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Hale-Bopp:

The Great Comet of 1997

by Martin J Powell


"Independent reports of the visual discoveries of a new comet have been received from Alan Hale and Thomas Bopp.

All observers note the comet to be diffuse with some condensation and no tail, motion towards the west-northwest." 

- 'British Astronomical Association' Circular, 9th August 1995

 

 In June 1995 the American magazine Sky & Telescope published an article which discussed the likelihood of seeing unusual astronomical events such as meteor showers, supernovae and bright comets.1  A spectacular comet, it said, comes along typically every ten years or so.  Astronomers had long been aware that, as we approached the end of the century (and indeed, the end of the millennium) we were long overdue for a really bright naked eye comet.  The last bright comet that had attracted attention in recent times had been Comet West in 1976.  Compared to the large number of bright comets that had been seen during the 19th century (some could even be seen in daylight) the 20th century had fared rather poorly.

Sketch of the telescopic view of Comet Hale-Bopp on 9th June 1996

Author's sketch of the telescopic view of Comet Hale-Bopp as it appeared on 9th June 1996 at 0145 UT, at a magnification of 81x. The eyepiece field of view is 36.7 arcminutes.

The article also referred to the tendency for more than one bright, naked eye comet to appear over a short interval of time.  However, from a statistical viewpoint, the current dearth of comets was "unusual, but not exceedingly so."

The article could not have been more timely. As if by some Divine response, only a month after the Sky & Telescope article had gone into print, two amateur astronomers in America, Alan Hale and Thomas Bopp, were independently observing the globular cluster M70 through their telescopes when they spotted a fuzzy object in the same field of view. The discovery having been confirmed as a new comet, it was subsequently designated  C/1995 O1.  What was most notable about it was that, for its computed distance - 663 million miles (1067 million km) - it was remarkably bright.  Most comets at this distance (well beyond the orbit of Jupiter) would be very faint.  Hale-Bopp was a thousand times brighter than Comet Halley would be at the same distance. Either Hale-Bopp was in a state of outburst (only bright temporarily) or it was intrinsically bright (with a large nucleus).  No previous comet had ever been discovered by amateurs at such a great distance.  The comet was predicted to enter the inner solar system, putting on its best show, around the Spring time of 1997, almost two years hence.  There would be a long and anxious wait to see if Hale-Bopp would live up to expectations.

 

But there was another surprise in store.  Six months later, on 30th January 1996 the Japanese amateur Yuji Hyakutake discovered a comet through his giant pair of binoculars whilst on a regular mountain-top comet hunt.  It turned out that this comet would approach the Earth very closely and would likely provide a spectacle for both astronomers and the public alike. Comet Hyakutake did not disappoint.  It streaked past the Earth in the spring of 1996, becoming easily visible with the naked eye and displaying a magnificently long tail.  Unfortunately for observers in Britain, the weather during this time was not at all favourable, and most people missed out on the spectacle.  All too soon, Hyakutake was gone.  It seemed that perhaps the great comet we had been waiting for had arrived at short notice, and had disappeared just as quickly.

At the time of Hyakutake's passage, Hale-Bopp was around 8th magnitude - easily visible in telescopes and binoculars.  For British observers, however, Hale-Bopp was rather low down in the sky and often difficult to detect through the atmospheric haze and light pollution.  It was slowly moving Northwards, however, and seemingly brightening all the time.

The following account includes a selection of descriptions taken from my astronomical notebook.  I observed the comet on 33 separate occasions between 1996 and 1997, using three instruments: an 8 inch (200mm) Schmidt-Cassegrain reflecting telescope (i.e. having a mirror 8 inches across), a pair of 10x40 binoculars (i.e. 10x magnification with lenses 40mm across), and of course, the naked eye.  All except one of my observations took place from my home city of Cardiff in Wales, United Kingdom.

All times are given in Universal Time (UT) which is equivalent to GMT and is used as a world-wide standard for astronomical purposes.  Explanations of various astronomical terms used in this account will be found in the glossary by clicking on the underlined terms.

Sketch of the telescopic view of Comet Hale-Bopp on 7th July 1996

The telescopic view on 7th July 1996 at 0000 UT, at a magnification of 81x. The field of view is 36'.7.

My first observation of the comet took place almost a year before it became a public spectacle.  Moving towards the inner solar system at 302 million miles distant (between the orbits of Mars and Jupiter) it had not yet developed a tail but its nucleus was already surrounded by a cloud of gas and dust.

9 June 1996

0145 Comet is diffuse with a somewhat stellar nucleus, positioned at the SSW end of an oval-shaped coma. The best views of the comet were obtained at magnifications of 81x and 120x.

7 July 1996

0000 Comet at opposition. Detected easily under clear skies.  A bright condensed nucleus at the southern end of a halo orientated ca. North-South. The coma appears to be more extensive than on 9 June, using an LPR filter, and is fan-shaped, with perhaps three gradations of brightness.

0125 Sighted in binoculars as a faint hazy patch of light. The fact that it is visible in binoculars suggests that its magnitude is around 7.5 to 8.0.

15 July 1996

2330 Through binoculars, the comet appears as a faint blur in the constellation Scutum. At this point it is crossing the open cluster NGC 6649, which is too faint to discern with my binoculars. A rough estimate of the comet's brightness was around 6.0. A brief view was obtained using the telescope at 81x and 120x with the LPR filter. The North-South elongation of the coma is obvious.

 

Sketch of the outline of Comet Hale-Bopp on 19th March 1997

An outline sketch of Comet Hale-Bopp on 19th March 1997 at 2000 UT, using 10x40 binoculars.

Hale-Bopp had broken through the naked eye visibility threshold and was still brightening. It disappeared into the twilight in early December and then passed behind the Sun. The question on all astronomer's lips was: would it continue to brighten, or would it have "fizzled out" by the time it was detected again?

26 January 1997

0515 Observed for the first time since its period of invisibility close to the Sun. The comet was low down when first detected, perhaps 12° high in the East, and was immersed in sky-glow and some haze near the horizon. However it was quite easy to see thereafter, the coma appearing quite obvious through the binoculars, facing Northwards, i.e. away from the Sun. The nucleus was compact and bright. A rough estimate of its magnitude would place it around 3.7 ± 0.2. The predicted magnitude at this time is 1.2, although Sky & Telescope (January 1997) says it "should be somewhere between 2nd and 4th magnitude."

To the great relief of comet watchers world-wide, Hale-Bopp had continued to brighten.  Although slightly fainter than predictions had hoped for, it was nonetheless likely to put on a good show.

Sketch defining the approximate visual limits of Hale-Bopp's two tails as they appeared on 24th March 1997

On 9th March in central Asia, tens of thousands of people endured temperatures of -20° C to see the comet during a total solar eclipse - only the fourth time in recorded history that a bright comet had been seen during a total eclipse.

10 March 1997

1900 Comet observed through binoculars in bright twilight in the NW.  It appeared as a bright star with a slight diffusion towards the North.  The nucleus is very condensed and bright, its naked eye magnitude perhaps rivaling that of the star a Cygnus (= 1.3). At 1945 the tail was inclined to the horizontal by perhaps 20°.  The comet is now circumpolar, so for the next few weeks it will be visible all night long.

18 March 1997

1930 to 2000  The comet is clearly visible to the naked eye at around 21° high in the NW.  The tail can be traced about 1°.8 with the naked eye and a slight curvature is evident.  The central 'spur' of the tail is angled towards the NW.  The nucleus is bright and stellar in appearance.  The comet looked quite dramatic whenever it appeared through holes in the drifting cloud - just like an old painting!

19 March 1997

1930 to 2010  Moving swiftly through the constellation of Andromeda, Comet Hale-Bopp is now one of the most obvious objects in the night sky. Despite the presence of the gibbous Moon high up in the South, its tail could be seen with the naked eye, angled upwards at around 30° to the horizon and extending to about 1°.8 (with averted vision). Through binoculars, a very faint and wide tail could be detected with averted vision to at least 4°.5. The curvature in the tail was perhaps more evident on the lower edge, curving through about 5°. The tail section immediately behind the nucleus was very bright, resembling the beam of a search-light in the night sky, pointing to the NW. A much fainter hood was evident with averted vision to the North of the nucleus. The nucleus was a cream colour and it was stellar in appearance. The magnitude was estimated at around -0.5 or even brighter.

The comet did not pass by without controversy.  In America, a "leading astronomer" claimed that he had detected an "Earth-sized object orbiting around the comet's nucleus".  Theories then emerged that NASA was deliberately not releasing the latest pictures from the Hubble Space Telescope because they did not want the public to know "the truth".  The Astronomical League in America issued a quick rebuttal of these theories, saying that there was no object near Hale-Bopp and that Hubble's cameras had intentionally been switched off because the comet was now too close to the Sun for the telescope to safely image it.2

The lowest point in Hale-Bopp's apparition took place in America when 39 members of the Californian Heaven's Gate religious cult committed suicide, believing that the comet was harbouring a space-ship which had come to take them to paradise.

The comet made its closest approach to the Earth around midday on 22nd March, at a distance of 1.315
Astronomical Units (122 million miles /197 million km).

Sketch of the approximate visual limits of Hale-Bopp's two tails on 30th March 1997

23 March 1997

0300 Seen intermittently between clouds as it rose in the NNE. The nucleus was bright enough to break through the thinning cloud layers, and its tail stood vertically over the city. It was certainly a very obvious object, cutting through the light pollution like a knife through butter, and I estimated its tail length at about 1°.9.

At this tranquil time of night, the appearance of a comet in the Northern sky seemed to typify the ceaseless activity of nature - speeding through space whilst the clouds passed overhead, never sleeping, always moving. A new comet recalling the ghosts of comets past, seen by an 'enlightened' society but seeming just as ominous - a new Star of Bethlehem, a Comet of the Nineteen Nineties.

24 March 1997

2010 The two tails could be seen in binoculars, but only the dust tail can be seen with the naked eye under the sky conditions. The gas tail (on the East side) was by far the fainter of the two and it could only be seen with averted vision, extending about 3°.5 from the nucleus towards the North. The dark 'fork' between the tails was evident with averted vision. The gas tail was very obviously straight, in contrast to the dust tail, which curved most prominently on its lower edge. The comet is clearly brighter than Hyakutake was in the previous year (March 1996) which reached around -0.5 at its brightest. I would guess therefore that Hale-Bopp's current magnitude is around -1.0 or even a little brighter.

29 March 1997

0415 Observed rising in the NNE. The position of the nucleus was marked relative to its location on the previous evening at 2045. It had moved 0°.578 in 7.5 hours, a rate of motion of 0°.077 per hour. Through simple trigonometry, I estimated that it was moving through space at around 99,762 mph.Sky & Telescope magazine (April 1997) says that on April 1 (perihelion date) the comet would be moving through space at 98,425 mph (158,400 kmh).

2000 Comet observed under near-ideal conditions. The dust tail appears to be around 6° long using thumb measurements. The nucleus was observed with the telescope at 81x magnification, which revealed a remarkable amount of detail around the pseudo-nucleus.

Photo of Hale-Bopp taken on 31st March 1997, showing the second bluish ion tail

A number of curved dark lines (shells or envelopes) were seen ahead of the nucleus. By 2050 the comet had fallen to an elevation of 22°. A yellow filter was used for the later observations, although there was little marked difference in appearance. At 240x the coma appeared to have a much sharper edge to it on the Eastern side, and there was a curious oval-shaped 'halo' just ahead of the coma, detectable with averted vision.

A peculiar effect was noted with the nearby star w And (magnitude 4.8) which was located within the tail a short distance NNE of the nucleus. It appeared to twinkle much more vividly than the other stars around the comet, possibly the result of the intervening cometary dust particles. The effect was so vivid that a 3-D effect was sensed, the comet genuinely appearing to be a lot closer than the star.

30 March 1997

2015 to 2120  Observed from near Sigingstone in the Vale of Glamorgan. A relatively dark site, with a naked eye limiting magnitude of around 6.3 at the zenith. Impressive views were made with both binoculars and the naked eye.

With the naked eye the dust tail could be traced about 5°.6 and the gas tail was suspected (with great difficulty) using averted vision. The gas tail could be seen quite well with binoculars, although averted vision was necessary, and it could be traced to a distance of 6°. Interestingly the Eastern side of the gas tail could not be traced as far as on the Western side. It was unclear whether this was the result of its physical structure (i.e. tapering towards the Western edge) or whether it was simply too faint to detect along this section. Striations were suspected in the tail leading away from the coma.

31 March 1997

2000 Only seven hours ahead of the comet's perihelion at 0318, the nucleus was observed through the telescope at 81x and 162x magnifications. As on 29 March, shells were visible ahead of the pseudo-nucleus. A fountain appeared to spiral off the nucleus in a clockwise direction. The approximate 'height' of the fountain, measured from the pseudo-nucleus, was 1'.4.  Three shells were detected, the Southernmost only detectable with averted vision. The 'haze' observed on 29 March was also seen again, extending about 5'.8 from the SW edge of the coma. It appeared to have moved slightly Westwards since the previous observation. The Western side of the tail leading immediately from the coma appeared brighter and more diffuse than on the Eastern side. The pseudo-nucleus was golden yellow, and much of the coma had a yellowish hue, fading to grey along the tail.

Sketches of Hale-Bopp's coma and pseudo-nucleus on 31st March 1997

Telescopic views of the coma and nucleus (left) on 31st March 1997 at 2000 UT (magnification 81x; field of view 36'.7) and (right) at 2045 UT on the same day; 162x magnification.

1 April 1997

0318 Observed at the moment of perihelion. Although this is technically the comet's brightest moment, sky conditions were not ideal, with light pollution and some light haze interfering.

3 April 1997

2030 to 2100  The comet seemed to take on a rather different appearance on this night. The gas tail was more notable than on any other previous city observation, being traced to a distance of 4°.1. The most remarkable aspect of the comet, however, was the very obvious curvature of the dust tail, which curved through an arc of some 40°. Through binoculars the curvature was more notable on the Northern edge of the tail. It looked wider than on previous city observations, its widest traceable section spanning 2°.1.

On a hill-top North of Cardiff, South Wales, over a thousand curious members of the public gathered to catch a glimpse of Hale-Bopp through telescopes supplied by the local astronomical society. Similar such gatherings and "star parties" took place across the globe, often causing significant traffic tail-backs.

Sketch of the approximate visual limits of the comet's two tails on April 9th 1997

More adventurous outings generated by the comet included comet cruises and charter flights allowing enthusiasts to observe it from 41,000 feet.

10 April 1997

2030 to 2115  The dust tail looked impressive despite the presence of the Moon, stretching to an estimated distance of 6°. Much of the tail, however, was dim and had to be discerned using averted vision. Some structure was evident along the length of the gas tail. The central section appeared to be very tenuous, fanning out at its extremity to the NE. Just to the East of this section there appeared to be a very much fainter section which extended just to the NE of the coma. A curious 'dark band' seemed to separate the brighter and darker sections of the gas tail. Similarly the coma seemed to display two regions of brightness, its Eastern edge being very sharply defined.

12 April 1997

2045 Through the telescope, the pseudo-nucleus appeared much the same as in late March, with up to three curved shells ahead of it. No filters were used. The approximate diameter of the coma, by simple trigonometry, is 832,780 statute miles (1,340,229 km).

By this time the comet’s declination had fallen to that of the co-latitude (+38°.5) and hence from this time onwards the comet was no longer circumpolar.

Scientific results began to appear about Hale-Bopp. The
Hubble Space Telescope estimated the comet's nucleus to be 25 miles across (40 km), making it one of the largest comets to enter the inner the Solar System in recorded history. A team of French researchers studied the wobbling of the bright dust jet and from this they estimated the rotation period of the nucleus to be around 11.4 hours.  Like Hyakutake before it, X-rays were detected emanating from the coma region, although the exact mechanism which generates them is unclear. Spectroscopic observations identified over 30 different molecules in Hale-Bopp's atmosphere, some of which had never been detected in a comet before.

Comet Hale-Bopp's two tails as sketched on April 10th 1997

As summer approached and the comet continued to head Southwards, it became increasingly difficult to spot in the lengthening evening twilight. With the comet now receding and fading, the best of the show was now over.

29 April 1997

2110 Observed in binoculars through gaps in the high cloud. The dust tail extends to perhaps 3°.2 and its curvature does not appear as prominent as on previous occasions. The gas tail was not seen at all. The comet is perhaps equal in brightness to Castor in Gemini (magnitude 1.6), so it has almost certainly now fallen below magnitude 0.0. The twilight is now notably longer and even at 2110 a faint glow persists, masking some of the tail extremities. The comet stands in the WNW, around 15° high, at this time.

Hale-Bopp plunged Southward out of the solar system and receded into the depths of space.  It did, however continue to be monitored for several months by observers in the Southern hemisphere, who had largely missed out on the spectacle.

As Hale-Bopp faded from view, a debate began as to whether it deserved the status of "Great Comet".  Should it be ranked alongside those of the 19th century?

Much depends on the factors one applies when assessing a comet's 'greatness'.  A spokesman at NASA JPL said that a comet should be considered 'great' if it reached magnitude zero or brighter, and showed an impressive tail.3  Hale-Bopp certainly became brighter than magnitude zero and its tail was interesting, but not spectacular when compared to previous comets. Hyakutake had the second longest tail on record (after Halley in 1910) and although its nucleus was physically smaller than Hale-Bopp's, it came to within just 0.102 AU (9.5 million miles/15 million km) of the Earth - one of the closest approaches of any comet in history.  John Bortle of Sky & Telescope magazine ranked Hale-Bopp fifth of all the great comets he had seen, placing it just after Hyakutake.4

Whilst the visual impression of Hale-Bopp's coma and tail did not match those of Hyakutake, it was nonetheless an easy naked eye comet, and it was visible for a much longer period.  Where Hale-Bopp excelled was in its visible coma structure. Martin Mobberley, Vice-President of the British Astronomical Association (BAA), described it as "the cometary telescopic view of the century."5  Such distinct details around a comet's pseudo-nucleus had not been seen since Donati's Comet of 1858.

Over the months of its visitation, Hale-Bopp was variously described as "Comet of the Century" (almost certainly an exaggeration), "Comet of the Decade" (arguably so) and the "Great Comet of 1997" (true without any doubt). It also attained the unique status of being the most observed comet of all time.

BAA President Maurice Gavin:

"There can be no doubt that Hale-Bopp was a Great Comet…For modern society, often dismissive of the night sky and what it has to offer for free, the comet was a welcome 'beacon'." 6

Hale-Bopp will return in 2,380 years.

 


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Martin J Powell is a participant in the Amazon Europe S.ŕ r.l. Associates Programme, an affiliate advertising programme designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.co.uk, Amazon.de and Amazon.fr

REFERENCES

1.  "Great Comets, Novae and Lady Luck" by Mark Gingrich in Sky & Telescope, June 1995, pgs 86-9. The article also included a BASIC computer program to calculate the odds of seeing such events.

2.  "Earthwatch" by Graham W. Birdsall, in UFO Magazine (UK) Jan/Feb 1997, pgs. 18-9.

3.  Don Yeoman, cited in Astronomy & Space magazine, May 1997, pg. 21.

4.  John E. Bortle, cited in "The Great Comet of 1997" by Edwin Aguirre in Sky & Telescope, July 1997, pgs. 54-5.

5.  "Tails of the Twentieth Century" by Martin Mobberley, Journal of the British Astronomical Association, August 2000 (Vol 110, No. 4), pg. 200.

6.  "From The President" by Maurice Gavin, Journal of the British Astronomical Association, June 1997 (Vol 107, No. 3), pg. 111.

 


Copyright M J Powell October 2001

 

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