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Comet Hale-Bopp

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: C/1995 O1 (Hale-Bopp)

   Comet
   ( List of comets)
   Comet Hale-Bopp, shortly after passing perihelion in April 1997.
   Discovery
   Discoverer Alan Hale and
   Thomas Bopp
   Discovery date 1995
   Alternate
   designations The Great Comet of 1997,
   C/1995 O1
   Orbital elements ^A

                               Epoch 2450460.5

   Eccentricity (e) 0.995086
   Semi-major axis (a) 186 AU
   Perihelion (q) 0.91 AU
   Aphelion (Q) 371 AU
   Orbital period (P) 2537 a
   Inclination (i) 89.4°
   Last perihelion date April 1, 1997
   Next est. perihelion date 4380

   Comet Hale-Bopp ( formally designated C/1995 O1) was probably the most
   widely observed comet of the 20th century, and one of the brightest
   seen for many decades. It was visible to the naked eye for a record
   18 months, twice as long as the previous record holder, the Great Comet
   of 1811.

   Hale-Bopp was discovered on 23 July 1995 at a very large distance from
   the Sun, raising expectations that the comet could become very bright
   when it passed close to the Sun. Although comet brightnesses are very
   difficult to predict with any degree of accuracy, Hale-Bopp met or
   exceeded most predictions for its brightness when it passed perihelion
   on April 1, 1997. The comet was dubbed the Great Comet of 1997.

   The passage of Hale-Bopp was notable also for inciting a degree of
   panic about comets not seen for decades. Rumours that the comet was
   being followed by an alien spacecraft gained remarkable currency, and
   inspired a mass suicide among followers of the Heaven's Gate cult.

Discovery

   The comet was discovered by two independent observers, Alan Hale and
   Thomas Bopp, both in the United States. Hale had spent many hundreds of
   hours searching for comets without finding one, and was tracking known
   comets from his driveway in New Mexico when he chanced upon Hale-Bopp,
   shining at 11th  magnitude near a globular cluster, M70, in the
   constellation of Sagittarius just after midnight. Hale first
   established that there was no other deep-sky object near M70, and then
   consulted a directory of known comets, finding that no known objects
   were in this area of sky. Once he had established that the object was
   moving relative to the background stars, he emailed the Central Bureau
   of Astronomical Telegrams, the clearing house for astronomical
   discoveries.

   Bopp did not own a telescope. He was out with friends near Stanfield,
   Arizona observing star clusters and galaxies when he chanced across the
   comet while at the eyepiece of his friend's telescope. He realised he
   might have spotted something new when he checked his star atlases to
   find out what other deep-sky objects were near M70, and found that
   there were none. He actually contacted the Central Bureau of
   Astronomical Telegrams using a telegram. The following morning, it was
   confirmed that this was a new comet, and it was named Comet Hale-Bopp,
   with the designation C/1995 O1. The discovery was announced in
   International Astronomical Union circular 6187.

Early progress

   It was soon apparent that Hale-Bopp was no ordinary comet. For a start,
   when its orbit was calculated, it turned out to be 7.2  Astronomical
   Units (AU) from the Sun, placing it between Jupiter and Saturn and by
   far the greatest distance from Earth at which a comet had been
   discovered. Most comets at this distance are extremely faint, and show
   no discernable activity, but Hale-Bopp already had an observable coma.
   An image taken at the Anglo-Australian Telescope in 1993 was found to
   show the then-undiscovered comet some 13 AU from the sun, a distance at
   which most comets are essentially unobservable (Halley's Comet was
   50,000 times fainter at the same distance from the Sun). Analysis
   indicated that its nucleus was about 50  kilometres in diameter, nearly
   three times the size of Halley.

   Its great distance and surprising activity indicated that Comet
   Hale-Bopp might become very bright indeed when it reached perihelion in
   1997. However, comet scientists were wary – comets can be extremely
   unpredictable, and many have large outbursts at great distance only to
   diminish in brightness later. Comet Kohoutek in 1973 had been touted as
   a 'comet of the century' and turned out to be very unspectacular.

Hale-Bopp becomes a Great Comet

   Hale-Bopp became visible to the naked eye in May 1996 , and although
   its rate of brightening slowed considerably during the latter half on
   1996, scientists were still cautiously optimistic that it would become
   very bright. It was too close to the Sun to be observable during
   December 1996, but when it reappeared in January 1997 it was already
   bright enough to be seen by anyone who looked for it, even from large
   cities with light-polluted skies.
   The comet became a spectacular sight in early 1997.
   Enlarge
   The comet became a spectacular sight in early 1997.

   The Internet was a growing phenomenon at the time, and numerous
   websites that tracked the comet's progress and provided daily images
   from around the world became extremely popular. The Internet played a
   large role in encouraging the unprecedented public interest in
   Hale-Bopp.

   As the comet approached the Sun, it continued to brighten, shining at
   2nd magnitude in February, and showing a growing pair of tails, the
   blue gas tail pointing straight away from the Sun and the yellowish
   dust tail curving away along its orbit. On March 9, a solar eclipse in
   Mongolia and eastern Siberia allowed observers there to see the comet
   in the daytime. Hale-Bopp had its closest approach to Earth on March
   22, 1997.

   As it passed perihelion on April 1, 1997, the comet had developed into
   a spectacular sight. It shone brighter than any star in the sky except
   Sirius, and its two tails stretched 30-40 degrees across the sky. The
   comet was visible well before the sky got fully dark each night, and
   while many great comets are very close to the Sun as they pass
   perihelion, Comet Hale-Bopp was visible all night to northern
   hemisphere observers.

   As impressive as the comet was, it could have been much more
   impressive. Had it passed as close to Earth as Comet Hyakutake (C/1996
   B2) did in 1996, then the comet's tail would have spanned the entire
   sky and it would have been brighter than the full moon. However, even
   though its closest approach to Earth was at a distance of 1.315 AU, a
   distance which would have rendered many lesser comets totally
   invisible, Hale-Bopp still spanned half the sky with its two tails,
   although the longest reaches of the tails were too faint to be visible
   to the naked eye.

The comet recedes

   After its perihelion passage, the comet moved into the southern
   celestial hemisphere, and its show was over as far as most of the
   public of the Northern Hemisphere were concerned. The comet was much
   less impressive to Southern Hemisphere observers than it had been in
   the Northern Hemisphere, but southerners were able to see the comet
   gradually fade from view during the second half of 1997. The last
   naked-eye observations were reported in December 1997, which meant that
   the comet had remained visible without aid for 569 days, or about 18
   and a half months. The previous record had been set by the Great Comet
   of 1811, which was visible to the naked eye for about 9 months.

   As the comet receded it continued to fade, but it is still being
   tracked by astronomers. As of January 2005, the comet is further from
   the Sun than Uranus, at a distance from Earth of about 21 AU, but is
   still observable with large telescopes. Recent observations have found
   that it still displays a distinct tail.

   Astronomers expect that the comet will remain observable with large
   telescopes until perhaps 2020, by which time it will be nearing 30th
   magnitude. By this time it will become very difficult to distinguish
   the comet from the large numbers of distant galaxies of similar
   brightness. It will return around the year 4380.

Orbital changes

   The comet probably made its last perihelion 4,200 years ago. Its orbit
   is almost perpendicular to the plane of the ecliptic, which means that
   very close approaches to planets are rare. However, in March 1997 the
   comet passed within 0.77 AU of Jupiter, close enough for its orbit to
   be affected by Jupiter's gravity. The comet's orbit was shortened
   considerably to a period of 2,380 years, and it will next return to the
   inner solar system around the year 4380. Its greatest distance from the
   sun ( aphelion) will be about 360 AU, reduced from about 525.

Scientific results

   Comet Hale-Bopp was observed intensively by astronomers during its
   perihelion passage, and several important advances in cometary science
   resulted from these observations.

Sodium tail

   Comet Hale-Bopp's neutral sodium tail (the straight tail extending up
   to the left from the nucleus)
   Enlarge
   Comet Hale-Bopp's neutral sodium tail (the straight tail extending up
   to the left from the nucleus)

   One of the most remarkable discoveries was that the comet had a third
   type of tail. In addition to the well-known gas and dust tails,
   Hale-Bopp also exhibited a faint sodium tail, only visible with
   powerful instruments with dedicated filters. Sodium emission had been
   previously observed in other comets, but had not been shown to come
   from a tail. Hale-Bopp's sodium tail consisted of neutral atoms, and
   extended to some 50 million kilometres in length.

   The source of the sodium appeared to be in the inner coma, although not
   necessarily on the nucleus. There are several possible mechanisms for
   generating a source of sodium atoms, including collisions between dust
   grains surrounding the nucleus, and 'sputtering' of sodium from dust
   grains by ultraviolet light. It is not yet established which mechanism
   is primarily responsible for creating Hale-Bopp's sodium tail.

   While the comet's dust tail roughly followed the path of the comet's
   orbit and the gas tail pointed almost directly away from the Sun, the
   sodium tail appeared to lie between the two. This implies that the
   sodium atoms are driven away from the comet's head by radiation
   pressure.

Deuterium abundance

   The abundance of deuterium in Comet Hale-Bopp in the form of heavy
   water was found to be about twice as much as that in Earth's oceans.
   This implies that, although cometary impacts are thought to be the
   source of a significant amount of the water on Earth, they cannot be
   the only source if Hale-Bopp's deuterium abundance is typical of all
   comets.

   The presence of deuterium in many other hydrogen compounds was also
   detected in the comet. The ratio of deuterium to normal hydrogen was
   found to vary from compound to compound, which astronomers believe
   suggests that cometary ices were formed in interstellar clouds, rather
   than in the solar nebula. Theoretical modelling of ice formation in
   interstellar clouds suggests that Comet Hale-Bopp formed at
   temperatures of around 25–45  Kelvin.

Organic species

   Spectroscopic observations of Hale-Bopp revealed the presence of many
   organic chemicals, several of which had never been detected in comets
   before. These complex molecules may exist within the cometary nucleus,
   or might be synthesised by reactions in the coma.

Rotation

   Comet Hale-Bopp's activity and outgassing was not spread uniformly over
   its nucleus, but instead came from several large jets from specific
   points. Observations of the material streaming away from these jets
   (see movie of spiral waves here) allowed astronomers to measure the
   rotation period of the comet, which was found to be about 11 hours
   46 minutes. Superimposed on this rotation were several periodic
   variations over several days, implying that the comet was rotating
   about more than one axis.

A satellite?

   In 1999, a paper was published that hypothesised the existence of a
   binary nucleus to fully explain the observed pattern of Comet
   Hale-Bopp's dust emission. The paper was based on theoretical analysis,
   and did not claim an observational detection of the proposed satellite
   nucleus, but estimated that it would have a diameter of about 30 km,
   with the main nucleus being about 70 km across, and would orbit in
   about three days at a distance of about 180 km.

   The findings of this paper were disputed by observational astronomers,
   as even with the high resolution available with the Hubble Space
   Telescope, images of the comet reveal no trace of a double nucleus.
   Also, while comets have been observed to break up before, no case has
   previously been found of a stable binary nucleus. Given the very small
   mass of cometary nuclei, the orbit of a binary nucleus would be easily
   disrupted by the gravity of the Sun and planets.

   Observations using adaptive optics in late 1997 and early 1998 were
   claimed to show a double peak in the brightness of the nucleus.
   However, controversy still exists over whether any observations can
   only be explained by a binary nucleus.

Paranoia and superstition

   Comet Hale-Bopp over Zabriskie Point, Death Valley, USA
   Enlarge
   Comet Hale-Bopp over Zabriskie Point, Death Valley, USA

   In many cultures, comets have historically been viewed as bad omens and
   viewed with great suspicion. Perhaps because of the very long build-up
   to Hale-Bopp's passage, and its rare size and activity, the comet
   became the subject of many bizarre beliefs and theories.

   In November 1996, amateur astronomer Chuck Shramek of Houston, Texas
   took a CCD image of the comet, which showed a fuzzy, slightly elongated
   object nearby. When his computer sky-viewing program did not identify
   the star, Shramek called the Art Bell radio program to announce that he
   had discovered a "Saturn-like object" following Hale-Bopp. UFO
   enthusiasts, such as remote viewing proponent Courtney Brown, soon
   concluded that there was an alien spacecraft following the comet. In
   fact, the object was simply an 8.5-magnitude star, SAO141894, which did
   not appear on Shramek's computer program because the user preferences
   were set incorrectly. Reportedly, Shramek refused to admit to his
   mistake when this was pointed out to him.

   Later, Art Bell even claimed to have obtained an image of the object
   from an anonymous astrophysicist who was about to confirm its
   discovery. However, astronomers Olivier Hainaut and David J. Tholen of
   the University of Hawaii stated that the alleged photo was an altered
   copy of one of their own comet images .

   A few months later, in March 1997, the cult group Heaven's Gate chose
   the appearance of the comet as a signal for their mass cult suicide.
   They claimed they were leaving their earthly bodies to travel to the
   spaceship following the comet.

Hale-Bopp's legacy

   For almost everyone who saw it, though, Hale-Bopp was simply a
   beautiful and spectacular sight in the evening skies. Its lengthy
   period of visibility and extensive coverage in the media meant that the
   comet was probably the most-observed comet in history, making a far
   greater impact on the general public than the return of Halley's Comet
   in 1986 did, and certainly being seen by a greater number of people
   than witnessed any of Halley's previous appearances. It was a
   record-breaking comet: discovered the furthest from the Sun, with the
   largest cometary nucleus known, and it was visible to the naked eye for
   twice as long as the previous record-holder. It was also brighter than
   magnitude 0 for eight weeks, longer than any other comet in the past
   thousand years.
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