 Hyakutake picture from March 1996 Comet Hyakutake (formally C/1996 B2) was discovered by Yuji Hyakutake on January 30 1996. It was actually the second comet known by this name, the first being comet C/1995 Y1, which Hyakutake had discovered just a few weeks earlier. While he was re-observing his first comet (which never became visible to the naked eye), Hyakutake happened to look at the patch of sky where he discovered it. To his great surprise there was another comet there, in almost exactly the same position his first had been.
When the first calculations of the comet's orbit were made, it was realised that the comet was going to pass very close to the earth, just 0.1 astronomical units away. As Comet Hale-Bopp was already being talked up as a possible Great Comet, it took a while for the astronomical community to realise that Hyakutake too might become spectacular.
By mid-March, the comet was still fairly unremarkable, shining at 4th magnitude, and with a tail about 5 degrees long. However, as it neared its closest approach to the earth it became rapidly brighter, and its tail grew in length. By March 24th the comet was one of the brightest objects in the night sky, and its tail stretched an impressive 75 degrees across the sky. Visually the comet had a notably bluish-green colour.
Because Hyakutake was at its brightest for only a few days, it did not have time to permeate the public imagination in the way that Comet Hale-Bopp did the following year. Many European observers in particular did not see the comet at its peak because of unfavourable weather conditions. However, many people who saw both Hyakutake and Hale-Bopp at their peaks contend that Hyakutake was the more impressive.
The Ulysses spacecraft made an unplanned and unexpected pass through the tail of the comet on May 1 1996. Based on the Ulysses encounter, the comet's tail is known to have been at least 300 million miles long. Terrestrial observers found ethane and methane in the comet, the first time either of these gases had been detected in a comet. Hyakutake was also found to be emitting X-rays, the first time a comet had been seen to do so. Radar results from the Arecibo Observatory indicated that the nucleus of the comet was 2km across.
Hyakutake had previously passed through the inner solar system about 17,000 years ago; gravitational interactions with the gas giants during its 1996 passage stretched its orbit greatly, and it will not return to perihelion again for approximately 72,000 years.
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