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.
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