|
|
 |
 |
 |
 |
 |
 |
 |
 |
Alan Hale
|
In the early days of comet hunting amateurs were free to search as much of the sky they wanted with no clear competition from the professionals, they were visual searchers who were limited only by their determination and the field was wide open to them. Times have changed since those days and now amateurs face a real threat from a host of prolific professional search systems that have emerged onto the scene since the late 1990’s…and for good reason! These surveys have been employed to scan the skies for earth threatening asteroids and comets that could some day pose a serious threat to civilization. Their goal is to find 90% of |
Thomas Bopp
|
the potentially hazardous objects out there with a diameter of over 1km. As a consequence the number of amateur comet discoveries has declined since that time. A large number of these robotic patrollers of the sky are now in operation with names like LINEAR (Lincolin Institute Of Near Earth Asteroid Research), NEAT (Near Earth Asteroid Tracking), LONEOS (Lowell Observatory Near Earth Object Search), SPACEWATCH, CSS (Catalina Sky Survey), SWAN (Solar Wind Anisotropies) and SOHO (Solar And Heliospheric Observatory). Using examples of the research completed by Shigeki Murakami I hope to show that amateurs can still compete and continue making discoveries despite the threat from these automated surveys. |
 |
SWAN is one of the instruments on board the solar observing satellite SOHO to observe Lyman – alpha lines (UV) emitted by hydrogen. The images by SOHO cover most of the sky and are available to the public at its web site. Comets are visible in SOHO’s images because their main component is H2O. If a comet is brighter than magnitude 10 and has sufficient gaseous emissions then it can be picked up by SWAN however is this instrument as efficient as it sounds? A magazine article about the discovery of C/2002 O6 helped to increase the anxieties of comet hunters. It stated that comets of magnitude 11 are now observable by SWAN and that it's capability to |
cover most of the sky will make amateur comet hunters work increasingly difficult. Mr Seiichi writes that he has learned that all the visually discovered comets in the past year were captured by SWAN without exception….he continues 'it is not just a passing concern that comets may not be discovered visually any longer. Suddenly fears crossed my mind: are my observing method and observing instruments appropriate? Do I still have a chance if I look for fainter comets? I started thinking about strategies'.
- To sweep the skies at higher altitudes in order to find faint comets sacrificing the low altitude skies. This will make it possible to sweep the skies which do not overlap with LINEARS coverage (discussed later).
- To use magnifications twice the aperture in cm, as the minimum effective power would show objects of one or two magnitudes brighter than the limiting magnitude of the aperture.
- To observe from sites with low light pollution and at a high elevation, even if it means a longer driving time. It is reported that with every 3000 feet (914m) the visibility of the object will be increased by 0.5 to 1 magnitude.
The availability of SWAN images to the public is not a recent occurrence. It became well known after the observations of C/1973 E1 (Kohoutek) by rockets, that comets emit Lyman – alpha lines. Swan images are displayed by ecliptic coordinates. SOHO is stationary at the Laglangian point 1.5 million km from the earth towards the sun. The directions to the earth and the sun are excluded from observation of Lyman – alpha emissions, which correspond to areas of approximately 30* X 30* and 30* X 30*. The skies other than these areas are observed three times a week. Observation from the ground is affected by weather, latitudes, and the phase of the moon, but SOHO is an awesome new weapon to capture comets almost at any part of the sky regardless of these factors. Had this new instrument really captured new comets before they were discovered visually? I tried to find if SWAN images had caught all of the new comets which had been discovered since 2001. The following is the summary of the results of his research…
C/2001 Q2 Petriew – It is barely visible in the image of August 5. In the images of August 8 and 9 it is faintly visible, but it seems difficult to determine if it is a comet. It is also seen in the images of August 14 and 16, located in the area interfered with a lot of noise (or Lyman – alpha emissions). It is difficult to identify it as a comet.
153P Ikeya – Zhang – It is confirmed in the January 5 image, and in the images from January 8 onward it is clearly seen.
C/2002 E2 Snyder – Murakami – In the image of February 19 it is barely visible. From February 21 on it has become faint but discernible. Around February 28 it is still faint but relatively easy to identify. It is located slightly above the bright C/2000 WM1 LINEAR. In the images of March 9 it has become very difficult temporarily but in March 12 and 14 images it is discernable though faint. Around February 28 it is most noticeable but difficult to determine it's true nature.
C/2002 F1 Utsunomiya – At the time of the discovery it was in the area of the vicinity of the sun which lacks data. Even if bright light was emitted near the boundary of this area it would be difficult to determine if it is a comet or solar flare. On April 7 it is clearly seen.
C/2002 O4 Honig – In the images of July 16 and 18 a stain - like spot is seen shifting its position, but it is difficult to determine it from noise. In the image of July 20 it is not detectable. In the July 25 image it brightens suddenly. It is difficult, however, to identify the comet if you use only the July 16 and 18 images.
C/2002 X5 Kudo – Fujikawa – SWAN images are available only to November 13 as of December 15 and to November 23 as of December 22; therefore, there is no way to use images to find the comet before their visual discovery. The comets magnitude is estimated to be visually about 10. In the images of November 21 and 23 it is extremely faint and impossible to discover only from these two images.
With these results I have come to the conclusion that, out of 6 comets visually discovered since 2001, only 153P Ikeya – Zhang is possible to discover using SWAN images before its visual discovery! Many comets were certainly visible in the SWAN images before their visual discoveries, but it is not the same as to say they were discoverable as comets. I think this simply indicates the difference in views between observers and searcher. Trying to identify a comet with its position well known is totally different from finding a comet whose existence is completely unknown. It is the same with visual or photographic discoveries, or use of SWAN images. SWAN images contain a lot of noise or Lyman – alpha emissions. As these disappear and reappear with time, it is difficult to determine if the seen object is a comet. Visually discoverable comets are not necessarily discoverable from SWAN images…no need to be afraid of SWAN!
 |
The sky visible from the northern hemisphere has been exhaustively searched by LINEAR. Australia may be the last sanctuary left for visual comet searchers….is this correct? Mr Shigki considers the problem….The area of sky outside LINEAR’s coverage is fairly large. In spite of this, there have been hardly any visual discoveries from the northern hemisphere. This may be because comets usually enter LINEAR’s search area at least once before they attain a brightness possible for visual discovery. This will enable LINEAR to find comets. Although it is not impossible to discover comets visually from the northern hemisphere, the probability of visual discoveries must be now diminished to a fraction of what it used to be. |
 |
Consider the relation between the number of discoveries by LINEAR and solar elongations over a period between the beginning of the search by LINEAR and the end of 2002. The total number of discoveries is 99. Most comets were discovered at a solar elongation of more than 80 degrees. Those discovered at a solar elongation of less than 80 degrees count two in the morning and one in the evening. Among those were two discovered at a solar elongation of 75 degrees and 70 degrees. It indicates that the areas of sky within a solar elongation of 80 degrees are most likely outside LINEAR’s search coverage. After LINEAR started to operate at full capacity, four comets were visually discovered at solar elongations of more than 80 degrees; three in the morning and one in the evening. Of those four, three were discovered in the southern sky; C/1998 P1 Williams (Australian), C/1999 H1 Lee (Australian), and C/2000 W1 Utsunomyia – Jones (Japanese and New Zealander). We tend to think the southern sky is beyond the reach of the Japanese observers, but, as explained later, south of - 30 degrees in declination are outside LINEAR’s coverage. C/2000 W1 was discovered at a solar elongation of 83 degrees, but the declination -41 degrees at the time was outside LINEAR’s coverage. The last of the four comets discovered at solar elongations of more than 80 degrees is C/2002 O4 Honig. This comet was discovered within LINEAR’s search area. As explained later, when you closely examine LINEAR’s search area over a period of one month, you will notice that a considerable amount of sky has not been searched |
even though the solar elongations are more than 80 -90 degrees. It is likely caused by bad weather or other conditions. As the area of coverage by LINEAR for one night is limited, a comet can avoid detection by LINEAR depending on its direction of motion, speed, and pattern of brightening. C/2002 Honig was discovered because it literally went through LINEAR’s search nets. Note that most of the discoveries by amateurs were made at solar elongations of less than 80 degrees, outside the coverage by LINEAR. This has been even before LINEAR commenced its operation. As regards the search areas, visual search by amateurs and LINEAR’s coverage do not overlap and there is not much competition between them.
Incidentally, both discoveries by LINEAR and by amateurs show that there were twice more discoveries in the morning than in the evening. I have read such claims in a book that there are more transparent, less light polluted, and observers have better physical conditions after a night long sleep. However, this argument is conventional and unconvincing. LINEAR’s telescopes are installed at locations where meteorological conditions are excellent and very little light pollution exists. And, as they are telescopes, it is irrelevant if the observer has had a good sleep or not. It will be more logical to argue that more new comets appear in the pre dawn skies than in the evening. This author has no knowledge of convincing theories presented to explain this fact. A conventional thinking goes as follows: An observable part of the evening sky (or constellations) in the west at any particular time is continuously observable for several months prior to that time without any interference from the glow of the sun. On the other hand, an observable part of the predawn eastern sky at any particular time of year is not observable for about two months prior to that time hidden in the glow of the sun. Therefore, at pre dawn the part of the sky hitherto hidden continuously emerges out of the glow of the sun resulting in more discoveries. However, there is no way to know if this theory is correct. I have once heard that LINEAR has difficulty in detecting diffuse objects and that diffuse comets not detected by LINEAR had been discovered by amateur observers. When a stellar object and a comet of the same magnitude are compared, the stellar object with its light concentrated at a point is easier to detect than a comet with extended, diffuse light. This is the same photographically and visually. But the seemingly convincing argument that LINEAR cannot detect comets with extended light is simply nonsense.
Sky coverage by LINEAR for dark period 6 (June), 2001 |
About the issue of whether visual discoveries are possible under the gaze of LINEAR Mr. Tsutomu Seki, the head of OAA Comet section, has attempted an analysis based on the presumption that, of the six comets Mr. Seki has discovered, those at solar elongations of more than 90 degrees with magnitude 18 or brighter would be detectable by LINEAR. He concluded that LINEAR could have detected one comet without any doubt, one without certainty, and could not have discovered the other four. He has also analyzed C/1999 A1 Tilbrook (discovered on January 12 1999 UT) and C/1999 N2 Lynn (discovered on July 13 1999 UT). He points out that LINEAR was not able to discover C/1999 A1, in spite of the fact that on December 5, 1998 it was located at +78 degrees in declination with the magnitude being 12 and a solar elongation of 101 degrees. This implies that northern hemisphere comet hunters had a good chance of discovering this comet. He continues that LINEAR could not find C/1999 N2, which was located at - 36 degrees in declination in October 1998 at a solar elongation of 120 degrees with |
magnitude 16, though the solar elongation diminished after that. As I referred to earlier, LINEAR was probably unable to discover it because it could not cover the area south of - 30 degrees in declination.
Mr. Seki has also analyzed C/2000 W1 Utsunomiya - Jones (discovered on November 19 UT) and P/2001 Q2 Petriew (discovered on August 18 UT). He states that LINEAR could not discover C/2000 W1, although the comet was located at - 9 degrees in declination with a solar elongation of 89 degrees at magnitude 11. P/2001 Q2 had maintained small solar elongations up to the discovery and been outside LINEAR's search area.
Mr. Seki's analysis show that including the comets prior to the start of search by LINEAR there were many comets which were not discoverable by LINEAR and those which were discoverable but not discovered.
Sky coverage by LINEAR for dark period 12 (December), 2001 |
At the beginning of this article Mr. Yoshida's remarks: "The sky visible in the northern hemisphere has been exhaustively searched by LINEAR." How justifiable is his claim? How many of the comet hunters know the search areas that LINEAR covers? As I explained earlier, outside LINEAR's coverage is the area of sky of a solar elongation of less than 80-90 degrees. I would like to elaborate on the search area covered by LINEAR here. At LINEAR's web site the following facts are provided: Search by LINEAR is conducted on moonless dark nights to detect moving minor planets (comets) by photographing the same fields at several times per night. LINEAR's search areas over a period of one month at 6th dark period, 2001 (June) and the 12th dark period, 2001 (December). The vernal equinox is at the center of each illustration displayed in equatorial coordinates. The coloured area is LINEAR's search area with the ecliptic drawn in black. Lighter the color is, the fainter the limiting magnitudes of the search plots. The darkened areas are outside LINEAR's coverage showing that the part of sky south of - 30 degrees in declination is excluded from search. I heard from a number of people that the Milky Way was outside the coverage, but as shown in Figures 2 and 3, the Milky Way |
is within LINEAR's coverage. You can see this by comparing Figures 2 and 3 with star charts. I also heard that finding comets would be difficult in the Milky Way. Some people asked me if I had deliberately searched the Milky Way to find C/2002 E2 Snyder-Murakami thinking that LINEAR could not perform well in the Milky Way. It was not in my mind at all at the time of the discovery. However, I have a tendency to search the Milky Way, if I have a choice, because I can enjoy that part of the sky strewn with so many stars.
Regarding the remark that comet discoveries are difficult in the Milky Way, I asked Mr. Akimasa Nakamura, as I did not have much information on it. Mr. Nakamura replied:
"LINEAR's algorithim for detecting objects in motion is different from that of other surveys. Normally, detection of objects in motion is made from a number of frames. In the case of LINEAR five frames of the same field are stacked up and 'five stars in a straight line' are picked up as an object in motion. Because of this, new objects are not often missed, even if they overlap with stars. It can also search the summer Milky Way without any problem, while other surveys tend to avoid this part of the sky."
LONEOS and NEAT avoid the Milky Way in their search. The areas searched by these surveys including LINEAR for the latest one month period are made publicly available at the NEO page of the MPC (Minor Planet Center) (The data will be removed after one month.)
The boundary of search coverage by LINEAR at the end and start of twilight in June 2001 |
|
The boundary of search coverage by LINEAR at the end and start of twilight in December 2001 |
According to this, LINEAR's coverage extends roughly east and west almost symmetrically from the position of opposition. An example of search for a period of approximately one month shows that the search begins with a strip of the sky about 10 to 15 degrees wide (about 25 degrees wide at the northern most strip) per night parallel to the celestial equator between + 80 degrees and - 30 degrees in declination. After completing this, it searches a strip of the sky 30 degrees wide at most on both sides of the ecliptic parallel to the ecliptic. Once it has been completed, it resumes searching along the celestial equator. No search is conducted for about 5 days before and after full moon.
Figure 4 shows LINEAR's coverage over a period of one month at the 6th dark period (June), which is projected onto the celestial sphere at latitude 36 degrees north at the time of new moon (June 21, Japan Standard Time). The centre of the concentric circles is the zenith and the outermost circle represents the horizon. The area west (right) of the broken line is outside LINEAR's coverage at the end of twilight. The area east (left) of the solid line is outside the coverage at the beginning of twilight.
In other words, the area outside LINEAR's coverage at the end of twilight is the part of sky below 40 - 60 degrees in altitude (but occasionally near the zenith) west of the meridian. At the beginning of twilight it is the part of sky below 30 - 65 degrees east of the meridian. The boundary in the illustration is based on the aggregate of all the areas that LINEAR has searched approximately over a period of one month. Please note that the search area on any given day is naturally smaller than this.
LINEAR's search area is often determined by lines drawn along right ascension and declination. As a result, the search area on the celestial sphere is bordered by curved lines. In Figure 4, though, the boundary of the search area (Figure 2) is indicated by a series of straight lines joining the representative points on the boundary. I believe this is sufficient for the purpose of this paper.
Figure 5, like Figure 4, shows LINEAR's search area over a period of one month for the 12th dark period (December), based on Figure 3. On a night of new moon, the search area at dawn covers excessively low altitudes and the evening search reaches excessively high altitudes. As the information on the actual search area was not obtainable, I set the date for December 23, JST, with the first quarter moon so that the broken line and dotted line become symmetrical. At the end of twilight outside LINEAR's search area is the part of sky below 40 - 80 degrees in altitude west of the meridian and at the beginning of twilight the part of sky below 30 - 60 degrees east of the meridian.
Between summer solstice in Figure 4 and winter solstice in Figure 5, there is not much difference in the altitude distribution of LINEAR's search area at the beginning of twilight and the end of twilight. This applies to other seasons and there is almost no difference throughout the year. I think that many comet searchers spend one to two hours at a time searching the eastern morning or western evening sky below 40-50 degrees. This part of the sky does not overlap much with LINEAR's search area. Further, in the example of June 2001 a considerable amount of dark areas, which means unsearched, is seen within LINEAR's search area. C/2002 O4 Honig, which I referred to earlier, might have slipped through such areas or it might have fortunately moved through an area not covered by LINEAR. Alternatively, it may be the case that the summer when this comet was discovered was during the rainy season in the state of New Mexico where the telescope of LINEAR was installed and that it may have resulted in reduced operation.
 |
LINEAR is the most effective automatic search system at the moment, but a more effective system is expected to emerge in the future. It has been already announced that one of such new systems will start to operate in 2006 in Hawaii. An illustration of Pan-STARRS (left). It is expected to reach the limiting magnitude of 24 with a 30 - 60 second exposure covering an area of 3 degrees in diameter (7 square degrees). |
It's search area will amount to 3000 square degrees for one night and in two weeks it covers 10,000 square degrees three times. Because of Hawaii's low latitude, it can cover 70% (28,000 square degrees) of the whole sky.
When I received this information, I felt that this was a death knell to comet hunters. We were given only three years of life. Although what actually will happen is difficult to foresee until the new system starts to operate, I found the following passage in the paper I had downloaded from its home page:
"If we restrict observations to zenith distance of less than about 45 degrees then the total sky available from Hawaii is about 30,000 deg square. The visible sky on any night, say within four hours of opposition for concreteness, is then about 10,000 deg square, of which we can observe about 30% in a single night."
The total area of the sky is about 41,000 square degrees and at any given time half of the sky is visible from any place on the earth. Therefore, if we assume that about 10,000 square degrees is the area of the searchable sky, a considerable amount of low altitude skies will be left unsearched. In this respect there is not much cause for concern compared with LINEAR's impact.
However, the new system's limiting magnitude is 24 for stellar objects, 4 to 5 magnitudes fainter than LINEAR's. Therefore, cometary objects brighter than magnitude 22 are discoverable. This will hurt comet hunters' chances. According to Mr. Seki's analysis, quite a number of comets have not entered in LINEAR's search area. Some comets, like C/2002 O4 Honig, slipped through LINEAR's search area. Therefore, if Pan-STARRS and LINEAR adopt similar search patterns, there will be many comets which stay outside Pan-STARRS's search area or slip through the area of its search. For this reason I never believe that the chances for visual discovery of comets come to naught by the emergence of Pan-STARRS.
I wrote this article because I wanted to tell as many comet hunters as possible to pursue their dream of finding new comets and continue their search. This may increase competitors, but it would be great if I could share my dream with other comet hunters. In fact, through my discovery I got to know many people and was able to exchange views with them, which in turn helped me writing this article.
I hope I have convinced the readers that it is premature to abandon your dream now. Starting with the exchanges with Mr. Utsunomiya, I have argued that SWAN is not a threat to comet hunters. The division of the search areas by LINEAR and amateurs has naturally developed without much overlapping. There is still some hope for discovery by amateurs even after Pan-STARRS begins to operate.
I am fully aware that there are other factors which make comet hunting impossible, such as one's work, family, residence, etc. I myself can be transferred and forced into an impossible situation. However, in recent years technological advancement in CCDs and other areas has been remarkable, making spectacular photography of comets possible even at urban sites. With advanced technologies some day you may have a chance of discovery from cities if you continue to pursue your dream.
It is certain that comet hunters can survive for some time in the future and the chances of new discoveries remain strong.
The NEO sky survey and the amateur comet hunters can discover 68% to 76% of the near earth comets (perihelion distance, q<1.0). Most of the undiscovered comets are small ones with faint absolute magnitude. Note that the NEO sky survey misses many near earth comets. Without the amateur comet hunters, the discovery rate of near earth comets will decrease to 60%. The amateur comet hunters compete with the NEO sky survey, and also complement it.
Discovery rate of near earth comets by simulation - visual limit magnitude 10.0 |
|
Discovery rate of near earth comets by simulation - visual limit magnitude 12.0 |
Comet hunters can survive for the time being, though the discovery rate reduced to one third in the northern hemisphere after NEO sky survey. In the southern hemisphere if the search becomes active higher discovery rate is expected than the statistics to date.
When Kaoru Ikeya discovered his 6th comet 153P Ikeya-Zhang, 35 years had past since his last discovery (Sky & Telescope, July 2002, p.70-73). Shigehisa Fujikawa has spent 24 years to find his 5th comet that named him, C/2002 X5 Kudo-Fujikawa. Both of them continued comet hunting for decades of years without hesitation. Tsutomu Seki emphasizes importance of self confidence and dedication; "Strong rivals exist in any area and only comet hunters who fight bravely and attack openly are crowned with victory".
Unless complete sky coverage of the NEO sky surveys is realized, amateur comet hunters can survive but they have to swim with the sharks. Comet hunting goes on.
In his latest book David Levy (discoverer of 21 comets) summaries...
'Comet hunting has utterly changed in the 37 years since I started comet hunting in 1965. Back then, the threat to visual searches was from photographic surveys, which were succeeding in finding most of the comets. However, these efforts were concentrating on the sky at opposition to the Sun, and not terribly thoroughly at that, leaving many comets available for visual observers. Even after the enormous productivity of the great Palomar photographic surveys of the 1980s and 1990s - surveys specifically designed to discover comets and asteroids - until a few years ago it still seemed possible for comets to be found visually'.
'Although it is certainly still possible to discover a comet visually, it is far more difficult to do so than it was even a few years ago. Some comets happen to come in at a shallow angle to the Sun that hides them from the big surveys. But the date and time of the last visual comet discovery is approaching. I still believe that there will always be comets for the amateur to seek and find through his or her visual telescope. It's harder than it used to be, and many searchers will give up. In 1967, Robert Burnham Jr. who discovered comets both visually and photographically, advised me that "If you hunt long enough, stay away from the galaxies in Virgo, and never give up, some day you will find a comet." I think this advice still holds, although that "some day" could be decades into the future for most searchers'.
'As you hunt for comets, remember that it isn't a good idea to have the discovery of a new comet your only goal. In 1997, Leif Robinson, then-editor of Sky and Telescope, came down pretty hard on single-minded searchers. "I've never had any great admiration for comet hunters," he editorialized. "To spend hundreds of hours in failure for each minute of success never seemed like a good deal to me. ... For amateurs there's the allure of getting your name hitched to a star, albeit a hairy one. If you're very lucky, like Thomas Bopp, your name might appear in text books for years." If the only reason you spend all this time comet hunting is to find a comet, then Robinson has a point. So did the great Japanese comet hunter Minoru Honda, who discovered 12 comets and 11 novae during his lifetime, in the advice he gave Kaoru Ikeya before the young comet hunter made his first discovery. "If you desperately want to find a new comet, please stop your search because you may never be able to find a new comet. However, if you are content to search the sky without ever experiencing a new comet discovery, please keep searching because someday, you may be able to find a new one." Leslie Peltier, in Starlight Nights, perhaps said it best: “In spite of this increasing competition there always will be comets for the amateur to seek and, in some facets of this work, he still has an advantage. In a given time he can cover far more sky than can the camera, he can know within half an hour the true nature of a suspected object and he can search much closer to the sun in regions which would fog a photographic plate.”
I hope that after reading the above information any skeptical observers will be re – educated regarding the chances of further visual comet discovery for the time being in the northern hemisphere. I hope that anyone out there who has abandoned their dream of comet discovery will re consider their decision and take up this fabulous rewarding past time. With my previous two articles (visual comet hunting and observing comets) I hope that the reader will feel better equipped and more confident in the field of comet discovery. Happy hunting!
Martin McKenna
