C/1996 B2 (Hyakutake)
The discovery of Hyakutake’s second comet
On January 30, 1996 the Japanese amateur astronomer Yuji Hyakutake left his house in the village of Hayato, about 600 miles south-west of Tokyo, and set off for his normal observing site. After a journey of around 10 miles he stopped and set up his equipment ready for the night’s work. Since the previous July he had been avidly searching for comets using 25×150 binoculars. These had already proven themselves a month earlier when he had discovered the first Comet Hyakutake. That comet wasn’t particularly bright but he was planning to take some photos to record its progress. Frustratingly, the patch of sky where his comet would be was obscured behind some cloud so he started to scan the clear areas using his binoculars. Just before 5am, as he was sweeping through Libra, he noticed the 11th magnitude fuzzy object which was to make him famous throughout the world. The comet was officially named C/1996 B2 (Hyakutake) but at that time there was no indication that this was anything other than a run-of-the-mill, faint comet.
CCD astrometry of the comet quickly followed Hyakutake’s discovery and by February 3rd the first parabolic orbital elements were reported. These showed that the comet would reach perihelion on May 2nd at a distance of only 0.22 AU from the sun. A close perihelion distance was remarkable enough but the elements also indicated that the comet would approach to within 16 million km of the Earth in late March. Hyakutake’s comet had grabbed the attention of astronomers around the world. Given the brightness at discovery it was possible that this object could reach first magnitude at close approach.
The elements of Hyakutake’s orbit are shown in table 1.1. Shortly after these elements appeared I began to assess how this comet would appear from my location at 52° north. For a change those of us in the northern hemisphere would have the best view since the comet’s orbit and the position of the Earth were ideal. When Hyakutake first spotted his comet it was well below the ecliptic plane (fig 10.1). By 1996 March 12th the comet would pass through the ecliptic heading north and it would be 1.3 AU from the sun. Thirteen days later, on March 25, it would make the close Earth fly-by at a distance of only 0.102 AU and it would then appear near the zenith for northern observers. Following this close approach the comet would move further north reaching its maximum distance above the ecliptic plane on April 22nd, all the time being well seen by northern hemisphere observers. As the comet started back towards the ecliptic plane the elongation would rapidly decrease and northern observers would lose it in the evening twilight sometime around the end of April. The comet would then continue on to perihelion on May 1st at a solar distance of 0.23 AU. The southbound ecliptic crossing (descending node) would occur four days later and it would then be up to southern observers to recover the comet as it rose out of the morning twilight in the second week of May.
While the orbital circumstances were good there were more serious problems much nearer home. For much of the northern hemisphere the months of February and March are not particularly favourable for comet observers since cloud cover is a regular nuisance. We knew from the orbit that the best of the display would be in the short period between March 22-27 and it was entirely possible that we would be clouded out completely at the critical time. It was time to start making plans.
From the viewpoint of mid-northern observers the comet was initially low in the morning sky. Observers further south fared better and many images began to appear on the Internet. By February 8th the 1.5m ESO telescope at La Silla, Chile, had obtained a spectrum of the comet. This spectrum was dominated by reflected solar radiation but the expected cometary emission from Cyanogen (CN) and carbon molecules (C2 and C3) was present. Observers at Lowell Observatory reported that the comet’s water production rate was around 70% of comet Halley at an equivalent solar distance. This was encouraging news since the it implied an active nucleus.
By the second week of February 1996, the magnitude had increased to around 9 and the coma was around 6 arc minutes in diameter. The comet was still a southern hemisphere object at declination -24° and so it remained a difficult object for observers in northern Europe. All the same the comet was showing encouraging signs of activity and a tail was first detected in a CCD image obtained on February 16th using the Danish 1.54-m telescope at La Silla (fig 10.2).
The brightness of the comet was increasing rapidly and by February 20th it had reached seventh magnitude and the coma diameter was around 22 arcminutes with a 1° tail. For northern hemisphere observers the comet was south of declination -22°. By early March the comet was still well south of the equator but it had become a naked-eye object at fifth magnitude and binocular observers reported a short tail.
The rotation of the nucleus was inferred from images obtained at Pic-du-Midi on March 9. These images, taken with a 1.05m telescope, showed at least two curved jets rotating clockwise and changing orientation over periods of a few minutes. The observed jets were around 2,000km long and the observations implied that the nucleus had a rotation period of around 6.6 hours.
Radar contact with the comet was achieved on March 24. Powerful radio pulses were directed towards the comet from the Goldstone dish and echoes were received 107 seconds later. The radar results implied a very small nucleus of 1-3km diameter surrounded by a dense cloud of pebble-sized objects. Shortly afterwards the first ever detection of X-rays from any comet was made using the ROSAT satellite. The brightest parts of the comet in X-rays were diffuse, crescent shaped and offset sunwards by about 6 arcminutes from the nucleus corresponding to a distance of about 30,000km.
By March 20, when the comet passed into the northern hemisphere, it had brightened to second magnitude and was a beautiful naked eye sight to those lucky enough to have clear skies. The coma was already larger than 1° and an ion tail of up to 20° had been reported by some observers. The comet was now moving quickly across the sky as it approached the Earth. By this time I had decided that my only chance to see this comet was to flee the UK in search of better weather. In this regard Tenerife is a convenient location for UK observers since flights are cheap and plentiful, the holiday infrastructure is well developed and the weather prospects and latitude were ideal for the comet. A group of us combined our resources and booked the tickets.
Having decided to travel to a better observing site I had to decide what equipment to take. At the time of closest approach Hyakutake was going to be huge so simple SLR cameras and a driven mounting, such as those described in chapter 5, were all that was required. We took a combination of cameras, a home-made barndoor mount and a larger Vixen SP mounting for longer focal length shots along with lots of film, both colour and hypered Tech Pan. Airport X-ray machines can be quite a problem for travelling astrophotographers since high speed films can be affected if they are scanned. I would always suggest carrying rolls of film in your pocket so that they don’t pass through the machines. If you don’t have enough pockets you can buy special shielding bags which will protect the film as it is scanned.
For observers at mid-northern latitudes the comet was at its best in the early morning hours as it rose towards the zenith. From Tenerife, on the early morning of March 23rd the comet had the distinctive “spring-onion” appearance shown in figure 10.3. The star Arcturus was embedded in a tail which extended over at least 20°. In 11×80 binoculars the coma showed a stellar nucleus surrounded by a classic hood. That evening the tail was less well defined but the coma was slightly brighter than the previous day.
At its closest on the morning of March 25th the comet approached zero magnitude and the tail stretched from Ursa Major, through Boötes and possibly as far as the bowl of Virgo. Fortuitously, the comet become most active right at the time of closest approach and the tail detail that we could see on the night of March 24/25 was astounding. This time we were at an altitude of 1,600-m on Mount Teide and the sky was perfect from dusk until dawn. The visible tail extended for at least 25° with direct vision and well over 40° with averted vision (see fig 5.8 on page X). In 11×80 binoculars a bright, tailward pointing spike was very prominent and a major disconnection event was clearly visible to the naked eye. On that night the comet totally dominated the sky and it was easy to understand how ancient people must have been terrified by such objects.
The exact time of close approach was 7hr UT on March 25th. After this the comet began to recede from the Earth as it headed in towards the Sun. The viewing geometry meant that the apparent tail-length was expected to grow as the comet moved away from the Earth. In the early morning hours of March 26 we made our final trip to observe the comet from Tenerife. The tail had again changed considerably from the day before and it was longer, possibly up to 50° with averted vision. The comet was now near to Kochab in Ursa Minor.
On the night of March 26/27 the head passed within 4° of Polaris and various observers took advantage of this to produce some good fixed-camera photos (fig 5.4 on page X). By March 29 the moon had started to become a problem for visual observers but the comet was bright enough to allow relatively simple equipment to capture a spectrum. One such was obtained by Maurice Gavin on April 1 (fig. 10.4). The coma shows reflected sunlight and two prominent Swan Band emission lines in the cyan and green parts of the spectrum. Two fainter emission lines are visible in the blue and yellow and absorption lines due to Earth’s atmosphere are visible in the red. Since the tail was much fainter only the reflected solar continuum is visible.
By the end of the first week of April 1996, the comet was sinking lower into the north-west on its way to perihelion. A prominent dust tail finally began to appear around April 10 although it never surpassed the ion tail. Meanwhile jet activity in the coma continued. The tail now had a classic appearance with a sharp ion tail and a diffuse dust tail. Photographs taken with Schmidt cameras reveal exquisite detail and many streamers are visible in the tail (fig 10.5).
As the comet moved towards the sun it entered the field of the LASCO C3 coronograph on board the SOHO satellite. Images of the comet at perihelion were obtained by this instrument (fig 10.6) but terrestrial observers had to wait until May 9 for the comet to reappear from the sun’s glare. By this time it was a southern hemisphere morning object at around third magnitude. The comet faded rapidly as it moved away from the sun. Southern hemisphere observers picked it up on May 9th in a bright sky. By May 18th the comet was visible in a darker sky but the coma had faded to fourth magnitude. It faded past sixth magnitude around June 10th and by late August it was fainter than magnitude 10.
Since Hyakutake made such a close approach to the Earth it was possible to see considerable detail in the inner coma with amateur-sized telescopes. The comet showed many interesting features close to the nucleus. Of particular importance were a sunward fan and the very intense tailward-pointing jet that was seen around close approach. This was also the first bright comet to make a close approach since CCD cameras became widely available in the amateur community. Photographers had never been particularly successful in reproducing the details visible in the inner coma of comets since the wide variation in light levels exceeded the available dynamic range of printing papers. The ability of CCD cameras to operate over a very large range of brightness levels meant that they could record the fine detail in the inner coma which in former times was only available to visual observers. Advanced processing techniques such as unsharp masks and radial filters can be applied easily to electronic images (as described in chapter 8) and some of the amateur CCD results were stunning.
Terry Platt obtained a sequence of CCD images on April 1st using an SX camera. The exposures were kept short to ensure that the CCD did not saturate on the bright inner coma and each frame was processed with an unsharp mask. The nine frames clearly show the anti-clockwise rotation of various features (fig. 10.7) and the effect is particularly strong when the frames are processed into a movie (see the CD-ROM).
One of the most impressive features of this comet was its tail. Since Hyakutake made such a close approach its tail aspect changed rapidly from the end of March to mid-April. It was also possible to observe major changes in the tail structure over periods measured in hours. We do not normally have the opportunity to do this since comets with large tails are usually only visible in a dark sky for a short period. Hyakutake had the unusual property that, at the time of close approach, its massive tail was visible for the entire night.
Hyakutake was certainly a Great Comet but its discoverer was a particularly modest man. In a statement released shortly after the discovery he said: “I am a bit perplexed by all the attention paid to me, when it is the comet that deserves the credit.”