New Distant Comet Headed for Bright Encounter
How Impressive Will Comet Hale-Bopp Become in 1997?
25 August 1995
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.
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 .
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 .
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 it approaches the Sun in general depends on its orbital type. Comets with periods of a few hundreds to some thousands of years - which have therefore already been close to the Sun one or more times - brighten by the inverse-fourth, or even higher, power of the heliocentric distance and are often brighter and more spectacular after perihelion. Comets in almost-parabolic orbits with much longer periods (many of which make their first passage near the Sun) brighten more slowly, e.g., by the inverse cube of heliocentric distance.
If the orbital period is near 3000 years, this comet would belong to the first category and the extrapolation indicates that the brightness near perihelion in April 1997 could then reach magnitude -1.5. This means that Comet Hale-Bopp would become almost as bright as Jupiter which will also be visible in the night sky at that time .
What is even more exciting, the comet would stay this bright during several weeks and be visible from the Northern hemisphere during many hours each night. It will even be above the horizon all night for observers located north of geographic latitude 45 degrees. Thus, in addition to providing a wonderful opportunity to astronomers for learning more about comets, Hale-Bopp may become an object of great public interest.
This is especially so, because it is now almost 20 years since a comet has been this bright. The last one (Comet 1976 VI - which was discovered at ESO) attained this magnitude during a few days in early March 1976.
Recent observations from La Silla
At the present time, Comet Hale-Bopp is located in the southern constellation of Sagittarius and it is very well placed for observations from southern observatories. Irrespective of its performance later on, the unusual brightness at the present heliocentric distance already makes it a very interesting object for astronomers.
Consequently, Comet Hale-Bopp was observed soon after its discovery from the ESO La Silla Observatory with no less than seven telescopes. The overall goal has been to characterize this object and to try to determine what is causing its unusual brightness.
The ESO observations are of many different types and have involved many observers. At the 15-metre Swedish-ESO Submillimetre Telescope (SEST), Albert Nummelin, Anne-Marie Lagrange and Thierry Forveille searched on August 3-4 and 9-10 for emissions from the CO molecule. According to one theory, CO gas may possibly be the driving agent that is responsible for `lifting' dust particles off a comet's nucleus when it is more than about 750 million kilometres from the Sun. However, no emission from CO was seen to the sensitivity limit of these observations, thus placing important constraints on the proposed mechanism.
Normally, CN is one of the first gaseous molecules to be detected in the coma of comets approaching the Sun. For instance, in Comet Halley, emissions from CN were first seen at a heliocentric distance of about 725 million kilometres. It would therefore be of great interest to learn whether CN is already now present in the coma of Comet Hale-Bopp. Spectroscopic observations with the ESO 1.5-metre telescope were performed by Anne-Marie Lagrange, Jean Luc Beuzit, Stephane Guisard and Pierpaolo Bonfanti on August 3-4 and 9-10. They have now been reduced and do not show any such emission. At the present distance of the comet from the Sun, the temperature is too low for water ice (the major component of cometary nuclei) to evaporate efficiently, and with the non-detection of CO and CN, the driving gas that has produced the well visible dust cloud around the nucleus of Comet Hale-Bopp is still unknown.
Direct images of Comet Hale-Bopp were made in many wavebands. Of special interest are the high-resolution, infrared J, H and K-band images obtained on August 6 and 16 with the ADONIS adaptive optics instrument at the ESO 3.6-metre telescope by Anne Marie Lagrange, Jean Luc Beuzit and Patrice Bouchet. One of these images is available as eso9522a. This is the sharpest image of the central condensation obtained so far and shows that the dense dust cloud around the nucleus has a diameter of almost 8000 kilometres (FWHM = 1.7 arcsec).
During a run at the IRAC2 instrument at the MPI/ESO 2.2-metre telescope on August 4-5, Chris Lidman and Patrice Bouchet obtained several infrared images, one of which is reproduced as eso9520a. It is a J-band image (wavelength 1.5 micron) that very clearly shows the extended, asymmetric dust cloud around the nucleus. It measures more than 1 arcminute in diameter.
At optical wavelengths, many CCD frames through U, B, V, R and I filtres were obtained by Herve Geoffray and Anne-Marie Lagrange on August 10 with the Dutch 0.9-metre telescope. At the Swiss 70-cm telescope, Pierre Bratschi and Stephane Paltani made similar observations during every night from August 3-16; they found a slow brightening of the innermost coma during the first nights.
Finally, a brief attempt was made to obtain infrared spectra with the IRSPEC instrument at the 3.5-metre New Technology Telescope, but the comet was still too faint for this type of observation.
Continuation of the ESO observations
Because of its unusual brightness, and in order to follow the future evolution of Comet Hale-Bopp, coordinated observations at La Silla are now being planned for the next two months. Thereafter, the comet will move behind the Sun as seen from the Earth and observations with the large telescopes will only become possible again in March 1996. There is no doubt, however, that from then on, its further development will be followed with great care by the observers. Better predictions for its brightness near perihelion will also become available. If further positions show that the 3000-year period is correct, it will also then be possible to confirm it independently of the presumed 1993 observation.
The comet will cross the celestial equator in early December 1996. At least until then, the observers at ESO will have a front-row seat and will continue to observe this most welcome object. Due to the comet's high northern location in March-April 1997, observations from La Silla will be virtually excluded at that time. But after perihelion, Comet Hale-Bopp will again move south and from late June 1997, it will again be situated in the southern part of the sky.
In this connection it is of some interest to note that Comet Hale-Bopp may be similar to the great comet of the year 1811 (orbital type; very bright at large heliocentric distance). That comet was still spectacular three months after perihelion, so perhaps southern observers will then still witness a fine display of Comet Hale-Bopp!
Future information about Comet Hale-Bopp
A special Comet Hale-Bopp area has been set up at ESO on which further news about this exciting object and the latest images from La Silla will be displayed.
Note also that further information about this comet will become available on the IAU Circulars (by subscription only) and the Astronomical Headlines, both published by the IAU Central Bureau for Astronomical Telegrams.
 Another relatively bright pre-discovery image of Comet Hale-Bopp (magnitude 11.7) has since been identified on a photograph taken on May 23, 1995, by Terry Dickinson, a Canadian publicizer of astronomy observing in Arizona. Moreover, Robert McNaught has found another Schmidt plate from Siding Spring, obtained on September 1, 1991, which shows the sky region where the comet was located at that time, but no image can be seen. The ESO collection has also been checked and no such plates were found, although two ESO Schmidt plates obtained in May 1993 and August 1994 very narrowly `miss' the comet.