1997

“Great Comet” Hale-Bopp

Comets—small rocky and icy bodies that evaporate in spectacular fashion when they get close to the Sun—have excited and even alarmed people for ages. Some are visible in the sky at predictable times, like Halley’s Comet, which returns every 76 years. Many others, however, appear suddenly and at unpredictable times. Perhaps the most spectacular of these unexpected comets was comet Hale-Bopp, which was visible to most of the world for many months in the early evening skies during the spring of 1997.

Comet Hale-Bopp was discovered by the American amateur astronomers Alan Hale and Thomas Bopp in July 1995 as it was heading toward the Sun, but when it was still way out past the orbit of Jupiter. Tracking of the comet’s orbit revealed that it is on a long, elliptical path that takes more than 2,500 years to circle the Sun. At its farthest distance, the comet is more than 370 times farther from the Sun than the Earth is. Astronomers calculated that prior to 1997 the last time Hale-Bopp had passed by the Earth was in the summer of the year 2215 BCE. They classify this comet as “young” because its complement of highly volatile ices shows that it has spent most of its time in the cold outer solar system, rather than in the warm environs closer to the Sun.

Several years’ advance warning of the comet’s pass by Earth gave astronomers a rare opportunity to study such a young comet up close for the first time. Using spectroscopy and other methods, astronomers discovered rocky dust, water ice, sodium, and other molecules in the comet’s ion and dust tails—including some complex organic molecules never before detected in comets. The brightness of the head, or coma, of Hale-Bopp implied that the rocky and icy nucleus must be big for a comet—about 37 miles (60 kilometers) in diameter, or about six times the size of Halley’s comet. While Hale-Bopp is not a direct threat to us, a comet that size impacting the Earth at a velocity of more than 50 kilometers per second would wipe out civilization and potentially kill most life on our planet.

Billions of people saw Hale-Bopp because, for several months, it was bright enough to see with the naked eye just after sunset. There was a lot of media coverage about the “Great Comet of 1997,” and there was even some unfounded speculation and hype about possible contamination of the Earth by comet debris. Beautiful heavenly bodies or possible portents of doom? Comets have always been a bit of both.

An April 4, 1997, wide-field camera photo of comet Hale-Bopp, taken by astronomers at the Johannes Kepler Observatory in Linz, Austria. This 10-minute exposure shows the comet’s beautiful blue ion tail pointing away from the Sun, and the yellow-white dust tail pointing back along the comet’s curved trajectory.