星,木星的父亲和农业之神。
过高的观星的彗星观测与闪闪发光的尾巴,和流星
或流星显然是从天上掉下。

科学蓬勃发展在欧洲文艺复兴时期。
执政行星运动的基本物理定律是
发现,并围绕太阳的行星的轨道都
计算。 在17世纪,天文学家指出了新设备
所谓望远镜在天空,并提出惊人
发现。

但自1959年以来的年已达到的黄金时代
太阳系探索。 在第二次世界大战后,火箭的进步
II使我们的机器打破地球引力的抓地力和
前往月球和其他行星。

美国已经晚了自动化太空船,那么人类
载人探险,探索月球。 我们的自动化机器
已进入轨道,并降落在金星和火星; 探索太阳
环境; 观测彗星,并取得了近距离的调查,而
飞过水星,木星,土星,天王星和海王星。

这些旅行者在我们的知识带来了质的飞跃,并
了解太阳系。 通过电子视线
与我们的自动航天器,色彩等“感官”和
肤色havebeen给世界的是长久以来出现
到地球上的眼睛,模糊的磁盘或光点模糊的。
而此前数十个不明物体被发现。

未来的历史学家可能会查看这些开创性的航班
在太阳系的一些最显着
20世纪的成就。

AUTOMATED航天器

美国国家航空和航天管理局(NASA的)
自动化航天器太阳系探索有许多
形状和大小。 虽然他们是专为满足独立和
指定任务目标,工艺有很多共同之处。

每个航天器由各种不同的科学仪器
选择一个特定的任务,通过基本的子系统支持
为电力,轨迹和方向控制,以及
作为用于处理数据和与地球通信。

电力是一家以经营飞船所需
仪器和系统。 美国航天局使用从阵列困扰太阳能
光伏电池和小型核电机组供电,其
太阳能系统排放。 可再充电电池被用于
备份和补充力量。

想象一下,一个航天器已成功远航百万
通过空间里飞翔,但有一次一个星球附近,只
有其相机等遥感仪器指出错误
方式,因为它的速度超过目标! 为了帮助预防此类事故,一
的小型推进子系统用于控制航天器。

该推进器与维持一个设备链接
不断凝视选定的明星。 正如地球早期海员
用星导航海洋,航天器使用明星
保持其在太空中的轴承。 随着锁定到子系统
固定参考点,飞行控制器可以保持
飞船的科学仪器对准目标身上和
飞船的通信天线指着地球。
推进器也可用于微调飞行轨迹和速度
飞船,以确保目标体在遇到
计划的距离和在适当的轨迹。

1959年至1971年,美国航空航天局的航天器被派往
研究月球和太阳的环境; 他们还扫描
内行星比地球其他 - 水星,金星和火星。 这些
三个世界,和我们自己的,被称为类地行星
因为它们共享一个固体岩石组成。

对于早期的行星侦察任务,美国航空航天局
采用一个非常成功的系列飞船称为
水手。 他们的航班帮助塑造外侧规划
任务。 在1962年至1975年,七水手任务传导
我们在空间的行星邻居第一次调查。

所有的水手使用的太阳能电池板作为他们的主要动力
源。 空间飞行器的第一和最后的版本有两个
翅膀覆盖着光伏电池。 其他水手们
装有四个太阳能面板扩展fromtheir八角
紧身衣。

虽然从水手的水手2介于金星
宇宙飞船,在203千克克(447磅)称重,以
水手3月9日轨道飞行器,在974公斤克称重(2.147
英镑),其基本设计仍遍布颇为相似
节目。 水手金星5号飞船,例如,有
最初是为水手3月4日飞掠备份。 水手
10号飞船下旬到金星和水星使用的组件剩
从水手3月9日轨道飞行器计划。

1972年,美国宇航局发射先驱者10号,木星的航天器。
兴趣转移到四个外行星 - 木星,
土星,天王星和海王星 - 稠密气体巨球相当
不同于我们已经调查了地面世界。

四NASA航天器在所有 - 两位先驱和两个旅行者号 -
晚了上世纪70年代参观我们的太阳的外部区域
系统。 由于所涉及的距离,这些带着游客
任何地方从20个月12年到达目的地。
除非更快飞船,它们最终会变成第一
人类文物旅途遥远的恒星。 由于太阳
光变成了外太阳系,这些旅行者这样淡淡
不要使用太阳能发电,而是电力操作
通过加热从放射性同位素的衰变产生的。

美国航空航天局研制高估高度专业化的飞船重温
我们在三月和金星在邻居中间和70年代末。 双胞胎
海盗兰德斯装备作为地震和天气
站并作为生物实验室。 两个先进的人造卫星 -
水手工艺的后裔 - 所承载的海盗从兰德斯
地球再研究火星的特点从上​​面。

两个鼓形飞船先锋金星参观于1978年
先锋金星轨道器配备了一个雷达仪器
允许它“看”,通过行星的密集云盖
研究表面特征。 先锋金星多功能携带四
这探针穿云下降。 探针和
主体 - 所有这些都包含的科学仪器 -
关于收音行星的大气信息在其
血统向表面。

新一代的自动化航天器 - 包括
麦哲伦,伽利略,尤利西斯,火星观测和卡西尼 - 正在
发达国家和后期伸到太阳系进行详细
thatwill提高我们的理解我们的考试
邻居和我们自己的星球。

THE SUN

在太阳系中的对象的讨论必须启动
随着太阳。 太阳相形见绌的其他机构,代表
大约有百分之99.86的所有质量在太阳系;
所有的行星,卫星,小行星,彗星,尘埃和气体加起来
只有约百分之12:14。 这代表了百分之0:14
材料来自太阳的阵型吃剩的。 一百零九
土将需要以适合整个太阳的磁盘,和ITS
内饰缓缴能130万地球。

作为一个明星,阳光透过的过程中产生能量
融合。 在太阳核心的温度为15万度
摄氏度(27万度华氏温度),压力有
340十亿倍地球的空气压力在海平面。 太阳
5500摄氏度的地表温度(10,000度
华氏度)似乎是寒冷相比,它的核心温度。
在太阳的核心,氢可融合成氦,产
能源。 太阳照常产生强磁场和流
带电粒子,既延长远远超出了地球。

太阳似乎已经主动4.6十亿年
有足够的燃料继续进行另外五个十亿年左右。
它的生命结束,太阳将开始融合成氦
较重的元素并开始膨胀起来,最终使越来越多
大,它会吞噬地球。 经过十亿年作为“红
巨人“,它会突然坍缩成一个”白矮星“ - 最后的
像我们这样的明星终端产品。 这可能需要一万亿年
冷静下来彻底。

许多航天器探索了Sun的环境中,但没有
已得到任何接近它的表面比大约两
从地球到太阳的距离的三分之二 先锋5-11时,
先锋金星轨道器,航海1,2和其他航天器有
所有采集的太阳能环境。 伊利亚斯航天器,
发起于1990年10月6日,是美国宇航局的一项联合太阳能的使命和
欧洲航天局。 在1992年2月8日,尤利西斯飞近
木星和使用木星的引力投掷下来的下方
飞机的行星。 虽然它仍然会在很远的距离
来自太阳,尤利西斯将飞越太阳极地地区在
1994年和1995年将执行采用九广泛的研究
板载科学仪器。

我们很幸运,太阳正好是现在这个样子。 如果IT
在几乎所有的方式都不同,生活几乎肯定
从来没有在地球上的发展。

MERCURY

获得第一的特写意见水星是主
水手10号飞船,推出11月3日的目的
1973年,从肯尼迪航天中心在佛罗里达州。 之后的旅程
近五个月,其中包括金星的飞掠时,
航天器通过太阳能的703公里(439英里)
系统的最内层的行星3月29日,于1974年。

直到水手10号,很少有人了解水星。 即使是
从地球上最好的望远镜看法呈水星作为一个模糊的
对象缺乏任何表面细节。 这颗行星是如此接近
太阳,它通常是失去了在太阳眩光。 当飞机是
地球的地平线日落之后或黎明前可见,这是
在我们的大气灰霾和尘埃遮蔽。 只有雷达
望远镜给了水星表面条件的任何暗示之前
水手10号的航程

这些照片的水手10通过无线电传回地球透露了一个
古代,大量陨石坑的表面,类似具竞争力我们自己
月亮。 这些照片还显示巨大的悬崖纵横交错
飞机。 这些显然是被创造当水星的内部
冷却和收缩,扭曲地球的地壳。 岩壁上的
高3公里(2英里),只要500千米(310
英里)。

在水手10号发现仪表水星具有弱
磁场和一丝气息 - 1000000000000分之1的
地球大气的密度和组成主要的氩,氖,
和氦气。 当行星的轨道需要它离太阳最近,
表面温度范围从467摄氏度(872度
华氏)关于水星的阳光一面-183摄氏度(-298
在黑暗的一面华氏度)。 这个范围到面
温度 - 650摄氏度(1,170华氏度) - 冰
最大的太阳能系统中的单个机构。 水星
从字面上烘烤并冻结在的Sametime。

天夜长水星。 的组合
旋转相对于星级(59个地球日),并迅速慢
围绕太阳(88个地球日)的革命意味着一个水星
太阳日需要176个地球日或两个水星年 - 一次
以最里面的行星,完成绕太阳轨道2!

水星似乎有轻硅酸盐岩石类地壳
这地球。 科学家认为水星有一个沉重的富铁
核心组成稍低于其体积的一半。 这会
水星丈夫的核心有益的,按比例,比月球的核心
或者它们的任何行星。

初次邂逅水星后,水手10号做了两个
额外的飞越 - 9月21日,1974,和1975年3月16日 -
控制用于定向飞船气前精疲力竭,
调查团得出的结论。 每个飞掠发生在当地的萨米
水星时间当飞机的一半相同是
照明; 作为结果,我们还没有看到一半的
地球表面。

VENUS

离我们最近的行星 - 密集云层覆盖,遮掩木星
邻居 - 是第一个行星进行探索。 水手2
飞船,推出8月27日,1962是第一更
十几个成功的美国和苏联的任务,研究
神秘的飞机。 由于飞船飞过或送入轨道金星,暴跌
到大气中,或轻轻地降落在金星表面,浪漫
神话和猜测我们的邻居都安息。

12月14日1962年,水手2号内34.839传递
金星公里(21.648英里),并成为第一个航天器
扫描另一面; 测量金星42板载仪器
分钟。 水手5号,1967年6月推出,飞到更接近
飞机。 金星对4.094公里(2.544英里)内的传递
美国第二次飞掠,水手5号上的仪器测得的
地球的磁场,电离层,辐射带和
温度。 在它的途中水星,水手10号飞抵金星和
紫外线传输图片显示地球云
循环模式在金星大气。

在1978年的春季和夏季,两个航天器
发起了进一步揭开金星的奥秘。 12月4日
萨米年,先锋金星轨道器成为第一
飞船进入friendlyness围绕地球轨道。

五天后,五个独立的部件构成
第二飞船 - 先锋金星多功能 - 走进
金星气氛中在平面上方的不同位置。
四个小的,独立的探针和主体用无线电
大气数据传回地球在其向下降
表面。 虽然旨在探讨气氛,之一
幸存下来的探针与表面的影响,并继续
发送数据为1小时。

金星类似于地球的大小,物理成分和
密度比其他任何已知的行星更具竞争力。 然而,
飞船已经发现显著的区别。
例如,金星的自转(西向东)是逆行
相对于地球的东到西自旋和其他大部分
飞机。

大约有96.5%的金星的大气层(95倍
致密地球的)是二氧化碳。 的主要成分
地球的大气层是氮气。 金星的大气层就像一个
温室,允许太阳辐射到达地面,但
捕获这通常会被辐射回热
空间。 其结果是,该行星的表面平均温度的冰
482摄氏度(900华氏度),热到足以熔化
铅。

在先锋金星轨道器的无线电高度表提供的
通过行星的密集云层看到的第一手段,
确定表面特征几乎超过了整个地球。 美国航空航天局的
麦哲伦航天器,推出5月5日,1989已经进入预定轨道
环绕金星自8月10日,1990年用于航天器雷达
映射技术,提供了98%的高清晰图像
表面。

麦哲伦的雷达发现的火山为主的景观
功能,故障和撞击坑。 表面的大面积
显示泛滥与熔岩流多期的证据
躺在以前的顶部。 升高的区域伊什塔尔命名
兵马俑是一个熔岩填充盆大如美国。 在一
这个高原座麦克斯韦蒙特斯,一座山大小的结束
珠穆朗玛峰。 疤痕山的侧翼是100公里
(62英里)宽,2.5公里(1.5英里)深命名的撞击坑
克娄巴特拉。 (几乎所有的金星上的功能被命名为妇女;
麦克斯韦蒙特斯阿尔法区和Beta雷吉奥是例外)。
陨石坑金星上可能存活400亿年,因为
没有水,很少风蚀。

广泛的故障线路网络覆盖了地球,可能是
产生板块的地壳萨米弯曲的结果
在地球上。 但金星的表面温度足以
削弱巨岩裂隙几乎无处不在,预防
各大板块和大地震的断层像的形成
圣安德烈亚斯断层在加利福尼亚州。

金星的主要天气模式是高海拔,高
高速循环云层containe硫酸。 速度
达到高达360公里(225英里)的时速,在
云绕地球的只有四个地球日。 循环
冰在同一个方向 - 西向东 - 金星“缓慢转动
地球243天,而地球上的风吹在两个方向 -
西东,东方到西方 - 六交替带。 金星“
气氛充当的研究简化我们的实验室
天气。

EARTH

从空间看,我们的世界的区别
特点是它的湛蓝的海水,棕色和绿色陆地
白云。 我们是通过空气的海洋笼罩组成
78%的氮,21%的氧气和1%的其他
成分。 已知怀有太阳系中唯一的行星
生活中,地球绕太阳以150万美元的平均距离
公里(93000000英里)。 土是从第三行星
Sun和第五大太阳系的直径
就在几百公里比金星大。

我们这个星球的快速旋转和熔化的镍铁内核产生
一个广泛的磁场,这个磁场,非常久远的气息,
我们避开几乎所有的从有害辐射到来
太阳和其他恒星。 地球的大气层保护我们免受
流星以及,其中大部分烧起来,才可以罢工
表面。 活跃​​的地质过程没有留下证据
急雨地球几乎可以肯定获得它很快形成之后 -
约4.6十亿年前。 非常久远的其它新成立的
行星,它在的初期沐浴空间碎片
太阳能系统。

从我们的旅程进入太空,我们都已经非常了解我们
地球家园。 美国第一个卫星 - 资源管理器1 - 是
从卡纳维拉尔角发射在佛罗里达1958年1月31日和
发现了一个强烈的辐射区,现在被称为范艾伦
辐射带,地球周围。

此后,其它研究卫星已经发现,我们
地球的磁场是扭曲到由泪滴形状
太阳风 - 带电粒子的气流喷射持续
从太阳 我们已经了解到,磁场不褪色
关进空间,但有明确的界限。 我们现在knowthat
我们的束状高层大气,一度被认为冷静和平静,
seethes与活动 - 白天和承包晚上肿胀。
由太阳活动变化的影响,高层大气
促进天气和气候在地球上。

除了影响地球的气候,太阳活动引起
在我们的气氛戏剧性的视觉现象。 当少收
从太阳能wind've颗粒变得困在地球磁
现场,他们碰撞与空气分子上面我们星球的磁场
极。 这些空气分子然后开始发光,并称为
极光或北部和南部灯。

卫星约35.789公里(22.238英里)伸到
论坛发挥日常当地天气预报了重要作用。 这些
警惕电子眼警告美国危险的风暴。 连续
全球监测提供数据和有用的大量
有助于更好地了解地球的复杂天气
系统。

从他们独特的有利位置,卫星可以探测
地球上的海洋,土地使用和资源,并监视地球上的
有益健康。 这些眼睛已经在太空拯救了无数生命,提供
极大的便利,并告诉我们,我们可能会改变我们的
星球以危险的方式。

月亮

月球冰地球的天然卫星单。 人类第一次
在一个陌生的世界的脚步声是由美国宇航员上进行
我们的无气,无生命的伴侣的尘土飞扬的表面。
为人类载人阿波罗探险,美国宇航局准备
派出自动游侠,测量师和月球轨道器
飞船研究月球之间的1964年和1968年。

NASA的阿波罗计划离开月球物质的大笔遗产
和数据。 六两宇航员船员降落在月球和探索
1969年和1972年之间的表面,携带岩石后面的集合
与土壤共382公斤克(842磅),体重
由2000多个独立的样本。

由这种材料和其他研究中,科学家们
建造月球的历史,包括它的起步阶段。 岩石
从距今约4.0-4.3十亿月球高地收集
岁。 第一个几百万年前的月亮的存在
如此猛烈的这一时期的痕迹依然存在。 作为熔融
外层逐渐冷却并凝固成不同种类的
岩石,月球是由轰击小行星庞大和更小
对象。 有些小行星都是一样大的罗得岛或
特拉华及其与月球创建盆碰撞
数百公里的跨越。

这种灾难性的轰击逐渐减少约4
十亿年前,在离开月球高地长满了巨大的,
重叠陨石坑和破碎的破碎岩石的深层。
通过放射性​​元素的衰变所产生的热量开始熔化
月亮在水深约200公里内(125
英里)的表面之下。 然后,在接下来的7亿年 -
从大约3.8到3.1十亿年前 - 熔岩从内上涨
月球。 熔岩逐渐扩散出过面,洪水
大撞击盆地,形成暗区伽利略
伽利略,意大利文艺复兴时期的天文学家,叫玛丽亚,
这意味着海洋。

据我们所知,目前还没有显著火山
活动在月球上超过三十亿年。 因为
然后,在月球表面已经改变只能由陨石,
由太阳和星星,原子粒子的调节
大陨石和飞船宇航员和影响。 如果
我们的宇航员在月球上一个十亿年前降落,他们
wouldhave见过的风景非常相像的今天。
千百年来从现在开始,留下的阿波罗的脚步
机组人员将保持锐利和清晰。

月球的起源仍然是一个谜。 四论
尝试了解释:月球形成近地球作为一个单独的
片; 它是距离地球塔; 它形成了别的地方,是
我们的行星的引力捕获,或者这是一个结果
地球和火星大小的小行星之间的碰撞。
过去的理论有一些很好的支持,但还远远没有certainement。

MARCH

所有的行星,火星一直被认为太阳
该系统的主要候选对象窝藏外星生命。
天文学家通过望远镜研究这颗红色行星看见了什么
似乎是直线纵横交错其表面。 这些
观察 - 后来确定要问幻觉 - 点
流行的观点,即智慧生物建造了一个
地球上的灌溉渠系统。 1938年,当奥森
威尔斯播出了广播剧的基础上的科幻小说
经典世界大战HG威尔斯的,足够多的人相信
在火星入侵的故事引起恐慌附近。

另一个原因,科学家们期望寿命火星上不得不
做这个星球上的明显季节性颜色变化
表面。 这种现象一点猜测可能的条件
支持火星植被盛开在温暖的月份和
引起植物的生活变得休眠在寒冷的时期。

到目前为止,美国六个火星任务havebeen进行。
四水手飞船 - 三飞过地球一
friendlyness进入火星轨道 - 广泛调查之前,地球
海盗轨道器和兰德斯赶到。

水手4,在1964年年底推出,飞奔过去三月7月14日,
1965年,即将到来的表面9.846公里(6.118英里)之内。
发射地球地球22的特写照片,
飞船发现了许多环形山和自然发生的通道,但
没有证据表明人工运河或流水。 水手6号和
7其次withtheir飞越在1969年夏天和
返回201的图片。 水手4,图6和7显示出的分集
面的条件,以及一个薄的,寒冷,干燥的气氛
二氧化碳。

5月30日1971年,水手9号航天飞机发射于一
使命,使一年之久的火星表面的研究。
飞船到达五个半月后剥离,只
查找在三月的一个行星范围内沙尘暴这使得中间
土地拍摄了几个星期是不可能的。 但在后
风暴清除,水手9号开始返回第一的7.329
图片; 这些发现以前未知的火星功能,
包括证据表明,水大量流入一次跨越
从表面上看,蚀刻河谷冲积平原和。

在八月和1975年9月,维京1和2号飞船 -
每一个由轨道器和着陆器 - 从升空
肯尼迪航天中心。 访问团的目的是要回答几个
关于这个红色星球的问题,包括是否有生命存在?
没有人预料到飞船发现火星的城市,但它是
希望在对海盗兰德斯excellant生物学实验
至少找到原始生命的证据 - 过去或现在。

海盗着陆器1。成为第一艘成功
着陆在另一个星球上,当它降落在1976年7月20日
而美国正在庆祝其二百周年。 照片
从克里斯平原(“黄金平原”),很晚才回来呈
暗淡,生锈的红色风景线。 由返回的全景图像
着陆器显示滚动平原,到处散落着石块和标记
沙丘起伏。 从火星土壤细红尘给人的
鲑鱼的天空的色调。 当海盗2着陆器​​降落在乌托邦
Planitia于1976年9月3日,它认为一个越滚景观
比一个看到它的前身 - 一个没有明显的沙丘。

结果后期通过在每个海盗着陆器实验室检查
尚无定论。 红色的火星土壤的小样品
经过测试,在三个不同的实验设计来检测
生物过程。 虽然一些测试结果似乎
表明生物活性,后来分析证实thatthis
活性是无机性质的,有关地球的土壤
化学。 是火星上有生命? 没有人知道,但
海盗任务发现任何证据表明有机分子存在
那里。

海盗着陆器成了气象站,风记录
的速度和方向,以及大气温度和
压力。 一些天气变化进行观察。 最高
温度记录无论是工艺是-14摄氏度(7
华氏度)的海盗着陆器1个网站,在盛夏。

最低温度,摄氏-120度(-184度
华氏),录得较偏北海盗着陆器2
这里的冬季期间。 近飓风的风速测量了
在火星的两个气象站在全球沙尘暴,但
由于气氛是如此之薄,风力很小。 海盗
着陆器2拍下霜轻补丁 - 也许水冰
- 在其第二个冬天在这个星球上。

火星大气,像金星,冰的主要
二氧化碳。 氮和氧的存在仅在小
百分比。 火星空气中含有只有约1 / 1,000片为多水
因为我们的空气,但即使这样少量的可凝结出来,形成
云层乘坐高,在大气中或涡流周围的山坡
高耸的火山。 清晨大雾局部补丁可以形成
进入山谷。

有证据表明,在过去的火星倾向性
气氛可能已经允许水流在行星。 物理
竞技特点类似的海岸线,峡谷,河床和
岛屿建议大河一旦标志着星球。

3月以来两颗卫星,火卫一和火卫二。 他们是小和
不规​​则形状和古养着,坑坑洼洼的表面。 这是
可能的月亮原本那太冒险小行星
靠近火星和它的引力被俘。

海盗轨道器和兰德斯超过了利润大
120天和90天,其设计寿命分别。 第一
失败是海盗轨道器2,它停止运营7月24日,
1978年当泄漏耗尽其姿态控制气体。 海盗着陆器
2经营直到1980年4月12日,当它是因为关闭
电池变性。 海盗轨道器1退出8月7日,1980
当它的姿态控制气体的最后用完。 海盗
兰德1停止运作1983年11月13日。

尽管海盗生物学不确定的结果
实验中,我们知道更多关于3月比其他任何行星,除了
地球。 美国宇航局的火星观测飞船,推出9月25日,
1992年,将扩大我们的火星环境的知识和
帮助导致人类探索这颗红色星球的。

小行星

该太阳能系统拥有大量的岩石和金属
这对象是围绕太阳运行的轨道,但太小问
认为羽翼丰满的行星。 这些对象被称为
小行星或行星的地雷。 大多数,但不是所有的,被发现在一个带
或火星和木星的轨道之间带。 有些轨道
跨越地球的路径,有证据表明,地球已经被
在过去的小行星击中。 其中最不被侵蚀,最好的
保存完好的例子是巴林杰陨石坑附近温斯洛,
亚利桑那州。

小行星留下的材料,从以上的形成
太阳能系统。 有一种理论认为,他们是一个遗迹
那是在一个巨大的碰撞不久前摧毁这个星球。 更多
有可能,小行星是物质从未凝聚成行星。
事实上,如果所有小行星估计总质量都聚集
成一个单一的对象,该对象将小于1500
千米(932英里)跨越 - 小于直径的一半我们
月亮。

数以千计的小行星havebeen地球鉴定。 这是
估计100,000足够明亮最终要
通过地面望远镜拍摄。

我们更多的了解关于小行星来自
研究空间碎片的那件案件的表面
地球。 这小行星是在碰撞过程与地球的
所谓的流星。 当流星体撞击我们的大气高
速度,摩擦导致此块的空间物质焚烧
光被称为流星连胜。 如果注释不流星
烧起来完全,还剩下什么撞击地球表面和冰
被称为陨石。 一个寻找陨石的最佳场所
冰南极冰盖。

检查的所有陨石,92.8%的相关的法庭
硅酸盐(石),以及5.7%的组成的铁和镍的;
其余的是三种材料的混合物。 石陨石
是最难确定,因为它们看起来很像
地球的岩石。

由于小行星是材料从非常早期的太阳能
系统,科学家们感兴趣的是他们的作文。 宇宙飞船
that have flown through the asteroid belt have found that the belt
is really quite empty and that asteroids are separated by very
large distances.

Current and future missions will fly by selected asteroids
for closer examination. The Galileo spacecraft, launched by NASA
in October 1989, investigated the main-belt asteroid Gaspra on
October 29, 1991 and will encounter Ida on August 28, 1993 on its
way to Jupiter. One day, space factories will mine the asteroids
for raw materials.

JUPITER

Beyond Mars and the asteroid belt, in the outer regions of
our solar system, lie the giant planets of Jupiter, Saturn, Uranus
and Neptune. In 1972, NASA dispatched the first of four spacecraft
slated to conduct the initial surveys of these colossal worlds of
gas and their moons of ice and rock. Jupiter was the first port of
call.

Pioneer 10, which lifted off from Kennedy Space Center in
March 1972, was the first spacecraft to penetrate the asteroid
belt and travel to the outer regions of the solar system. In
December 1973, it returned the first close-up images of Jupiter,
flying within 132,252 kilometers (82,178 miles) of the planet's
banded cloud tops. Pioneer 11 followed a year later. Voyagers 1
and 2 were launched in the summer of 1977 and returned spectacular
photographs of Jupiter and its family of satellites during flybys
in 1979.

These travelers found Jupiter to be a whirling ball of liquid
hydrogen and helium, topped with a colorful atmosphere composed
mostly of gaseous hydrogen and helium. Ammonia ice crystals form
white Jovian clouds. Sulfur compounds (and perhaps phosphorus) may
produce the brown and orange hues that characterize Jupiter's
atmosphere.

It is likely that methane, ammonia, water and other gases
react to form organic molecules in the regions between the
planet's frigid cloud tops and the warmer hydrogen ocean lying
below. Because of Jupiter's atmospheric dynamics, however, these
organic compounds — if they exist — are probably short-lived.

The Great Red Spot has been observed for centuries through
telescopes on Earth. This hurricane-like storm in Jupiter's
atmosphere is more than twice the size of our planet. As a high-
pressure region, the Great Red Spot spins in a direction opposite
to that of low-pressure storms on Jupiter; it is surrounded by
swirling currents that rotate around the spot and are sometimes
consumed by it. The Great Red Spot might be a million years old.

Our spacecraft detected lightning in Jupiter's upper
atmosphere and observed auroral emissions similar to Earth's
northern lights at the Jovian polar regions. Voyager 1 returned
the first images of a faint, narrow ring encircling Jupiter.

Largest of the solar system's planets, Jupiter rotates at a
dizzying pace — once every 9 hours 55 minutes 30 seconds. The
massive planet takes almost 12 Earth years to complete a journey
around the Sun. With 16 known moons, Jupiter is something of a
miniature solar system.

A new mission to Jupiter — the Galileo Project — is under
way. On December 7, 1995, after a six- year cruise that takes the
Galileo Orbiter once past Venus, twice past Earth and the Moon and
once past two asteroids, the spacecraft will drop an atmospheric
probe into Jupiter's cloud layers and relay data back to Earth.
The Galileo Orbiter will spend two years circling the planet and
flying close to Jupiter's large moons, exploring in detail what
the two Pioneers and two Voyagers revealed.

GALILEAN SATELLITES

In 1610, Galileo Galilei aimed his telescope at Jupiter and
spotted four points of light orbiting the planet. For the first
time, humans had seen the moons of another world. In honor of
their discoverer, these four bodies would become known as the
Galilean satellites or moons. But Galileo might have happily
traded this honor for one look at the dazzling photographs
returned by the Voyager spacecraft as they flew past these planet-
sized satellites.

One of the most remarkable findings of the Voyager mission
was the presence of active volcanoes on the Galilean moon Io.
Volcanic eruptions had never before been observed on a world other
than Earth. The Voyager cameras identified at least nine active
volcanoes on Io, with plumes of ejected material extending as far
as 280 kilometers (175 miles) above the moon's surface.

Io's pizza-colored terrain, marked by orange and yellow hues,
is probably the result of sulfur-rich materials brought to the
surface by volcanic activity. Volcanic activity on this satellite
is the result of tidal flexing caused by the gravitational tug-of-
war between Io, Jupiter and the other three Galilean moons.

Europa, approximately the same size as our Moon, is the
brightest Galilean satellite. The moon's surface displays a
complex array of streaks, indicating the crust has been fractured.
Caught in a gravitational tug-of-war like Io, Europa has been
heated enough to cause its interior ice to melt — apparently
producing a liquid-water ocean. This ocean is covered by an ice
crust that has formed where water is exposed to the cold of space.
Europa's core is made of rock that sank to its center.

Like Europa, the other two Galilean moons — Ganymede and
Callisto — are worlds of ice and rock. Ganymede is the largest
satellite in the solar system — larger than the planets Mercury
and Pluto. The satellite is composed of about 50 percent ice or
slush and the rest rock. Ganymede's surface has areas of different
brightness, indicating that, in the past, material oozed out of
the moon's interior and was deposited at various locations on the
surface.

Callisto, only slightly smaller than Ganymede, has the lowest
density of any Galilean satellite, suggesting that large amounts
of water are part of its composition. Callisto is the most heavily
cratered object in the solar system; no activity during its
history has erased old craters except more impacts.

Detailed studies of all the Galilean satellites will be
performed by the Galileo Orbiter.

SATURN

No planet in the solar system is adorned like Saturn. Its
exquisite ring system is unrivaled. Like Jupiter, Saturn is
composed mostly of hydrogen. But in contrast to the vivid colors
and wild turbulence found in Jovian clouds, Saturn's atmosphere
has a more subtle, butterscotch hue, and its markings are muted by
high-altitude haze. Given Saturn's somewhat placid-looking
appearance, scientists were surprised at the high-velocity
equatorial jet stream that blows some 1,770 kilometers (1,100
miles) per hour.

Three American spacecraft have visited Saturn. Pioneer 11
sped by the planet and its moon Titan in September 1979, returning
the first close-up images. Voyager 1 followed in November 1980,
sending back breathtaking photographs that revealed for the first
time the complexities of Saturn's ring system and moons. Voyager 2
flew by the planet and its moons in August 1981.

The rings are composed of countless low-density particles
orbiting individually around Saturn's equator at progressive
distances from the cloud tops. Analysis of spacecraft radio waves
passing through the rings showed that the particles vary widely in
size, ranging from dust to house-sized boulders. The rings are
bright because they are mostly ice and frosted rock.

The rings might have resulted when a moon or a passing body
ventured too close to Saturn. The unlucky object would have been
torn apart by great tidal forces on its surface and in its
interior. Or the object may not have been fully formed to begin
with and disintegrated under the influence of Saturn's gravity.
third possibility is that the object was shattered by collisions
with larger objects orbiting the planet.

Unable either to form into a moon or to drift away from each
other, individual ring particles appear to be held in place by the
gravitational pull of Saturn and its satellites. These complex
gravitational interactions form the thousands of ringlets that
make up the major rings.

Radio emissions quite similar to the static heard on an AM
car radio during an electrical storm were detected by the Voyager
spacecraft. These emissions are typical of lightning but are
believed to be coming from Saturn's ring system rather than its
atmosphere, where no lightning was observed. As they had at
Jupiter, the Voyagers saw a version of Earth's auroras near
Saturn's poles.

The Voyagers discovered new moons and found several
satellites that share the same orbit. We learned that some moons
shepherd ring particles, maintaining Saturn's rings and the gaps
in the rings. Saturn's 18th moon was discovered in 1990 from
images taken by Voyager 2 in 1981.

Voyager 1 determined that Titan has a nitrogen-based
atmosphere with methane and argon — one more like Earth's in
composition than the carbon dioxide atmospheres of Mars and Venus.
Titan's surface temperature of -179 degrees Celsius (-290 degrees
Fahrenheit) implies that there might be water-ice islands rising
above oceans of ethane-methane liquid or sludge. Unfortunately,
Voyager's cameras could not penetrate the moon's dense clouds.

Continuing photochemistry from solar radiation may be
converting Titan's methane to ethane, acetylene and — in
combination with nitrogen — hydrogen cyanide. The latter compound
is a building block of amino acids. These conditions may be
similar to the atmospheric conditions of primeval Earth between
three and four billion years ago. However, Titan's atmospheric
temperature is believed to be too low to permit progress beyond
this stage of organic chemistry.

The exploration of Saturn will continue with the Cassini
mission. Scheduled for launch in the latter part of the 1990s, the
Cassini mission is a collaborative project of NASA, the European
Space Agency and the federal space agencies of Italy and Germany,
as well as the United States Air Force and the Department of
Energy. Cassini will orbit the planet and will also deploy a
probe called Huygens, which will be dropped into Titan's
atmosphere and fall to the surface. Cassini will use radar to peer
through Titan's clouds and will spend years examining the
Saturnian system.

URANUS

In January 1986, four and a half years after visiting Saturn,
Voyager 2 completed the first close-up survey of the Uranian
system. The brief flyby revealed more information about Uranus and
its retinue of icy moons than had been gleaned from ground
observations since the planet's discovery over two centuries ago
by the English astronomer William Herschel.

Uranus, third largest of the planets, is an oddball of the
solar system. Unlike the other planets (with the exception of
Pluto), this giant lies tipped on its side with its north and
south poles alternately facing the sun during an 84-year swing
around the solar system. During Voyager 2′s flyby, the south pole
faced the Sun. Uranus might have been knocked over when an Earth-
sized object collided with it early in the life of the solar
system.

Voyager 2 found that Uranus' magnetic field does not follow
the usual north-south axis found on the other planets. Instead,
the field is tilted 60 degrees and offset from the planet's
center, a phenomenon that on Earth would be like having one
magnetic pole in New York City and the other in the city of
Djakarta, on the island of Java in Indonesia.

Uranus' atmosphere consists mainly of hydrogen, with some 12
percent helium and small amounts of ammonia, methane and water
vapor. The planet's blue color occurs because methane in its
atmosphere absorbs all other colors. Wind speeds range up to 580
kilometers (360 miles) per hour, and temperatures near the cloud
tops average -221 degrees Celsius (-366 degrees Fahrenheit).

Uranus' sunlit south pole is shrouded in a kind of
photochemical “smog” believed to be a combination of acetylene,
ethane and other sunlight-generated chemicals. Surrounding the
planet's atmosphere and extending thousands of kilometers into
space is a mysterious ultraviolet sheen known as “electroglow.”

Approximately 8,000 kilometers (5,000 miles) below Uranus'
cloud tops, there is thought to be a scalding ocean of water and
dissolved ammonia some 10,000 kilometers (6,200 miles) deep.
Beneath this ocean is an Earth-sized core of heavier materials.

Voyager 2 discovered 10 new moons, 16-169 kilometers (10-105
miles) in diameter, orbiting Uranus. The five previously known –
Miranda, Ariel, Umbriel, Titania and Oberon — range in size from
520 to 1,610 kilometers (323 to 1,000 miles) across. Representing
a geological showcase, these five moons are half-ice, half-rock
spheres that are cold and dark and show evidence of past activity,
including faulting and ice flows.

The most remarkable of Uranus' moons is Miranda. Its surface
features high cliffs as well as canyons, crater-pocked plains and
winding valleys. The sharp variations in terrain suggest that,
after the moon formed, it was smashed apart by a collision with
another body — an event not unusual in our solar system, which
contains many objects that have impact craters or are fragments
from large impacts. What is extraordinary is that Miranda
apparently reformed with some of the material that had been in its
interior exposed on its surface.

Uranus was thought to have nine dark rings; Voyager 2 imaged
11. In contrast to Saturn's rings, which are composed of bright
particles, Uranus' rings are primarily made up of dark, boulder-
sized chunks.

NEPTUNE

Voyager 2 completed its 12-year tour of the solar system with
an investigation of Neptune and the planet's moons. On August 25,
1989, the spacecraft swept to within 4,850 kilometers (3,010
miles) of Neptune and then flew on to the moon Triton. During the
Neptune encounter it became clear that the planet's atmosphere was
more active than Uranus'.

Voyager 2 observed the Great Dark Spot, a circular storm the
size of Earth, in Neptune's atmosphere. Resembling Jupiter's Great
Red Spot, the storm spins counterclockwise and moves westward at
almost 1,200 kilometers (745 miles) per hour. Voyager 2 also noted
a smaller dark spot and a fast-moving cloud dubbed the “Scooter,”
as well as high-altitude clouds over the main hydrogen and helium
cloud deck. The highest wind speeds of any planet were observed,
up to 2,400 kilometers (1,500 miles) per hour.

Like the other giant planets, Neptune has a gaseous hydrogen
and helium upper layer over a liquid interior. The planet's core
contains a higher percentage of rock and metal than those of the
other gas giants. Neptune's distinctive blue appearance, like
Uranus' blue color, is due to atmospheric methane.

Neptune's magnetic field is tilted relative to the planet's
spin axis and is not centered at the core. This phenomenon is
similar to Uranus' magnetic field and suggests that the fields of
the two giants are being generated in an area above the cores,
where the pressure is so great that liquid hydrogen assumes the
electrical properties of a metal. Earth's magnetic field, on the
other hand, is produced by its spinning metallic core and is only
slightly tilted and offset relative to its center.

Voyager 2 also shed light on the mystery of Neptune's rings.
Observations from Earth indicated that there were arcs of material
in orbit around the giant planet. It was not clear how Neptune
could have arcs and how these could be kept from spreading out
into even, unclumped rings. Voyager 2 detected these arcs, but
they were, in fact, part of thin, complete rings. A number of
small moons could explain the arcs, but such bodies were not
spotted.

Astronomers had identified the Neptunian moons Triton in 1846
and Nereid in 1949. Voyager 2 found six more. One of the new moons
– Proteus — is actually larger than Nereid, but since Proteus
orbits close to Neptune, it was lost in the planet's glare for
observers on Earth.

Triton circles Neptune in a retrograde orbit in under six
days. Tidal forces on Triton are causing it to spiral slowly
towards the planet. In 10 to 100 million years (a short time in
astronomical terms), the moon will be so close that Neptunian
gravity will tear it apart, forming a spectacular ring to
accompany the planet's modest current rings.

Triton's landscape is as strange and unexpected as those of
Io and Miranda. The moon has more rock than its counterparts at
Saturn and Uranus. Triton's mantle is probably composed of water-
ice, but the moon's crust is a thin veneer of nitrogen and
methane. The moon shows two dramatically different types of
terrain: the so-called “cantaloupe” terrain and a receding ice
cap.

Dark streaks appear on the ice cap. These streaks are the
fallout from geyser-like volcanic vents that shoot nitrogen gas
and dark, fine-grained particles to heights of 2 to 8 kilometers
(1 to 5 miles). Triton's thin atmosphere, only 1/70,000th as thick
as Earth's, has winds that carry the dark particles and deposit
them as streaks on the ice cap — the coldest surface yet found in
the solar system (-235 degrees Celsius, -391 degrees Fahrenheit).
Triton might be more like Pluto than any other object spacecraft
have so far visited.

PLUTO

Pluto is the most distant of the planets, yet the
eccentricity of its orbit periodically carries it inside Neptune's
orbit, where it has been since 1979 and where it will remain until
March 1999. Pluto's orbit is also highly inclined — tilted 17
degrees to the orbital plane of the other planets.

Discovered in 1930, Pluto appears to be little more than a
celestial snowball. The planet's diameter is calculated to be
approximately 2,300 kilometers (1,430 miles), only two-thirds the
size of our Moon. Ground-based observations indicate that Pluto's
surface is covered with methane ice and that there is a thin
atmosphere that may freeze and fall to the surface as the planet
moves away from the Sun. Observations also show that Pluto's spin
axis is tipped by 122 degrees.

The planet has one known satellite, Charon, discovered in
1978. Charon's surface composition is different from Pluto's: the
moon appears to be covered with water-ice rather than methane ice.
Its orbit is gravitationally locked with Pluto, so both bodies
always keep the same hemisphere facing each other. Pluto's and
Charon's rotational period and Charon's period of revolution are
all 6.4 Earth days.

Although no spacecraft have ever visited Pluto, NASA is
currently exploring the possibility of such a mission.

COMETS

The outermost members of the solar system occasionally pay a
visit to the inner planets. As asteroids are the rocky and
metallic remnants of the formation of the solar system, comets are
the icy debris from that dim beginning and can survive only far
from the Sun. Most comet nuclei reside in the Oort Cloud, a loose
swarm of objects in a halo beyond the planets and reaching perhaps
halfway to the nearest star.

Comet nuclei orbit in this frozen abyss until they are
gravitationally perturbed into new orbits that carry them close to
the Sun. As a nucleus falls inside the orbits of the outer
planets, the volatile elements of which it is made gradually warm;
by the time the nucleus enters the region of the inner planets,
these volatile elements are boiling. The nucleus itself is
irregular and only a few miles across, and is made principally of
water-ice with carbon monoxide, carbon dioxide, methane and
ammonia — materials very similar to those composing the moons of
the giant planets.

As these materials boil off of the nucleus, they form a coma
or cloud-like “head” that can measure tens of thousands of
kilometers across. The coma grows as the comet gets closer to the
Sun. Solar charged particles push on gas molecules and the
pressure of sunlight pushes on the cloud of dust particles,
blowing them back like flags in the wind and giving rise to the
comet's “tails.” Gases and ions are blown directly back from the
nucleus, but dust particles are pushed more slowly. As the nucleus
continues in its orbit, the dust particles are left behind in a
curved arc.

Both the gas and dust tails are blown away from the Sun;
effect, the comet chases its tails as it recedes from the Sun.
tails can reach 150 million kilometers (93 million miles) in
length, but the total amount of material contained in this
dramatic display would fit in an ordinary suitcase. Comets — from
the Latin cometa, meaning “long-haired” — are essentially dramatic
light shows.

Some comets pass through the solar system only once, but
others have their orbits gravitationally modified by a close
encounter with one of the giant outer planets. These latter
visitors can enter closed elliptical orbits and repeatedly return
to the inner solar system.

Halley's Comet is the most famous example of a relatively
short period comet, returning on an average of once every 76 years
and orbiting from beyond Neptune to within Venus' orbit. Confirmed
sightings of the comet go back to 240 BC This regular visitor to
our solar system is named for Sir Edmond Halley, because he
plotted the comet's orbit and predicted its return, based on
earlier sightings and Newtonian laws of motion. His name became
part of astronomical lore when, in 1759, the comet returned on
schedule. Unfortunately, Sir Edmond did not live to see it.

A comet can be very prominent in the sky if it passes
comparatively close to Earth. Unfortunately, on its most recent
appearance, Halley's Comet passed no closer than 62.4 million
kilometers (38.8 million miles) from our world. The comet was
visible to the naked eye, especially for viewers in the southern
hemisphere, but it was not spectacular. Comets have been so
bright, on rare occasions, that they were visible during daytime.
Historically, comet sightings have been interpreted as bad omens
and have been artistically rendered as daggers in the sky.

Several spacecraft have flown by comets at high speed; the
first was NASA's International Cometary Explorer in 1985. An
armada of five spacecraft (two Japanese, two Soviet and the Giotto
spacecraft from the European Space Agency) flew by Halley's Comet
in 1986. Additional comet missions are being examined in the
United States and abroad.

CONCLUSION

Despite their efforts to peer across the vast distances of
space through an obscuring atmosphere, scientists of the past had
only one body they could study closely — Earth. But since 1959,
spaceflight through the solar system has lifted the veil on our
neighbors in space.

We have learned more about our solar system and its members
than anyone had in the previous thousands of years. Our automated
spacecraft have traveled to the Moon and to all the planets beyond
our world except Pluto; they have observed moons as large as small
planets, flown by comets and sampled the solar environment.
Astronomy books now include detailed pictures of bodies that were
only smudges in the largest telescopes for generations. We are
lucky to be alive now to see these strange and beautiful places
and objects.

The knowledge gained from our journeys through the solar
system has redefined traditional Earth sciences like geology and
meteorology and spawned an entirely new discipline called
comparative planetology. By studying the geology of planets,
moons, asteroids and comets, and comparing differences and
similarities, we are learning more about the origin and history of
these bodies and the solar system as a whole.

We are also gaining insight into Earth's complex weather
systems. By seeing how weather is shaped on other worlds and by
investigating the Sun's activity and its influence throughout the
solar system, we can better understand climatic conditions and
processes on Earth.

We will continue to learn and benefit as our automated
spacecraft explore our neighborhood in space. Missions to each
type of body in the solar system are in flight or under
development or study.

We can also look forward to the time when humans will once
again set foot on an alien world. Although astronauts have not
been back to the Moon since December 1972, plans are being
formulated for our return to the lunar landscape and for the human
exploration of Mars and even the establishment of martian
outposts. One day, taking a holiday may mean spending a week at a
lunar base or a martian colony!

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