Article

By Jean Manfroid.

FNRS Director of Research at the University of Liège's Department of Astrophysics, Geophysics and Oceanography. This article was published in the Decemeber 2012 issue of the Liège Astronomy Society's journal, 'Le Ciel'.

The Belgian comets

Few places in Belgium are suitable for deep sky observation and even less so for observation near the horizon where comets that are hitherto unknown can appear. This explains the paucity of comet missions up to the present day in our country.

In 1941, Eugène Delporte (1882®C1955), the Belgian astronomer who established the current limits of the constellations, co-discovered the comet 57P/du Toit-Neujmin-Delporte from the Uccle observatory. The sky was a lot darker at the time than it is now, given that light pollution had been largely eliminated due to the war in Europe. At that tragic time, communications in Europe were difficult and slow, which explains why three individuals are credited with discovering the same comet. The complicated history of this comet probably ended in 2002 when astronomers working on the 2m20 telescope in Hawaii noticed that it broke up into a chain of mini-comets covering half a degree in the sky which is equivalent to a million kilometers. This is not to forget the comet Shoemaker-Levy 9 which broke up while passing near Jupiter in 1992 with the resulting debris crashing into the giant planet two years later.

In 1951, a somewhat auspicious year, two periodic comets were discovered at Uccle: 
49P/Arend-Rigaux  and 50P/Arend. In 1956, it was again at Uccle that a new comet was discovered, the beautiful Arend-Roland.

Then there was a long period of inactivity before a new comet was found by a Belgian, the comet Heck-Sause (C/1973A1), which was named after Liege astronomer André Heck and his French assistant Gérard Sause. At the time astronomical observation was organized far from Belgium, and it was at the observatory of Haute-Provence that this discovery was made.

We mustn’t forget a sixth Belgian comet 133P/Elst-Pizarro which had the peculiarity of being a comet and asteroid at the same time, or neither one or the other and which recurrently displayed a spindly tail instead of the usual plume, 133P is part of the select category of the “main belt comets” (MBC). This discovery was made even further away, in the pristine skies of the Chilean Andes.

Tracking comets

It was only in the 18th century that it was established that apart from spectacular comets which are visible to the naked eye, there were others which were more numerous but so weak that the astronomers of the time, particularly Charles Messier (1730-1817), compiled lists of nebulous objects that could be confused with them. Tracking comets required a perfect knowledge of the sky which had to be tirelessly studied. Many amateurs have used this method which has become more and more neglected since the arrival of electronic sensors. Today, telescopes on the ground and in space comb the sky and comparison with databases quickly reveals the presence of a new “intruder” no matter how faint it may be. 

The transition between these two methods was photographic imaging, and it was by means of this technique that all the Belgian comets were found. The advantage of photographic plates with an appropriate telescope was that a large field could be recorded. However, developing the photos took time and the comparison of negatives with atlases or previous negatives was very laborious. At that happy time, amateurs could still nurture the hope of visually discovering a beautiful comet while the professionals more or less systematically combed the sky with a Schmidt camera making it possible to record fields of several degrees at a time.  Telescopes such as the Schmidt of the Mount Palomar observatory were abundantly used for this type of work with real success. Other lesser-known telescopes occasionally carried out such observations among other tasks which were imposed by the various research programs.

A comet for Liege

One such case was that of the Franco-Belgian Schmidt (Grand Schmidt) telescope at the observatory of Haute-Provence, where an important part of the observations were dedicated to the study of known comets, but where new comets could sometimes be searched for. This opportunity yielded results as it led to the discovery, forty years ago, of a “Liege” comet and the only one to date, the C1973A1/Heck-Sause. This is also the only comet ever to have been found at the observatory of Haute-Provence.

In the interest of initiating studentse into astronomical observation, series of fields were chosen which combined astronomical interest and possibility in order to find a comet. The aesthetic aspect was not neglected and fields showing galaxies, nebulae or clusters were prioritized as a motivating factor. The Virgo cluster of galaxies was one of the targets and the presence of many diffuse stars tested the shrewdness of the competing observers. The act of finding a comet cannot be improvised. If chance has an important role to play it is better to lend it a helping hand.   Thus, it is not very useful to search for brilliant new comets in the middle of the sky at night; it is highly unlikely that they would be allowed to arrive there unnoticed. On the other hand, weaker objects might well have been neglected. Brilliant objects can appear near the horizon at dawn or dusk which until then were hidden by the light of the sun.  However, relatively bright comets could be more easily found by the visual observation of amateurs who rapidly studied the sky with open binoculars or telescopes. It would have been pointless to try to compete with them using an ill-adapted photographic technique. The researchers from Liege therefore applied a comprehensive strategy in order to maximize their chances of success with the Schmidt telescope by concentrating on faint comets. The exposures, which were several tens of minutes long only made it possible to comb a very small part of the sky each night, but this could be done in depth.

These long exposures required careful and continuous guidance of the eyepiece in the absolute darkness of a dome that was often freezing accompanied by the incessant ticking of sidereal and universal time clocks.  One the exposure was finished, the photo was developed then examined while still wet in case something appeared. An in-depth study followed the next day when the film was completely dry.

Comparison of the images with existing atlases couldn’t be done by computer as is the case nowadays. It was a laborious process which required starting with maps that were not very detailed such as those of the Star Atlas of David Phillip Norton or the Coeli Atlas of Antonin Bečvář in order to identify the general field, then to progress with more detailed atlases (for example Bečvář’s Borealis, Eclipticalis and Australis Atlases) to finally produce the large photographs of the Palomar Sky Survey. Another technique consisted in repeating the same fields at intervals of one hour and comparing them with an apparatus that made it possible to rapidly sequence the images. A mobile object then seemed to jump between two positions. This effect is currently reproduced much more conveniently with the “/blink/” function of astronomical instruments. Calculations of scale, interpolation of coordinates and precessions all had to be done manually, with all the risk of error that this entails while working in haste. The positions were traced onto tracing paper but sometimes directly onto negatives and maps. Thus the hesitations and hopes of our predecessors can sometimes be detected in the archives.

The research strategy with the Grand Schmidt telescope proved worthwhile when, on the night of the tenth to the eleventh of January 1973, André Heck and Gérard Sause noticed a diffuse-looking intruder among the Virgo cluster of galaxies. Two extra negatives taken the same night showed that the object had moved. Therefore it was indeed an object of the solar system, it was a comet. It was still necessary to verify that this was not a comet that was already known and which had been discovered very recently and not yet officially catalogued. These verifications required some phone calls and the precious help of François Dossin (1927®C1998), it was established that it was indeed a new comet. A telegraph was then sent to the CBAT. It was still nighttime in the US and François Dossin and Jean-Pierre Swings were able to warn the observatory at Mount Palomar where the astronomer Wallace L.W. Sargent (1935®C2012) succeeded in capturing an image with the famous 1m22 Grand Schmidt telescope, barely 12 hours after it was discovered.

As it was the first comet found in 1973, it received the provisional designation 1973a, in accordance with the system in operation at the time. Now that comets are being discovered in greater numbers the designation was changed to C/1973 A1 (first comet of the first half of the first month of 1973). It was also given the designation 1972 VIII in accordance with a system that has since been discontinued (the eight comet to pass the perihelion in 1972).

Without being spectacular, the Heck-Sause comet is quite an imposing object. Its size which was hastily announced to be 12 was seen to be closer to 9 or 10 when the remaining negatives were analysed. If we consider that the comet was far from the sun (r=2.72 astronomical units) and the Earth (Δ=2.25 AU) we can say that it was intrinsically brilliant. 

By taking account of the geometric effects alone, the magnitude of a comet must be corrected to 5(log(r) +log (Δ)). This relation expresses the idea that an object that is ten times further away from us appears to be a hundred times less bright and therefore loses 5 magnitudes. At the same time, if it is ten times farther than the sun it receives one hundred times less light and loses another five magnitudes. The application of this formula gives a magnitude of 5 for our comet at the distances r and Δ of 1 AU, ensuring that it is visible to the naked eye. As the activity of a comet increases considerably on approaching the sun, its brightness will be greater. To take account of this we empirically correct ®C and locally ®C, the above formula with an extra empirical term n*log(r), the factor n which can be as high as 10 or 20 which would signify an additional gain of 4 to 8 magnitudes.  It is therefore likely that, if it approached Earth’s orbit, the comet would have been spectacular.

The perihelic distance of the comet was more than 2.5 AU, and this point had already been passed at the moment of its discovery. Heck-Sause was moving away inexorably. Analyis of the orbit indicates a very long period. Undoubtedly the comet was making its first appearance in the vicinity of the sun having come from the Oort cloud, that vast reservoir of frozen heavenly bodies on the edge of the solar system.

Spectrums were rapidly taken at the observatory of Haute-Provence and they only showed the light reflected by the dust. The dilution of the solar radiation at these distances was not sufficient to set off the activity of the ®C gases, the factor n as mentioned above takes account as much as possible of this activation of gases near the sun. Not all comets behave like this. Astronomers had noticed the analogy with the comet C/1950 K1 (Minkowski) which, twenty years earlier, at an equivalent distance from the sun, showed practically no gas fluorescence in contrast with C/1961 R1 (Humason) which only showed a little dust and a lot of gas.

On the first negatives, the comet was seen to have a tail of half of one degree. The observations carried out over the following months revealed a tail extending for more than one and a half degrees in March 1973 (E.I. Shchukin at Irkoutsk with an astrograph of 50 cm). The comet was at its nearest to Earth on 11 February. It is a pity that it was not possible to use the deep negatives taken by the Franco-Liege Schmidt at this period. However, a hard copy of one of them (page 384), taken in the night of 29 to 30 January remains showing the considerable extension of the comet.

After the announcement of the discovery, the comet was found on negatives taken on the 4th of January by M. Koishikawa of the municipal observatory of Sendai, which shows the importance of a careful examination of images in order to avoid missing a discovery. Perhaps this observer had adopted the same strategy as André Heck, without carrying it through to the end.

The comet passed the perihelion on the 5th of October 1972 in the southern hemisphere. Drowned out by the brightness of the Sun, and at two AU behind it, it was unobservable.  Climbing rapidly into the sky in December and January 1973, it became a target for comet hunters. In summer, observations were prevented due to a weak solar elongation, but from September the comet was photographed with Grand Schmidt telescope by François Dossin and it was seen to be astonishingly bright despite its distance away.

The last images were taken on the 25th and 26th of January 1974 at more than 5 AU from the Earth and the Sun (François Dossin with the Grand Schmidt of Haute-Provence, and Elisabeth Roemer with the 2m29 telescope of the Steward Observatory).

The condensed object was given a magnitude of 19.6 by Roemer, but the image taken by the Liege Schmidt a few hours earlier seem to indicate a much brighter object than this estimate. The absence of reference stars is the main reason for these differences.

The comet is now an insignificant point in Pisces at 63 AU, two times further than Pluto and Neptune (at 30 and 32 UA respectively) and Halley’s comet (33 AU) which passed the perihelion in 1986. By comparison, the voyager probes launched in 1977 have been much faster and are now at 123 and 100 AU. As for the contemporary of Heck-Sause, the comet Kohoutek, it took an entirely different direction and is now moving through the Gemini constellation at around the same distance, 64 AU.

Though the scientific contribution of the Liege comet was not so important, we can be certain that it was educationally very beneficial to the students who participated in these observations many of whom have become teachers. This episode constituted the astronomical experience of their lives and undoubtedly they still speak to their own students about it: real astronomy in a real observatory, the routine of observations, the excitement of the discovery, the doubts and the verifications not to forget the media aspect.

The Schmidt telescope of Haute-Provence.

The Schmidt telescope of the observatory of Haute-Provence (the « Grand Schmidt » as opposed to another more modest telescope from the same observatory) is a large field telescope made up of a primary spherical mirror of 87 cm in diameter and a focal length of 209 cm (cf A. Heck, L’astronomie, 1973, p 241). A corrector plate 62 cm in diameter is placed at the center of the curve of the apparatus which gives it an opening of 3.36. The scale of the images is around 100 arcseconds per millimeter. Their diameter of 16.5 cm corresponds to a field of 4.6 degrees. Such a field is still out of reach of CCD sensors due to the fact that the focal surface is spherical. Photographic films existed in such dimensions and could easily follow the focal surface without difficulty. In order to benefit from a better stability of the emulsion, they were replaced by photographic plates after the perfection, ®C following a lot of damage, ®C of a system making it possible to cause them to swell in order to follow the spherical shape of the focal surface of the Schmidt telescope.

Modern instruments approaching the same field characteristics have to use more complex optics, mosaics of sensors, complicated observation strategies, and all of this backed up by very heavy computer equipment.

The idea of this Franco-Belgian telescope dates from 1959, and the definitive installation took place in 1970. The essential element of the Liege observation program consisted in tracking comets with a view to studying their shape: structure of the tail, the comas, and their development. Monitoring heavenly objects was done with an eyepiece by means of a guiding glass fixed to the telescope. This solution which works well for fixed stars is imprecise where comets are concerned as their core is often diffuse or too faint. An ingenious system was perfected making it possible to program a movement of the eyepiece which compensates for the movement of the object. The guidance could then be done, not on comets but on a neighboring star.

The death of photographic emulsions signified the end of wide-field imaging on this telescope and lead to a reconversion of sorts with a small-dimension to a sensor.