When Alan Hale in New Mexico and Thomas Bopp in Arizona independently, within half an hour of each other, discovered a comet in July 1995, it did not appear to be anything special at first. As time passed and positional observations started to reveal the orbit, some idea of how special this comet might turn out to be, started to become clear.
The first surprise was the distance at which the comet was discovered - further than the orbit of Jupiter - which set a new record for the furthest a comet was ever discovered visually. It soon became apparent that for it to be visible at such a distance it either had to be very active or very big and it might just be the one to break the 20-year `drought' of great comets since comet West in 1976. Unfortunately this honour was robbed from Hale-Bopp by a brief but glorious sweep past the Sun of comet Hyakutake a year before Hale-Bopp could start its grand display. Hyakutake almost stole the show because of its close approach to Earth (1/10 astronomical unit (AU) i.e. 1/10 of the distance between the Earth and the Sun) which made it very bright (about magnitude 0) and allowed it to sport an enormous tail spanning as much as 60° on the sky.
On its way out to us, Hale-Bopp had an interesting trip. Its first encounter was with Jupiter when it passed within 0.77 AU of this largest member of our solar system during April 1996. This close approach caused Hale-Bopp's orbit to change such that instead of re-visiting us again in 4200 years it will now only be 2380 years until we see it again. On the 8th of May 1996, Hale-Bopp was occulted by the Moon, when as seen from certain locations on Earth, the Moon moved in front of the comet, which is a rare event in itself. The Moon also played a role in two other interesting events. When the Moon was covering the Sun's disk during the solar eclipse on 9 March 1997, observers in Siberia managed to get a glimpse of the comet during `daytime'. As the comet brightened during the latter half of March 1997, so did the Moon, but some relief was briefly brought by the lunar eclipse of 23-24 March 1997 for observers in the northern hemisphere.
The orbit of Hale-Bopp brought it closest to Earth on 22 March 1997 at a distance of 1.3 AU (200 million km) and the furthest north in the sky on March 25. When it went round the Sun on 1 April 1997 at 0.9 AU (137 million km) it was moving at a speed of 44km/s with respect to the Sun. On June 26, 1997, Hale-Bopp will cross the equator moving back to the southern half of the celestial sphere.
When the show finally started, Hale-Bopp lived up to expectations and became brighter than magnitude 0. It compares well with comets Hyakutake, West, Bennett and the 1910 apparition of comet Halley as far as brightness is concerned - in fact it ended up being about 4 times brighter than comet Hyakutake. Even though the physical length of the ion tail is expected to grow to just over 1 AU (150 million km) during April/May 1997, it will not be as impressive as that of Hyakutake which had the advantage of passing very close to the Earth. Foreshortening (the fact that we do not have a full side-on view of the tail) as well as the comet's distance from Earth produces a tail of 10-15 degrees in length. By employing modern tools such as the Hubble Space Telescope (HST), astronomers found that the nucleus is huge ; 30-40km in diameter compared to Halley's 10km (which is considered to be large for a comet) and 5km for an `average' comet. The nucleus is also extraordinarily active, producing more dust than Hyakutake and about 10 times more water than Halley - a water production rate of 250 tons per second was recently measured. All of this makes Hale-Bopp intrinsically the brightest visitor to pass inside the Earth's orbit since the great comet seen by Tycho Brahe in 1577 and the third intrinsically brightest comet of all time. For comparison, Hale-Bopp is a thousand times brighter than Halley's comet seen at the same distance.
In the past comets have been looked upon as the bearers of bad fortune, powerful omens and fearsome gods manifesting themselves. In modern times these views have largely been supplanted by scientific knowledge of the nature and true origin of comets. Though less mystical in the modern view, comets retain their fascination for scientists and lay persons alike.Great naked eye comets still retain their dramatic impact on the general public and therefore afford excellent opportunities for scientists to raise public awareness of astronomy in particular and science in general. These majestic shows of nature are simply too impressive to ignore and excite the imaginations of people from all walks of life.
By studying these ancient relics from the creation of the solar system, scientists are provided with excellent clues to what conditions were like in the early stages of the formation of the Sun and planets. Because Hale-Bopp was visible for such a long time before it rounded the Sun, scientists had an ideal opportunity to study its development in great detail as it approached the Sun. Using such tools as the Hubble Space Telescope, the International Ultraviolet Explorer and two NASA observatories, provided them with the most revealing portrait of the workings of a cometary nucleus since the spacecraft missions to comet Halley in 1986. The results of this study changed our understanding of the way the frozen nucleus is arranged. By looking at the ratios of the rates at which water is released compared to dust and other trace components, the complicated arrangement of the nucleus could be studied. Brief outbursts as far back as September 1996, lasting little more than an hour but producing increased dust production rates of at least eight-fold, was measured with HST. This new information suggests that the different components of the nucleus are contained in separate regions of the nucleus instead of a more homogeneous distribution as previously thought.
The comet is now heading away from the Sun and therefore getting dimmer, but will still be fairly bright through May for northern hemisphere observers. By late May Hale-Bopp will be too close to the line sight as viewed from the northern hemisphere and the comet will become a southern hemisphere object from about the beginning of May. It is expected to be a naked-eye object until about the end of June, after which it will be lost in the glare of the Sun as it gets too close to the Sun in the sky. Later in the year, telescope users in far southern latitudes should still be able to follow the comet throughout 1998 as it moves away from us.
While Hale-Bopp is still very bright, one only needs to look low in the sky to the north-west soon after sunset to find the comet. The map in Fig.2 shows the position of the comet against the sky background from 2 April to 9 October 1997 in 5-day intervals; the dates are in American format (MM/DD/YY). The comet symbols on the chart do not depict the true direction and length of the tail. The orientation of the map is as seen by observers in the southern hemisphere; northern hemisphere observers have to turn it upside-down. While Orion is still prominent in the evening skies, it is easy to orientate the map to the sky. By holding it up to the sky, rotate the map until the direction of the bright constellations matches those on the sky. By looking at the dates, find the position of the comet on the map and them look for it in the sky.
Always look from as dark a place as possible with as few lights as possible in the direction of the comet. Bright moonlight also prevents one from seeing the dimmer tail parts of the comet. Because comets are such extended objects, the best tool to aid one in getting a better glimpse of the comet is an ordinary pair of binoculars.