Text by Andy Turner, researched and written at the time (April 1997)

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The photograph

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Date: 2111 hours (UT) 3rd April 1997.
Location: Lion Creek near Wallasea Island, Essex.
Film: Fujichrome 1600 Provia 35mm slide film.
Exposure: 1 minute at f2.8
Equipment: Olympus OM40 with 50mm lens on a homemade “Scotch” mount equatorial tracker.

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Halfway between the top left corner and top centre is the star Mirfak in Perseus. Halfway between top centre and the top right corner and about one fifth of the way down the photograph from the top edge is the Double Cluster showing only as a barely noticeable concentration of stars. About one sixth of the way along a line running from the nucleus of the comet towards the top left corner is a faint smudge which is the open cluster M34. Extend the same line beyond M34 and the next bright star is the eclipsing binary, Algol. The bright star immediately below the comet is Almach.

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The comet

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DISCOVERERS: Thomas Bopp - aged 47, from Glendale, Arizona (amateur astronomer) and Alan Hale - aged 39, from Cloudcroft, New Mexico (professional astronomer)

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DATE OF DISCOVERY: 22 July 1995.

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CLOSEST APPROACH TO EARTH: 22 March, 1997 - 1.32 A.U. (122,701,665 Miles or 197,469,188 kms.)

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DISTANCE FROM EARTH AT TIME OF PHOTOGRAPH: 1.378 A.U. (128,093,101 miles or 206,145,864 kms.)

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PERIHELION: (Closest approach to Sun): 0314 (U.T.) 1 April 1997 - 0.914 A.U. (84,589,784 Miles or 136,134,062 kms.)

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NEXT PERIHELION: Estimated to take place in the year 4377.

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SIZE OF NUCLEUS: Estimated at about 25 miles (40 kms.) in diameter. This is far too small to have registered on the photograph even if it had been exposed to view instead of completely shrouded within the coma. Even an object the size and brightness of the Moon (2,154 miles or 3,476 kms.) would only barely have shown up at that distance in a 50mm lens.

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DIAMETER OF VISIBLE COMA: Estimated at about 1,000,000 miles ( about 1,600,000 kms)

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When the comet was discovered it was still beyond the orbit of Jupiter which averages 5.1 A.U. from the Sun. At discovery the comet was probably at a distance of about 6.2A.U. The fact that it was observable in amateur instruments, albeit fairly large amateur instruments, at such a great distance was due entirely to the fact that Hale-Bopp is so large. It is approximately 4 times the diameter of comet Halley and is thus around 64 times it's volume and mass.

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Even at that vast distance the surface was beginning to warm up and the frozen gases of which all comets are largely made became volatile and began to sublime. A sublimation progressed, some of the vast amount of dust that had been frozen into the ice was released. A particles of dust are released from cometary nuclei by the subliming gas, they initially have no great tendency to go anywhere but remain instead clinging around the nucleus, gradually forming an atmosphere, or coma as it is known in relation to comets, around it In the case of Hale-Bopp the coma has surrounded the nucleus out to a distance of about 500,000 miles giving the coma a diameter of about 1,000,000 miles. Slowly but surely, however, the pressure exerted by sunlight starts to blow particles away from the coma thus forming the bright dust tail which shines by reflecting the very sunlight that is responsible for dispersing it. It was as long ago as the beginning of the seventeenth century that Johannes Kepler first suggested that the pressure of sunlight was responsible for a comets tail. With modern physics we are able to go much further than Kepler and actually calculate the extent of the pressure exerted by solar radiation, and thus to predict it' effects on various sizes of particles.

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Particle sizes in cometary dust tails are typically between 10 and 100 micrometres (1 micrometre = 1/1,000 mm) and at such relatively large sizes the particles are somewhat slow to respond to the effects of radiation pressure but do nevertheless almost always point somewhat away from the Sun regardless of whether the comet itself is approaching the Sun or receding from it. The exact direction in which the tail points gives an indication of the particle sizes involved. It is a compromise between trailing directly in the wake of the comet and streaming directly away from the Sun in the direction of the solar radiation, with the smaller particles tending to follow the solar radiation most closely.

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In contrast, the particles of the ion, or plasma tail as it is also known, are exclusively ionised molecules, mainly positively ionised carbon monoxide and carbon dioxide, which are much smaller - nanometre sizes - than the dust tail particles and so respond much more readily to the radiation pressure. However, calculations show that solar radiation pressure cannot account for the rectilinear ion tails of comets. In 1951 the German astronomer Ludwig Biermann proposed that the action of a solar plasma - the solar wind - moving radially out from the Sun at high velocity and violently repulsing the ions formed in the comets atmosphere was responsible. Thus, the principle force directing the ion tail is actually the Sun' magnetic field which is carried on the solar wind. The ionised particles are propelled along magnetic field lines at speeds between 10 and 100 kms / sec.

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The luminosity of these particles is only partly due to reflection of sunlight, the greater part being due to molecular and atomic emission. The typical blue colour is because the principal emission is due to the carbon monoxide ions emitting at about 420 nanometres which lies in the blue part of the spectrum. Recently a British team observing from the Canary Islands discovered a third tail composed of sodium atoms. This is a phenomenon never observed before in this or any other comet.