Comet Hyakutake Close to the Sun -- Observations with LASCO Coronograph

6/12/1996 - 73-96r
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For the first time, scientists were able to observe a close encounter of a long orbital period (10,000 years) comet with the Sun. "This cannot be done from the ground, because of the scattering of sunlight in the Earth's atmosphere. Even in space, in the absence of the Earth's atmosphere, a special telescope, called a coronagraph, must block out all the stray light generated in the instrument itself, to make the comet visible. Such an instrument is the Naval Research Laboratory's (NRL's) Large Angle Spectrometric Coronagraph (LASCO) instrument on board the Solar and Heliospheric Observatory (SOHO) satellite," says Dr. Guenter Brueckner, NRL's principal investigator of the LASCO program.

NRL's LASCO is orbiting in deep space aboard the ESA-NASA SOHO satellite. Comet Hyakutake entered the field of view of the LASCO coronagraph on April 29, 1996 and left the field of view on May 5, 1996. During this passage the comet went through its perihelion on May 1, 1996. At that time, its closest distance to the Sun was approximately 20 million miles. "During the four days that the comet was close to the Sun, the structure of its tail underwent major changes" says Dr. Brueckner.

Hyakutake showed three distinct tails. One of them remained aligned with the orbital plane. It presumably consisted of newly emitted heavier particles, which cannot be moved by either radiation pressure or the solar wind. Indeed, radar observations suggest that heavier particles had broken off from the nucleus of the comet.

According to Dr. Brueckner, the ion tail, which consists of atomic particles and electrons, is always closely aligned with the direction to the Sun. It rotates in the plane of the sky, as Hyakutake swings around the Sun. The interaction with the solar wind streaming away from the Sun causes this leeward direction. However, the picture is more complicated. In the last image taken on May 5, the tail had split into at least two distinct structures.

The classical dust tail also rotates in the plane of the sky, but much slower than the ion tail. It also becomes wider. Radiation pressure from the Sun pushes the dust particles away from the Sun. Because this does not happen instantly, the heavier particles stay behind the lighter ones. This may be a clue to the mass distribution of dust particles in a comet's tail.

Finally, the brightness of the coma decreased during the four days of observations. This shows the interaction of the solar wind with the coma, blowing away atomic particles from the coma into the ion tail.

"If we come up with a plausible explanation for all these effects during the comet's close encounter with the Sun, the intriguing question remains: Was this the first visit of Hyakutake or has the comet passed through our solar system many times before? Can the comet regenerate or are the solar forces strong enough to destroy it over long periods of time?" asks Dr. Brueckner.

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