This wonderful comet was visible to the naked eye for several months in 1997. My first sighting was in the predawn sky in February, when it was already quite a bright object. As the weeks progressed it continued to brighten and move westwards across the northern sky. In doing so it became a more convenient object to view, so much so that by April it was visible nearly all night from as soon as the sky got dark.
By early April Hale-Bopp had brightened to magnitude -1. With careful scrutiny it was visible to the naked eye in the north west almost immediately after sunset, in a sky that would normally be considered much to bright to see a nighttime object. With binoculars it could be seen before sunset.
This sort of performance puts Hale-Bopp into the premier league of great comets. Although others have exceeded this brightness, albeit it briefly on a very close approach to the Earth or Sun, Hale-Bopp has the second greatest intrinsic brightness ever recorded, of magnitude -0.7 (beaten only by Comet Sara bat of 1729, at -3). The intrinsic brightness is a relative measure of a comet's brightness after removing the effects caused by differing comet - Earth and comet - Sun distances. By setting both to 15AU (1 Earth - Sun distance = 93 million miles = 150 million km), comets with very different orbital characteristics can be compared on an equal basis. Hale-bopp was magnitude 0 to -1 for several weeks around perihelion when its distance from the Sun was 0.9AU and from the Earth 1.3AU. To put this in context, Comet Hyakutake reached this brightness a year earlier but only for a few days: its intrinsic magnitude was +5.3 (some 250 times fainter than Hale-Bopp) but made up for it by passing just 9 million miles from Earth, 13 times nearer than Hale-Bopp, so giving it a distance advantage of 170 (remember that this component of the brightness is governed by the square of the distance)
|I was never able to photograph Hale-Bopp in a truly dark sky, with either the lights of London or moonlight interfering. This photo is unfortunately affected by London sky glow. Longer exposures gained no advantage as any further detail that should have become visible was swamped by increasing fogging of the film.|
10 Apr 1997, 50mm, 30s at f/2 on Kodacolor 100 (detail)
Note the faint blue ion tail, which points directly away from the sun, and the brighter dust tail offset to the right. The camera was unguided so even this relatively short exposure shows a slight trailing.
I turned my 22cm reflector onto Hale-Bopp for the first time on or about 1 Apr 1997. It was the first time it had been used since relocating to Oxted and I immediately suspected that something was wrong with the alignment of the optics, for the comet's nucleus was surrounded by a series of concentric rings. A quick check on some nearby bright stars did not reveal anything amiss, so this curious sight was evidently not a diffraction pattern caused by the telescope but something genuinely associated with he comet.
On scrutinising the image carefully under various magnifications it seemed as if the rings were parts of a spiral rather than strictly concentric. It dawned on me that perhaps what I was looking at was something akin to a Catherine wheel, with a jet of bright material being expelled from the nucleus and being swept out into a spiral as it rotated. No movement was visible just by watching, though over an hour or more it looked as if the scene was changing. Unfortunately the telescope was not set up to take photos at the time so I was unable to take pictures to be sure.
|A search on the internet soon confirmed this to be the case, as this picture taken by the Pic du Midi Telescope in the Pyrenees confirms. Somewhere I also found a movie created by time-lapse photography which shows the rotation, and if I ever locate it again, I will put it here.|
Close up of Hale-Bopp nucleus, end Mar 1997
Note the jets of material being ejected by the nucleus being swept into a spiral as the nucleus rotates anticlockwise. The arrow top right indicates the direction of the Sun. The actual nucleus is lost in the bright cloud of gas and dust at the centre.
The comet remained visible until into June, by which time it was starting to head south and towards the sun (as seen from the Earth), and became lost to northern observers. By August it was re-emerging from behind the sun for the benefit of those in the southern hemisphere, where large telescopes can still track it to this day. It will be another 4,000 years before it returns to the inner solar system again.
|Link to NASA Hale-Bopp news archive|
|Link to JPL / NASA Hale-Bopp picture archive|
|Link to European Southern Observatory updates and explanations|