Comet Hyakutake

The comet was found by Yuji Hyakutake, a Japanese amateur astronomer, on January 31, 1996. It was given the designation C/1996 B2. The reason for the designation was that it was the second comet (2) discovered in the second half-month (B) of 1996.

Quite soon after its discovery, it was realized that  in late March/early April 1996, Hyakutake was going to pass close to the Earth (on cosmic scales). As it also seemed to be an active comet, this meant that people could be treated to the sight of a really bright comet.

This excited cometary scientists, who'd been waiting for a bright comet to come along for many years. Comet Hale-Bopp had already been discovered, but would not be at its brightest for another year. Many astronomers rushed to prepare observations in the few weeks they had between discovery and the closest approach to the Earth.

Hyakutake didn't disappoint at all. For a brief period around its closest approach of 0.1 AU (15 million km), it was brighter than Comet Hale-Bopp would be the following year. For observers north of the equator, the comet was particularly easy to observe in late March. As it passed almost directly over the Earth's North Pole, the comet remained visible all night in the Northern Hemisphere.

Because  of its close approach to the Earth, a lot could be learnt about things happening very close to its nucleus. As well as optical  and infrared observations, radar was used to probe the nucleus. These observations indicated that it was around 2-3 km across.

(Right) An animation of the region surrounding the nucleus of Comet Hyakutake for several hours on April 1/2, 1996. The sequence was obtained using the 0.6m Radcliffe Telescope at the University of London Observatory, by Geraint Jones (then of the Mullard Space Science Laboratory), and Steve Fossey of the University of London Observatory. The nucleus (which can't be resolved on this scale) is at the centre of the image. The Sun was towards the lower right. Curved jets dust can be seen leaving the sunward side of the nucleus. Short vertical lines are background stars. Bright points are cosmic ray hits on the CCD detector.

Hyakutake's nucleus was found to be fragmenting - a train of mini-nuclei was found, showing that sizeable pieces were breaking off the nucleus.

Left is an enhanced image of the region near Hyakutake's nucleus, acquired between 00:08 and 00:14 UT on March 28, 1996 at the Nordic Optical Telescope, La Palma, Canary Islands. The Sun was towards the left, and the image shows a region around 36000km across. The nucleus is located in the largest white region around a quarter of the way along the image. Curved jets of gas and dust emanating at the nucleus are visible on the sunward side. Nine nuclear fragments are detectable with careful analysis, the largest of which appears at the centre of this image. The faint diagonal streaks are star trails. The image was enhanced by smoothing using a Gaussian filter, and then subtracting the logarithm of  the smoothed image from the logarithm of the original.

As the nucleus was fragmenting, gas was being released both from the nucleus itself and from the fragments (large and small) which had been broken away from it. The interaction of the flows from these two sources formed a structure of higher neutral gas number density on the anti-sunward side of the nucleus. When viewed from the side (around March 27), this structure appeared as an arc.

The image on the right shows the development of the CN (cyanogen) arc over several days, as recorded at the Nordic Optical Telescope. The Sun is towards the left in each frame, and the white scale bar measures 1000km at the comet. Note that the phase angle (Earth-Comet-Sun) was changing during this time, from 71° on March 26, 91°  on March 27 to 111°  on April 5.

Other observations of the neutral gas in the comet, using optical and radio observations, revealed several molecules that hadn't been detected at comets previously.

One of the biggest discoveries made was the detection of X-rays from the comet by the Rosat spacecraft. Comets had not been previously regarded as being sources of X-rays.

The part of the comet which most interests Ulysses scientists is the ionized (plasma) component. During the close approach to the Earth, Hyakutake's ion tail was spectacular; stretching tens of degrees across the sky.

The image below is of Hyakutake obtained with the 0.6m Radcliffe telescope at the University of London Observatory at 23:36 UT on April 1, 1996, covering a region around 159000 by 106300km at the comet. A continuum (dust) image was subtracted from the (H2O+ & dust) image to isolate the light from the water ions. The Sun was towards the lower right, so the solar wind was flowing towards the upper left. The short bright and dark lines are star trails. The broad, diffuse lines are cometary tail rays - a phenomenon which so far has not been satisfactorily explained, but are generally believed to trace the magnetic field lines of the solar wind as it is convected through the comet.

As of April 6, 2000, Hyakutake is around 13.3 AU (about 2 billion km, or 1.2 billion miles) from the Earth, and 13.8 AU from the Sun (about 2.1 billion km, or 1.3 billion miles). In the sky, it's in the Southern Hemisphere constellation of Circinus (the compasses), near Centaurus. It is expected to return to the inner solar system in around 29,500 years.

• How Yuji Hyakutake found his comet (Sky & Telescope)

• Comet Hyakutake Home Page, Jet Propulsion Laboratory

• First X-rays from a comet discovered

• University of London Observatory - Radcliffe Telescope

• Nordic Optical Telescope