SOHO Images Comet Hyakutake's Close Encounter with the Sun


5/16/1996 - 64-96r
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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.



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