The discovery of Hale-Bopp

During the winter and early spring of 1997 the northern sky was dominated by an extraordinary object. Comet C/1995O1 Hale-Bopp had been discovered two years earlier and it was one of the largest cometary bodies to enter the inner solar system in recorded history. The geometry of the apparition was such that northern hemisphere observers had the opportunity to observe the comet for an extended period. In contrast to its illustrious predecessor of the previous year, C/1996 B2 Hayakutake¹, the extended apparition of Hale-Bopp coincided with unusually clear weather in the UK. This meant that practically everyone, from dedicated comet observers to the most disinterested member of the public, saw it and it was probably seen by more people than any comet in history.

On the night of 1995 July 22/23, around six months before Hyakutake’s second discovery, Alan Hale and Thomas Bopp made independent discoveries of the comet which would become one of the most famous astronomical objects in history. At the time the comet was near to M70 in the star-clouds of Sagittarius and it was visible as a faint tenth magnitude fuzz. It was quickly designated C/1995 O1 (Hale-Bopp) but it was a while before a good orbit was available. At discovery Hale-Bopp was moving very slowly to the NW and one possibility was that the comet was heading straight for us. This led to “Doomsday Comet” headlines in the newspapers but when the preliminary orbital elements were announced on July 27^th  they showed that, far from being a close-by object moving towards us, Hale-Bopp was a far-away monster that would not approach our planet any closer than 1.2 AU.

The orbital elements are listed in table 10.2. At the time of its discovery Hale-Bopp was over 7 AU from the Sun and 6.2 AU from the Earth. If it was tenth magnitude at that distance how bright would it be at perihelion? Now that he had a preliminary orbit Rob McNaught at the Anglo-Australian Observatory could search old Schmidt plates for prediscovery images. The oldest image of the comet the he could find was on a plate taken on 1993 April 27 using the UK Schmidt. At that time the comet was 13.1 AU from the Sun at a magnitude of around 18. There was now no doubt that this was a huge comet. McNaught’s old positions helped to improve the orbit and they showed that Hale-Bopp had last visited the inner solar system around 4,200 years ago. Perturbations this apparition would change the orbit so that we would have to wait only 3,400 years until its next visit.

The circumstances of Hale-Bopp’s apparition were especially favourable for those of us in the northern hemisphere. When it was discovered Hale-Bopp was further away than Jupiter and well south of the ecliptic (fig 10.8) at a declination of -32°. The plane of the orbit is almost at right angles to the ecliptic and the comet would move slowly towards its northbound crossing (the ascending node). It reached this on 1996 February 29 by which time it was 5.2 AU away from the Sun and 5.8 AU from the Earth but still at a declination of -22°. At around this time, in January 1996, it went behind the Sun as seen from the Earth (conjunction) but it was recovered in the morning sky in early February. The next opposition occurred in 1996 June when the comet was in Scutum at a declination of -20° and the distance to the Sun and Earth was 3.9 and 2.9 AU respectively. During the autumn and winter of 1996 the comet gradually became brighter and it moved north passing over the celestial equator in early December. This was about the time of the next conjunction but the elongation never became less than 26° and so observations were able to continue without a break as the comet was followed into the morning sky.

In early 1997 the comet moved rapidly north and it reached the furthest distance above the ecliptic on March 6th when it was in Cygnus. By this time the comet was best seen as an evening object. At perihelion on April 1st Hale-Bopp was 0.91 AU from the Sun, 1.35 AU from the Earth and at a declination of +44°. The comet was then moving rapidly south and northern hemisphere observers lost it at the end of May. Hale-Bopp crossed the ecliptic at the descending node on 1997 May 7 but by this time the Earth was almost on the opposite side of the Sun. The comet crossed back into the southern celestial hemisphere at the end of June and it reached a declination of -64° by the end of the 1997.

Observations

It was clear that Hale-Bopp was going to be impressive. Throughout the latter part of 1995 and the early part of 1996 the comet steadily brightened. Professionals were busy observing this new object and the first detection of CO took place in September 1995 when it was 6.7 AU from the Sun. CO is the dominant molecule in the coma this far from the Sun since water only starts to play a significant part in the activity of a comet at around 3 AU.

By the summer of 1996 Hale-Bopp was visible to the naked eye. From the time of its discovery a considerable amount of detail had been visible in the coma and observers became perplexed that many of the radial features appeared to be constant from night to night. Some even suggested that the nucleus rotation period was greater than a year! Much of this detail was seen visually but it was also well recorded on CCD images. The bizarre appearance was explained by Zdenek Sekanina of NASA’s Jet Propulsion Laboratory. He said that the jets could be explained by dust emission from three or four discrete active sources on the rotating nucleus which were periodically switched on and off as they rotated in to and out of the sunlit side of the comet (fig 10.9).

From careful analysis of the jets it seemed that the nucleus had a rotation period of around 11.5 hours. This was confirmed by observation of the concentric dust shells in the inner coma which originated from the active jets on the nucleus. These were separated by around 12,000 km and they were expanding outwards at 0.3 km/s which was consistent with the 11.5 hr spin period.

The last solar conjunction prior to perihelion occurred around Christmas 1996 but by this time Hale-Bopp was at such a high northern declination that many observers followed it all the way through the winter. In 1997 January the show really started. Hale-Bopp began to dominate the morning sky with its growing tails and extraordinary coma detail. By March it was moving into the evening sky as an unmissable object visible to anyone who bothered to look. It had a total magnitude of around -0.6 and its bright dust tail was even visible from light-polluted cities. From the countryside the entire comet including its long, straight ion tail was a stunning sight. The pictures on these pages demonstrate that the comet was an amazing object at whatever scale we looked. It had fantastic features that were visible in the centre of the coma with a large telescope (fig 10.10) but it also had a stunning beauty that was only visible in wide-angle views (figs 10.11).

A very interesting observation was made in early April 1997 when astronomers using the James Clerk Maxwell telescope on Mauna Kea, Hawaii detected a special form of water, HDO, in Hale-Bopp. Most cometary activity comes from normal water, H₂O, but in some cases the one of the Hydrogen atoms in the water molecule gets replaced by an isotope called Deuterium. The ratio of Deuterium to Hydrogen is a very powerful indicator of where the water came from rather in the manner of a DNA fingerprint. The team found that the ratio in Hale-Bopp was one part of Deuterium to 10,000 parts of Hydrogen which is comparable to ratio found in the Earth’s oceans but different to the ratio seen in the interstellar medium. This lends support to the theory that all of the water on our planet originally came from impacting comets.

The tails

Hale-Bopp had no fewer than three tails. In addition to the expected type I (ion) and type II (dust) tails a team using the Isaac Newton Telescope on La Palma detected a large tail of neutral Sodium atoms. This Sodium tail was seen as a straight feature 6.6° long and less than 10 arcminutes wide with parallel edges over its entire length. No such tail had ever been seen in previous comets. Spectra showed that the tail was pure Sodium with none of the normal cometary components such as water or Carbon present. Theory showed that the Sodium could not have been released directly from the nucleus but it must have arisen from other molecules which were dissociated after ejection.

The dust tail in most comets is normally rather featureless but, as mentioned in chapter 5, some large comets display interesting features called synchronic bands. These bands are thought to be caused by episodic releases of dust from the comet’s nucleus as sources turn on and off when they rotate in to and out of sunlight. The dust particles released are then affected by solar radiation pressure in different ways depending on their size. Smaller particles get accelerated by more than larger ones and a single release of dust gets spread out along a line called a synchrone to form a well-defined linear feature called a synchronic band. Dust which is released continuously from the nucleus moves along an arc called a syndyne and this defines the normally rather bland and featureless background of the dust tail.

Only a few comets have exhibited synchronic banding. Prior to Hale-Bopp synchronic bands had been seen most prominently in the glorious dust tail of Comet West in March 1976. The bands in that comet were especially prominent because the nucleus had broken up. No such damage afflicted Hale-Bopp’s nucleus but since the comet was so large and active there were plenty of episodic dust releases to generate the telltale bright diagonal striations of synchronic banding.

During the early part of 1997 the position of the comet relative to the Earth and Sun meant that Hale-Bopp’s dust tail was pointing away from us and so very little detail was to be seen. As the geometry became more favourable during the early spring synchronic bands became visible in the dust tail as it lengthened and broadened. Glyn Marsh and Denis Buczynski managed to image these features using various astrographic lenses of 0.5 to 1.2 metre focal length and large format (5-inch ´ 4-inch and 10-inch ´ 8-inch) high resolution sheets of Hydrogen hypered Kodak Technical Pan film (fig 10.12).

Professional workers at Calar Alto also detected at least 12 straight bands in the tail on March 17/18 ranging from 1.5° to 3° from the nucleus. These bands were 1 to 3 arcminutes wide and some of them extended more than 1° across the tail. The position angle of these bands indicated that they were not in exactly the place expected for a synchrone assuming direct emission from the nucleus and so the dust grains had probably come from secondary processes in the outer coma or the dust-tail.

On its way out

By August 1997 Hale-Bopp was only visible from the southern hemisphere and its had faded to third magnitude. By the end of 1997 it had fallen back below naked-eye visibility but it did develop a short anti-tail for a while as it crossed the ecliptic.

As the comet faded most people stopped observing it and this is all the encouragement that a comet needs to put on an unexpected show. By 1998 December the comet was more than 7 AU from the Sun and the total magnitude was around 11. Gordon Garradd was still monitoring it from Australia and he reported a remarkable 3-magnitude brightening of the nuclear region. The total magnitude of the comet was not much affected but this nuclear outburst gradually enlarged and dispersed through the coma. This was very similar to the outbursts seen in comet 29P/Schwassmann-Wachmann 1 at comparable solar distances of 6-7 AU and it shows how important it is to continue monitoring comets for as long as possible as they retreat from the Sun.

Hale-Bopp remains visible as it travels back to the outer Solar System (fig 10.13). At the end of 2001 it was 15 AU from the Sun but still had a total magnitude of 15. It was then in the far south constellation of Mensa but large telescopes were still tracking it. It will reach 30 AU out at the end of 2010 when it will only be a few degrees from the southern pole. Even then the comet should still be visible to CCD equipped amateur telescopes at a predicted predicted magnitude of 19. A full twenty years after its discovery it should fall below magnitude 20 but it will remain visible in large telescopes for decades to come.