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 March 25, 1997. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

Please note that during the hectic phase in March - April 1997, additional, very recent information may be displayed on the front page of the ESO Comet Hale-Bopp Homepage.

A brief introduction to this Update

Some of the news items presented below have already been displayed during one of the preceding days as Latest News at the ESO Comet Hale-Bopp Homepage. However, since some interested visitors to this site may have been away (the Christian Easter holidays were celebrated in many countries during the last weekend), they are here re-edited and combined with other items of the same general category with the aim of providing a better overview of the current situation.

The information in this Update is presented under the following headings (click to jump):

1. Hale-Bopp passes perihelion

2. Where to observe the comet

3. Observing conditions

4. The intrinsic brightness

5. About the tails

6. The comet's orbit

7. Lunar eclipse on March 23-24, 1997

8. No splitting of the nucleus observed

9. Grains in the coma

10. More molecules observed

11. How much water does Hale-Bopp produce?

12. List of detected molecules

13. Rocket launch successful

14. More images and how to make them

15. Hale-Bopp on the Web

16. Various research papers published

17. More space-based observations to follow

18. Replies to common questions

19. Miscellaneous information

1. Hale-Bopp passes perihelion

Comet Hale-Bopp passed its perihelion - the orbital point closest to the Sun - early this morning at about 03:19 UT. At that time the distance to the Sun was about 137 million km and the orbital speed 44 km/sec. The distance to the Earth was about 202 million km while the comet was moving away from us at a speed of 13 km/sec.

It is now located about 43 o from the Sun in the sky and is moving at a rate of about 1.7 o /day.

The next perihelion passage will take place some 2380 years from now, in the late 44th century.

2. Where to observe the comet

During the past week, the comet's magnitude has been stable around -0.7.

Hale-Bopp is now a wonderful evening object, high above the northwestern horizon. Don't miss this great opportunity to see a really bright comet!

Another Sky & Telescope Newsletter (March 28, 1997) is available with information about Hale-Bopp and also the recent Lunar Eclipse.

There has been a tremendous interest in the animated finding chart for Comet Hale Bopp by the Soenderborg group announced in the last Update! As a result, the line appears to have been completely overloaded, and we have now installed a copy [GIF; 8k] of this file at the ESO site.

On March 25-26, Hale-Bopp passed a few degrees north of the Great Andromeda Nebula , a galaxy similar to the Milky Way galaxy in which we live, and consisting of hundreds of billions of stars like our Sun. It is about 2 million light-years distant, i.e. its light takes about 2 million years to reach us, travelling at a speed of 300,000 kilometres per second. The 'fuzzy spot' seen south of the comet on those days by many observers with binoculars was therefore most certainly this galaxy.

3. Observing conditions

The present, optimal observing period will last until new moon on April 7, after which moonlight will begin to interfere in the evening. The very best time will probably be during the first days in April when the comet is expected to be at its maximum brightness and the tails are the longest and most impressive.

As examples, here are two excerpts from recent, typical reports by experienced observers, reflecting the importance of the observing conditions:

Mar. 26.85: excellent conditions, Zodiacal light is glaringly obvious; comparison basically with Alpha Aur, Beta Ori, and Sirius; the comet rivals Mars, but it wasn't used because of the difference in color; to the naked eye, strongly-clockwise-curved dust fan spanning p.a. 340-350 deg; 10-deg gas tail near p.a. 5 deg (dust tail much brighter than gas tail) [Alfredo Pereira, Cabo da Roca, Portugal].

Mar. 28.07: to the naked eye, the comet looks like `Rigel with a tail', although the star is about 7 deg higher up, and it is dramatically inferior to Sirius in brightness; there is a 10-deg ion tail in p.a. 3 deg and a broad 16-deg dust tail that is initially directed toward p.a. 348 deg [J. Bortle, Stormville, NY].

The ion and dust tails, now seen on a competely dark sky background, have lengths of up to about 20 o and 15 o , respectively.

The full span of the tails in the sky is only visible at completely dark sites and after full night adaptation of your eyes - this may take from 15 - 30 minutes. It should therefore be no surprise if you do not see a 20 o tail the moment you step out of your house! Experienced observers, whose reports indicate tail lengths of this order always prepare themselves carefully and know where to find the `darkest' sites before they estimate the tail lengths.

4. The intrinsic brightness

As mentioned in the Update of March 17 , Comet Hale-Bopp is definitely one the intrinsically brightest comets ever observed.

Jacques Sauval (Belgium) has commented on this point: Comet 1729 (Sarabat) with perihelion distance 4.05 AU and apparent magnitude m1 = 5 is also one of the most luminous comets (absolute magnitude H 10 around -3; a probable diameter of about 100 km (?) as found in the literature), but it remained barely visible to the naked eye, in addition to the famous and very bright comet of 1577.

5. About the tails

The following email has been received from Andrew Fastow in response to an earlier comment, posted under `Latest News': After reading your web site [March 30], I was walking along looking at the comet from 8,000 ft up in Utah [USA]. I recalled that your web site said the comet is 125 million miles away and its tail is 60 million miles long. It suddenly struck me that the tail should look much longer than it actually does with the naked eye. If for example I was looking at a 60 ft wide building from 125 ft away it would obscure much more of my field of vision than the tail of comet did. I would estimate that the visible tail of the comet appears to be about 2 million miles assuming it's 125 million miles away. Is this because the visible part of the tail to the human eye only a small fraction of the total length of the tail, or is the comet and thus its tail moving at an angle such that the tail appears to be a lot smaller than it actually is? Probably a combination of the two? Please could you enlighten me on this issue; how much of the tail is visible to the naked eye and what angle is the comet moving relative to a mountain top in Utah? Thank you.

The answer is indeed a combination of the two effects - see also the remark above about the need to dark-adapt your eyes. Once you have estimated the angle spanned by the tail in the sky, you may calculate the `true' length of the tail.

Important addition on April 2: Knud Jepsen, physics teacher at Haslev Gymnasium (Denmark) and member of the Executive Council of the European Association for Astronomy Education (EAAE) has pointed out that the tail is even longer than indicated by the simple formula given in the February 26 Update , which is in fact only valid when the tail is very short. For a full explanation with the correct trigometric formulae, please look at the explanation prepared by him. With an angular extent of the plasma tail of 20 o , it results in a true tail length of 148 million km, or almost 1 AU! (My apologies for having overlooked this and many thanks to Knud Jepsen for calling attention to this problem!)

Deep exposures with wide-angle cameras would most certainly show an even greater extent.

6. The comet's orbit

New orbital elements and an ephemeris by Don Yeomans have just become available.

7. Lunar eclipse on March 23-24, 1997

Bob Yen announces the availability at his website of images from the March 23-24 lunar eclipse. He was able to get clear skies for the lunar eclipse, and shot the comet during near-totality. He had to drive 350 miles to dodge the local weather -- searching around in desert at night for my site. `Real fun', he says!

8. No splitting of the nucleus observed

Various comments have been received about the photo in the March 25 Update , possibly showing a splitting event. For instance, Mark McCaughrean (Max-Planck-Institut fuer Radioastronomie, Bonn, Germany) finds that there are enough bright star trails in the wide-field image to see that they all have small `companions' at exactly the same distance and position angle as the apparent fragment does from the body of Hale-Bopp. Also Thierry Pauwels (Royal Observatory of Belgium, Brussels) had a look and there is now no doubt that the effect is due to a `guiding error' (a `jump') during the exposure, or possibly a `double' exposure. Thus it is not real and no splitting took place.

Another obvious example of this effect is a `double' image obtained by Natale Suardi (Circolo Astrofili Bergamaschi, Bergamo, Italy). He writes: I would like to draw your attention, and possibly to have your comments, on the image [GIF; 27k] taken on March 16... [with] 180 mm f/2.8 Nikon. Image processing: gamma correction. The anomalous feature on the left is not present in the images taken some minutes before and after.... You may wish to look at several other very nice images on their Hale-Bopp homepage Sara la cometa del secolo? (Will it be the comet of the century?).

So if you happen to obtain such an image with your equipment, be careful to check how the other objects in the field look like!

9. Grains in the coma

An infrared image of Hale-Bopp obtained with the 200-inch telescope at Mt. Palomar on February 19 is now available at a Cornell University site. It shows (at wavelength 10.3 microns) the comet and submicron-size dust grains being ejected from the surface as the comet is warmed by the Sun. Some of the grains are crystalline, in contrast to the more amorphous structure of the rest, indicating that the grains were subjected to strong heating sometime in their history, before they were incorporated into the frozen comet nucleus about 4.5 billion years ago. This photo was taken using a combination spectrograph-camera designed and built at Cornell for the 200-inch telescope at Mt. Palomar. The photo is part of a Press Release referring to a corresponding article in Science , see also below.

In this connection, note also the information about olivine found in Comet Hale-Bopp.

10. More molecules observed

Laura Woodney (University of Maryland) and collaborators have announced on IAU Circular 6607 (March 29) that In observations at the NRAO 12-meter on March 23 UT we detected H 2 CS for the first time in Hale-Bopp....Assuming a rotational temperature of 80 K we derive a production rate of 5.4 x 10. OCS J(12-11) [was] measured on March 22 UT... [and] using the same temperature the production rate of OCS is 6.1 x 10 27 mol/sec. We also observed H 2 S and mapped CS during this period.

N. Dello Russo (NASA) and colleagues report (IAU Circular 6604, March 26) the detection of water in Comet Hale-Bopp on Jan. 21.5, Feb. 23.9-24.1, and Mar. 1.7-1.9 UT, using the NASA Infrared Telescope Facility (+ CSHELL cryogenic infrared spectrometer) on Mauna Kea....The water-production rate (x 10 30 molecules/sec) was 2.7 on Jan. 21.5, 4.3 on Feb. 23.9, and 4.0 on Mar. 1.8, assuming a rotational temperature of 70 K and an ortho-para-ratio of 2.45. After detailed data reduction, we report that spectral lines of HDO (100 vibrational band) were not detected, contrary to the report on IAUC 6573. The preliminary reductions were adversely affected by high and variable telluric HDO. (`Telluric' means in the Earth's atmosphere).

An account by astronomers from Ohio State University and Lowell Observatory (USA) was published in the journal Science (see also below ) which confirms the existence of cyanogen gas in Comet Hale-Bopp while the comet was still beyond the orbit of Jupiter. At some moment, it was thought that cyanogen gas is released only after a comet has entered the H 2 O sublimation zone (approximately when the temperature increases to about the freezing point), i.e. at about 3.0 AU (450 million km) from the Sun. However, these observations were made when Hale-Bopp was 6.5 AU from the Sun. Sublimation or evaporation at this distance may be driven by things other than water - carbon dioxide or carbon monoxide, for example. In this connection, the detection of cyanogen gas in the asteroid (2060) Chiron which was made some years ago at an even greater distance, is also of significance.

On IAU Circular 6607 (March 29), M. K. Bird (University of Bonn, Germany) and collaborators at the Max-Planck-Institut fuer Radioastronomie (MPIfR, Bonn), report the detection of ammonia (NH 3 ) in comet Hale-Bopp using the 100-m MPIfR radio telescope at Effelsberg near Bonn. They measured the three individual K-band lines of this molecule, i.e. (3,3) at 23.870 GHz, (2,2) at 23.723 GHz, and (1,1) at 23.695 GHz and found that the lines are centered symmetrically on the velocity of the cometary nucleus. The full width at half-maximum of the (3,3) line is 1.6 km/s. No noticeable changes in the strengths of the lines could be detected between the two observation dates on Mar. 13.4 and 25.5. The observation of this molecule in a comet is a rare event.

11. How much water does Hale-Bopp produce?

In official announcements as those above, the productions rates of various molecules are normally expressed in molecules per second. By what do they mean in units that are closer to daily life? Here is some information in reply to this question posed by Earl Crockett (USA).

In order to calculate the corresponding production rates Q H 2O (tons/sec), remember that 1 gramme (g) (= 10 -6 tons) = 6 x 10 23 atomic mass units (a.m.u.) (Avogadro's number) and that one water molecule (H 2 O) weighs 2 x 1 + 16 = 18 a.m.u.. Thus you have to use the following formula:

Q H 2O (tons/sec) = Q H 2O (molecules/sec) x 18 x 10 -6 / (6 x 10 23 )

The above measured water production rate of Q H 2O = 4.0 x 10 30 molecules/sec on March 1.8 therefore corresponds to approx. 120 tons/sec. You may use the same formula for other molecules by substituting `18' with their respective atomic masses. For instance, the above indicated production rate for OCS of 6.1 x 10 27 mol/sec (atomic mass 16 + 12 + 32 = 60) corresponds to Q OCS = 0.6 tons/sec = 600 kg/sec.

12. List of detected molecules

Here is an updated list of molecules whose detection has so far been announced in IAU Circulars or in research papers:

and the following isotopes:

Is this list complete? Would anybody who has additional information kindly contact the author via the email address given at the bottom of this page? Thank you in advance.

13. Rocket launch successful

Information is available from Andrew Yee (Toronto, Canada) about the appearance on the web of an article by Eduardo Montes (Associated Press Writer) who on Monday night watched the successful lift-off at White Sands (USA) of the first rocket of four which will collect data on the Comet's composition, including gas emissions and dust particles. Each carries a different set of instruments.

It appears that no further information about this event is available at the moment of publication of the present Update.

14. More images and how to make them

Jost Jahn calls attention to the well-organised Hale-Bopp picture site now available with German and English texts. And here is another site with great pictures in Northern Bavaria.

The New York Institute of Photography has issued guidelines for photographing Comet Hale-Bopp. Roy Weinstein of that Institute writes that they have been amazed about the public interest in this subject. Visit their special Hale-Bopp page and learn all the details.

A series of recent new fine images from the Crni Vrh observatory (Slovenia) is now on the Web.

Another fine view has been published at the IAC site (Canary Islands, Spain). And here is an image [GIF; 48k] obtained by Aldo Macri in the early evening of March 25 when the comet was seen from a little hill near Napoli (Italy) called Camaldoli.

While we are in Italy, don't miss the impressive image by Matteo Maturi who characterizes himself as a young italian boy (18 years), who hopes you will like the image [JPEG; 82k] that I took by the morning of 16/03/97 on the sky resort of Madonna di Campiglio on the Brenta Dolomites with a Doi lens 500mm f/8 on TP 2415 HP . Well done, Matteo!

On March 26, the question was posed here whether anybody had taken a photo with the reflection of the comet's image from a water surface? A quick response was received from Sebastian Fleischmann and Frank Lungenstrass (Sternwarte Solingen, Germany): One of our members has taken such an image two weeks ago on the North Sea coast. It shows Hale-Bopp over a lake there. In fact, it is a very beautiful view and we wanted to present this picture in the Web, but we had some technical problems. We expect that it will now appear at our site (see the Other Sites' page ) next weekend .

Meanwhile, a beautiful image of the comet, with its reflection in the calm surface of a lake and taken by Bill Schwittek (Peachland, North Carolina, USA) is now available the JPL Hale-Bopp site.

15. Hale-Bopp on the Web

There has been another, dramatic increase in the number of 'hits' at the many Hale-Bopp websites all over the world, reflecting the still growing interest in the events around this unusual object.

Ron Baalke at JPL reports that his site passed the million mark on March 29, receiving no less than 1,243,749 hits in one 24-hour period. With the exception of some of the major news servers, few other sites in the world receive so many visitors. This is most obviously also an all-time high for any astronomy-related site!

The ESO Hale-Bopp web-area is now receiving up to 65,000 hits per day, of which over 10,000 are on the front page with the Latest News alone.

16. Various research papers published

A series of seven research papers about Hale-Bopp observations have been published in the latest issue of the journal Science (note that the abstracts and full texts are available to subscribers only). However, more information about some of these papers has already become available in the form of Press Releases.

A Press Release from the Space Telescope Science Institute about observations with the Hubble Space Telescope (HST) and the International Ultraviolet Explorer (IUE) is now available on the Web, with information from detailed imaging and spectroscopic studies carried out during the past year by an international group of scientists, headed by Harold Weaver of the Space Telescope Science Institute (Baltimore, USA). The findings include, in particular, violent eruptions from the surface of the nucleus, important non-uniformities on this surface and a new estimate of the size of the nucleus (30 - 40 km in diameter). More information is now available in a condensed version of the science paper.

The IUE spacecraft was a most successful joint NASA/ESA project which delivered a wealth of ultraviolet spectroscopic data between 1978 and until it was turned off in September 1996.

Information about the HST is also available from the joint ESA/ESO Space Telescope European Coordinating Facility (ST/ECF) which is located at the ESO Headquarters in Garching (Bavaria, Germany).

17. More space-based observations to follow

News about observations of Hale-Bopp with NASA's Polar spacecraft has been placed on the Web. The text also contains an explanation why the HST cannot observe Hale-Bopp at this moment. More specific details about these observations and this spacecraft may be found at another webpage: Observations of Comet Hale-Bopp with the Imagers on the Polar spacecraft.

18. Replies to common questions

The ESO Hale-Bopp site continues to receive emails from numerous visitors. Unfortunately, many forget to indicate their email address, thus making it impossible to provide a direct answer to their questions.

Some of these questions are very detailed and it would require a significant effort to provide answers, others are of a more general nature. Here are a few replies to the 'most asked qustions' of the latter category in the hope that they will be useful, not only for those who asked them.

The ESO Hale-Bopp website is based in Garching near Munich, in southern Germany; it is run by the European Southern Observatory (ESO) , the major European astronomy organisation supported by eight European countries. ESO operates a major astronomical observatory at La Silla in Chile and is building the Very Large Telescope (VLT) that will become the world's largest optical facility a few years from now.

As is usual on the European continent, the distances at this site are given in kilometres (km) and, for astronomical measures, in Astronomical Units (AU) . 1 AU (the mean distance from the Earth to the Sun) is about 149,600,000 km. One mile = 1.609 km, so in order to convert a distance from km to miles, as used in the UK and the USA, you must divide the indicated km-distances by this number. For instance, today at noon (April 1, 12:00 UT), the distance from the Earth to Hale-Bopp is 1.355 AU according to the ephemeris (the value of Delta ); this corresponds to 202.7 million km, or 126.0 million miles.

The diameter of the nucleus is believed to be no smaller than 20 km and no larger than about 40 km, but it has not yet been possible to make an exact measurement. If it ressembles the nucleus of Comet Halley that was observed in 1986, it consists probably of about 80 percent water ice; the rest is made up of other ices (frozen gases, mostly CO and CO 2 ) and dust grains of various sizes, of which many contain silicates and many are `organic', i.e. they contain molecules with carbon atoms.

When the surface of the nucleus is heated by strong sunlight, the ices sublimate (their molecules pass straight into the gaseous form), so that an `atmosphere' (the gaseous coma ) is produced around the nucleus. Again mainly due to the action of the sunlight, especially the high-energy ultraviolet part, some of these molecules are broken down into smaller molecules or even individual atoms. Some of the particules become electrically charged (and are then referred to as ions ). They are accelerated in the direction away from the Sun by the ambient magnetic field, thereby producing the ion tail . A gas of ions is also called a plasma , hence the ion tail is also known as the plasma tail . The current length of Hale-Bopp's plasma tail is at least 100 million km, or more than 60 million miles.

When gas escapes from the nucleus, it carries along many of the dust grains and they form a dust coma , a dust cloud around the nucleus. Thus the coma is a mixture of gas and dust. It is possible to 'separate' the two by obtaining images of the coma through different optical filters. For instance, images obtained in violet and blue light mostly show the gas and images in red light mostly show the dust. The dust in the coma is pushed away by the pressure of the sunlight and forms a broad dust tail . In Comet Hale-Bopp, the reddish dust tail is seen to the right of the more blueish ion tail at this moment.

Another frequently asked question is why Comet Hale-Bopp is often referred to as C/1995 O1 ? This is a technical designation used by professional astronomers. The `C' serves to indicate that this object is a long-period comet ; an `A' in this place would refer to an asteroid; `P' is a comet with a period shorter than 200 years; `D' is a `dead' comet, e.g. Shoemaker-Levy 9 that collided with Jupiter in July 1994. `1995 O1' indicates that it was the first comet (`1') found in the second half of July 1995 (`O'); the count starts with `A' for the first half of January, `B' in the second half of that month and the letter 'I' is not used. Thus, the first comet to be discovered in the first half of April 1997 would get the designation `C/1997 G1'.

19. Miscellaneous information

In the morning of Sunday, March 30, at 02:00 Central Europe Time (CET), summer time - also known as Daylight Saving Time - was introduced in many countries on the northern hemisphere, including almost all in Europe, when the clocks were advanced by one hour. The Central European Summer Time (CEST) is two hours ahead of Universal Time (UT) , i.e. UT = CEST - 2h . Observers of Hale-Bopp are reminded that time indications in their reports must always be given in UT. The summer time in Europe will continue until the last weekend in October.

And finally, a friendly warning! It will not have gone unnoticed that this splendid comet is passing its perihelion on April 1. In many countries (but not in all!) this particular day is traditionally associated with jokes which may range from kind and innocent, really funny ones to those which are less easy to recognize as such. A number of jokes connected to Hale-Bopp have appeared on the Web - most of them are quite obvious (e.g., the sender is an obscure society with the remarkable acronym F.O.O.L.S. !) - others less so. What is clearly a joke for people of one culture may not be so for those of another. So it may not be a bad idea to be a little critical if you come across somewhat strange messages on the Web in these days. Just to be sure: to avoid the risk of any misunderstandings, the ESO Hale-Bopp site will not bring any such messages , and you may safely assume that all information posted here, also today, contains news of real substance.