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Article: <[email protected]>
From: [email protected](Nancy )
Subject: Re: Nancy and DejaNews
Date: 26 Mar 1997 16:14:19 GMT
In article <[email protected]> Chris Franks writes:
> It is not barely outgassing, it is so bright that even you could
> see it if you looked at it now.
> Chris Franks <[email protected]>
That tiny tail? Compared to what it was supposed to be doing in 1995? In 1995 it was estimated to be 10 TIMES the size of Halley's comet, 250 BRIGHTER than Halley's at a comparable distance. So what happened to what has become, comparatively, a Hale-Bleep
..........
Posted on sci.astro by [email protected] (Ron Baalke)
PR 10/95 25 August 1995
European Southern Observatory
For immediate release
NEW DISTANT COMET HEADED FOR BRIGHT ENCOUNTER
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!
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.