2月7日,ESA-NASA SOHO太空船上两套不同的仪器发现有一颗彗星正冲向太阳,这使得天文学家们能够更好的观测它。由于有了LASCO日冕观测仪和在太空中观察太阳的有利的位置,使得这仅仅是SOHO自1996年以来发现的300颗彗星中的一颗。




    同时,伴随着这张LASCO的紫外线区照片产生的是由SAO的UVCS小组的Michael Uzzo拍下的两张紫外照片。这两张照片是分别在彗星的彗核距太阳2,700,0000和1,600,000千米的时候拍下的。而LASCO表面保护日冕仪不受直射的阳光的干扰的白环显示了太阳的大小和位置。

    而在2月6日,德国的Sebastian Hoenig和中国的周兴明通过英特网上每天都为彗星猎手们发送的SOHO中发现了这颗彗星。而Brian Marsden从由LASCO小组提供的数据中,想要算出这颗彗星的轨道,以便向国际天文联合会正式宣布这颗彗星——C/2001 C2 (SOHO)的发现。就像以往发现的SOHO一样,这颗彗星属于“掠日彗星”的家族。人们相信,这些小彗星是很久以前的一颗较大的彗星的碎片。对C/2001 C2 (SOHO)彗星来说,和太阳遭遇是“致命”的。

    UVCS的照片展示了被太阳的热量从彗星中蒸发出的水蒸气的分解出的氢原子的紫外线照片。SAO的John Raymond估计,这颗彗星正在以每秒100千米的速度向外散发水蒸气,而这颗彗星的彗核不过10~20米宽。而别的彗星的彗核比如哈雷慧星的彗核要用千米来计算。

    在太阳外270万公里处,彗星在纤细的太阳风中运动,而当彗星越来越靠近太阳时,太阳风的密度就成十倍的增长。这就像是彗星从速度很快的 太阳风中出来而进入了高密度的慢速太阳风的效果一样。更为精密的分解也许能够将这些估计的精确度提高。


   译自 ESA-NASA 2001年2月23日 

SOHO watches Kamikaze comet as it plunges into Sun
Chinese Version

  A comet that fell into the Sun on February 7 was tracked by two different instruments on the ESA-NASA SOHO spacecraft, enabling scientists to characterize it quite precisely. This was just one of nearly 300 comets discovered by SOHO since 1996, thanks mainly to the privileged view of the sky around the Sun given by the visible-light coronagraph LASCO. 

  On this occasion SOHO's ultraviolet coronagraph UVCS also observed the comet repeatedly. It gave valuable additional information, both about the comet and about the solar wind close to the Sun. 

  The accompanying picture shows, superimposed on a LASCO visible-light image, two of the ultraviolet images obtained by Michael Uzzo of the UVCS team at the Smithsonian Astrophysical Observatory (SAO) in Cambridge, Massachusetts. They were timed about an hour apart, when the comet's head was 2.7 and 1.6 million kilometres from the Sun's surface. The blow-up of the first image shows a wide and well-defined gas tail more than 500 000 kilometres long. The white ring on the LASCO coronagraph mask, which shields the instrument from the glare of direct sunlight, denotes the size and position of the visible Sun. 

  Sebastian Hoenig in Germany and Xing Ming Zhou in China discovered the comet on 6 February in the LASCO images that are available every day to comet hunters via the Internet. Data from successive observations, supplied by the LASCO team, enabled Brian Marsden at SAO to compute the comet's orbit and to make the discovery official on behalf of the International Astronomical Union by designating it as Comet C/2001 C2 (SOHO). Like most of the comets found by SOHO it belonged to a family of small "sungrazers" that are believed to be fragments of a large comet that broke up long ago. For C/2001 C2 (SOHO) the encounter with the Sun was fatal. 

  The UVCS images show ultraviolet light from hydrogen atoms, made by the break-up of water vapour released from the comet by the Sun's heat. John Raymond of SAO estimates that the comet was letting off steam at about 100 kilograms per second, and that the comet nucleus was only 10-20 metres wide. In large objects like Halley's Comet the nucleus is measured in kilometres. 

  At 2.7 million kilometres out (as in the first of the two UVCS images) the comet was flying through a relatively tenuous solar wind but, closer in, the density seems to have increased almost tenfold. This is interpreted as an effect of the comet passing out of the region of a fast solar wind into a slower windstream of higher density. Further analysis may refine all of these estimates.


   From ESA-NASA February 23, 2001