The Finding of Comet SOHO 2002 C4:
A Comet for New York

(a revised version of a story that appeared in the April 2002 Eyepiece)

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Comet SOHO C/2002 C4 (the modest dot to which the arrow points), which I found
on Feb. 11, 2002 on images taken by the SOHO (Solar and Heliospheric Observatory)
spacecraft. The image is from the blue-filtered LASCO C3 coronagraph. Venus is the
bright object at the left edge (the line to its right is a diffraction spike). To the
upper right, you can see a bit of the occulting disk that blocks the sun, in effect
creating an artificial eclipse. You can see some streaming of the corona; the dark streak
is the pylon that holds the occulting disk in place. (Images courtesy of the SOHO/LASCO
consortium. SOHO is a project of international cooperation between ESA and NASA.)

An animation showing the comet's motion over
an eight-hour period. The comet appears at the
lower left corner (beneath the star) and tracks
diagonally up across the field of view. The star
is the same one that is to the comet's lower left
in the first photograph.

By celestial measure it was a tiny thing, a chunk of ice and dirt perhaps the size of a bus, and its lifetime—which extended back to the dawn of our solar system—was fast nearing an end. Locked in an Icarian plunge almost straight towards the sun, it was already within 5 million miles of the solar furnace, and each hour brought it a few hundred thousand miles closer. The comet’s already depleted core—whittled away by successive encounters with our star over millennia—steadily melted, releasing its last reserves of gases, water vapor, sand, and dust. For a few hours after I became the first to detect the comet now designated SOHO C/2002 C4, its smudge of light held steady, then it rapidly faded as the last of its material vaporized. Within 12 hours it was gone, though its scattered molecules will continue circling the sun in roughly the same orbit.

Ever since I got started in amateur astronomy in the early 1970s, it had been a dream of mine to discover a new comet, but I never could have imagined in those early years that I would find it on a rainy night, from my New York City apartment. The comet was visible on images taken by the SOHO (Solar and Heliospheric Observatory) spacecraft that are regularly posted to the Web. SOHO, a joint project between NASA and the European Space Agency, was launched in late 1995 to study the sun and its vicinity. Along with the solar phenomena such as flares, prominences, and coronal mass ejections that the probe was designed to study, some of SOHO’s images contain comets—over 500 to date. NASA and ESA professionals found the first SOHO comets; Terry Lovejoy of Australia became the first amateur astronomer to find one, soon after the images were first made available to the public on the Web in late 1999. Soon a trickle of amateur discoveries became a flood, and now almost all SOHO discoveries are found by a far-flung band of enthusiasts who scour the latest images on their home computers.

Although SOHO’s bounty includes the smallest comets ever discovered (it is generous to even count some of them as comets, as they can be as small as 5 meters in diameter), most are fragments of the brightest known comets, a family of objects known as Kreutz sungrazers (after Heinrich Kreutz, who noticed the similarities in their orbits, and theorized that they had all come from the same parent body, a “mega-sungrazer” that is believed to have progressively fragmented into smaller). These comets—which take between 400 and 2000 years to complete an orbit—come within a fraction of a solar diameter of the sun’s surface, and some are believed to even collide with our star (with no apparent harm to the sun)—if they don’t vaporize first. The brightest of them, which include 1965’s Comet Ikeya-Seki and the Great Comets of 1843 and 1882, are easily visible in broad daylight even when very near the sun. (The exceptionally brilliant comet of 372 BC—which was so bright that it reputedly “Cast shadows like the moon” at night—as well as a daylight comet that appeared in 1106 AD, are also believed to be members of the Kreutz family.) Although the largest of the Kreutz fragments seen in SOHO images have become about as bright as Sirius—the brightest star in the night sky—their proximity to the sun has rendered them invisible other than in the satellite images, and none have survived the solar passage. Only one SOHO sungrazer, a sporadic not belonging to any known comet family, was later observed visually; the rest have never been seen directly, not even through a telescope.

Hunting for SOHO’s comets has become very competitive, with the vast majority found by a core group of about a dozen highly skilled and experienced observers, many of whom have distinguished themselves through their dedication to finding, observing, and studying comets, online and in many cases in the night sky as well. Kazimieras Cernis of Lithuania, finder of 10 SOHO comets, also discovered three comets telescopically over a decade ago. China’s XingMing Zhou, who has found 26 SOHO comets (including the two brightest found during the time I’ve monitored the site), searches the night skies through his telescope for new comets for over 1600 hours, so far without success. Three Germans have made important contributions. Maik Meyer manages the Catalog of Comet Discoveries website (a list of all known comet discoveries), as well as the Yahoo comets mailing list. He and Rainer Kracht have each discovered new classes of sungrazing comets whose members all travel in similar orbits. Sebastian Hönig hosts a website with a wealth of information to help beginners get started in searching for SOHO comets. Along with his 33 SOHO comet finds and nearly 500 asteroid discoveries in online data, on July 22, 2002 he discovered a comet through his telescope, becoming the first German amateur to do so since 1946. Jonathan Shanklin, finder of six SOHO comets, heads the British Astronomical Association’s comets section and publishes its biannual newsletter, The Comet’s Tale. (In his job with the British Antarctic Survey, he was one of three scientists to first report the Antarctic ozone hole in 1985.) Michael Oates, a father of three from Manchester, England, pioneered the technique of using advanced photo-processing software to search through archived images from SOHO’s early years before its data was adequately monitored. He has found over 140 SOHO comets, about a tenth of all known comets found throughout history. His observations were instrumental in determining that the number of Kreutz comet fragments was far greater than previously imagined, around 200,000 by conservative estimates.

SOHO itself has been an overachiever. Apart from some trouble with its gyroscopes and a three-month period when radio contact was lost, the spacecraft has experienced few serious problems, and most of the instruments are still in excellent shape. SOHO’s original mission was to last through 1998; it has been extended until 2007, by which time another mission named STEREO is due to take its place. STEREO is designed to watch the sun using two satellites placed far enough apart to create a stereoscopic image of our star, its flares and plasma storms, and any comets that approach it.

Analysis of SOHO’s trove of comets is providing astronomers with a better understanding of how comets split apart, and how the Kreutz and other comet groups formed. Fragmentation is not unique to sungrazing comets—few will forget the two dozen fragments of Shoemaker-Levy 9 that collided, one by one, with Jupiter; the comet had split after a previous close approach to the giant planet. The Nineteenth Century had Biela’s Comet, which split into two before eventually disintegrating, leaving a meteor shower in its stead. Two of the recent LINEAR comets, 1999 S4 and 2001 A2, both underwent significant fragmentation; the splitting of the latter comet boosted its brightness by a factor of about 100, no doubt due to the large amount of dust and gas released as several fragments broke from the nucleus. Sungrazing comets, though, are particularly prone to fragmentation, and astronomers have a unique opportunity to study such a large sample of fragments.

Although sungrazers most frequently break apart when close to the sun and heat and tidal forces are the greatest, fragmentation can occur at any point in their orbit, and the point at which the split occurs can often be determined by examining the orbital characteristics of the fragments. (Many of SOHO’s Kreutz comets appear in pairs, or have orbital elements that are virtually identical to others.) It is now apparent that the Kreutz group is very young for a comet family, with the initial split occurring perhaps as recently as two millennia ago. (It is likely that the brilliant comet of 372 BC, which was reported by the Greek astronomer Ephorus to have split in two, was the parent body of all the Kreutz fragments seen today.) The Kreutz family is itself made up of two major subgroups, one whose largest known component is the Great Comet of 1843, and another that includes the comets of 1882 and 1965 (believed to be among the largest pieces of the daylight comet of 1106, which was also observed to split). In early 2002, three additional families of sungrazing comets were identified; they appear to be much older than the Kreutz group. Only 3% of SOHO’s sungrazers remain unassociated with any known comet family.

I’ve had a longtime love of comets, and make every effort to get to a relatively dark-sky site when one is in easy binocular range; I’ve observed close to 20 to date. I was drawn to comets because of their surprising nature; their often unexpected arrival out of the depths of space, their tendency to travel in orbits that take them far beyond the bounds of the zodiac, and their dramatic variations in brightness and appearance. As noted comet hunter David Levy has said, “Comets are like cats. They have tails, and they do precisely what they want.” Comet hunting and observing is an area in astronomy in which amateurs have traditionally made significant contributions.

In the summer of 1973, I took an observational astronomy class. One night I showed my teacher a pale streak of light in my telescope, which I hoped was a new comet. He agreed it was a comet, and suggested I call Brian Marsden at the Harvard/Smithsonian Center for Astrophysics the next day to report my “discovery” Dr. Marsden was quite skeptical, but very patient; he even took the trouble to track my number down and call me back when we got disconnected. It turned out that “Hoffman’s Comet” was a reflection from a bright star in the tube of my telescope. I did a small amount of hunting for comets in the years to follow, but never lived in a place that was conducive to it. Moving to New York City in 1980 made it very difficult to observe comets, let alone search for them. Then in late 2000, I read about amateur astronomers finding new comets on photos posted to the Internet, a project that even a citybound cometophile could engage in.

I started my SOHO comet search on June 21, 2001, examining the images from two cameras—the narrow-field C2, which shows the solar vicinity through an orange filter, and the wider-field, blue-filtered C3—posted to the SOHO website as often as two or three times an hour. The cameras are called coronagraphs; the sun is masked, in effect creating an artificial eclipse that reveals the great strands and loops of the solar corona. I look for specks or teardrops of light that move in a predictable manner, usually in a beeline for the sun from frame to frame. I particularly like C3; the sky is a deeper-than-daytime blue yet it is filled with stars ten times as faint as you can see from a rural night sky. More comets are discovered in C2, which has greater sensitivity, but the brighter ones are usually first picked up in C3, when they are still farther from the sun and have more of a chance to brighten. The technology I used at first was very simple, just a computer with a dial-up connection, a program for measuring the position of objects (Microsoft Paint will do in a pinch), and a web browser. By opening a series of images in different browser windows I could blink between them by hand, looking for objects that moved in a predictable manner over several frames. I have since gotten a faster connection and a higher-end graphics program, but the principle is the same.

I set myself the task of finding a comet in 2001, which seemed reasonable considering the number of comets being found and my experience as an observer. I soon discovered that it wasn’t as easy as I had figured, and I missed my goal by a little over a month. Despite spending many hours a week downloading and poring over SOHO images, only about once a month did I find an object whose behavior appeared cometlike enough to report to the clearinghouse for SOHO comet claims . They all turned out to be noise, chance alignments of cosmic ray hits, or artifacts of solar activity. Other observers had already reported all the true comets before I saw them.

The vast majority of SOHO comets are found by Europeans; I usually confine my own search to the early evening hours on the East Coast when it’s well after midnight in Europe and presumably most of the keen-eyed comet hunters there are asleep. Apart from Canada’s Mike Boschat, who has found 32 SOHO comets, North America has been underrepresented. Recent months have seen an increase in American SOHO finds, inparticular by Rob Matson and John Sachs, but in the 16 months between October 2000 and February 2002, although over 150 SOHO comets were found, only one was first spotted by a U.S. amateur. On a Monday night, six weeks into my own search, to read that David Johnson of Chicago had reported a new SOHO comet. I quickly located it, and as new images arrived, I watched its streak of light sail across the field of view, gradually fading into the greater glare of the sun as it headed towards perihelion at about 5 a.m. the following morning: September 11, 2001.

Once my phone and e-mail were restored after the attack (they were intermittent for about a week), I resumed scouring the SOHO images for comets—it beat watching CNN’s endless replays of planes crashing into buildings and the other horrors of that time, and allowed me to try to focus for a little while on something far removed from our world and its troubles—but my reasons for looking had totally changed. Any personal thrill or glory in finding a comet no longer seemed of much importance. I realized that it wasn’t every day that someone in New York City finds a comet—indeed, that it was quite possible that it had never been done before—and saw that I had a chance to help demonstrate in a small way that new and extraordinary things could still be done here despite the terrible reality we had been thrown into, that the life of the city would go on. Hunting for a comet became even more of a mission than before—though I wasn’t looking just for myself anymore, but for my city as well.

In early February, SOHO experienced a technical problem, the spacecraft went into “safe mode,” and the flow of images stopped. When it resumed on February 10, my comet—already halfway across the C3 field-of-view in route to the sun—was visible; I reported it, and other observers easily confirmed it. It was officially given the utilitarian name Comet SOHO C/2002 C4; comets found in data from spacecraft or automated searches (such as LINEAR) are named for the source of the data rather than the person making the find. As challenging and time-consuming as it is to find a comet in satellite images, it pales in comparison to telescopic comet hunting, which requires braving the elements to spend countless hours scanning at the optimal times in the evening and before dawn, sifting through the numerous objects in the sky that can be mistaken for comets, being at the mercy of the weather and the phases of the Moon while competing with numerous other amateurs as well as the automated surveys for a far more modest number of comets than are located through SOHO.

Still, I was told by John Pazmino, former President of the Amateur Astronomers Association of New York, that mine is the first comet ever found from New York City by any method, a mind-blowing possibility. I’ve done some investigating to try to confirm this. The one other candidate I’ve come across is the Great Comet of 1843, ironically a Kreutz sungrazer that is the closest known major relative to my little fuzzball—though it shone a million times brighter at its best. One of the first reports of the comet of the comet (a sighting on Feb. 5, 1843) ran in a New York City publication, but because of the comet’s 30-degree southern declination it is unlikely that the sighting was actually made in the New York area; the report was perhaps relayed by a sailing ship returning from the Southern Hemisphere, where most comet experts put the discovery. Nonetheless, the finder’s identity and location are lost to history.

Today, the pervasiveness of light pollution (skyglow due to excessive and misdirected lighting) makes finding a comet by telescope from major metropolitan areas unimaginable, but there were still dark skies in New York and in other large cities in the mid-1800s, and at least one observatory in lower Manhattan. The 1850s saw the discovery of two comets by Robert Van Arsdale from across the Hudson River in Newark. After 1879, when Thomas Edison commercialized the carbon-filament light, the days of dark skies in cities were numbered, though even in 1910—when Halley’s Comet came through—the Milky Way was still visible from Manhattan. Today, even if Governor Pataki finally were to agree to sign a light-pollution bill here, it would still not reduce sky glare enough to give urban astronomers any real chance of finding a new comet telescopically, so New York City will probably have to be content with the likes of SOHO comet discoveries. Here, it’s often an adventure simply trying to observe known comets; their fuzzy glow is easily obliterated in a sky that never truly darkens.

Ten days before I found my fuzzball, two Asian amateur astronomers—Kaoru Ikeya of Japan and DaQing Zhang of China—discovered a new comet almost simultaneously in their telescopes that quickly brightened to naked-eye visibility, at least for people outside of large cities. (Ikeya is the same man who co-discovered the aforementioned great 1965 sungrazer, one of five comets he found in the mid-1960s. He ended a 35-year comet-hunting drought with the co-discovery of Comet Ikeya-Zhang.) Ikeya-Zhang was the first of four comets found telescopically between February and July 2002, three of them coming in a six-week stretch—a rare flurry of visual discoveries at a time when astronomy is becoming increasingly automated (In each of the previous two years, only a single such discovery had been made.). Computerized sky surveys with acronyms like LINEAR, BATTERS, and NEAT, which use high-resolution CCD imaging to locate asteroids that could conceivably collide with Earth, are also discovering comets in large numbers, many of which (unlike the sungrazers found through SOHO) would likely have been found by amateur astronomers in their telescopes. LINEAR (short for Lincoln Near-Earth Asteroid Research) alone has found 90 comets in five years, including several that brightened to naked-eye visibility. SOHO itself has another instrument called SWAN (for Solar Wind Anisotropy) that is able to detect comets. It produces a small image of the entire sky in which bright points in ultraviolet light—some of them comets—can be seen. Although only three comets have been discovered with SWAN, it was recently determined that most of the comets found visually by amateurs could have been seen earlier on SWAN images, which will undoubtedly be getting much more attention in the future, and could provide additional competition to visual observers. Still, it’s heartening to see comets still slip through this automated detection grid to reward diligent amateur skywatchers.

Whether the recent spate of visual discoveries will be a continuing trend or is merely a reprieve before additional and more powerful sky surveys (several of which are in the works) all but close the door on amateur comet discoveries is an open question. (Even if the sky surveys win out, perhaps the day will come when our part of the solar system is catalogued well enough that they are less necessary and once more comet discoveries will largely be the domain of backyard skywatchers.) Yet while technology has diminished the odds of visual discovery of comets, it has also provided the means for cyber-savvy observers to find them online by using the “eye” of a spacecraft to monitor the solar vicinity. In homes on at least five continents, SOHO comet hunters search through freshly downloaded images, trying to be the one to first detect the next cosmic iceberg that strays too close to the sun. On June 27, 2002, I found my second sungrazing comet. As if to underscore the fact that SOHO’s images are relayed from a spacecraft far beyond Earth’s atmosphere, I found it—as with my first find—on a night of pouring rain.

E-mail to tonyhoffman [at] earthlink [dot] net