Astronomy

Comet Hyakutake was first discovered on January 30, 1996 by Yuji Hyakutake, an amateur astronomer from southern Japan. He had been searching for comets for years and had moved to Kagoshima for the dark skies in the nearby rural areas. He was using a powerful set of binoculars with six-inch objective lenses to scan the skies on the night that he discovered the comet.

This comet he saw was actually the second Comet Hyakutake; Hyakutake had discovered comet C/1995 Y1 several weeks earlier. While he was re-observing his first comet and the surrounding patch of sky, Hyakutake was quite surprised to find another comet in almost the same position as the first had been. Not quite believing that he could find a second discovery so soon after the first, Hyakutake reported his observation to the National Astronomical Observatory of Japan the following morning. Later that day, the discovery was confirmed by independent observations.

When the first calculations of the comet’s orbit was made, scientists discovered that the comet was going to pass just 0.1 AU from the Earth on 25 March. Only three comets in the previous century had passed closer. Comet Hale-Bopp was already being discussed as a possible “great comet”; the astronomical community eventually realised that Hyakutake might also become quite a site to see because of its close approach.

The comet’s orbit showed that it had last returned to the inner solar system approximately 17,000 years earlier. Because the comet most likely had passed close to the Sun several times before, the approach in 1996 would not be a maiden arrival from the Oort cloud, a place where comets with orbital periods of millions of years came from.

Comets entering the inner solar system for the first time may brighten rapidly before fading as they near the Sun, as a layer of highly volatile material evaporates. This was the case with Comet Kohoutek in 1973; it was initially touted as potentially spectacular, but only appeared moderately bright. Older comets show a more consistent brightening pattern. Thus, all indications pointed that Comet Hyakutake would be bright.

Besides approaching close to the Earth, the comet would also be visible throughout the night to northern hemisphere observers at its closest approach because of its path, passing very close to the pole star. This would be an unusual occurence, because most comets are close to the Sun in the sky when the comets are at their brightest, leading to the comets appearing in a sky not completely dark.