[meteorite-list] SOHO Analyses A Kamikaze Comet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:11 2004
Message-ID: <200102231708.JAA04810_at_zagami.jpl.nasa.gov>

ESA Science News
http://sci.esa.int

23 Feb 2001

SOHO analyses a kamikaze comet

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.

USEFUL LINKS FOR THIS STORY

* SOHO mission
  http://sohowww.estec.esa.nl/
* More about SOHO
  http://sci.esa.int/soho/
* Hot shots from SOHO
  http://soho.nascom.nasa.gov/hotshots/

IMAGE CAPTION:
[http://sci.esa.int/content/searchimage/searchresult.cfm?aid=14&cid=12&oid=26188&ooid=26198]
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.
Received on Fri 23 Feb 2001 12:08:26 PM PST