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Comet Hale-Bopp (January 28, 1997)
MPI/ESO 2.2-m + EFOSC II (May 14, 1996)
This is a summary of recent developments around this comet; the previous was published on the ESO Web on January 08, 1997. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.
Richard M. West (ESO)
Munich, January 28, 1997; 17:00 UT
Hale-Bopp still going strong
The latest estimates of the brightness of Comet Hale-Bopp indicate a magnitude between 2 and 3, supporting the general feeling that this Comet is developing quite `normally', i.e. as predicted by the usual formula.
Nevertheless, a few amateur measurements have been received since yesterday which are somewhat on the low side, that is closer to magnitude 3 than 2. However, it is quite likely that these estimates have been influenced by strong moonlight and they should not give any reason for concern at this moment.
Thus the previous prediction of a maximum brightness somewhere between 0 and -1 still holds.
The really interesting time will come in early February when the Moon is no longer in the morning sky and the Comet will be well visible towards East in a completely dark sky some hours before sunrise.
Several people have asked about the apparent absence of a significant tail. This question is quite natural, since most comets observed in the past at magnitude 2-3 have had an impressive tail which could easily be seen with the unaided eye. But at this time the tail of Comet Hale-Bopp points more or less away from the Earth and since we therefore see it from the front, it does not appear as large and impressive as it really is. In fact, an experienced American observer, John Bortle, who also writes for Sky and Telescope, did see a 1.5 degree long tail about a week ago.
The coma diameter has been estimated at about 0.5 degrees, that is about as large as the Moon in the sky. This is one more indication that the Comet will become a fine sight just over a week from now, after the New Moon on 7 February. I have no doubt that we shall then begin to receive a flood of enthusiastic reports from happy observers.
In this connection, I would recommend all interested readers to have a look at the fine Austrian Hale-Bopp site at http://www.ast.univie.ac.at/~wuchterl/hale-bopp_at.html which contains much interesting information that is regularly updated. Here is an example of the lightcurve by Herbert Raab , as well as a recent image by Gerald Rhemann (Vienna) (obtained with a 225/255/435mm Schmidt camera at Ebenwaldhöhe, Austria, on 18 January 1997; Exposure 3 minutes on Kodak Pro Gold 400). See the above mentioned website for more details.
You may also like to visit the interesting site of the Swabian Observatory in Southern Germany.
Note also the weekly reports from Sky & Telescope.
New orbital elements have just been published by Don Yeomans which indicate shorter orbital periods than before, 4200 years before the present perihel passage, and 2380 years afterwards.
New ephemerides for observers are available at this Website since today.
The coma structure
Further information about the coma structure, as discussed in the previous summary of January 8 has been announced by Zdenek Sekanina (Jet Propulsion Laboratory, Pasadena, USA) and Hermann Boehnhardt (University of Munich, Germany), on IAU Circular 6542 (January 21). They find in particular that: The comet's persisting porcupine-like appearance, consisting of 6-8 nearly-straight jets in various directions, is conceptually understood as a product of dust emission from 3-4 discrete active sources on the rotating nucleus. The jet pairs (one pair per source) are interpreted as boundaries of fan-shaped formations..... One of several tentative solutions suggested by our computer simulation experiments that fit the observed jet-orientation pattern on several R-filter images taken with the large telescopes at the European Southern Observatory in Aug.-Nov. 1996, places the sunlit rotation pole near R.A. = 30 deg, Decl. = +45 deg (equinox 2000.0), which implies an obliquity of the comet's orbit plane to its equatorial plane of 140 deg. The three most active areas have polar distances of about 20, 55, and 130 deg. These results are insensitive to both the comet's rotation sense and its spin rate.
More high-resolution images obtained from ESO have been prepared by Ivo Saviane from Padova, Italy who has prepared a special page at http://obelix.pd.astro.it/~ivo/hb/. He refers to the already published image and writes: I set up a small W3 page showing one of the images, which I think could be interesting. In my opinion, an 8th jet can be seen, besides the already known 7 ones. I put there also the ESO image which shows the 7 jets, approximately rotated as my frame; the new jet is indicated by an arrow. Indeed, it's a very faint feature. The original frame has been processed in order to enhance faint features, in a way described within the same page.
In the same IAU Circular, M. Womack and collaborators report observations of comet C/1995 O1 with the NRAO 12-m telescope at Kitt Peak of H 2 CO at 218 GHz on 1996 Dec. 15 and 1997 Jan. 17. They also find a weighted average ortho-to-para H 2 CO ratio of 1.5 +/- 0.3 for this comet over the range of heliocentric distances 1.6-2.0 AU. This ratio is lower than that reported for H 2 O in comets 1P/Halley and C/1986 P1 (Wilson), and is similar to what is observed for H 2 CO in cold, dense interstellar clouds.
The latter finding, however, is being contested by D. Bockelee-Morvan on behalf the Meudon group in France . They write that it is impossible to measure the ortho-to-para ratio (OPR) using observations of only two lines. The intensity ratio depends not only on the OPR, but also on the populations of the rotational levels involved. These populations depend on the temperature of the gas and other parameters. Several lines are thus needed to get a good description of the rotational population distribution. They also refer to observations they have made themselves in mid-november 1996 of the same ortho and para H 2 CO lines Womack and colleagues observed and they see no reason for an anomalous OPR.
A Hale-Bopp school project
Astronomy educators and their students may be interested in a school project around Comet Hale-Bopp that is organised by the European Association for Astronomy Education (EAAE). You may find a description at http://www.algonet.se/~sirius/eaae/newsl4/hb.htm.
Note also the Comet Hale-Bopp Collaborative project of the Astronomy On-Line Programme.
During the past two weeks, I have received many E-mails from visitors to this Web page requesting various information about Comet Hale-Bopp in particular and comets in general.
I am sorry that I am unable to answer these questions individually, but I summarize below answers to some of the most common.
Many E-mails deal with the visibility of the Comet, that is when and where it may be observed in the sky. You will find several fine sky charts with the path of the Comet indicated, for instance a map for February , published by Sky & Telescope .
At the end of January, the Comet is about 15 degrees North of the celestial equator which means that it is easier to observe from the northern hemisphere than from the southern. As mentioned above, the best time is to go out in the early morning after February 7 and to look towards North-East. With a magnitude of about 2 or perhaps even brighter, even not very experienced observers will find it comparatively easy to see it as a diffuse small cloud . At that time, a tail may also be visible which points away from the horizon.
Influence on the Earth?
Quite a few have asked about the possible influence of the Comet on the Earth and ourselves.
Don't worry! First of all, Comet Hale-Bopp will never come very close to the Earth; the closest passage is in late March at a distance of nearly 200 million km, that is about 500 times the distance to the Moon or 1.3 times that to the Sun. Also, the Earth will not pass through the Comet's tail and it is unlikely that the dust particles which are being left behind by the Comet in its orbit will be seen as meteors now in 1997. Still, there is a certain possibility that some of them may later, perhaps some years from now, enter the Earth's atmosphere and then be seen as shooting stars .
Likewise, the Comet will not influence the atmosphere and the magnetic field of the Earth. It is simply too small and it is too far away.
The origin of Comet Hale-Bopp
Some have asked about the origin of Comet Hale-Bopp.
There is little doubt that this comet has passed several times around the Sun; see the orbital periods indicated above. Following the present passage, it will come back about 2400 years from now.
According to the current theory, it is most likely that this Comet was born about 4.5 billion years ago somewhere just outside the orbit of distant planet Pluto in the so-called Kuiper Belt (named after a famous astronomer of Dutch origin). Its nucleus, the dirty snowball , consists of icy and dusty material from the interstellar nebula that gave birth to the Sun and the planets.
Another possibility is that Comet Hale-Bopp was born somewhat closer to the Sun, between the orbits of Uranus and Neptune. In this case it may first have been expelled from this region of the solar system towards the so-called Oort Cloud of Comets (after another famous Dutch astronomer) which surrounds the solar system at a very large distance. From here, it may later have been pushed in the direction of the Sun and the planets due to the gravitational effect of a passing star, or, more likely a passing giant molecular cloud.
We do not know for sure which of the two possibilities is the correct one, but most astronomers are inclined to place the origin of Comet Hale-Bopp in the Kuiper Belt. Back to ESO Hale-Bopp Homepage