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Comet Hyakutake

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   CAPTION: C/Hyakutake (1996 B2)

                       Comet
   ( List of comets)
               Image:Hyakutake 2.jpg
                     Discovery
   Discoverer                Yuji Hyakutake
   Discovery date            1996
   Alternate
   designations              C/1996 B2
                Orbital elements ^A

                               Epoch 2450400.5

   Eccentricity (e)          0.999902
   Semi-major axis (a)       2349.02 AU
   Perihelion (q)            0.230204 AU
   Aphelion (Q)              4698.77 AU
   Orbital period (P)        70,000-114,000 a (?)
   Inclination (i)           124.9°
   Last perihelion date      May 1, 1996
   Next est. perihelion date 72,000-116,000 (?)

   Comet Hyakutake ( Japanese: 百武彗星 Hyakutake suisei, IPA [çakɯtake
   sɯiseː]; formally designated C/1996 B2) is a comet that was discovered
   in January 1996, and passed very close to the Earth in March of that
   year. It was dubbed The Great Comet of 1996, and was one of the closest
   cometary approaches to the Earth in the previous 200 years, resulting
   in the comet appearing very bright in the night sky and being seen by a
   large number of people around the world. The comet temporarily upstaged
   the long-awaited Comet Hale-Bopp, which was approaching the inner solar
   system at the time, although Hyakutake was at its brightest for only a
   few days.

   Scientific observations of the comet led to several notable
   discoveries. Most surprising to cometary scientists was the discovery
   of X-ray emission from the comet, the first time a comet had been found
   to be emitting X-rays. This emission is believed to be caused by
   ionised solar wind particles interacting with neutral atoms in the coma
   of the comet. The Ulysses spacecraft also unexpectedly crossed the
   comet's tail at a distance of more than 500 million  km from the
   nucleus, showing that Hyakutake had the longest tail yet known for a
   comet.

   Hyakutake is a long period comet. Before its most recent passage
   through the solar system, its orbital period was about 15,000  years,
   but the gravitational influence of the giant planets has now increased
   this to 72,000 years.

Discovery

   Comet Hyakutake captured by the Hubble Space Telescope on April 4, 1996
   with an infrared filter.
   Enlarge
   Comet Hyakutake captured by the Hubble Space Telescope on April 4, 1996
   with an infrared filter.

   The comet was discovered on January 30, 1996 by Yuji Hyakutake, an
   amateur astronomer from southern Japan. He had been searching for
   comets for some years and had moved to Kagoshima partly for the dark
   skies in the nearby rural areas. He was using a powerful set of
   binoculars with six- inch objective lenses to scan the skies on the
   night of the discovery.

   The comet he found was actually the second Comet Hyakutake, the first
   being comet C/1995 Y1, which Hyakutake had discovered just a few weeks
   earlier. While he was re-observing his first comet (which never became
   visible to the naked eye), Hyakutake happened to look at the patch of
   sky where he discovered it. To his great surprise there was another
   comet there, in almost exactly the same position his first had been.
   Although he could hardly believe he had discovered a second comet so
   soon after the first, Hyakutake reported his observation to the
   National Astronomical Observatory of Japan the following morning. Later
   that day, the discovery was confirmed by independent observations.

   At the time of its discovery the comet was shining at magnitude 11.0
   and had a coma about 2.5  arcminutes across. It was about 2
   astronomical units (AU) from the Sun. Later, a pre-discovery image of
   the comet was found on a photograph taken on January 1, when the comet
   was about 2.4 AU from the Sun and had a magnitude of 13.3.

Orbit

   When the first calculations of the comet's orbit were made, scientists
   realised that the comet was going to pass very close to the Earth on 25
   March, just 0.1 AU away. Only three comets in the previous century had
   passed closer. As Comet Hale-Bopp was already being discussed as a
   possible "great comet", it took a while for the astronomical community
   to realise that Hyakutake too might become spectacular; its close
   approach to Earth meant it was very likely to become a great comet.

   Also encouraging for the comet's chances of becoming bright was that
   its orbit showed it had last returned to the inner solar system about
   17,000 years ago. This meant it was likely to have passed close to the
   Sun several times before, rather than being a fresh arrival from the
   Oort cloud, which contains comets having orbital periods of millions of
   years. Comets entering the inner solar system for the first time may
   brighten rapidly at first before fading as they near the sun due to a
   layer of highly volatile material being evaporated away: this was the
   case with Comet Kohoutek in 1973, which was initially touted as a comet
   of the century, but only appeared modestly bright. Older comets show a
   more consistent and predictable brightening pattern.

   Besides approaching the Earth closely, the comet's path also meant that
   it would be visible throughout the night to northern hemisphere
   observers at its closest approach, passing very close to the pole star.
   Most comets are close to the Sun in the sky when they are at their
   brightest and thus may not be seen in a completely dark sky.

The comet passes the Earth

   The comet on the evening of its closest approach to Earth on 25 March
   1996.
   Enlarge
   The comet on the evening of its closest approach to Earth on 25 March
   1996.

   Hyakutake became visible to the naked eye in early March. By mid-March,
   the comet was still fairly unremarkable, shining at 4th magnitude with
   a tail about 5  degrees long. However, as it neared its closest
   approach to Earth it rapidly became brighter, and its tail grew in
   length. By March 24, the comet was one of the brightest objects in the
   night sky; its tail stretched an impressive 35 degrees across the sky.
   The comet had a notably bluish-green colour.

   The comet made its closest approach to Earth on 25 March, when it was
   moving across the night sky so rapidly that its movement could be
   detected against the stars in just a few minutes. It raced across the
   sky, moving about the diameter of a full moon (half a degree) every
   30 minutes. Observers estimated its magnitude as around 0, and tail
   lengths of up to 80 degrees were reported. Its coma, now close to the
   zenith for observers at mid-northern latitudes, appeared some 1.5 to
   2 degrees across, roughly four times the diameter of the full moon.
   Even to the naked eye, the comet's head appeared distinctly green, due
   to strong emission from diatomic carbon (C[2]).

   Because Hyakutake was at its brightest for only a few days, it did not
   have time to permeate the public imagination in the way that Comet
   Hale-Bopp did the following year. Many European observers in particular
   did not see the comet at its peak because of unfavourable weather
   conditions. However, many people who saw both Hyakutake and Hale-Bopp
   at their peaks contend that Hyakutake was more impressive.

Perihelion and afterwards

   The SOHO satellite captured this image of Hyakutake as it passed
   perihelion, with a nascent coronal mass ejection also visible to the
   left of the Sun.
   Enlarge
   The SOHO satellite captured this image of Hyakutake as it passed
   perihelion, with a nascent coronal mass ejection also visible to the
   left of the Sun.

   After its close approach to the Earth, the comet faded to about 2nd
   magnitude. It reached perihelion on May 1, 1996, brightening again and
   exhibiting a dust tail in addition to the gas tail well seen as it
   passed the Earth. By this time, though, it was very close to the Sun
   and was not seen as easily. It was observed passing perihelion by the
   SOHO Sun-observing satellite, which also recorded a large coronal mass
   ejection being formed at the same time. Its distance from the Sun at
   perihelion was 0.23 AU, well inside the orbit of Mercury.

   After its perihelion passage, Hyakutake faded rapidly and was lost to
   naked-eye visibility by the end of May. Its orbital path carried it
   rapidly into the southern skies, and following perihelion it became
   much less monitored than before. The last known ground-based
   observation of the comet took place on October 24, 1996, when it was
   shining at magnitude 16.8 and no longer had an observable coma.

   Hyakutake had previously passed through the inner solar system about
   17,000 years ago; gravitational interactions with the gas giants during
   its 1996 passage stretched its orbit greatly, and it will not return to
   the inner Solar System again for approximately 72,000 years. However,
   there are conflicting orbital data suggesting a revolutionary period
   closer to 114,000 years.

Scientific results

Spacecraft passes through the tail

   The Ulysses spacecraft made an unplanned and unexpected pass through
   the tail of the comet on May 1, 1996. The encounter was not noticed
   until 1998, when astronomers analysing old data found that Ulysses'
   instruments had detected a large drop in the number of protons passing,
   as well as a change in the direction and strength of the local magnetic
   field. They realised that this implied that the spacecraft had crossed
   the 'wake' of an object, most likely a comet, but did not identify the
   object responsible.

   Two years later, two teams independently analyzed the same event. The
   magnetometer team realized that the changes in the direction of the
   magnetic field mentioned above agreed with the "draping" pattern
   expected in a comet's ion, or plasma tail. The magnetometer team looked
   for likely suspects. No known comets were located near the satellite,
   but looking further afield, they found that Hyakutake, some 500
   million km away, had crossed Ulysses' orbital plane on April 23. The
   solar wind had a velocity at the time of about 750 km/s, at which speed
   it would have taken about eight days for the tail to be carried out to
   where the spacecraft was situated at 3.73 AU, approximately 45 degrees
   out of the ecliptic plane. The orientation of the ion tail inferred
   from the magnetic field measurements agreed with the source lying in
   Comet Hyakutake's orbital plane.

   The other team, working on data from the spacecraft's ion composition
   spectrometer, discovered a sudden large spike in detected levels of
   ionised particles at the same time. The relative abundance of chemical
   elements detected indicated that the object responsible was definitely
   a comet.

   Based on the Ulysses encounter, the comet's tail is known to have been
   at least 570 million km (360 million  miles; 3.8 AU) long. This is
   almost twice as long as the previous longest-known cometary tail, that
   of the Great Comet of 1843, which was 2.2 AU long.

Composition

   Terrestrial observers found ethane and methane in the comet, the first
   time either of these gases had been detected in a comet. Chemical
   analysis showed that the abundances of ethane and methane were roughly
   equal, which is thought to imply that its ices formed in interstellar
   space, away from the Sun, which would have evaporated these volatile
   molecules. Hyakutake's ices must have formed at temperatures of 20  K
   or less, indicating that it probably formed in a denser than average
   interstellar cloud.

   The amount of deuterium in the comet's water ices was determined
   through spectroscopic observations. It was found that the ratio of
   deuterium to hydrogen (known as the D/H ratio) was about 3×10^−4, which
   compares to a value in Earth's oceans of about 1.5×10^−4. It has been
   proposed that cometary collisions with Earth might have supplied a
   large proportion of the water in the oceans, but the high D/H ratio
   measured in Hyakutake and other comets such as Hale-Bopp and Halley's
   Comet have caused problems for this theory.

X-ray emission

   X-ray emission from Hyakutake, as seen by the ROSAT satellite.
   Enlarge
   X-ray emission from Hyakutake, as seen by the ROSAT satellite.

   One of the great surprises of Hyakutake's passage through the inner
   solar system was the discovery that it was emitting X-rays, with
   observations made using the ROSAT satellite revealing very strong X-ray
   emission. This was the first time a comet had been seen to do so, but
   astronomers soon found that almost every comet they looked at was
   emitting X-rays. The emission from Hyakutake was brightest in a
   crescent shape surrounding the nucleus with the ends of the crescent
   pointing away from the Sun.

   The cause of the X-ray emission is thought to be a combination of
   several mechanisms. Reflection of solar X-rays is seen in other solar
   system objects such as the Moon, but is not likely to be able to
   explain the whole flux from Hyakutake, as the diffuse coma would be an
   inefficient X-ray reflector. Interactions between energetic solar wind
   particles and cometary material is also likely to contribute
   significantly to this effect. Observations of Comet LINEAR with the
   Chandra satellite in 2000 determined that X-rays observed from that
   comet were produced predominantly by collisions between nitrogen and
   oxygen ions in the solar wind and neutral hydrogen in the comet's coma.

Nucleus size and activity

   The region around the nucleus of Comet Hyakutake, as seen by the Hubble
   Space Telescope. Some fragments can be seen breaking off.
   Enlarge
   The region around the nucleus of Comet Hyakutake, as seen by the Hubble
   Space Telescope. Some fragments can be seen breaking off.

   Radar results from the Arecibo Observatory indicated that the nucleus
   of the comet was about 2 km across, and surrounded by a flurry of
   pebble-sized particles ejected from the comet at a few metres per
   second. This measurement of the nucleus size corresponded well with
   indirect estimates using infrared emission and radio observations.

   The small size of the nucleus ( Halley's Comet is about 15 km across,
   while Comet Hale-Bopp was about 40 km across) implies that Hyakutake
   must have been very active to become as bright as it did. Most comets
   undergo outgassing from a small proportion of their surface, but most
   or all of Hyakutake's surface seemed to have been active. The dust
   production rate was estimated to be about 2×10^3 kg/s at the beginning
   of March, rising to 3×10^4 kg/s as the comet approached perihelion.
   During the same period, dust ejection velocities increased from 50 m/s
   to 500 m/s.

   Observations of material being ejected from the nucleus allowed
   astronomers to establish its rotation period. As the comet passed the
   earth, a large puff or blob of material was observed being ejected in
   the sunward direction every 6.23  hours. A second smaller ejection with
   the same period confirmed this as the rotation period of the nucleus.
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