Computer Science – Sound
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996eso..pres....6.&link_type=abstract
ESO Press Release, 03/1996
Computer Science
Sound
Scientific paper
Astronomers Prepare for a Rare Event In the early morning of January 31, 1996, Japanese amateur astronomer Yuji Hyakutake made his second comet discovery within five weeks. He found the new comet near the border between the southern constellations of Hydra (The Water-Snake) and Libra (The Scales), amazingly just three degrees from the position where he detected another comet on December 26, 1995.
After two weeks of hectic activity among amateur and professional astronomers all over the world, much interesting information has now been gathered about the new comet which has been designated C/1996 B2 (Hyakutake) . In particular, it has been found to move in a near-parabolic orbit that will bring it unusually close to the Earth next month. It is then expected to become bright enough to be seen with the unaided eye and to remain so during several weeks thereafter.
Preparations are now made to observe the celestial visitor with a large number of telescopes, on the ground and in space. This event offers a rare opportunity to study the immediate surroundings of a cometary nucleus in detail and the specialists intend to make the most of it. Discovery and orbit Yuji Hyakutake, of profession photoengraver and a well-known amateur astronomer, announced his new discovery without delay, and within 24 hours, it had been sighted by several other observers in Japan and Australia. Experienced comet-watchers described its appearance as `diffuse with central condensation and of magnitude 11-12', i.e. a little more than 100 times fainter than what can be seen with the unaided eye.
This brightness is not unusual for a comet discovered by an amateur, although it would probably have been missed, had it been just a little fainter. In the present case, the decisive factors for Hyakutake's success were undoubtedly his very powerful equipment (25 x 150 binoculars) and the advantageous combination of the comet's southern position in the sky and his location in Kagoshima, the southernmost prefecture of Japan.
Within three days only, nearly 120 positional measurements of the comet were obtained, mostly by amateur observers in Australia, PR China, the Czech Republic, France, Japan, Spain and the U.S.A. This allowed Brian Marsden of the Central Bureau for Astronomical Telegrams of the International Astronomical Union (Cambridge, Mass., U.S.A.) to compute a preliminary orbit. It showed that the comet moves along a parabola - or at least an extremely elongated ellipse - and that it must therefore have come from far away and may never have been near the Sun before. At the time of discovery, the comet was about 280 million km from the Earth and outside the orbit of Mars.
Moreover, the motion of the comet is such that it will continue to approach the Earth with a speed of about 58 km/sec during the next weeks and will pass within 15 million kilometres of our planet in late March. This corresponds to one tenth of the distance between the Earth and the Sun (0.1 AU) and, in cosmical terms, the passage is therefore a very close one. Information about some earlier comet encounters may be found in the Appendix at the end of this Press Release.
Continued observations have confirmed this and have also allowed to fix the moment of closest passage as Monday, March 25, at about 7h UT. At that time, the comet will be moving northwards through the northern constellation of Draco (The Dragon) at the exceptional rate of 0.77 deg/hour. The event will be best observable from the northern hemisphere. Two days later, the comet passes within a few degrees of the northern celestial pole.
The perihelion (the orbital point closest to the Sun) is reached on May 1, 1996, at a distance of 35 million kilometres from the Sun, far inside the orbit of the innermost planet, Mercury. From then on, the comet will rapidly move south, crossing the celestial equator in mid-May and reaching 70 degrees south in late July. Recent observations Comet Hyakutake obviously comes from far away, maybe even from the very distant `Oort Cloud' of comets that surrounds the solar system. In this sense it is different from the periodical comets which move in closed orbits around the Sun with revolution periods between a few years and some decades. Its `dirty snowball' nucleus of ices and dust has therefore not been heated by the Sun for a very long time, perhaps never, if this is its first visit to the inner regions of the solar system. Hence it is particularly difficult to predict its future performance. Nevertheless, the available observations seem to indicate that it is a quite `active' comet and that it may therefore become comparatively bright when it approaches the Earth and later at perihelion. But how bright ?
Imaging as well as spectroscopic observations have been performed in order to better characterize Comet Hyakutake. On CCD-frames obtained of the comet in early February with telescopes at the ESO La Silla Observatory and elsewhere, an elongation is clearly visible (cf. ESO Press Photo 11/96 ) in the anti-sunward direction of the coma (the cloud of gas and dust that surrounds the cometary nucleus). A real tail has not yet developed, but this is expected to happen soon. The size of the coma was measured as at least 7 arcmin, corresponding to a projected diameter of nearly 500,000 kilometres.
It is also of interest that until recently the coma otherwise appeared absolutely symmetrical - there was no indication of `jets', i.e. no large vents on the surface of the nucleus had yet become active. However, on images obtained with the ESO 3.6-metre telescope in the morning of February 13, a `jet'-like feature is seen which emerges south-east of the nucleus (i.e. from the sunlit side) and curls counter-clockwise towards the opposite side (the `tail'-direction). This is probably the first evidence of localized dust production on the surface of the nucleus.
CCD observations were made on February 9 at the Lowell Observatory (Flagstaff, U.S.A.) through special optical filters which isolate the light from different components of the coma, e.g. the light emitted by the OH-, C2- and CN-molecules in gaseous form and also the reflected sunlight from the dust grains. They show that the gas production rates are almost as high as those measured at famous Comet Halley when it was at about the same distance from the Sun during its approach in late 1985. The dust production of Comet Hyakutake also seems to be quite impressive.
The first spectra of the new comet were obtained at La Silla with the Boller and Chivens spectrograph at the ESO 1.52-metre telescope on February 8; they show comparatively strong emission of CN, C2 and C3 molecules, cf. ESO Press Photo 12/96. This is not unusual for a comet at the corresponding heliocentric distance. In conclusion, the recent observations show Comet Hyakutake to be an `active' comet. The evaporation of the ices on the surface of its nucleus, due to the heating of the Sun, is well underway and much dust is being ejected during this process. It is quite likely that this comet will put on a fine display, starting in mid-March and lasting until soon after the perihelion passage in early May. Nevertheless, there have been some cases [1] in recent times when the activity level of new comets did not develop as expected, so some caution is necessary. The encounter on March 25 By a straightforward extrapolation of the current brightness, it would appear that Comet Hyakutake will reach magnitude 1 on March 25, 1996, at the time of the closest approach to the Earth. This is almost as bright as the brightest stars in the sky. However, it is important to consider that this is the `integrated' brightness of the entire comet head which may fill an area of several degrees in diameter in the sky. Thus the comet will appear as a moderately bright, very diffuse object that is best visible in binoculars. There will be a central point of enhanced brightness, corresponding to the innermost part of the coma around the nucleus. The motion is sufficiently fast to be easily perceptible on the stellar background.
We do not know the size of the nucleus yet, but
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