Statistics – Computation
Scientific paper
Aug 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995eso..pres...10.&link_type=abstract
ESO Press Release, 08/1995
Statistics
Computation
Scientific paper
How Impressive Will Comet Hale-Bopp Become in 1997 ? A very unusual comet was discovered last month, on its way from the outer reaches of the solar system towards the Sun. Although it is still situated beyond the orbit of Jupiter, it is so bright that it can be observed in even small telescopes. It has been named `Hale-Bopp' after the discoverers and is already of great interest to cometary astronomers.
No less than seven telescopes have been used at the ESO La Silla observatory for the first observations of the new object. Together with data gathered at other sites, their aim is to elucidate the nature of this comet and also to determine whether there is reason to hope that it will become a bright and beautiful object in the sky from late 1996 and well into 1997.
Further observations are now being planned at ESO and elsewhere to monitor closely the behaviour of this celestial visitor during the coming months. Discovery circumstances The comet was discovered on 23 July 1995, nearly simultaneously by two American amateur astronomers, Alan Hale of Cloudcroft (New Mexico) and Thomas Bopp of Glendale (Arizona). Although the chronology is slightly uncertain, it appears that Hale first saw it some 10 - 20 minutes before Bopp, at 06:10 - 06:15 UT on that day. In any case, he informed the IAU Central Bureau for Astronomical Telegrams (CBAT) in Cambridge (Massachussetts) about his discovery by email already at 06:50 UT, while Bopp's message was filed more than 2 hours later, after he had driven back to his home, 140 km from where he had been observing.
Upon receipt of these messages, Brian Marsden at the CBAT assigned the designation `1995 O1' (indicating that it is the first comet found in the second half of July 1995). After further sightings had been made by other observers, and according to the venerable astronomical tradition, the new object was named after the discoverers.
The magnitude, reported as 10.5 by Hale, is not unusual for a comet that is discovered within one or two hundred million kilometres from the Earth. It corresponds to a brightness that is about 60 times fainter than what can be seen with the naked eye and according to the statistics, a few comets with this brightness are normally discovered every year. However, some astronomers early remarked that the comet appeared to be moving rather slowly in the sky, indicating that it were possibly situated farther away. A near-parabolic orbit with perihelion passage in April 1997 Within less than three days after the announcement of the discovery, more than 60 accurate positions had been measured, many by advanced amateur astronomers equipped with modern CCD-detectors and the appropriate computer programmes. On this basis, Dan Green of the CBAT published a first, highly uncertain parabolic orbit. To some surprise, it showed that the comet was located at a heliocentric distance of no less than 1,000 million kilometres, well beyond the orbit of Jupiter! It was immediately obvious that it must therefore be intrinsically very bright. Indeed, it was about 250 times brighter than Comet Halley when this famous object was observed at the same distance in late 1987!
During the next few days, observers all over the world obtained additional positions which allowed Brian Marsden to calculate a more accurate orbit. Thus, it also became possible to trace the comet's motion backwards in time with some confidence. As a result, Robert McNaught at Siding Spring Observatory (Australia) soon found a possible image of Comet Hale-Bopp on a photographic plate obtained in late April 1993 with the 1.2-metre Schmidt telescope at that site, i.e. more than two years before the discovery. The estimated magnitude of this object was about 18. It has not yet been possible to establish with absolute certainty that this image is indeed of Comet Hale-Bopp, which was at that time nearly 2,000 million kilometres from the Sun, but if the identification is correct, this would again indicate a most unusual brightness at this enormous distance [1].
Subsequent orbital calculations depend heavily on this assumption and for that reason, there is still some uncertainty about the comet's true orbit. When the 1993 position is included in the computations, it appears that Comet Hale-Bopp moves in a near-parabolic orbit with a revolution time of about 3000 years [2].
According to this orbit, it will pass about 120 million kilometres from Jupiter in April 1996, and it will approach the Sun to about 140 million kilometres when it passes perihelion in early April 1997.
At the time of perihelion, the comet's geocentric distance will be about 200 million kilometres, the angular distance (`elongation') in the sky from the Sun about 45 degrees and it will be located in the northern sky at declination +45 degrees. It will actually by `circumpolar' in Northern Europe and therefore well observable all night from there. Why is Comet Hale-Bopp now so bright ? One possible cause for the unusual brightness of Comet Hale-Bopp at its present location, more than 200 million kilometres outside the orbit of Jupiter, is that it possesses a very large nucleus, that is the `dirty snowball' of dust and ice at the centre of a comet. The larger the diameter of the nucleus, the more sunlight will be reflected from its surface and the brighter will it appear. A corresponding estimate indicates that the diameter of its nucleus would be nearly 100 kilometres, as compared to about 10 kilometres for Comet Halley.
However, it is also important to consider that - due to the heating action of the sunlight on its surface - the nucleus of a comet that is not too far from the Sun will emit dust particles of which many assemble as a cloud around it (the `dust coma'). These particles are moved outwards by the pressure of gas molecules emanating from the melting ice(s) in the nucleus.
That this is indeed the case for Comet Hale-Bopp can be clearly seen on the first high-resolution images from ESO which confirm the presence of a dense dust cloud around the nucleus. It is in fact likely that most of the light observed from the central condensation in the comet's head is sunlight reflected from the particles in this cloud. The nucleus is probably completely hidden from view inside this cloud and we do not see it at all.
When we cannot observe the nucleus of a comet directly, we can only judge its size indirectly from the amount of dust it produces; a larger dust production will normally correspond to a larger nucleus. However, a temporarily high dust production rate during an outburst from the nucleus will lead to an overestimate of its size. In this case, the comet's brightness will begin to fade after a while, as the dust particles ejected during the outburst slowly disperse into space.
A main goal of future observations is therefore to decide whether or not Comet Hale-Bopp has just undergone an outburst. For this, the brightness of the central condensation and the size and shape of the dust cloud must be carefully monitored as long as possible. In this connection, the relatively bright pre-discovery images from April 1993 and May 1995 (see above) seem to argue against a recent outburst. How bright will the comet be at perihelion ? The main question now asked from many sides is obviously how bright the comet will be when it passes perihelion in 1997. Will it, as some headlines have already stated, and in view of its current brightness, become the `comet of the century, if not of the millennium' ?
From the above, it is clear that no firm prediction can be made before we have learned whether the present brightness is `stable' or whether it undergoes important variations which indicate that there has been a recent outburst. Astronomers are therefore very reluctant to express themselves on this point until further observations become available.
However, if the comet did not undergo a recent outburst and the nucleus is indeed as large as the current brightness would appear to indicate, then the comet may possibly become very bright at perihelion. Experience has shown that the evolution of a comet's brightness as
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