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彗星的基础知识

一、彗星的发现和命名

彗星,俗称扫帚星,“彗”字即扫帚之意。外文中的彗星一comet一词来自希腊文,意思是有“尾巴”或“毛发”的星。天文学中形象地以符号代表彗星。古代人偶然看到形貌奇怪的彗星出现,感到恐惧,看作灾祸的征兆,其实彗星出现只是一种自然现象,天文观测研究逐步揭开了彗星之谜。

  历史上有很多彗星出现的记录,以我国古书上的记录为最早和最多,有时记为孛星、星孛、妖星、异星、蓬星、长星等。《淮南子》中有“武王伐纣··· ···彗星出”,据我国著名天文学家张钰哲推算,这是哈雷彗星在公元前1056年的回归,这是天文学对历史年代考证的重要贡献。

  西方人长期受亚里士多德的错误看法的影响,认为彗星是地球大气中的一种燃烧现象,甚至哥白尼也认为“希腊人所谓的彗星,诞生在高层大气”。直到16世纪末,第谷才首次观测证明1577年大彗星比月球远得多,我国早在《晋书天文志》就有“彗星无光,傅日而为光。故夕见则东指,晨见则西指。在日南北皆随日光而指,顿挫其芒,或长或短。”古代只把彗星作为偶然出现的,直到17世纪,英国天文学家哈雷才计算彗星轨道,他发现1682、1607、1531年出现的彗星有相似的轨道,断言这是同一颗彗星的三次回归,并预言它在1758年底或1759年初会再次出现,虽然他逝世于1742年而未亲自见到,但这颗彗星果然在1759年出现了,为了纪念他,这颗彗星称为“哈雷彗星”。哈雷一生计算出24个彗星轨道。

  哈雷预言的应验激起人们去搜寻发现新彗星。按照国际惯例,新发现的彗星以最先发现者(至多三人,1994年后改为最多两人)命名。

  虽然肉眼见到的亮彗星很少,但现代望远镜每年平均可看到20一25颗彗星,其中约1/3是新发现的,业余目视发现2至3颗。已有观测记载的彗星有1800 多颗,去掉重复回归的,仅有1600多颗。实际上,彗星只有运行到离我们较近时才被观测到,而它们远离太阳时就观测不到了,据统计估算太阳系有 1012(万亿)1013(十万亿)颗彗星,它们绝大部分在太阳系外部。

二、彗星的轨道特性

1、彗星轨道

  通过多次观测的资料,可以推求出彗星的绕太阳公转轨道要素,即近日距、过近日点时刻、偏心率、轨道面对黄道面的倾角、升交点(在轨道上由南向北经黄道面上的点)黄经、近日点与升交点的角距,进而可以推算出彗星的历表,即不同时刻在天球上的视位置(赤经与赤纬)。很多彗星的轨道是扁长椭圆形、抛物线乃至双曲线。显然,沿抛物线或双曲线轨道运动的彗星是非周期彗星,它们会一去不返、逃离太阳系,那么它们是否真正属于太阳系成员呢?实际上,这些彗星是在过近日点前后被观测的,算出的是其吻切轨道,而且在这段时间里三种轨道差别较小,观测精度不够可导致算出的轨道误差,更重要的是彗星运行中受大行星的引力摄动等影响而发生轨道变化,大多数彗星在改正这些影响后算出的轨道仍是偏心率略小于1的扁长椭圆,因而它们是太阳系成员,也有少数彗星会一去不返。既使很扁长椭圆轨道的彗星,其公转周期也很长,要几百年乃至几万年才回归太阳系一次,在人类文明史中只有短周期的彗星(公转周期小于200年)才被多次观测到,绝大多数短周期彗星是顺向公转达的(即跟行星公转方向相同),它们的轨道面相对黄道面的倾角小于45度,有少数(如哈雷彗星)逆向公转,而长周期彗星和非周期彗星的轨道面倾角是随机分布的,顺向公转和逆向公转的都很多。

2、彗星族

  约2/3的短同期彗星的远日距小于7天文单位,即它们在远日点时临近木星轨道,称它们为“木星族彗星”。一般认为,近抛物线(偏心率e约等于1)轨道的彗星接近木星时,因受木星引力摄动大,其轨道改变而被俘获为短周期彗星。典型例子是Brooks(1889V)彗星,它接近木星后,公转周期从约29年变为 7年。此外,还有些彗星的远日距靠近土星、天王星、海王星轨道,分别称作“土星族彗星”、“天王星族彗星”、“海王星族彗星”,但数目少,是否来自“俘获”尚有疑问。

3、彗星群

  除了过近日点时刻不同之外,其余五个轨道要素都很接近的一些彗星称为“彗星群”。已确认出10个彗星群,各群的彗星数目有多有少。有一种看法认为,同群的彗星是由一颗大彗星分裂出来的。确实观测到一些彗星分裂的事例。最著名的是“掠日彗星群”,至少有16颗彗星,其近日距小于0.01天文单位,可以穿越日冕,其中池谷一关彗星(1965VIII)在1965年10月20日过近日点后两星期内分裂为三颗。1993由休梅克夫妇(E.Shoemaker,C.Shoemaker)和(D.Levy)发现的Shoemaker一Levy9彗星在1992年7月接近木星时可能发生多次分裂,1993年先观测到5个子彗核,后增至11、17直到21个子彗核,在照片上排列成一串,成为“天空中的项链”。

4、奥尔特云与柯伊伯带

 1950年,荷兰天文学家奥尔特(J.H.Oort)作了彗星轨道的统计研究,发现轨道半径为3万至10万天文单位的彗星数目很多,他推算那里有个大致球层状的彗星储库,有上千亿颗彗星。早在1932年欧匹克(E.Opiek)也曾提出过类似看法,因而这个彗星储库称为“奥尔特云”或“奥尔特一欧匹克云”,那里的彗星绕太阳公转的周期长达几百万年。按照近年的更仔细研究,奥尔特云中有上万亿至十万亿颗彗星。当然,这些遥远的彗星绝大多数尚不能直接观测到,只有在恒星的引力摄影动下或彗星相互碰撞时,有的彗星发生很大的轨道变化,当它沿扁长轨道进入内太阳系时,才成为“新”彗星被观测发现。

 1951年,美国天文学家柯伊伯(G.Kuiper)研究彗星性质与彗星形成,认为在太阳系原始星云很冷的外部区里的挥发物凝聚为冰体一彗星,当外行星在冰体群中长大时,外行星的引力弥散作用使一些彗星驱入奥尔特云,但是冥王星之外没有行星形成,他提出冥王星之外有个彗星带一即柯伊伯带,那里有很多彗星,它们的轨道近于圆形,轨道面对黄道面倾角不大。1964年,惠普尔(F.Whipple)等提出,冥外彗星带会引起外行星及彗星引力摄动,若此带在40天文单位处,则彗星总质量约为地球质量的80%;若在50天文单位处,则总质量为地球的1.3倍。1988年邓肯(M.Duncan)证明,柯伊伯带是短周期彗星的主要源,而奥尔特云不是它们的源区。(右图:据说是柯伊伯带星体)

  正如前面所述的,近年新发现的冥外天体1992QB1(Smiley)和1993FW应是柯伊伯带内边界区的彗星(尽管现在以小行星方式命名),而离太阳 32至35天文单位的1993RO、1993RP、1993SB、1993SC可能是从柯伊伯带摄动出来、处在向短周期演变的天体。柯伊伯带从离太阳40 天文单位外延到几百天文单位(其外界尚不知道),估计此带中的彗星有上万颗,它们是太阳系形成时期的原始冰体残留下来的,这些彗星保存着太阳系原始物质的信息。欧洲空间局将在2003年发射罗赛达(Rosetta)飞船会合由柯伊伯带来的短周期彗星,揭示彗星性质及太阳系形成的奥秘。

天文台观测发现一颗掠日彗星飞向太阳将被吞噬

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这张由太阳及日球层天文台2010年3月12日拍摄的照片,清晰地显示了一颗掠日彗星正朝太阳飞去,很可能将葬身“火海”。照片是由SOHO的大角 度分光日冕仪(LASCO)拍摄的,照片中太阳的光芒被一个圆盘遮蔽。

北京时间3月15日消息,据美国太空网网站报道,一颗新近发现的彗星正朝太阳飞去,预计最终将难逃葬身炽热的太阳的命运。

由于这颗彗星同太阳的距离过于接近,如果不借助专业观测工具,肉眼很难看到。据SpaceWeather.com网站报道,太阳及日球层天文台 (Solar and Heliospheric Observatory,SOHO)拍摄的最新太阳照片,清晰地显示了一颗彗星飞向太阳的“死亡坠落”。

据了解,彗星近距离掠过太阳的现象并不罕见。天文学家估计,超过1600颗彗星定期会穿越太阳系的核心区域,而掠日彗星的数目在2005年就已 经跨越1000大关。

这颗飞向太阳的彗星属于克鲁兹族掠日彗星,它们的轨道使得自己非常接近于太阳表面,最近只有数十万英里。无一例外,这种遭遇都会导致彗星因太阳 极高的温度而蒸发,但大多数彗星由于太小而难以被观察到。这一次,SOHO探测器上的照相机终于让科学家们如愿以偿。

据悉,所有的克鲁兹掠日彗星都是数百年前解体的一颗大彗星的残骸。克鲁兹彗星是根据德国天文学家海因里希-克鲁兹(Heinrich Kreutz)命名的,克鲁兹是第一位确定这种联系的科学家。

太阳及日球层天文台是欧洲航天局及美国国家航空航天局共同研制的无人太空船,于1995年发射升空。该天文台共安装了十二台主要的科学仪器,每 一个都能够独立的观察太阳或者太阳的某个局部。据悉,太阳及日球层天文台是通过日冕观测仪拍摄到克罗伊策掠日彗星即将被太阳吞噬的照片的。日冕观测仪可以 遮挡图像中最明亮的物体,从而令科学家可以观测到太阳周围的活动,例如彗星靠近。

太阳及日球层天文台在发现掠日彗星上可谓功不可没。在太阳及日球层天文台发射升空之前,其他太空望远镜只发现过16颗掠日彗星。而自太阳及日球 层天文台1995年发射以来,天文学家迄今已通过这一仪器发现数百颗克鲁兹掠日彗星,一些直径甚至只有几米。(新浪科 技)

1.3万年前彗尾碰撞地球导致微型冰河时期(图)

据英国每日邮报报道,日前,科学家最新研究显示,13000年前一场持续1小时的太空彗星雹暴袭击地球,导致地球进入微型冰河时期。

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13000年前一场持续1小时的太空彗星雹暴袭击地球,导致地球进入微型冰河时期

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这项研究将解释大约公元前11000年神秘的地球物种大规模灭绝之谜

这场灾难是由于彗星进入地球大气层分裂,向地球表面散布数以千计的冰灰尘巨块,每个这样的碎裂巨块碰撞力相当于一颗超级原子弹。此次碰撞事件摧 毁了遍布全球的大量动物物种,严重扰乱了当时石器时代早期人类的生活,使地球生态环境进入一个持续1000多年的冰冻期。

这一观点的提出者是英国加的夫大学比尔-纳皮尔(Bill Napier)教授,他指出该理论将解释大约公元前11000年神秘的地球物种大规模灭绝之谜。

长期以来,科学家一直迷惑不解,在冰河时期末期地球开始逐渐暖和,但突然全球性温度下降8摄氏度。地球突然降温导致已逐渐消退的冰川再度扩散, 该现象将解释同时期北美洲地区35种人类所熟悉的哺乳动物灭绝之谜。

一些地质学家曾认为,导致13000年前地球温度骤降的原因可能是一颗较大的小行星碰撞地球,它要比0.65亿年前导致恐龙灭绝的小行星更小一 些。

科学家之所以判断彗星碰撞地球,是由于在地质岩石层发现了线索,其中包括陆地野火燃烧形成的12.5厘米厚的煤烟层。此外,通过显微镜观测还发 现猛烈碰撞形成的陨石和碰撞陨坑中有“纳米级钻石”微粒,这些微粒形成的时期与地球物种灭绝时间相吻合。

这项发现起初认为,13000年前4公里长的彗星碰撞在现今加拿大和美国北部境内的大冰原上。但其他科学家称,13000年前这样大小的彗星碰 撞地球的概率仅千分之一,并且这样一颗彗星碰撞无法解释地球全球性的火灾。

纳皮尔教授的观点认为,远古时期物种灭绝发生于地球与一颗较大彗星尾部相碰撞,数以千计的冰灰尘巨块像雨点般地落在地面上,每一个巨块可释放出 百万吨级原子弹的爆炸力。碰撞灾难使大气层弥漫着烟雾,地面上落了一层厚厚的烟煤层,并且遮住了太阳光线。

这颗超大质量彗星是于2-3万年前进入太阳系内部,尾部与地球发生碰撞直至完全消失。纳皮尔说:“在过去3万年里,这颗超大质量彗星逐渐分解, 数以千计的碎片不止一次地碰撞在地面上。”目前,他的这项研究理论发表在《皇家天文协会月刊》杂志上,认为彗星分裂形成的冰雹碎片碰撞地球持续了仅1个小 时。

这可能导致数以千次的碎片碰撞,产生全球性的火灾,并在地面上沉积了纳米级的钻石颗粒,此次碰撞对地球产生的标志性影响是大量物种消亡。

近代在地球表面形成的彗星碰撞事件是塔吉什湖陨星碰撞,2000年1月一颗彗星坠落在加拿大育空地区,碰撞产生了大量的纳米级晶体颗粒,科学家 现已对这些颗粒进行了研究。