Over The Horizon

Hail to Hale-Bopp!

Alan Gilmour and Pam Kilmartin

The discovery of a bright comet far from the Sun makes a public relations nightmare for astronomers -- if the comet will eventually be easily seen in the night sky, then people want to know about it, but a comet that is bright far away won't always brighten proportionately as it moves nearer the Sun. Premature predictions of the "Comet of the Century" don't help the image of astronomy as an exact science. Such a dilemma has arisen with a comet recently found. It could become one of the brightest comets ever seen. Comet Hale-Bopp was discovered in July by two amateur astronomers, Alan Hale and Thomas Bopp, working independently. The comet was a fuzzy spot of light, the brightness of an 11th-magnitude star slightly out of focus (about 100 times fainter than the faintest star a naked human eye can see).

The comet was found to be nearly seven times Earth's distance from the Sun, lying between Jupiter and Saturn. To be seen at such a distance, it had to be extraordinarily bright. Adding to the excitement was the prediction that the comet would pass less than Earth's distance from the Sun in April 1997.

A rule-of-thumb formula links a comet's brightness to its distance from the Earth and the Sun; Hale-Bopp should have a brightness of magnitude -2 in April 1997. That's the same brightness as Jupiter, presently the brightest "star" in the evening sky.

At first it was assumed that the comet had flared, that some explosion or impact on its icy nucleus had thrown a cloud of dust into the surrounding space. As the dust dispersed, the comet would fade. Or perhaps it was was a "new" comet, one that had not passed near the Sun before. New comets are rich in very volatile ices of methane and carbon monoxide that sublimate at large distances from the Sun. They appear bright far off but don't perform closer to the Sun, as with Kohoutek in 1973-4.

These notions of temporary brightening were quickly challenged when a probable image of the comet was found on a photo taken two years earlier, enabling a much more accurate orbit to be calculated. This showed that the comet was not new but had visited the Sun around 3,000 years ago. Its great brightness is therefore real and not temporary. However, this all presently hangs on the reality of one tiny spot on one photo.

Unfortunately, whatever its performance brightness-wise, Comet Hale-Bopp will be awkwardly placed on the far side of the Sun when it is at its best. It will also be in the northern sky, and so below New Zealand's horizon or very near it, but it could well provide northern hemisphere viewers with an impressive sight.

Alan Gilmore works at the Mt John Observatory in Tekapo
Pam Kilmartin is at Mt John Observatory.