NASA, the European Space Agency
(ESA) and the Naval Research Laboratory (NRL) are releasing today
a set of unprecedented images representing a time lapse movie
of the bright Comet Hyakutake making its close approach to the
Sun.
The observations were made during
April 29 - May 6, 1996 with the NRL-built Large Angle Spectrometric
Coronagraph (LASCO) instrument on the Solar and Heliospheric
Observatory (SOHO) spacecraft. The comet came within about 20
million miles of the Sun and is seen in the images against the
background of the million-degree hot outer atmosphere of the
Sun, the corona.
"Such observations require
a special instrument in space to suppress the glare of the Sun
and reveal the comet and its tails," says Dr. Guenter Brueckner,
NRL's principal investigator for LASCO. Scattering of sunlight
in the Earth's atmosphere prevented good views from the ground
during the comet's "perihelion passage," when it was
closest to the Sun.
The orbital period of Comet Hyakutake
has been estimated to be 10,000 years. Hyakutake is called a
"new" comet because it was not seen when, and if, it
last visited the solar system. As Hyakutake approaches the Sun,
it is being heated enormously. If this is the first visit of
the comet, it could be broken into pieces, according to scientists.
Images captured by the LASCO instrument have shown that this
did not happen when the comet was in LASCO's field of view, which
is approximately the size of the constellation Orion.
"Comet Hyakutake could have
passed through the solar system many times before," says
Dr. Brueckner, who is also head of the NRL's Solar Physics Branch.
"How many times remains a mystery."
Hyakutake's orbit carries it
back into the so-called "Oort Cloud," a vast collection
of billions of comets that is located 1.4 light years away from
the solar system. These comets are presumably the remnants of
the cloud from which the stars were formed billions of years
ago.
When the comet enters the outer
atmosphere of the Sun, it begins to react with the Sun's environment
and can be used as a "probe" of the solar corona. LASCO
images show the head of the comet, and clearly visible are three
separate tails that behave differently as Hyakutake swings around
the Sun. These tails are made of different material; dust of
different sizes, perhaps chunks of ice and atomic particles,
each of which reacts differently with their environment. The
heavy particles follow the comet in its orbit without being redirected
by an outside force while the light dust particles are lining
up away from the Sun and are driven by the Sun's intensive radiation.
Finally, the atomic particles are repelled from the comet by
the solar wind and presumably line up with the magnetic field
of the solar corona.
As the comet speeds through the
corona at 37 miles per second, these forces have direct influence
on its tails, which could clearly be seen changing their relative
direction over the seven day observation period.
Coronal mass ejections were also
observed by LASCO, in which hot gases were expelled and accelerated
by the corona's magnetic field to travel through the interplanetary
medium. A
strong reaction between such a solar high-speed cloud and the
portion of the comet's tails made of atomic particles are expected
when Hyakutake crosses the equatorial plane of the Sun. The comet
was out of LASCO's field-of-view during this crossing, but the
scientists will have another opportunity when Hyakutake reappears
from behind the Sun and can be seen later in the southern hemisphere's
night sky with ordinary telescopes.
Researchers expect to learn more
about the tails of the comet and the surrounding solar corona
with more detailed analysis.
LASCO is a joint project between
NRL (USA), the Max Planck Institut für Aeronomie (Germany),
the Laboratoire d'Astronomie Spatiale (France), and the School
of Physics and Space Research at the University of Birmingham
(UK). SOHO is a project of international cooperation between
ESA and NASA.
More information can be found
on the LASCO Comet Hyakutake page on the World Wide Web @ URL:
http://lasco-www.nrl.navy.mil/b2-1996.html.
The U.S. Naval Research Laboratory is the Navy's full-spectrum corporate laboratory, conducting a broadly based multidisciplinary program of scientific research and advanced technological development. The Laboratory, with a total complement of nearly 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 85 years and continues to meet the complex technological challenges of today's world. For more information, visit the NRL homepage or join the conversation on Twitter, Facebook, and YouTube.
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