J. Brit. Astron. Assoc., 106, 3, 1996
Comet C/1996 B2 (Hyakutake) photographed by Brian Manning at Kidderminster on 1996 April 5. A 10 minute exposure at approx. 21.00 UT with a 200mm, 1.3m f.l. Wright camera. Hypered TP4415 film, 2½×2-inch format. B. G. W. Manning.
Comet Hyakutake was an even more impressive object than expected. Whilst, at the time of writing, it does not look as if the most optimistic perihelion predictions will be attained, the comet was a spectacular sight at Earth close approach. At its closest on March 25 the comet sported a naked-eye tail in excess of 50 degrees from dark sites and rivalled Arcturus as one of the brightest objects in the sky. The noted comet observer John Bortle remarked that this was one of the great cometary displays of the last thousand years; he suggested that the view on March 25 may have been similar to the P/Halley apparition in 837 AD. With binoculars a tremendous amount of intricate detail was visible in the tail and wide-field photographs show multiple disconnection events.
In early March the comet was still well south of the equator, but it was already a naked-eye object at fifth magnitude and binocular observers reported a short tail in PA 280. 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 dark skies. The head was already larger than 1 degree and an ion tail of up to 20 degrees had been reported by some observers.
Unfortunately, weather conditions over the UK during early to mid-March ranged from patchy to downright dreadful. The most favoured parts of the country were the south-west and the south coast but even here the weather was not good. The poor forecasts forced several BAA members to leave the country. John Rogers fled the 'miasmic swamp' of his native East Anglia for the sunnier climes of southern Spain. Mike Maunder was conveniently already on an observatory tour in Tenerife and Martin Mobberley, Glyn Marsh and the author left at short notice for Tenerife courtesy of Mark Kidger at the Instituto Astrofisica de Canarias.
Spectra from the IUE satellite reported in IAUC 6355 indicated that Hyakutake's molecule production rate was significantly higher than for Halley. A number of emission lines were seen and the most important were those for C2, CS, CO2+, NH, C, O, CO and S. The IUE results indicated that water production reached a temporary maximum around closest approach (IAUC 6370) as small pieces broke off the nucleus exposing fresh surfaces. The Hubble Space Telescope (HST) tracked these small fragments as they moved away from the nucleus at around 11.5m/s.
On the nights around closest approach the comet put on a spectacular show which left most observers short of adjectives. On March 25 the magnitude reached 0.0 and the tail stretched from Ursa Major, through Bootes and possibly as far as the bowl of Virgo. The tail was very bright for an ion tail and showed multiple disconnection events which changed in form over periods measured in minutes. No significant dust tail activity was seen at this time.
Radar contact with the comet was achieved by the Goldstone X-band (8.5GHz) dish on March 24. A few dozen 480kW pulses were directed towards the comet and the echoes were received 107 seconds later. Steve Ostro of JPL reported that the radar results implied a very small nucleus of 1–3km. The inability of the HST to image an extended nucleus despite a pixel resolution of 7.5km tends to confirm the radar observations although they are still preliminary. Earlier, on March 19–20 optical observations by the NTT at La Silla with a resolution of 18km/pixel had shown that the dense central gas and dust condensation had a diameter of 70km but these observations did not resolve the nucleus.
At the end of March as the Moon waxed to full the weather cleared and UK observers had many opportunities to view the comet. By this time it was still spectacular but the best of the show was over. At the beginning of April it had faded to around second magnitude. A movie of the central condensation was made by Terry Platt on April 1. He used an SXL8 CCD camera to take a short exposure of the nuclear region at half hourly intervals from 1940 to 2340. This movie clearly shows the nuclear rotation and the overall impression is rather similar to water jets from a rotating garden sprinkler. Observations from Pic du Midi had indicated a most likely nucleus rotation period of around 6.2 hr. This is around eight times faster than Halley's.
One of the most surprising results was obtained by the X-ray satellite ROSAT. Observations of the comet in X-rays showed levels around 100 times those expected. Possibly solar X-rays were absorbed by molecules in the coma causing them to fluoresce. Alternatively violent collisons between solar wind particles and the coma may have been causing the X-ray emissions. Much work remains to be done to interpret these observations.
By April 10 Comet Hyakutake was within the orbit of Venus and the magnitude had stabilized at around 2.5. Further small fragments had broken from the nucleus but the object was not brightening as expected. The coma diameter had shrunk to 10' and it now looks unlikely that the comet will be brighter than second magnitude at perihelion. Brian Marsden computes that Hyakutake's original orbital period was 8,000 years, but that after passing through the inner solar system gravitational perturbations have changed this to 14,000 years (IAUC 6359). We shall not see this comet again!
Various images of the comet including Terry Platt's movie (in MPEG format) are available on the TA WWW site at http://www.demon.co.uk/astronomer/comets.html.
Nick James (Comet Section CCD Adviser)
1996 April 14
For the weekend of Comet Hyakutake's closest approach to Earth, photographers Martin Mobberley, Nick James and Glyn Marsh fled the clouds of England for Tenerife in the Canary Islands. Here are Martin's impressions of their experience.
1996 March 22/23: 0300 UT. Punta Hidalgo on the NE corner of Tenerife. On the sea shore.
The comet's tail was at least 20 degrees long to the naked eye in PA 230. The head was roughly mag 0.5, very similar to Vega. The head was remarkable. A distinctly bluish colour in 11×80 binoculars. A starlike nucleus surrounded by a classic hood and a larger tear-shaped coma 1.5 degrees in diameter. The tail stretched well beyond Arcturus, which was certainly brighter than the head.
1996 March 23/24: 0000 UT. Hyakutake beachside star party, Puerto de Güimar on the eastern side of the island.
The Mayor had ordered the city lights turned off for 90 minutes. There were 5,000 people on the beach just looking at the Comet...
0300 UT: 2300m near Izana Observatory; 0400 UT: 1800m at Chipeque.
At this altitude the tail could be traced to at least 40 degrees. The head was slightly fuzzier tonight and not as sharply defined. Still around mag 0.5 though and still 1.5 degrees in diameter and distinctly blue.
1996 March 24/25: Chipeque (1800m) above La Laguna.
A 9-hour observing session from 2200 to 0700 UT (dawn). An absolutely crystal clear and freezing night at 5500 ft. The Milky Way looked irritatingly like cloud. The comet totally dominated the night sky and caught your eye when you were just walking around, despite being at the zenith.
The head was even more fuzzy tonight, especially when compared to Night 1. At 2240 UT we spotted a tailward pointing spike about 2 minutes long within the coma in the 11×80s. The tail was 20–25 degrees long with direct vision and well over 40 deg. with averted vision. After moonset, at times I was sure I could see 60 deg. plus of tail! A tail disconnection event was easily visible in binoculars.
1996 March 25/26, 0240–0530 UT. Back on the seashore at Punta Hildago, facing north.
With the comet in the far northern sky this was a magnificent sight. Polaris was 28 deg. above the sea and the head of the comet at approx. mag 0.9 was close to Kochab in Ursa Minor. The tail stretched through the gap between Alcor/Mizar and Alioth and was visible for 20 deg. with direct vision and at least 50 deg. with averted vision.
The temperature was remarkably mild and we were sitting against the beach wall with the waves breaking on the north shore, looking out to sea at this awesome sight. Now I know what the ancients must have seen in centuries and millennia gone by: I can understand their terror!
We acknowledge the invaluable help of Andy Sefton, who serviced the Vixen GP equatorial mount on the night before the trip, and Mark Kidger, who showed us the best observing sites.
Return to Journal 1996 June contents page