SOHO Analyzes a Kamikaze Comet
23 Feb 2001
(Source: European Space Agency)
A comet that fell into the Sun on 7 February 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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Comet C/2001 C2 (originally designated SOHO-294) was discovered in LASCO C3 images on 6 February 2001 and has been confirmed as a member of the Kreutz family of Sun-grazing comets These comets are believed to be fragments of a parent body which was torn apart some time in the distant past. The image above is a composite of LASCO C2 data, the white light corona at 19:54 UT, and two UVCS observations of the comet itself. The UVCS images represent observations taken at 4.82 and 3.32 Solar Radii (Ro) over a time span of 19:11-20:02 UT and 20:11-20:45 UT respectively. Each UVCS image is a time sequence of several 200 second exposures as the comet passed across the entrance aperture. The UVCS comet images have been approximately positioned and scaled to the LASCO corona image. A wide, well-defined tail can be seen in the magnified version of the 4.82 Ro observation.
Solar radiation heats the comet which in turn causes the outgassing of the water molecules and dust. The dust scatters sunlight at visible wavelengths, making the comet bright in LASCO images. The water molecules break down into oxygen and hydrogen atoms, and the hydrogen atoms interact with the coronal plasma. The hydrogen atoms scatter Lyman Alpha (1215 Angstroms) radiation from the Sun. The UVCS images represent the brightness at this wavelength.
Several properties of the comet and the encountered coronal environment can be determined from this observation. The outgassing rate of water molecules was calculated at ~100 kg/s and the size of the cometary nucleus was on the order of 10 meters during the UVCS observations. The comet also acted as a probe for the determination of coronal conditions. Densities of about 10,000 particles per cubic centimeter at 4.82 Ro and 86,000 particles per cc at 3.32 Ro were found. This sharp increase suggests that the comet passed from a fast solar wind region to a denser slow solar wind area. All of the results presented here are preliminary estimates.
Credits: Thanks to Doug Biesecker (LASCO) for his early reporting and position estimates, Brian Marsden for the orbit prediction, and the UVCS ops and science teams (Michael Uzzo, Kuen Ko, Rai Wu, John Raymond) for making this observation possible.