|This page is a web version of the article "Can Comet Hunters survive?" written by Shigeki Murakami and published in the August 2003 issue of the Oriental Astronomical Association Journal, the Heavens.|
Can comet hunters survive?
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 Utsunomiya-Jones (Japanese and New Zealander). We tend to think the southern sky is beyond the reach of Japanese observers, but, as explained later, south of -30 degrees in declination are outside LINEAR's overage. C/2000 W1 Utsunomiya-Jones was discovered at a solar elongation of 83 degrees, but the declination -41 degrees at the time of discovery 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 and 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 O4 Honig was discovered because it literally went through LINEAR's searching nets.
If you compare Figure 1(a) and 1(b), most of the discoveries by amateurs were made at solar elongations of less than 80 degrees, outside the coverage by LINEAR. This had been so 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 (Figure 1(a)) and by amateurs (Figure 1(b)) 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 discoveries in the morning than in the evening because predawn skies 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 predawn 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 of year 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 (or constellations) 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 predawn the part of 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 hope that those adept in mathematics conduct a quantitative analysis or simulation based on the theory of statistics and probability.
I have once heard that LINEAR has a difficulty in detecting diffuse objects and that diffuse comets not detected by LINEAR had been discovered by amateur observers. Because I couldn't understand why LINEAR had such a difficulty, I asked Mr. Akimasa Nakamura. He explains: "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 and diffuse light is simply nonsense."
Analysis by Mr. Tsutomu Seki
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.(5) 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 analysed C/1999 A1 Tilbrook (discovered on January 12 1999 UT) and C/1999 N2 Lynn (discovered on July 13 1999 UT) (6). 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 analysed C/2000 W1 Utsunomiya-Jones (discovered on November 19 UT) (7) and P/2001 Q2 Petriew (discovered on August 18 UT) (8). 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.
|Figure 2: Sky coverage by LINEAR for dark period 6 (June), 2001
Reprinted width permission of MIT Lincon Laboratory, Lexington, Massachusetts.
LINEAR's search area
At the beginning of this article I quoted 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 website (9) 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. Figures 2 and 3 show an example of 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 colored 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.
|Figure 3: Sky coverage by LINEAR for dark period 12 (December), 2001
Reprinted with permission of MIT Lincon Laboratory, Lexington, Massachusetts.
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:
|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 search
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 center 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 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.
A new threat
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.(11)
|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).
Its 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 deg2. The visible sky on any night, say within 4 hours of opposition for concreteness, is then about 10,000 deg2, of which we can observe about 30% in a single night." (12)
The total area of the sky is about 41,000 square degrees and at any given time a 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' chance. 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.
A big threat
There is something else to pose a threat to comet hunters. It is from other comet hunters. Out of five comets visually discovered in 2002, three were discovered by more than one comet hunter. I myself became a threat to other comet hunters twice in December, 2002.
During a morning search on December 15, I was within about 10 degrees horizontally from C/2002 X5 Kudo-Fujikawa. (Mr. Kudo discovered it on December 14; Mr. Fujikawa on December 15, JST.) I was sweeping the sky vertically by a telescope with a true width of field of 1.3 degrees. If I had continued sweeping for another 30 minutes vertically, I would have discovered this comet. Unfortunately, I was clouded out, though I had enough time before the beginning of twilight. When I researched C/2002 Y1 Juels-Holvorcem, which was discovered by amateurs using a CCD camera on December 28 UT, I found that the comet was either within my search area around the time of discovery or very close to it. I certainly had a chance of discovery. I will show you the results of analysis below of my search based on my records on star clusters and nebulae I came across during my comet search excluding obvious open clusters.
-December 15: Transparency 4/5
I began searching at a point 10 degrees horizontally from C/2002 Y1 and did sweeping in a direction away from that point. Affected by scattered clouds, I moved toward C/2002 X5 Kudo-Fujikawa.- December 18: Transparency 1/5→2/5
I was within about 5 degrees horizontally from C/2002 Y1 or the comet was in the area I searched.-December 31: Transparency 2/5→3/5
C/2002 Y1 was definitely in the area I searched. (At my parents' home in Shiga Prefecture)
A transparency rating of 5/5 is the best and 1/5 is the worst. C/2002 Y was discovered at CCD magnitude 15, but it was estimated to be at visual magnitude between 12.5 and 13. Actually I saw C/2002 Y1 on January 13, 2003 for the first time. Sky transparency was 5/5. Intuitively I estimated it to be 11.5 magnitude and it was fairly diffused. However, if transparency had been better than or equal to 4/5, I felt it would have been possible to discover it, even if it had been a little fainter. On December 18 even if it had been in the area I had searched, it would not have been discovered because of poor transparency. It may be for the same reason that the comet was not found on December 31, in spite of its discovery already being announced. Solar elongations of C/2002 X5 and C/2002 Y1 at the time of their discoveries were 72 degrees and 84 degrees respectively, probably barely outside LINEAR's coverage. The chance of visual discovery still exists.
I have never had an "almost discovered" experience, but realized that races for discovery among amateurs were world-wide.
A real threat
There is a far more serious threat to comet hunters. It is from comet hunters themselves when they abandon their dream of discovery and stop searching. This is the biggest threat that comet hunters face.
I remember that, concerned about LINEAR's search area, I checked its coverage at the beginning of 2000, prior to my surrender to LINEAR. I discovered that there was an unexpectedly large area outside the coverage by LINEAR, as shown in Figures 2 - 5. In spite of that, I abandoned comet search, as mentioned earlier. Looking at visual discoveries from the summer of 1998 to the end of 2000, I came to believe firmly that visual discoveries would be only possible in the southern hemisphere, such as in Australia, "the last sanctum for visual comet hunters", even though visual discovery in the northern hemisphere might not be impossible.
My record shows that I searched for comets only once between July 2001, when I stopped searching, and the time I resumed search. It was on August 24 (transparency: 3/5), when I went to see P/2001 Q2 Petriew with a 20cm reflector. It was immediately after the comet had been discovered. By the 20cm scope at 31x, the comet, at an altitude of about 30 degrees with its discovery magnitude of 11, could not have been discovered visually. With a power of 42 it would have been difficult and even at an altitude of 40 degrees it would have been barely visible. Even if the comet was placed at the center of the eyepiece field, the chances for discovery would be less than 50%. The fact that I would like to emphasize here is that even from a remote area in Niigata Prefecture, which many Japanese amateurs believe has little light-pollution problem, an 11th-magnitude comet is difficult to discover by a 20cm telescope. This further strengthened my impression that visual discovery of comets was not possible under ever-worsening light-pollution unless they were bright.
The poor appearance of P/2001 Q2 Petriew reassured me of my feeling of resignation and I started again observing nebulae and clusters with my 18-inch (46cm) telescope. Winter came to Niigata Prefecture and there were hardly any starry nights. While snow fell continuously day after day, it suddenly occurred to me that I was not getting a sense of accomplishment from observing nebulae and clusters. I was even a little depressed with the feeling of emptiness.
My mind was completely occupied with various thoughts about comet hunting.
With a large-aperture telescope I can beat light pollution and find fainter comets, can't I ? But doesn't the large cumbersome Dobsonian tube only leave me exhausted ?
By chance this resolution was made on my birthday. And coincidentally 153P/Ikeya-Zhang was discovered in the evening sky that day. Soon after, on my third attempt, I discovered C/2002 E2 Snyder-Murakami. No matter how I look at it, it is too perfect to believe. I wonder how it was possible.
Going back to what I was discussing earlier, I realized that the act of searching itself was the true enjoyment in which I could pursue my boyhood dream and find a meaning in life. This was the very reason for resuming comet searching. I have no intention to boast my "strong will power" or "blood-sweating toil." Such phrases are irrelevant. I have only come to the decision to return to comet hunting after exhaustive soul searching trying to find out what I really wanted to pursue.
My feeling at the time may have been similar to what Mr. Minoru Honda had once said: "If you really want to find a new comet in any way possible, you should stop searching because you may never be able to find one. However, if you are still content even without a new discovery, continue your search. You may be able to find a new comet."
Mr. Tsutomu Seki said: "Stars will become visible only to a serene mind and a mind undisturbed by desire for fame." It is not that I had reached the philosophical level of Mr. Honda's and Mr. Seki's, but I was simply honest to myself.
It may be no more than just luck that I had discovered a new comet after resolving to restart comet search, but if I had given up searching, there would have been no discovery. Giving up comet hunting, this is a real threat to comet hunters.
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 someday 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.
I am grateful to Dr. Yusuke Ezaki, who encouraged me to write this two-part article. I am also grateful to Mr. Akimasa Nakamura for information on LINEAR.
1. Seiichi Yoshida. 1999. Kugatsu no kometto topikkusu (September Comet
topics). Monthly Sky Watcher, p.91. Rippu Shobo Publishers.
2. Atsuyoshi Nishina. 2000. Watashi no aiki (My favorite telescope).
Gekkan Tenmon Gaido, November, p. 188. Seibundo Shinkosha Publishers.
3. Kriege, D., and Berry, R. 1998. The Dobsonian Telescope, p.475.
Willmann-Bell, Richmond, VA, USA.
Maik Meyer's home page.
5. Tsutomu Seki. 2000. OAA Comet Section Monthly Report.
Tenkai 81 (The Heavens), (901), June, pp. 423-425.
6. Tsutomu Seki. 2000. OAA Comet Section Monthly Report.
Tenkai 81 (The Heavens), (905), October, pp. 701-703.
7. Tsutomu Seki. 2001. OAA Comet Section Monthly Report.
Tenakai 82 (The Heavens), (909), February, pp. 116-118.
8. Tsutomu Seki. 2001. OAA Comet Section Monthly Reports.
Tenkai 82 (The Heavens), (917), October, pp. 688-689.
LINEAR home page.
MPC, Sky Coverage in The NEO Page.
Pan-STARRS home page.
The original article was written by Shigeki Murakami in the Japanese language.
This web version in English was translated by Eiji Kato.
Shigeki Murakami's web-page:
Translator: Eiji Kato
An amateur astronomer living in Australia. He runs the following web sites: