Hale-Bopp

For a year and a half across 1996 and 1997, a single brilliant point of light hung in the skies of both hemispheres — bright enough to be seen from the heart of a lit city and unmistakable from any dark hillside. Catalogued as C/1995 O1 but known to almost everyone as Hale-Bopp, it became the Great Comet of 1997, the most widely observed comet of the twentieth century.

It was discovered on 23 July 1995 — remarkably, independently and on the same night — by two American observers, Alan Hale in New Mexico and Thomas Bopp in Arizona. What made it extraordinary was how far away it was when first spotted: beyond the orbit of Jupiter, and already bright, which meant its nucleus had to be enormous.

A giant nucleus

Hale-Bopp's nucleus is estimated at about 60 km across — roughly six times the size of Halley's, and one of the largest cometary nuclei ever measured. That sheer scale is why it shone so brightly for so long: even far from the Sun it had enough surface to vent huge quantities of gas and dust, and it stayed a naked-eye object for a record 18 months, far longer than any comet in living memory.

The Great Comet of 1997

Hale-Bopp reached perihelion on 1 April 1997, glowing at around magnitude −1.8, as bright as the brightest stars. It displayed two distinct tails — a whitish, curved dust tail and a straight, bluish ion tail sculpted by the solar wind — and astronomers even detected a faint third tail made of sodium. For months it was simply part of the evening sky, seen by more people than perhaps any comet before it.

A long, steep orbit

Hale-Bopp falls toward the Sun on an orbit tilted almost perpendicular to the plane of the planets and stretching far out into the outer Solar System. Its passage through the inner system slightly reshaped its path: where it once took roughly 4,200 years to circle the Sun, gravitational tugs shortened that to about 2,500 years. It will not return until around the year 4385.

A lasting mark

Beyond the spectacle, Hale-Bopp was a scientific bonanza, studied across the spectrum for the better part of two years and yielding detailed measurements of a comet's composition and behaviour. For a whole generation it was simply "the comet" — the one that hung in the sky through the seasons and turned casual sky-watchers into comet-watchers.