[1] It was dubbed the Great Comet of 1996; its passage to within 0.1 AU (15 Gm) of the Earth on 25 March was one of the closest cometary approaches of the previous 200 years.

Reaching an apparent visual magnitude of zero and spanning nearly 80°, Hyakutake appeared very bright in the night sky and was widely seen around the world.

[6] He had been searching for comets for years and had moved to Kagoshima Prefecture partly for the dark skies in nearby rural areas.

[2] Because it had probably passed close to the Sun several times before,[5] the approach in 1996 would not be a maiden arrival from the Oort cloud, a place from where comets with orbital periods of millions of years come.

Comets entering the inner Solar System for the first time may brighten rapidly before fading as they near the Sun, because a layer of highly volatile material evaporates.

[5] Its coma, now close to the zenith for observers at mid-northern latitudes, appeared approximately 1.5 to 2 degrees across, roughly four times the diameter of the full moon.

[5] The comet's head appeared distinctly blue-green, possibly due to emissions from diatomic carbon (C2) combined with sunlight reflected from dust grains.

It was observed passing perihelion by the SOHO Sun-observing satellite, which also recorded a large coronal mass ejection being formed at the same time.

Astronomers analysing old data found that Ulysses' instruments had detected a large drop in the number of protons passing, as well as a change in the direction and strength of the local magnetic field.

The magnetometer team realized that the changes in the direction of the magnetic field mentioned above agreed with the "draping" pattern expected in a comet's ion, or plasma tail.

No known comets were located near the satellite, but looking further afield, they found that Hyakutake, 500 million km (3.3 AU) away, had crossed Ulysses' orbital plane on 23 April 1996.

The solar wind had a velocity at the time of about 750 km/s (470 mi/s), at which speed it would have taken eight days for the tail to be carried out to where the spacecraft was situated at 3.73 AU, approximately 45 degrees out of the ecliptic plane.

The orientation of the ion tail inferred from the magnetic field measurements agreed with the source lying in Comet Hyakutake's orbital plane.

[21] The other team, working on data from the spacecraft's ion composition spectrometer, discovered a sudden large spike in detected levels of ionised particles at the same time.

Chemical analysis showed that the abundances of ethane and methane were roughly equal, which may imply that its ices formed in interstellar space, away from the Sun, which would have evaporated these volatile molecules.

[29] Radar results from the Arecibo Observatory indicated that the comet nucleus was about 4.8 km (3 mi) across, and surrounded by a flurry of pebble-sized particles ejected at a few metres per second.

As the comet passed the Earth, a large puff or blob of material was observed being ejected in the sunward direction every 6.23 hours.