Ten years ago, astronomers were monitoring the departure of Comet Halley from the inner solar system while the public grumbled that they had been had.
In fact, the public had had unrealistically high expectations of the 1986 return of this most famous of comets.
Several of its previous appearances had been sufficiently bright to attract attention without the benefit of publicity, and been recorded in weaving or paint. In 1910, the comet’s last appearance, it was singularly favourably placed and relatively close to the earth, so that its brilliant tail extended right across the night sky.
In 1986, interest was swelled by publicity from the astronomical community, for this was to be the first time that space probes would investigate the coma (the veil of gas and dust which is the visible part of a comet) and nucleus, revealing that which is forever invisible from Earth. To many an entrepreneur all this added up to the marketing opportunity of a lifetime, and it was exploited to the full with all the hype and misinformation that typifies such exercises.
So it was that Instamatic cameras were loaded with colour film in the expectation of frame-filling supersaturated colour images and great was the disappointment when they did not materialise.
Comet Halley actually performed very much as the astronomers had predicted, and their efforts to probe the coma were amply rewarded by photographs of the nucleus, as well as other data sent back by the probe Giotto. The dirty snowball turned out to be a 12kilometre-long peanut with
an unexpected jet-black skin pockmarked with vents from which evaporating material poured into space. Additional results came from two Vega probes and two Japanese probes monitoring the coma. But the hucksters had done their job only too well, and to this day one hears that Halley “was a flop,” that the scientists “had it wrong.”
Although there are hundreds of known comets, and a dozen are discovered or rediscovered each year, Hale-Bopp immediately attracted a good deal of attention. When it was discovered on July 23, 1995 simultaneously by Alan Hale in New Mexico and Thomas Bopp in Arizona, C/1995 01 proved to be rather more than seven astronomical units (AUthe distance of the Earth from the Sun) away from us. This placed it about halfway between the orbits of Jupiter and Saturn.
Comets are normally invisible out there, for it is so cold that such coma as there may be is of hydrogen, which is quite invisible, and the tiny nucleus is a minute black spot on a black background. Not until they have approached to within about 3 AU of the Sun does the Sun heat the surface enough for water ice to start sublimating and so develop a visible coma. For Comet HaleBopp to be visible at such a distance suggested that it might be spectacular when it approached Earth.
Some suggested that it could be the brightest comet since the great comet of 1577; unlikely to be visible in daylight, but bright enough to be immediately seen at dusk as a rival in brilliance to Sirius and Canopus.
But don’t bet your shirt on it. Many comets have shown great promise before perihelion their closest approach to the Sun only to reappear from that encounter as cinders rather than Cinderellas, with their available ice virtually all evaporated, and the splendour of their pre-perihelion tail but a pathetic remnant.
One reason suggested for Hale-Bopp’s visibility is that the solid icy nucleus may be unusually large, 10-40 kilometres. Other reasons could be that it is particularly dusty and contains a lot of frozen carbon monoxide, which sublimates at a lower temperature than ice does.
Analysis of its very elongated orbit reveals that Hale-Bopp is a long-period comet that was last here 4200 years ago, at about the end of the Old Kingdom when the Great Pyramid was young and pharaoh Pepi II ineffectually struggled to maintain his authority against his increasingly independent barons.
The comet’s orbit is almost perpendicular to that of Earth and the other planets in our solar system. However, this time the positions of the giant planets Jupiter and Saturn will change the comet’s orbit, shortening its period to 2400 years
Constantly under observation since its detection, Hale-Bopp has not behaved as expected. Normally, comets appear to brighten in a fairly regular manner as they approach the Earth and the Sun. Hale-Bopp’s increase in brightness as it swept towards perihelion was irregular, making the always dicey game of predicting maximum brightness even more fraught with difficulty than usual.
Atypical phenomena bring the wild-eyed out of the woodwork to meet the bug-eyed, and Hale-Bopp hs been no exception. Chuck Shramek of Houston, an amateur astronomer, captured an image of the comet on the night of November 14 which appeared to show a “Saturn-like object” close by. Since the star-charting computer programme that he was using for reference did not show any star in this position, Shramek assumed that he had discovered a new planet. Rather than seek independent confirmation, Shramek trumpeted his news on the nationally syndicated Art Bell Show, known for its credulity over reports of UFO abductions, spacecraft landing in the back paddock or any of the more dramatic types of paranormal event.
A quick check by a number of astronomers, including Alan Hale, immediately revealed that there was a star just where the “new planet” was reported, and that the “ring” was no more than a diffraction spike generated by the optical system. Far from abashed, Shramek maintained the non-stellar reality of the Saturn-Like Object (SLO), and was joined by a number of other observers, who claimed that they had recorded “changes in course,” and see the SLO as an alien space craft using Hale-Bopp as cover. Thirty-nine members of the Heaven’s Gate movement used this information as their “ticket to ride” when they committed mass suicide this Easter.
Comet Hale-Bopp’s closest approach to Earth was on March 22, but it was then still 197 million kilometres distant. Perihelion was on April 1, when it was 136 million kilometres from the Sun. At that time the comet was travelling at 44 kilometres per second.
For New Zealanders, viewing Hale-Bopp will not be easy, for its closeness to the Sun means that when the comet is nearest and brightest, it will be in the early evening sky. As this article goes to press, with the comet drawing away from the Sun, it will be visible for longer after sunset, and then after dusk, but by the same token it will be becoming fainter as its surface cools.
The diagram, adapted from that in The New Zealand Astronomical Yearbook, 1997, Grant Christie & Stan Walker, shows the position of the comet at various dates just after sunset. Along the Ecliptic, the Sun is shown at the appropriate dates together with the line of the horizon at Auckland (diagonal lines), and the comet is shown sized according to its estimated brightness, and also dated.
Because the figure shows half the celestial equator and its constellations (i.e. 180° or 12 hours), and the Earth’s axis of rotation is at right angles to the celestial equator, distances on the diagram measured parallel to the celestial equator can be read off as times. 200 mm = 12 x 60′ so 1 mm = 3.6 minutes.
Do not count on seeing the comet immediately after sunset, for in April civil twilight is 30 minutes after sunset, and nautical twilight getting seriously darknot for another 30 minutes. To further confound us, the moon is full on April 23, May 22, June 21 and July 20, which means as we wait for the sky to darken the rising moon frustrates us. Note that before the full moon, the moon rises earlier than sunset; after full moon it rises progressively later than sunset, so it is these latter evenings which are our window of opportunity.