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| image=[[Image:Comet Hale-Bopp.jpg|200px|Comet Hale-Bopp, shortly after passing [[perihelion]] in April 1997.]]
| image=[[Image:Comet Hale-Bopp.jpg|200px|Comet Hale-Bopp, shortly after passing [[perihelion]] in April 1997.]]
| discoverer=[[Alan Hale (astronomer)|Alan Hale]] and<br />[[Thomas Bopp]]
| discoverer=[[Alan Hale (astronomer)|Alan Hale]] and<br />[[Thomas Bopp]]
| discovery_date=[[23 July]] [[1995]]
| discovery_date=23 July 1995
| designations=The Great Comet of 1997,<br />C/1995 O1
| designations=The Great Comet of 1997,<br />C/1995 O1
| epoch=2450460.5
| epoch=2450460.5
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| period=2537 [[Julian year (astronomy)|a]]
| period=2537 [[Julian year (astronomy)|a]]
| inclination= 89.4°
| inclination= 89.4°
| last_p=[[April 1]], [[1997]]
| last_p=April 1, 1997
| next_p=4380
| next_p=4380
}}
}}


'''Comet Hale-Bopp''' ([[Astronomical naming conventions#Comets|formally designated]] '''C/1995&nbsp;O1''') was probably the most widely observed [[comet]] of the twentieth century, and one of the brightest seen for many [[decade]]s. It was visible to the [[naked eye]] for a record 18&nbsp;months, twice as long as the previous record holder, the [[Great Comet of 1811]].
'''Comet Hale-Bopp''' ([[Astronomical naming conventions#Comets|formally designated]] '''C/1995&nbsp;O1''') was arguably the most widely observed [[comet]] of the twentieth century, and one of the brightest seen for many [[decade]]s. It was visible to the [[naked eye]] for a record 18&nbsp;months, twice as long as the previous record holder, the [[Great Comet of 1811]].


Hale-Bopp was discovered on [[July 23]], [[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|Great Comet]] of 1997'''.
Hale-Bopp was discovered on July 23, 1995 at a great distance from the [[Sun]], raising expectations that the comet would brighten considerably by the time it passed close to Earth. Although predicting the brightness of comets with any degree of accuracy is very difficult, Hale-Bopp met or exceeded most predictions when it passed [[perihelion]] on April 1, 1997. The comet was dubbed '''the [[Great comet|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 [[extraterrestrial life|alien]] [[spacecraft]] gained remarkable currency, and inspired a [[mass suicide]] among followers of a [[cult]] named [[Heaven's Gate (religious group)|Heaven's Gate]].
The passage of Hale-Bopp was notable also for inciting a degree of comet-related [[panic]] not seen for decades. Rumours that an [[extraterrestrial life|alien]] [[spacecraft]] was following the comet gained remarkable currency, and inspired a [[mass suicide]] among followers of a [[cult]] named [[Heaven's Gate (religious group)|Heaven's Gate]].


== Discovery ==
== Discovery ==


The comet was discovered by two independent observers, [[Alan Hale (astronomer)|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, with an [[apparent magnitude]] of 10.5, near the [[globular cluster]] [[Globular Cluster M70|M70]], in the constellation of [[Sagittarius (constellation)|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 the sky. Once he had established that the object was moving relative to the background stars, he [[email]]ed the [[Central Bureau for Astronomical Telegrams]], the clearing house for astronomical discoveries.
The comet was discovered by two independent observers, [[Alan Hale (astronomer)|Alan Hale]] and [[Thomas Bopp]], both in the [[United States]].<ref>{{cite journal |title = The comets of 1995 | last = Shanklin |first = Jonathan D. | journal = Journal of the British Astronomical Association | year= 2000 | volume =110 |issue = 6 |pages= p. 311 |url = http://articles.adsabs.harvard.edu//full/2000JBAA..110..311S/0000311.000.html}}</ref> Hale had spent many hundreds of hours searching for comets without success, and was tracking known comets from his driveway in [[New Mexico]] when he chanced upon Hale-Bopp just after midnight. The comet had an [[apparent magnitude]] of 10.5 and lay near the [[globular cluster]] [[Globular Cluster M70|M70]] in the constellation of [[Sagittarius (constellation)|Sagittarius]].<ref name="Circ6187" /> Hale first established that there was no other [[deep-sky object]] near M70, and then consulted a directory of known comets, finding that none were known to be in this area of the sky. Once he had established that the object was moving relative to the background stars, he [[email]]ed the [[Central Bureau for Astronomical Telegrams]], the clearing house for astronomical discoveries.<ref name="TimeDiscovery">{{cite news|title=COMET OF THE DECADE, PART II| url = http://www.time.com/time/magazine/article/0,9171,986055,00.html | publisher = Time magazine | date=March 17, 1997 | first = Michael D. | last = Lemonick | accessdate=2008-10-30}}</ref>


Bopp did not own a [[telescope]]. He was out with friends near [[Stanfield, Arizona]] observing star clusters and [[galaxy|galaxies]] when he chanced across the comet while at the eyepiece of his friend's telescope. He realized 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 contacted the Central Bureau of Astronomical Telegrams via [[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&nbsp;O1. The discovery was announced in [[International Astronomical Union]] circular 6187.
Bopp did not own a [[telescope]]. He was out with friends near [[Stanfield, Arizona]] observing star clusters and [[galaxy|galaxies]] when he chanced across the comet while at the eyepiece of his friend's telescope. He realized he might have spotted something new when, like Hale, he checked his star maps to determine if any other deep-sky objects were known to be near M70, and found that there were none. He telegraphed the Central Bureau of Astronomical Telegrams, and, the following morning, it was confirmed that this was a new comet, and it was named Comet Hale-Bopp, with the designation C/1995&nbsp;O1. The discovery was announced in [[International Astronomical Union]] circular 6187.<ref name="Circ6187">{{cite web|last=Hale|first=A.|coauthors= Bopp, T.; Stevens, J.|title=IAU Circular No. 6187|publisher=[[International Astronomical Union|IAU]] |date=July 23, 1995|url=http://www.cfa.harvard.edu/iauc/06100/06187.html#Item1|accessdate=2008-10-29}}</ref><ref>{{cite journal | title = Amateur Contributions in the study of Comet Hale-Bopp | journal = Earth, Moon, and Planets | volume =79 | issue = 1&ndash;3 | year = 1997 | pages = pp. 307–308 | first =Thomas | last =Bopp | doi = 10.1023/A:1006262006364}}</ref>


==Early observation==
== Hale-Bopp becomes a Great Comet ==
Hale-Bopp's orbital position was calculated as 7.2&nbsp;[[astronomical units]] (AU) from the Sun, placing it between [[Jupiter (planet)|Jupiter]] and [[Saturn (planet)|Saturn]] and by far the greatest distance from Earth at which a comet had been discovered.<ref>{{cite journal|title=COMET C/1995 O1 (HALE-BOPP) | url=http://www.cfa.harvard.edu/mpec/J95/J95P01.html | last=Marsden | first= B. G. | year = 1995 | journal = Minor Planet Electronic Circular | volume=1995-P05}}</ref><ref>{{cite journal|title=Evolution of a Spiral Jet in the Inner Coma of Comet Hale-Bopp (1995 O1)| url=http://www.journals.uchicago.edu/doi/full/10.1086/310008 | first= M. R. | last =Kidger | coauthors=Serra-Ricart, M.; Bellot-Rubio, L. R.; Casas R. | doi=10.1086/310008 | journal = The Astrophysical Journal Letters | year=1996 | volume=461| pages=L119&ndash;L122}}</ref> Most comets at this distance are extremely faint, and show no discernible activity, but Hale-Bopp already had an observable [[coma (cometary)|coma]].<ref name="Circ6187"/> An image taken at the [[Anglo-Australian Telescope]] in 1993 was found to show the then-undiscovered comet some 13&nbsp;AU from the sun,<ref>{{cite web|last = McNaught| first = R. H. | coauthors = West, R. M.| title = Circular No. 6198| url=http://www.cfa.harvard.edu/iauc/06100/06198.html|publisher=[[International Astronomical Union|IAU]] | date = August 2, 1995 |accessdate=2008-10-29}}</ref> a distance at which most comets are essentially unobservable ([[Comet Halley|Halley's Comet]] was 50,000 times fainter at the same distance from the Sun). Analysis indicated later that its nucleus was 60&plusmn;20&nbsp;[[kilometre]]s in diameter, six times the size of Halley.<ref name="Fernandez2002" />


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.<ref name="TimeDiscovery"/>
⚫
Hale-Bopp became visible to the [[naked eye]] in May 1996, and although its rate of brightening slowed considerably during the latter half of 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 pollution|light-polluted]] skies.

== Perihelion ==

⚫
Hale-Bopp became visible to the [[naked eye]] in May 1996, and although its rate of brightening slowed considerably during the latter half of that year,<ref name="Kidger2004"/> 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 pollution|light-polluted]] skies.<ref>{{cite news|title = Comet Holds Clues to Birth of Time | url = http://query.nytimes.com/gst/fullpage.html?res=9B03E6DF1639F93AA35750C0A961958260&sec=&spon=&pagewanted=1 | last = Browne | first = Malcolm R. | date = March 9, 1997 | publisher = The New York Times}}</ref>


[[Image:Comet-Hale-Bopp-29-03-1997 hires adj.jpg|thumb|right|150px|The comet became a spectacular sight in early 1997.]]
[[Image:Comet-Hale-Bopp-29-03-1997 hires adj.jpg|thumb|right|150px|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.
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 comet Hale-Bopp.<ref>{{cite web| title= The Trail of Hale-Bopp | url= http://www.sciam.com/article.cfm?id=the-trail-of-hale-bopp | publisher= Scientific American | date= 1997-03-17 | accessdate =2008-10-23}}</ref>


As the comet approached the Sun, it continued to brighten, shining at 2nd&nbsp;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 the comet approached the Sun, it continued to brighten, shining at 2nd&nbsp;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 [[China]], [[Mongolia]] and eastern [[Siberia]] allowed observers there to see the comet in the daytime.<ref>{{cite journal |title = The total solar eclipse of 1997 March 9 | last = McGee | first= H. W. | coauthors=Poitevin, P. | journal = Journal of the British Astronomical Association | year= 1997 | volume =107 |issue = 3 |pages= p. 112&ndash;113 |url = http://adsabs.harvard.edu/abs/1997JBAA..107..112M}}</ref> Hale-Bopp had its closest approach to Earth on March 22, 1997.<ref name="stardust" />


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 [[degree (angle)|degree]]s 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 it passed perihelion on April 1, 1997, the comet developed into a spectacular sight. It shone brighter than any star in the sky except [[Sirius]], and its dust tail stretched 40–45 [[degree (angle)|degree]]s across the sky.<ref>{{cite web | url = http://www.eso.org/public/events/astro-evt/hale-bopp/comet-hale-bopp-summary-apr13-97-rw.html | title = Comet Hale-Bopp (April 13, 1997) | accessdate=2008-11-23 | last = West | first = Richard M. | authorlink = Richard Martin West | date = April 13, 1997 | publisher=European Southern Observatory}}</ref><ref>{{cite journal | first=B. S. | last= Shylaja| journal=Bulletin of the Astronomical Society of India | volume=25 | pages=p. 155&ndash;156 | year=1997 | title=What's New With Hale Bopp? | url=http://adsabs.harvard.edu/abs/1997BASI...25..155S}}</ref> 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.<ref>{{cite web | url = http://www.eso.org/public/events/astro-evt/hale-bopp/comet-hale-bopp-summary-mar07-97-rw.html | title = Comet Hale-Bopp (March 7, 1997) | accessdate=2008-11-25 | last = West | first = Richard M. | authorlink = Richard Martin West | date = March 7, 1997 | publisher=European Southern Observatory}}</ref>


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 (0.1 [[astronomical unit|AU]]), 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&nbsp;AU,<ref>{{cite web |url=http://stardust.jpl.nasa.gov/science/hb.html |title=Comet Hale-Bopp |accessdate=2008-10-09 |work=Stardust|publisher=Jet Propulsion Laboratory, NASA |date= November 26, 2003}}</ref> 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.
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 (0.1 [[astronomical unit|AU]]), 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&nbsp;AU,<ref name="stardust">{{cite web |url=http://stardust.jpl.nasa.gov/science/hb.html |title=Comet Hale-Bopp |accessdate=2008-10-09 |work=Stardust|publisher=Jet Propulsion Laboratory, NASA |date= November 26, 2003}}</ref> a distance which would have rendered many lesser comets totally invisible, Hale-Bopp still spanned half the sky{{fact|date=November 2008}} 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 perihelion==
⚫
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.<ref name="Kidger2004">{{cite journal | last = Kidger| first = M.R. | coauthors = Hurst, G; James, N. | year=2004 | journal = Earth, Moon, and Planets | volume = 78 | issue=1&ndash;3 | pages = 169&ndash;177 | title = The Visual Light Curve Of C/1995 O1 (Hale-Bopp) From Discovery To Late 1997 | url = http://www.springerlink.com/content/h72381014307x661/ | doi=10.1023/A:1006228113533}}</ref> The previous record had been set by the [[Great Comet of 1811]], which was visible to the naked eye for about 9 months.<ref name="Kidger2004"/><!-- 10.1023/A:1021545031431-->
[[Image:Laurie Larson Hale Bopp Sedona 97.jpg|thumb|right|A view of the comet from [[Sedona]], [[Arizona]]]]
⚫
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.
<!-- Unsourced image removed: [[Image:Lshb.jpg|thumb|250px|"Hale-Bopp over Lake Story"]] -->


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 (planet)|Uranus]], at a distance from Earth of about 21&nbsp;AU, but is still observable with large telescopes. Recent observations have found that it still displays a distinct tail.
The comet continued to fade as it receded, but is still being tracked by astronomers. As of January 2005, the comet is further from the Sun than [[Uranus (planet)|Uranus]], at a distance from Earth of about 21&nbsp;AU, but is still observable with large telescopes. In October 2007, 11 years after the perihelion and at distance of 25.7 AU from Sun, the comet was still active as indicated by the detection of the CO-driven coma.<ref>{{cite journal|last=Szabó |first= Gy. M.|coauthors= Kiss, L. L.; Sárneczky, K.| journal=Astrophysical Journal Letters |year=2008|volume=677|pages=p. 121 | title=Cometary Activity at 25.7 AU: Hale-Bopp 11 Years after Perihelion|doi=10.1086/588095 | url=http://adsabs.harvard.edu/abs/2008ApJ...677L.121S}}</ref>


Astronomers expect that the comet will remain observable with large telescopes until perhaps 2020, by which time it will be nearing 30th&nbsp;[[apparent magnitude|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 4377.
Astronomers expect that the comet will remain observable with large telescopes until perhaps 2020, by which time it will be nearing 30th&nbsp;[[apparent magnitude|magnitude]].<ref>{{cite web | url = http://www.eso.org/public/events/astro-evt/hale-bopp/comet-hale-bopp-summary-feb07-97-rw.html | title = Comet Hale-Bopp (February 7, 1997) | accessdate=2008-11-1 | last = West | first = Richard M. | authorlink = Richard Martin West | date = February 7, 1997 | publisher=European Southern Observatory}}</ref> By this time it will become very difficult to distinguish the comet from the large numbers of distant galaxies of similar brightness.


== Orbital changes ==
== Orbital changes ==
The comet likely made its last [[perihelion]] 4,200&nbsp;years ago.<ref name=perturb>{{cite web
⚫
The comet probably made its last [[perihelion]] 4,200&nbsp;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 1996 the comet passed within 0.77&nbsp;AU of [[Jupiter (planet)|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&nbsp;years, and it will next return to the inner solar system around the year 4377. Its greatest distance from the sun ([[aphelion]]) will be about 360&nbsp;AU, reduced from about 525 AU.
|date=1997-04-10
|title=Comet Hale-Bopp Orbit and Ephemeris Information
|publisher=JPL/NASA
|author=Don Yeomans
|url=http://www2.jpl.nasa.gov/comet/ephemjpl8.html
⚫
|accessdate=2008-10-23}}</ref> Its orbit is almost [[perpendicular]] to the plane of the [[ecliptic]], which ensures that close approaches to planets are rare. However, in April 1996 the comet passed within 0.77&nbsp;AU of [[Jupiter (planet)|Jupiter]], close enough for its orbit to be affected by the planet's [[gravity]].<ref name=perturb/> The comet's orbit was shortened considerably to a period of roughly 2,380&nbsp;years, and it will next return to the inner solar system around the year 4377. Its greatest distance from the sun ([[aphelion]]) will be about 370&nbsp;AU, reduced from about 525 AU.<ref>{{cite journal | title=Orbit Determination and Evolution of Comet C/1995 O1 (Hale-Bopp) | url=http://adsabs.harvard.edu/abs/1997EM%26P...79....3M | last=Marsden | first=B. G. | journal=Earth, Moon, and Planets | volume=79 | issue=1 | page=p. 3&ndash;15 | year=1997| doi=10.1023/A:1006268813208}}</ref>


== Scientific results ==
== Scientific results ==
[[Image:Hale-Bopp sodium tail.gif|thumb|left|200px|Comet Hale-Bopp's neutral sodium tail (the straight tail extending up to the left from the nucleus)<ref name="Cremonese1997" />]]
[[Image:Hale-Bopp Companion.jpg|thumb|right|150px|Hubble Telescope image, Late October]]

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


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 [[electric charge|neutral atoms]], and extended to some 50 million kilometres in length.
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 [[electric charge|neutral atoms]], and extended to some 50 million kilometres in length.<ref name="Cremonese1997">{{cite journal | journal = The Astrophysical Journal Letters | volume = 490 | pages =L199–L202 | year = 1997 | doi = 10.1086/311040 | title = Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail| author = G. Cremonese, H. Boehnhardt, J. Crovisier, H. Rauer, A. Fitzsimmons, M. Fulle, J. Licandro, D. Pollacco, G. P. Tozzi, and R. M. West}}</ref>


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.
The source of the sodium appeared to be the inner coma, although not necessarily 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, and the narrow<ref name="Cremonese1997" /> and diffuse<ref>{{cite journal | last=Wilson |first=J. K.| coauthors= Baumgardner, J.; Mendillo, M.| title=Three tails of comet Hale-Bopp | url = http://adsabs.harvard.edu/abs/1998GeoRL..25..225W | journal=Geophysical Research Letters | volume=25 | issue=3 | pages=p. 225&ndash;228 | doi=10.1029/97GL03704}}</ref> components of the tail may have different origins.<ref>{{cite journal | first=G. | last=Cremonese | coauthors=Fulle, M. | title=Sodium In Comets | url=http://adsabs.harvard.edu/abs/1997EM%26P...79..209C | journal=Earth, Moon, and Planets | volume=79 | issue=1 | pages=p. 209-220 | year=1997 | doi=10.1023/A:1006245619568}}</ref>


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]].
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]].<ref name="Cremonese1997" />


=== Deuterium abundance ===
=== 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 [[ocean]]s. 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 abundance of [[deuterium]] in Comet Hale-Bopp in the form of [[heavy water]] was found to be about twice that of Earth's [[ocean]]s. 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.<ref name="Cometwater">{{cite journal | title = Cometary Deuterium | journal = Space Science Reviews | volume = 90 | issue = 1&ndash;2 | year = 1999 | doi = 10.1023/A:1005269208310 | pages = 33&ndash;43 | first = Roland | last = Meier | coauthors = Owen, Tobias C.}}</ref>


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 cloud]]s, 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&nbsp;[[kelvin]].
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 cloud]]s, 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&nbsp;[[kelvin]].<ref name="Cometwater"/>


=== Organic species ===
=== Organics ===


Spectroscopic observations of Hale-Bopp revealed the presence of many [[organic chemistry|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 comet.
Spectroscopic observations of Hale-Bopp revealed the presence of many [[organic chemistry|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 comet.<ref name="Cometorganic">{{cite journal | title = Organic synthesis in the coma of Comet Hale–Bopp?| journal = Monthly Notices of the Royal Astronomical Society| volume = 320 | issue = 4 | year = 2002| doi = 10.1046/j.1365-8711.2001.04208.x | pages = L61&ndash;L64 | first = S. D. | last = Rodgers| coauthors = Charnley, S. B.}}</ref>


=== Rotation ===
=== Rotation ===
⚫
[[Image:Comet Hale-Bopp Death Valley.jpg|thumb|right|150px|Comet Hale-Bopp over [[Zabriskie Point]], [[Death Valley]], USA]]


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,<ref>{{cite web |url=http://www2.jpl.nasa.gov/comet/anim30.html |title=Bergeron Comet Hale-Bopp Animation |accessdate=2008-10-14 |work=Stardust|publisher=Jet Propulsion Laboratory, NASA}}</ref> 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 of rotation|axis]].
Comet Hale-Bopp's activity and outgassing were not spread uniformly over its nucleus, but instead came from several specific jets. Observations of the material streaming away from these jets,<ref>{{cite web |url=http://www2.jpl.nasa.gov/comet/anim30.html |title=Bergeron Comet Hale-Bopp Animation |accessdate=2008-10-14 |work=Stardust|publisher=Jet Propulsion Laboratory, NASA}}</ref> 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 of rotation|axis]].<ref>{{cite journal | journal = The Astrophysical Journal Letters |volume = 501 |pages =L221&ndash;L225 | year = 1998 | doi = 10.1086/311465 | title = The Rotation Period of C/1995 O1 (Hale-Bopp)
| author = Javier Licandro, Luis R. Bellot Rubio, Hermann Boehnhardt, Ricard Casas, Benedikt Göetz, Angel Gómez, Laurent Jorda, Mark R. Kidger, David Osip, Nanci Sabalisck, Pablo Santos, Miquel Serra-Ricart,Gian Paolo Tozzi,Richard West}}</ref>


=== A satellite dispute ===
===Satellite dispute ===


In 1999, a paper was published that hypothesised the existence of a binary [[comet nucleus|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&nbsp;km, with the main nucleus being about 70&nbsp;km across, and would orbit in about three days at a distance of about 180&nbsp;km.
In 1999, a paper was published that hypothesised the existence of a binary [[comet nucleus|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&nbsp;km, with the main nucleus being about 70&nbsp;km across, and would orbit in about three days at a distance of about 180&nbsp;km.<ref>{{cite journal | title = Detection of a Satellite Orbiting The Nucleus of Comet Hale–Bopp (C/1995 O1) | journal = Earth, Moon, and Planets | volume = 77 | issue = 3 | year = 1997 | doi = 10.1023/A:1006230712665 | pages = 155&ndash;163
| first = Z. | last = Sekanina}}</ref>


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 [[planet]]s.
The findings of this paper were disputed by observational astronomers, as even with the [[Hubble Space Telescope]] images of the comet revealed no trace of a double nucleus.<ref>{{cite journal| last=McCarthy | first=D. W. |coauthors=Stolovy, S. R.; Campins, H.; Larson, S.; Samarasinha, N. H.; Kern, S. D. | title=Comet Hale–Bopp in outburst: Imaging the dynamics of icy particles with HST/NICMOS | url=http://adsabs.harvard.edu/abs/2007Icar..189..184M | year=2007 | journal=Icarus | volume = 189 | issue = 1 | pages = p. 184&ndash;195| doi=10.1016/j.icarus.2007.01.019}}</ref><ref>{{cite journal | last=Weaver | first=H. A.| coauthors= Feldman, P. D.; A'Hearn, M. F.; Arpigny, C.; Brandt, J. C.; Stern, S. A.| title = Post-Perihelion HST Observations of Comet Hale-Bopp (C/1995 O1) | url=http://adsabs.harvard.edu/abs/1999Icar..141....1W | year=1999 | journal=Icarus | volume=141 | issue =1 | doi=10.1006/icar.1999.6159 | pages=p. 1&ndash;12}}</ref> Also, while comets have been observed to break up before,<ref>{{cite journal | last=Sekanina | first = Z. | title = The problem of split comets revisited | url = http://adsabs.harvard.edu/abs/1997A%26A...318L...5S | journal = Astronomy and Astrophysics Letters | year = 1997 | volume = 318 | pages = p. L5&ndash;L8 }}</ref> 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 [[planet]]s.


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.
Observations using [[adaptive optics]] in late 1997 and early 1998 were claimed to show a double peak in the brightness of the nucleus.<ref>{{cite journal | last = Marchis | first = F. |coauthors = Boehnhardt, H.; Hainaut, O. R.; Le Mignant, D. | journal = Astronomy & Astrophysics | year = 1999 |
title = Adaptive optics observations of the innermost coma of C/1995 O1. Are there a "Hale" and a "Bopp" in comet Hale-Bopp? | url = http://aa.springer.de/papers/9349003/2300985.pdf | volume = 349 | pages = 985&ndash;995}}</ref> However, controversy still exists over whether such observations can only be explained by a binary nucleus.<ref name="Fernandez2002" />


== Unexplained ==
== UFO claims ==
⚫
In November 1996, amateur astronomer [[Chuck Shramek]] of [[Houston, Texas]] took a [[charge-coupled device|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. [[unidentified flying object|UFO]] enthusiasts, such as [[remote viewing]] proponent [[Courtney Brown (researcher)|Courtney Brown]], soon concluded that there was an [[extraterrestrial life|alien]] [[spacecraft]] following the comet.<ref name="time">{{cite news|title=THE MAN WHO SPREAD THE MYTH | url = http://www.time.com/time/magazine/article/0,9171,986171,00.html | publisher = Time magazine | date=April 14, 1997 | first = Leon | last = Jaroff | coauthors = Willwerth, James | accessdate=2008-10-30}}</ref> Some astronomers claimed 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.<ref>{{cite book|title=Great Comets|first=Robert|last= Burnham|coauthors= Levy, David H. |year=2000 | publisher=Cambridge University Press | isbn=9780521646000 | pages=p. 191}}</ref>


⚫
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.<ref>{{cite web |date=January 15, 1997 | title=Fraudulent use of a IfA/UH picture |url=http://www.sc.eso.org/~ohainaut/Hale_Bopp/hb_ufo_tholen.html | publisher=European Organisation for Astronomical Research in the Southern Hemisphere |accessdate=2008-10-14| first = David J. | last = Tholen}}</ref>
⚫
[[Image:Comet Hale-Bopp Death Valley.jpg|thumb|right|150px|Comet Hale-Bopp over [[Zabriskie Point]], [[Death Valley]], USA]]
In many cultures, comets have historically been regarded as bad [[omen]]s and viewed with great suspicion. Perhaps because of the very long build-up to Hale-Bopp's passage, its rare size and activity, and [[millenarianism]] linked to worries over the upcoming [[Y2K]], the comet became the subject of many bizarre beliefs and theories.


⚫
A few months later, in March 1997, the cult group [[Heaven's Gate (religious 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.<ref>{{cite journal | title = Heaven's Gate: The End | first = Wendy Gale | last = Robinson | journal = Journal of Computer-Mediated Communication | volume = 3 | issue = 3 | pages =0–0 | doi = 10.1111/j.1083-6101.1997.tb00077.x | doi_brokendate = 2008-11-16 }}</ref>
=== Companion ===


==Legacy ==
⚫
In November 1996, amateur astronomer [[Chuck Shramek]] of [[Houston, Texas]] took a [[charge-coupled device|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. [[unidentified flying object|UFO]] enthusiasts, such as [[remote viewing]] proponent [[Courtney Brown (researcher)|Courtney Brown]], soon concluded that there was an [[extraterrestrial life|alien]] [[spacecraft]] following the comet. Some professionals in the astronomy community claimed 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. There was indeed a star thereabouts, but nothing of the magnitude of the object, as seen in one frame of Chuck's photos of the comet Hale-Bopp{{Fact|date=July 2008}}. Additionally, they claimed the "spires" of light which seem to extend from the object at a 45 degree angle were "diffractions" of light, and offered a few images of stars showing a similar effect (though the image provided had four spires of diffracted light, not two){{Fact|date=July 2008}}.


⚫
Its lengthy period of visibility and extensive coverage in the media meant that Hale-Bopp was probably the most-observed comet in history, making a far greater impact on the general public than the return of [[Comet Halley|Halley's Comet]] in 1986, and certainly seen by a greater number of people than witnessed any of Halley's previous appearances. It was a record-breaking comet&nbsp;&mdash; the furthest comet from the Sun discovered by amateurs,<ref name="stardust" /> with the largest cometary nucleus known after [[95P/Chiron]],<ref name="Fernandez2002">{{cite journal | last = Fernández | first=Yanga R. | title = The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity | url = http://www.springerlink.com/content/pr2756540kx94403/ | year = 2002 | journal = Earth, Moon, and Planets | volume = 89 | issue=1 | pages = 3&ndash;25| doi = 10.1023/A:1021545031431}}</ref> and it was visible to the naked eye for twice as long as the previous record-holder.<ref name="Kidger2004"/> It was also brighter than magnitude&nbsp;0 for eight&nbsp;weeks, longer than any other recorded comet.<ref name="stardust" />
⚫
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.<ref>{{cite web |date=January 15, 1997 | title=Fraudulent use of a IfA/UH picture |url=http://www.sc.eso.org/~ohainaut/Hale_Bopp/hb_ufo_tholen.html | publisher=European Organisation for Astronomical Research in the Southern Hemisphere |accessdate=2008-10-14}}</ref>


== Notes ==
⚫
A few months later, in March 1997, the cult group [[Heaven's Gate (religious 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.
⚫
{{Reflist|2}}

{{See also|Courtney Brown (researcher)#Hale-Bopp on Art Bell}}

== Hale-Bopp's legacy ==

⚫
For almost everyone who saw it, 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 [[Comet Halley|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&nbsp;&mdash; 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&nbsp;0 for eight&nbsp;weeks, longer than any other comet in the last thousand years.

== Footnotes ==
⚫
{{Reflist}}


== References ==
== References ==
Line 113: Line 119:
* Cremonese G., Boehnhardt H., Crovisier J. ''et al'', ''Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail'', Astrophysical Journal Letters, v. 490, p. L199
* Cremonese G., Boehnhardt H., Crovisier J. ''et al'', ''Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail'', Astrophysical Journal Letters, v. 490, p. L199
* Hale, A., & Bopp, T. 1995, IAU Circular, 6187
* Hale, A., & Bopp, T. 1995, IAU Circular, 6187
* Marchis F., Boehnhardt H., Hainaut O.R., Le Mignant D. (1999), PIMP ''Adaptive optics observations of the innermost coma of C/1995 O1. Are there a "Hale" and a "Bopp" in comet Hale-Bopp?'', Astronomy and Astrophysics, v.349, p.985
* Marchis F., Boehnhardt H., Hainaut O.R., Le Mignant D. (1999), ''Adaptive optics observations of the innermost coma of C/1995 O1. Are there a "Hale" and a "Bopp" in comet Hale-Bopp?'', Astronomy and Astrophysics, v.349, p.985
* Newcott, William R. (Dec. 1997). "The age of comets". ''National Geographic'',p. 100.
* Newcott, William R. (Dec. 1997). "The age of comets". ''National Geographic'',p. 100.
* Rodgers S.D., Charnley S.B. (2001), ''Organic synthesis in the coma of Comet Hale-Bopp?'', Monthly Notices of the Royal Astronomical Society, v. 320, p. L61-L64.
* Rodgers S.D., Charnley S.B. (2001), ''Organic synthesis in the coma of Comet Hale-Bopp?'', Monthly Notices of the Royal Astronomical Society, v. 320, p. L61-L64.
* Sekanina Z. (1999), ''Detection of a Satellite Orbiting The Nucleus of Comet Hale-Bopp (C/1995 O1)'', Earth, Moon, and Planets, v.77, p.155
* Sekanina Z. (1999), ''Detection of a Satellite Orbiting The Nucleus of Comet Hale-Bopp (C/1995 O1)'', Earth, Moon, and Planets, v.77, p.155
* Warell J., Lagerkvist C.-I., Lagerros J.S.V. (1999), ''Dust continuum imaging of C/1995 O1 (Hale-Bopp):. Rotation period and dust outflow velocity'', Astronomy and Astrophysics Supplement, v.136, p.245
* Warell J., Lagerkvist C.-I., Lagerros J.S.V. (1999), ''Dust continuum imaging of C/1995 O1 (Hale-Bopp):. Rotation period and dust outflow velocity'', Astronomy and Astrophysics Supplement, v.136, p.245
* Yeomans, Don. (1997) ''[http://eiger.physics.uiowa.edu/~vis/hale-bopp-ephem.html Orbit and Ephemeris Information for Comet Hale-Bopp (1995 O1)]''. Retrieved [[February 24]], [[2005]].
* Yeomans, Don. (1997) ''[http://eiger.physics.uiowa.edu/~vis/hale-bopp-ephem.html Orbit and Ephemeris Information for Comet Hale-Bopp (1995 O1)]''. Retrieved February 24, 2005.


== External links ==
== External links ==
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Revision as of 20:02, 25 November 2008

C/1995 O1 (Hale-Bopp)
Comet Hale-Bopp, shortly after passing perihelion in April 1997.
Discovery
Discovered byAlan Hale and
Thomas Bopp
Discovery date23 July 1995
Designations
The Great Comet of 1997,
C/1995 O1
Orbital characteristics
Epoch2450460.5
Aphelion371 AU
Perihelion0.91 AU
Semi-major axis186 AU
Eccentricity0.995086
Orbital period2537 a
Inclination89.4°
Last perihelionApril 1, 1997
Next perihelion4380

Comet Hale-Bopp (formally designated C/1995 O1) was arguably the most widely observed comet of the twentieth 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 July 23, 1995 at a great distance from the Sun, raising expectations that the comet would brighten considerably by the time it passed close to Earth. Although predicting the brightness of comets with any degree of accuracy is very difficult, Hale-Bopp met or exceeded most predictions 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 comet-related panic not seen for decades. Rumours that an alien spacecraft was following the comet gained remarkable currency, and inspired a mass suicide among followers of a cult named Heaven's Gate.

Discovery

The comet was discovered by two independent observers, Alan Hale and Thomas Bopp, both in the United States.[1] Hale had spent many hundreds of hours searching for comets without success, and was tracking known comets from his driveway in New Mexico when he chanced upon Hale-Bopp just after midnight. The comet had an apparent magnitude of 10.5 and lay near the globular cluster M70 in the constellation of Sagittarius.[2] Hale first established that there was no other deep-sky object near M70, and then consulted a directory of known comets, finding that none were known to be in this area of the sky. Once he had established that the object was moving relative to the background stars, he emailed the Central Bureau for Astronomical Telegrams, the clearing house for astronomical discoveries.[3]

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 realized he might have spotted something new when, like Hale, he checked his star maps to determine if any other deep-sky objects were known to be near M70, and found that there were none. He telegraphed the Central Bureau of Astronomical Telegrams, and, 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.[2][4]

Early observation

Hale-Bopp's orbital position was calculated as 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.[5][6] Most comets at this distance are extremely faint, and show no discernible activity, but Hale-Bopp already had an observable coma.[2] An image taken at the Anglo-Australian Telescope in 1993 was found to show the then-undiscovered comet some 13 AU from the sun,[7] 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 later that its nucleus was 60±20 kilometres in diameter, six times the size of Halley.[8]

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.[3]

Perihelion

Hale-Bopp became visible to the naked eye in May 1996, and although its rate of brightening slowed considerably during the latter half of that year,[9] 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.[10]

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 comet Hale-Bopp.[11]

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 China, Mongolia and eastern Siberia allowed observers there to see the comet in the daytime.[12] Hale-Bopp had its closest approach to Earth on March 22, 1997.[13]

As it passed perihelion on April 1, 1997, the comet developed into a spectacular sight. It shone brighter than any star in the sky except Sirius, and its dust tail stretched 40–45 degrees across the sky.[14][15] 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.[16]

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 (0.1 AU), 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,[13] a distance which would have rendered many lesser comets totally invisible, Hale-Bopp still spanned half the sky[citation needed] with its two tails, although the longest reaches of the tails were too faint to be visible to the naked eye.

After perihelion

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.[9] The previous record had been set by the Great Comet of 1811, which was visible to the naked eye for about 9 months.[9]

The comet continued to fade as it receded, but 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. In October 2007, 11 years after the perihelion and at distance of 25.7 AU from Sun, the comet was still active as indicated by the detection of the CO-driven coma.[17]

Astronomers expect that the comet will remain observable with large telescopes until perhaps 2020, by which time it will be nearing 30th magnitude.[18] By this time it will become very difficult to distinguish the comet from the large numbers of distant galaxies of similar brightness.

Orbital changes

The comet likely made its last perihelion 4,200 years ago.[19] Its orbit is almost perpendicular to the plane of the ecliptic, which ensures that close approaches to planets are rare. However, in April 1996 the comet passed within 0.77 AU of Jupiter, close enough for its orbit to be affected by the planet's gravity.[19] The comet's orbit was shortened considerably to a period of roughly 2,380 years, and it will next return to the inner solar system around the year 4377. Its greatest distance from the sun (aphelion) will be about 370 AU, reduced from about 525 AU.[20]

Scientific results

Comet Hale-Bopp's neutral sodium tail (the straight tail extending up to the left from the nucleus)[21]

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

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.[21]

The source of the sodium appeared to be the inner coma, although not necessarily 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, and the narrow[21] and diffuse[22] components of the tail may have different origins.[23]

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.[21]

Deuterium abundance

The abundance of deuterium in Comet Hale-Bopp in the form of heavy water was found to be about twice that of 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.[24]

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.[24]

Organics

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 comet.[25]

Rotation

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

Comet Hale-Bopp's activity and outgassing were not spread uniformly over its nucleus, but instead came from several specific jets. Observations of the material streaming away from these jets,[26] 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.[27]

Satellite dispute

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.[28]

The findings of this paper were disputed by observational astronomers, as even with the Hubble Space Telescope images of the comet revealed no trace of a double nucleus.[29][30] Also, while comets have been observed to break up before,[31] 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.[32] However, controversy still exists over whether such observations can only be explained by a binary nucleus.[8]

UFO claims

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.[33] Some astronomers claimed 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.[34]

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.[35]

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.[36]

Legacy

Its lengthy period of visibility and extensive coverage in the media meant that Hale-Bopp 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, and certainly seen by a greater number of people than witnessed any of Halley's previous appearances. It was a record-breaking comet — the furthest comet from the Sun discovered by amateurs,[13] with the largest cometary nucleus known after 95P/Chiron,[8] and it was visible to the naked eye for twice as long as the previous record-holder.[9] It was also brighter than magnitude 0 for eight weeks, longer than any other recorded comet.[13]

Notes

  1. ^ Shanklin, Jonathan D. (2000). "The comets of 1995". Journal of the British Astronomical Association. 110 (6): p. 311. {{cite journal}}: |pages= has extra text (help)
  2. ^ a b c Hale, A. (July 23, 1995). "IAU Circular No. 6187". IAU. Retrieved 2008-10-29. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ a b Lemonick, Michael D. (March 17, 1997). "COMET OF THE DECADE, PART II". Time magazine. Retrieved 2008-10-30.
  4. ^ Bopp, Thomas (1997). "Amateur Contributions in the study of Comet Hale-Bopp". Earth, Moon, and Planets. 79 (1–3): pp. 307–308. doi:10.1023/A:1006262006364. {{cite journal}}: |pages= has extra text (help)
  5. ^ Marsden, B. G. (1995). "COMET C/1995 O1 (HALE-BOPP)". Minor Planet Electronic Circular. 1995-P05.
  6. ^ Kidger, M. R. (1996). "Evolution of a Spiral Jet in the Inner Coma of Comet Hale-Bopp (1995 O1)". The Astrophysical Journal Letters. 461: L119–L122. doi:10.1086/310008. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ McNaught, R. H. (August 2, 1995). "Circular No. 6198". IAU. Retrieved 2008-10-29. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ a b c Fernández, Yanga R. (2002). "The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity". Earth, Moon, and Planets. 89 (1): 3–25. doi:10.1023/A:1021545031431.
  9. ^ a b c d Kidger, M.R. (2004). "The Visual Light Curve Of C/1995 O1 (Hale-Bopp) From Discovery To Late 1997". Earth, Moon, and Planets. 78 (1–3): 169–177. doi:10.1023/A:1006228113533. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ Browne, Malcolm R. (March 9, 1997). "Comet Holds Clues to Birth of Time". The New York Times.
  11. ^ "The Trail of Hale-Bopp". Scientific American. 1997-03-17. Retrieved 2008-10-23.
  12. ^ McGee, H. W. (1997). "The total solar eclipse of 1997 March 9". Journal of the British Astronomical Association. 107 (3): p. 112–113. {{cite journal}}: |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  13. ^ a b c d "Comet Hale-Bopp". Stardust. Jet Propulsion Laboratory, NASA. November 26, 2003. Retrieved 2008-10-09.
  14. ^ West, Richard M. (April 13, 1997). "Comet Hale-Bopp (April 13, 1997)". European Southern Observatory. Retrieved 2008-11-23.
  15. ^ Shylaja, B. S. (1997). "What's New With Hale Bopp?". Bulletin of the Astronomical Society of India. 25: p. 155–156. {{cite journal}}: |pages= has extra text (help)
  16. ^ West, Richard M. (March 7, 1997). "Comet Hale-Bopp (March 7, 1997)". European Southern Observatory. Retrieved 2008-11-25.
  17. ^ Szabó, Gy. M. (2008). "Cometary Activity at 25.7 AU: Hale-Bopp 11 Years after Perihelion". Astrophysical Journal Letters. 677: p. 121. doi:10.1086/588095. {{cite journal}}: |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. ^ West, Richard M. (February 7, 1997). "Comet Hale-Bopp (February 7, 1997)". European Southern Observatory. Retrieved 2008-11-1. {{cite web}}: Check date values in: |accessdate= (help)
  19. ^ a b Don Yeomans (1997-04-10). "Comet Hale-Bopp Orbit and Ephemeris Information". JPL/NASA. Retrieved 2008-10-23.
  20. ^ Marsden, B. G. (1997). "Orbit Determination and Evolution of Comet C/1995 O1 (Hale-Bopp)". Earth, Moon, and Planets. 79 (1): p. 3–15. doi:10.1023/A:1006268813208. {{cite journal}}: |page= has extra text (help)
  21. ^ a b c d G. Cremonese, H. Boehnhardt, J. Crovisier, H. Rauer, A. Fitzsimmons, M. Fulle, J. Licandro, D. Pollacco, G. P. Tozzi, and R. M. West (1997). "Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail". The Astrophysical Journal Letters. 490: L199–L202. doi:10.1086/311040.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. ^ Wilson, J. K. "Three tails of comet Hale-Bopp". Geophysical Research Letters. 25 (3): p. 225–228. doi:10.1029/97GL03704. {{cite journal}}: |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ Cremonese, G. (1997). "Sodium In Comets". Earth, Moon, and Planets. 79 (1): p. 209-220. doi:10.1023/A:1006245619568. {{cite journal}}: |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ a b Meier, Roland (1999). "Cometary Deuterium". Space Science Reviews. 90 (1–2): 33–43. doi:10.1023/A:1005269208310. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  25. ^ Rodgers, S. D. (2002). "Organic synthesis in the coma of Comet Hale–Bopp?". Monthly Notices of the Royal Astronomical Society. 320 (4): L61–L64. doi:10.1046/j.1365-8711.2001.04208.x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  26. ^ "Bergeron Comet Hale-Bopp Animation". Stardust. Jet Propulsion Laboratory, NASA. Retrieved 2008-10-14.
  27. ^ Javier Licandro, Luis R. Bellot Rubio, Hermann Boehnhardt, Ricard Casas, Benedikt Göetz, Angel Gómez, Laurent Jorda, Mark R. Kidger, David Osip, Nanci Sabalisck, Pablo Santos, Miquel Serra-Ricart,Gian Paolo Tozzi,Richard West (1998). "The Rotation Period of C/1995 O1 (Hale-Bopp)". The Astrophysical Journal Letters. 501: L221–L225. doi:10.1086/311465.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  28. ^ Sekanina, Z. (1997). "Detection of a Satellite Orbiting The Nucleus of Comet Hale–Bopp (C/1995 O1)". Earth, Moon, and Planets. 77 (3): 155–163. doi:10.1023/A:1006230712665.
  29. ^ McCarthy, D. W. (2007). "Comet Hale–Bopp in outburst: Imaging the dynamics of icy particles with HST/NICMOS". Icarus. 189 (1): p. 184–195. doi:10.1016/j.icarus.2007.01.019. {{cite journal}}: |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  30. ^ Weaver, H. A. (1999). "Post-Perihelion HST Observations of Comet Hale-Bopp (C/1995 O1)". Icarus. 141 (1): p. 1–12. doi:10.1006/icar.1999.6159. {{cite journal}}: |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  31. ^ Sekanina, Z. (1997). "The problem of split comets revisited". Astronomy and Astrophysics Letters. 318: p. L5–L8. {{cite journal}}: |pages= has extra text (help)
  32. ^ Marchis, F. (1999). "Adaptive optics observations of the innermost coma of C/1995 O1. Are there a "Hale" and a "Bopp" in comet Hale-Bopp?" (PDF). Astronomy & Astrophysics. 349: 985–995. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  33. ^ Jaroff, Leon (April 14, 1997). "THE MAN WHO SPREAD THE MYTH". Time magazine. Retrieved 2008-10-30. {{cite news}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  34. ^ Burnham, Robert (2000). Great Comets. Cambridge University Press. pp. p. 191. ISBN 9780521646000. {{cite book}}: |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  35. ^ Tholen, David J. (January 15, 1997). "Fraudulent use of a IfA/UH picture". European Organisation for Astronomical Research in the Southern Hemisphere. Retrieved 2008-10-14.
  36. ^ Robinson, Wendy Gale. "Heaven's Gate: The End". Journal of Computer-Mediated Communication. 3 (3): 0–0. doi:10.1111/j.1083-6101.1997.tb00077.x. {{cite journal}}: Unknown parameter |doi_brokendate= ignored (|doi-broken-date= suggested) (help)

References

  • Cremonese G., Boehnhardt H., Crovisier J. et al, Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail, Astrophysical Journal Letters, v. 490, p. L199
  • Hale, A., & Bopp, T. 1995, IAU Circular, 6187
  • Marchis F., Boehnhardt H., Hainaut O.R., Le Mignant D. (1999), Adaptive optics observations of the innermost coma of C/1995 O1. Are there a "Hale" and a "Bopp" in comet Hale-Bopp?, Astronomy and Astrophysics, v.349, p.985
  • Newcott, William R. (Dec. 1997). "The age of comets". National Geographic,p. 100.
  • Rodgers S.D., Charnley S.B. (2001), Organic synthesis in the coma of Comet Hale-Bopp?, Monthly Notices of the Royal Astronomical Society, v. 320, p. L61-L64.
  • Sekanina Z. (1999), Detection of a Satellite Orbiting The Nucleus of Comet Hale-Bopp (C/1995 O1), Earth, Moon, and Planets, v.77, p.155
  • Warell J., Lagerkvist C.-I., Lagerros J.S.V. (1999), Dust continuum imaging of C/1995 O1 (Hale-Bopp):. Rotation period and dust outflow velocity, Astronomy and Astrophysics Supplement, v.136, p.245
  • Yeomans, Don. (1997) Orbit and Ephemeris Information for Comet Hale-Bopp (1995 O1). Retrieved February 24, 2005.

External links

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