Comet
Hyakutake, formally designated C/1996 B2) is
a comet, discovered on 31 January 1996, that passed very close to
Earth in March of that year. It was dubbed The Great Comet of
1996;
its passage near the Earth was one of the closest cometary approaches
of the previous 200 years. Hyakutake appeared very bright in the night
sky and was widely seen around the world. The comet temporarily
upstaged the much anticipated Comet Hale–Bopp, which was
approaching the inner Solar System at the time.
Scientific
observations of the comet led to several discoveries. Most
surprising to cometary scientists was the first discovery
of X-ray emission from a comet, believed to have been caused
by ionised solar wind particles interacting with
neutral atoms in the coma of the comet.
The Ulysses spacecraft unexpectedly crossed the comet's
tail at a distance of more than 500 million kilometres
(3.3 AU or 3×108 mi) from the nucleus, showing that
Hyakutake had the longest tail known for a comet. Hyakutake is
a long-period comet. Before its most recent passage through the
Solar System, its orbital period was about 17,000 years, but
the gravitational perturbation of the giant
planets has increased this period to 70,000 years
The
comet was discovered on 30 January 1996, by Yuji Hyakutake,
an amateur astronomer from southern Japan.He had been
searching for comets for years and had moved to Kagoshima
Prefecture partly for the dark skies in nearby rural areas. He was
using a powerful set of binoculars with 150 mm
(6 in) objective lenses to scan the skies on the night
of the discovery.
This comet was actually the second Comet
Hyakutake; Hyakutake had discovered comet C/1995 Y1 several weeks
earlier. While re-observing his first comet (which never became
visible to the naked eye)
and the surrounding patch of sky, Hyakutake was surprised to find
another comet in almost the same position as the first had been. Hardly
believing 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.
At
the time of its discovery, the comet was shining
at magnitude 11.0 and had a coma approximately
2.5 arcminutes across. It was approximately
2 astronomical units (AU) from the Sun. Later,
a precovery image
of the comet was found on a photograph taken on January 1, when the
comet was about 2.4 AU from the Sun and had a magnitude of 13.3
When
the first calculations of the comet's orbit were made,
scientists realized that it was going to pass just 0.1 AU from
Earth on 25 March. Only four 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 spectacular because of its
close approach.
Moreover, Comet Hyakutake's orbit meant that it
had last been to the inner Solar System approximately
17,000 years earlier. Because it had probably 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 come
from. 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. 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 suggested Comet Hyakutake
would be bright.
Besides approaching close to 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 occurrence, 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.
Hyakutake became visible to the naked eye in early March 1996. By
mid-March, the comet was still fairly unremarkable, shining at 4th magnitude with a tail about 5 degreeslong.
As it neared its closest approach to Earth, it rapidly became brighter,
and its tail grew in length. By March 24, the comet was one of the
brightest objects in the night sky, and its tail stretched 35 degrees.
The comet had a notably bluish-green colour.
The
closest approach occurred on 25 March. Hyakutake was moving so
rapidly across the night sky that its movement could be detected
against the stars in just a few minutes; it covered
the diameter of a full moon (half
a degree) every 30 minutes. Observers estimated its magnitude as
around
0, and tail lengths of up to 80 degrees were reported. 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. Even to the naked eye, the comet's head appeared distinctly
green, due to strong emissions from diatomic carbon (C2).
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