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Extinction by planetoid/comet impacts.
THE ELECTRONIC JOURNAL OF
THE ASTRONOMICAL SOCIETY OF THE ATLANTIC
Volume 5, Number 8 - March 1994
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TABLE OF CONTENTS
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* ASA Membership and Article Submission Information
* A Personal Adventure in Home Computing: The Origin of Comet
Shoemaker-Levy 9 - Andrew J. LePage
* Planetoid Impacts: Devastating Shapers of Earth's History
- Nicholas M. Burk
* Hunting for Comets: One Observer's Success Story - Michael Janes
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A PERSONAL ADVENTURE IN HOME COMPUTING:
THE ORIGIN OF COMET SHOEMAKER-LEVY 9
Copyright (c) 1994 by Andrew J. LePage
The author gives permission to any group or individual wishing
to distribute this article, so long as proper credit is given
and the article is reproduced in its entirety.
Despite recent trends in our society, I never felt inclined to
purchase a Personal Computer (PC) for home use. This is not to say
that I do not use computers. Nearly every work day during the past
decade I have sat in front of a computer terminal or workstation of
one sort or another. I program them, transfer and analyze data with
them, send Electronic Mail (E-Mail), and use them to perform a
multitude of other useful tasks.
Still, with every cryptic error message that appears on the
screen, I cannot help but wonder what twist of fate sentenced me to a
career that requires my using these autistic minions of Mephistopheles.
I just do not like computers and the last thing I wanted to do was let
one of these little beasts into my home.
At work there exist those wizard-like individuals known as System
Managers. These saints of the late Twentieth Century are the ones who
have to deal with all the mundane and mystical tasks required to keep
computers and networks running. I knew just enough about their
activities to confidently state that I wanted nothing to do with it.
I wanted to be nothing more than a User. Owning a PC meant that I
would have to become, in some sense, a System Manager.
For half of my professional career I worked for a major computer
company where PC was a four-letter word. There were none to be found
in my lab whether I needed one or not. As a result, I had absolutely
no experience with PCs, which just added to their mystery. If there
was ever something I really wanted to calculate for an article or just
out of curiosity, I would use my handy-dandy programmable calculator.
If it were a rather large task, I would derive the needed equations,
write my own programs, and run them on the computers at work on my
own time. All in all, I had little need for a PC.
Things have changed over the past several years. I still tend to
hate computers to the point where it is almost a phobia. However, I
have changed jobs and now have access to PCs, so I have learned to do
the basics with them. Many members of my family are very PC-literate
and are more than willing to help me with my PC problems. Today's PCs
are as powerful as the minicomputers I was using in the lab ten years
ago. Combined with the array of quality software that is available on
the market today, one can do more on a PC than I could ever hope to do
if I were to program a workstation from scratch in my spare time. It
was beginning to look as though a PC might be necessary after all.
The Plunge
On July 29, 1993, I took the plunge and bought a PC. Within a
couple of days - with the help of my brother - we got the little beast
up and running. Around this same time word was spreading about the
impending impact of comet Shoemaker-Levy 9 (SL-9) on Jupiter in July
of 1994. The amateur orbital dynamicist in me could not resist. I
had to simulate this orbit, but it would take forever to write my own
program to do it. Fortunately, there is PC-compatible software that
will do the trick.
The first major software purchase I made was one of the new
commercially available solar system simulators. After I installed the
program, I immediately called up the preloaded orbital elements of
SL-9 and ran things forward in time. The little comet disappeared in
a pleasing flash of light as it plunged into the Jovian atmosphere on
the predicted day. The next day I loaded the latest orbital elements
I obtained from the Internet and performed the simulation again with
similar results.
Normally, this would have been the end of it. After all, from
how many angles or levels of magnification can one watch a simulated
impact, even if the graphics are pretty? However, the experimentalist
in me naturally wanted to do a thorough job. The only other thing
to do was to run the simulation backwards in time. I used two test
bodies, each using one of the two sets of elements I had in hand. I
ran the simulation backwards and encountered the bane of every classical
Newtonian determinist: Chaos. At first the two test bodies followed
similar paths. Then, as the years passed, they slowly diverged. By
the time both were free of Jupiter's influence, they were in similar
orbits but in totally different parts of the sky. After a few checks
to ensure that I was not observing some sort of program or machine
induced error, I became convinced that the orbit of comet SL-9 was
chaotic.
The Experiment
Despite this little setback, I was still curious as to where
SL-9 came from. Chaos would make it impossible for myself and even
professional astronomers to precisely determine the position of this
strange comet, particularly before its capture by Jupiter. However, I
thought perhaps that I could make some sort of statistical statement
as to the whereabouts of SL-9 at any given point in time. So I
designed an experiment.
First, I obtained Don Yeoman's and Paul Chodas' orbital element
solution number 22 (and later number 28) and their uncertainties and
produced a swarm of test bodies scattered fairly evenly in parameter
space. Hopefully, this would be a realistic sample of all the possible
orbits of SL-9. Second, I assumed that any computational errors would
be random and that over a long enough time span - with a large sample
of test objects - these errors would average out to be zero. Finally,
I had to assume that a chaotic system such as this could be meaningfully
quantified by what is essentially a Monte Carlo simulation.
After 1,500 hours of computer time (that's right, two months at
twenty-four hours a day) and numerous checks, I had a group of eighteen
test bodies which I felt were representative of all possible SL-9 orbits.
Being painfully aware of statistics, I realized that such a small test
population would be a pollster's nightmare and result in statistical
uncertainties of give-or-take about twenty-five percent. Thankfully,
though, the test bodies' orbits did follow a pattern and I felt some
meaningful conclusions could be drawn. At the very least, I was able
to glimpse the majesty and grandeur of the forces that shape the orbits
of the countless unseen bodies that inhabit the outer parts of our
solar system.
The Results - Capture
The first obvious question about comet SL-9 is when was it captured
by Jupiter? Based on the results of my simulation, I would say around
the year 1970. Actually, I should state that there is an eighty-three
percent probability that SL-9 was captured by 1970. Reading between
the lines of some of the professional papers and abstracts I have seen
on SL-9 orbital simulations, it seems that 1970 was about when the
capture likely took place. So it appeared that my simulation was on
the right track.
The following table lists the probabilities that SL-9 was in orbit
around Jupiter by a certain date:
Table 1
Year Probability
1980 100%
1975 94%
1970 83%
1965 39%
1960 39%
1940 28%
1920 17%
1900 6%
1880 0%
According to my simulation, capture could have occurred sometime
between the late 1880s and the late 1970s. This may seem to be a
large range of dates, but we are dealing with the unpredictability of
chaos. Still, given the statistically significant forty-four percent
transition that took place in the late 1960s (a 1.9 sigma change, to
use the technical jargon), it is statistically most likely the capture
took place around 1970.
The next question is what sort of orbit did SL-9 have before its
capture? There is an eleven percent probability that SL-9 was captured
from a low inclination orbit with a perihelion near Jupiter and an
apohelion near Saturn. With an eight-nine percent probability, it
seems more likely that SL-9 was captured from a low inclination orbit
with an apohelion near Jupiter and a perihelion buried in the main
Planetoid Belt no closer than about 2.5 astronomical units (AU) from
the Sun. One AU equals the average distance between the Sun and Earth,
or about 150 million kilometers (93 million miles).
This sort of orbit looks much like that of a short-period comet.
I naturally wondered if perhaps SL-9 was observed before its capture?
Unfortunately, unless the comet is fresh from the outer solar system,
they will rarely display any significant cometary activity at 2.5 AU.
The Sun's feeble heat at such a distance will vaporize only the most
volatile of ices. This makes discovery less likely but not impossible.
A search of a cometary orbit data base, however, showed no likely can-
didates. The average three-degree inclination of the precapture orbits
was just too low. If SL-9 graced our skies before its discovery, it
happened centuries or millennia ago, if ever.
The Results - The Past
For my statistical studies of SL-9's possible orbits, I decided to
make several broad classes of orbits and determine how their populations
change with time. These classes are as follows:
Class 1 - Jupiter-Planetoid Belt: Bodies in this class had a
perihelion of between one and about four AU and an
apohelion near Jupiter. The period of revolution ranged
between five and ten years.
Class 2 - Jupiter-Co-Orbital: Bodies in this class of orbit had
periods ranging from ten to fourteen years and orbited
near Jupiter's orbit. This class was very unstable and
was typically occupied by a test body for only a few
decades as it was making a transition from orbit Class 1
to Class 3, or vice versa.
Class 3 - Jupiter-Saturn: Bodies in this class of orbit had periods
ranging from fourteen to twenty-eight years. The perihelion
was near Jupiter and the apohelion was near the orbit of
Saturn.
Class 4 - Jupiter-Uranus: Bodies in this class had periods ranging
from twenty-eight to fifty-six years. As you have probably
already guessed, the perihelion is near Jupiter and the
apohelion near Uranus.
Class 5 - Jupiter-Neptune: Bodies in this class had periods ranging
from fifty-six to ninety-five years. I know you can figure
out the rest.
Class 6 - Saturn-Co-Orbital: Like Class 2, these bodies circle near
the orbit of Saturn with periods in the twenty-four to
thirty-four-year range. Also like Class 2, this orbit is
unstable and occupied for only a few centuries at most.
This orbit is typical for bodies making the transition
from Class 3 to Class 7, or vice versa.
Class 7 - Saturn-Uranus: Bodies in this class had periods ranging
from thirty-four to sixty-eight years. As expected, the
perihelion is near Saturn and the apohelion near Uranus.
Of course there are many other possible classes of orbits. I just
did not observe them during my simulation. With more test bodies and
simulated run times on the order of one million years, I am sure I
would have seen a fair number of bodies in Uranus-Kuiper Belt orbits.
Unfortunately, due to time and program constraints, I could only
simulate back about seven thousand years.
Still, let us see what results I did get. Below is an abridged
list of test body population distributions going back to 5000 B.C.:
Table 2
Orbit Class
Year 1 2 3 4 5 6 7
1900 AD 72% 0% 22% 0% 0% 0% 0%
1800 AD 61% 0% 39% 0% 0% 0% 0%
1700 AD 50% 0% 50% 0% 0% 0% 0%
1600 AD 50% 0% 50% 0% 0% 0% 0%
1500 AD 50% 0% 50% 0% 0% 0% 0%
1000 AD 17% 17% 61% 6% 0% 0% 0%
500 AD 28% 0% 56% 17% 0% 0% 0%
1 BC 28% 0% 33% 28% 0% 6% 6%
500 BC 28% 6% 17% 39% 0% 6% 6%
1000 BC 22% 0% 33% 33% 6% 0% 6%
2000 BC 17% 0% 39% 17% 17% 0% 11%
3000 BC 11% 6% 28% 33% 11% 6% 6%
4000 BC 11% 11% 11% 39% 6% 6% 17%
5000 BC 6% 0% 33% 39% 11% 0% 11%
It should be remembered that the survey represents just a series
of population snapshots. Looking at the trends in the data, one might
think that bodies move orderly from one class of orbit to the next.
For example, starting at 1000 B.C., a particular test body is in a
Jupiter-Neptune orbit, followed by a Jupiter-Saturn, then Jupiter-
Planetoid Belt, then finally capture. The process is actually much
more complex. A body could start in a Jupiter-Neptune orbit, changing
to a Jupiter-Co-Orbital, then Jupiter-Saturn, Jupiter-Planetoid Belt,
then back to Jupiter-Saturn, and so on. There is a randomness about
it all.
The Results - The Distant Past and the Origin
Despite the randomness of individual bodies, there appear to
be trends in the population data. The number of bodies in Jupiter-
Planetoid Belt orbits, for example, seems to decrease logarithmically
as we go back in time. Extrapolating back, one can imagine a time
around 8000 B.C. when there would be no bodies left in this class
orbit. As time progresses, the population seems to move from one
class of orbit to the next higher one. We can easily see that as
the population in Jupiter-Planetoid Belt orbits decreases, the
population in Jupiter-Saturn orbits increases. As this population
reaches its apparent peak and starts to decline, we see an increase
in populations in Jupiter-Uranus orbits followed by Jupiter-Neptune
orbits.
Unfortunately, one has to be very careful about the conclusions
drawn. The uncertainties in these measurements is about plus or
minus twenty-five percent. Statistically speaking, twenty percent is
indistinguishable from fifty percent at a one sigma confidence level.
Scientists typically want something closer to three sigma confidence
level to be sure of their results. To that level of confidence, this
simulation can only tell the difference between zero percent and one
hundred percent. The observed population trends could be nothing
more than statistical noise.
The simulation's population of test bodies is too small to tell
the difference between pure random scattering, deliberate trends, or
some combination of the two. In reality, this is nothing more than a
detail in the grand scheme of things. The maximum observed orbital
inclination was ten degrees. Most are less than one-third of that.
According to the latest literature, this, along with a prograde orbit,
points to SL-9 originating in the Kuiper Belt, which is now believed
to be the source of the majority of periodic comets.
With this in mind, I now present the following simplified scenario
of the life of comet Shoemaker-Levy 9:
After the formation of our solar system about five billion years
ago, SL-9 orbited serenely around the Sun near the inner edge of the
Kuiper Belt beyond Pluto. As the years passed, the orbit slowly
changed under the gravitational influence of Neptune. After billions
of years of perturbations, SL-9 finally experienced a close encounter
with Neptune, which wrenched it permanently out of the Kuiper Belt.
During the hundreds of thousands to millions of years that
followed, the Jovian planets played pinball with the comet.
Eventually, Jupiter grabbed hold of it and gradually decreased the
period of the comet's orbit over thousands to tens of thousands of
years. A final encounter with Jupiter some hundreds to thousands of
years ago flung SL-9 into the inner solar system, where it may have
spent time as an active comet. About twenty-five years ago, SL-9 was
finally captured by Jupiter. After one dozen or so irregular orbits,
the twenty-one known parts of SL-9 will dive into the Jovian
atmosphere to their destruction.
So ends not only the life of comet Shoemaker-Levy 9 but my first
adventure with a PC. I am a bit more at ease with the little beast
now, but I am still concerned about what it might try to pull in the
future. In the mean time, I will muster up a little courage and try
something new. Stay tuned.
Related EJASA Articles -
"Cometary Conundrums", by M. Leon Knott - June 1993
"In Pursuit of Comet Swift-Tuttle", by Harry Taylor - September 1993
"Hunting for Comets: One Observer's Success Story", by Michael Janes
March 1994 (this issue)
"Planetoid Impacts: Devastating Shapers of Earth's History", by
Nicholas M. Burk - March 1994 (this issue)
About the Author -
Andrew J. LePage is a scientist at a small R&D company in the
Boston, Massachusetts area involved in space science image and data
analysis. He has written many articles on the history of spaceflight
and astronomy over the past few years that have been published in many
magazines throughout North America and Europe. Andrew has been a
serious observer of the Soviet/CIS space program for over one dozen
years.
Andrew's Internet address is: lepage@bur.visidyne.com
Andrew is the author of the following EJASA articles:
"Mars 1994" - March 1990
"The Great Moon Race: The Soviet Story, Part One" - December 1990
"The Great Moon Race: The Soviet Story, Part Two" - January 1991
"The Mystery of ZOND 2" - April 1991
"The Great Moon Race: New Findings" - May 1991
"The Great Moon Race: In the Beginning..." - May 1992
"The Great Moon Race: The Commitment" - August 1992
"The Great Moon Race: The Long Road to Success" - September 1992
"Recent Soviet Lunar and Planetary Program Revelations" - May 1993
"The Great Moon Race: The Red Moon" - July 1993
"The Great Moon Race: The Tide Turns" - August 1993
"The Great Moon Race: The Final Lap - November 1993
PLANETOID IMPACTS: DEVASTATING SHAPERS OF EARTH'S HISTORY
By Nicholas M. Burk
If there has been one catastrophic astronomical event which has
helped to shape the history of life on the planet Earth, it is the
impact of planetoids, also known as asteroids. This phenomenon,
which may have given birth to Earth's only natural satellite and
destroyed its most colossal life forms, may now be prevented by its
most advanced species - humanity.
In the embryonic stages of the solar system's formation, billions
upon billions of planetesimals aggregated into larger bodies. In time,
the largest of these mammoth planetoids became the predecessors of
today's planets. For millions of years, collisions with planetoidal
material were almost continuous. Today we can see the chaos of these
early impacts engraved on the faces of Mercury, our Moon, and many
other worlds in our solar system where little erosion has taken place
in millions and billions of years.
Astronomer William K. Hartmann has theorized that in the early
years of Earth, our planet was hit by a Mars-sized planetoid. Material
spewed out from Earth's mantle into space and formed a ring around the
planet. Just as embryonic material had aggregated before to form new
planets, this ring of debris eventually formed our Moon. [1]
Later, Earth was still bombarded by planetoids. Hartmann suggests
that primordial oceans may have been vaporized and with them early
forms of life never to be seen again. [2] As time progressed,
planetoid impacts occurred with lesser frequency.
Life moved from the sea onto land. Eventually amphibians and
reptiles surfaced. Then, in the time span just before the rise of
the dinosaurs, ninety percent of all species vanished. A definite
answer as to why this happened has yet to be uncovered, though a
devastating planetoid impact with Earth is one theory.
For the next two hundred million years, dinosaurs dominated
Earth's soils, skies, and oceans. Few species have ever prospered
so abundantly and for such a long period of time as these giant
creatures. Yet in an abrupt twist of geologic time, all dinosaurs
were wiped out in a quick ten million years.
Science grappled with this fantastic mystery for decades before
significant evidence emerged that hinted at a catastrophic ending for
these mighty creatures. In the early 1980s, a titanic impact crater
was uncovered near the Yucatan Peninsula.
The layer of rock which corresponds to the sixty-five million
year-old crater paints a grim picture of an apocalyptic catastrophe.
Blanketing our planet is a layer of metallic planetoidal residue, the
element iridium. Local shock quartz crystals hint at the intense heat
generated by an impact. Perhaps most compelling is a layer of soot
which is apparently the result of global forest fires caused by
planetoid debris raining down upon the rich Cretaceous forests.
Other estimates of the damage caused by the impact hint at massive
tidal waves and a blackening of the sky caused by airborne dust.
Although the dinosaurs were not immediately destroyed, the planetoid
set climatic and biological forces into motion which helped to forever
change Earth's history. [3]
The Cenozoic Era experienced minor extinctions and, perhaps not
uncoincidentally, layers of iridium at those levels have been found. [4]
Even in the age of human civilization - a tiny fraction of geologic
time - evidence of powerful planetoid impacts has been brought to light.
Perhaps the most famous event is the Tunguska impact which occurred
in the Siberian forests in 1908. Had this object struck a thickly
inhabited area, the results would have been far more disastrous.
Perhaps the hazards of planetoid impacts would have been permanently
engraved in the consciousness of humanity. [5]
As this millennium comes to a close, astronomers have confirmed
that there are between 750 and 1,000 Earth-crossing planetoids at
least 0.8 kilometer (0.5 mile) in size or larger roaming through the
solar system. [6] Probabilities say that the odds of an impact on
Earth occurring soon are minimal. Yet odds cannot predict chaos.
For example, in 1991, a ten-meter (thirty-three-foot) wide planetoid
dubbed 1991 BA quietly zipped halfway the distance between Earth and
the Moon. A similar incident occurred on March 15, 1994 with another
Earth-crossing planetoid.
For all of the potential catastrophic implications caused by
Earth-crossing planetoids, deflection has been deemed quite feasible.
In 1981, the Spacewatch Workshop, a meeting of invited astronomers,
planetary geologists, space mission specialists, and nuclear weapons
authorities from the United States Defense Department, addressed the
issue of deflecting planetoids that could pose a hazard to Earth. [7]
The majority of participants at the meeting agreed that not only
was the possibility of deflecting a planetoid feasible, but work on
systems to do so could begin right away. [8] Nuclear devices and even
TNT could be used to nudge a menacing planetoid off its impending
course, thereby preventing a tremendous disaster. In a statement to
the U.S. Congress, NASA's Dr. Wesley T. Huntress, Jr., stated that the
civilian space program will continue to monitor Near-Earth Asteroids
(NEA), but the issue of the actual deflection must be left up to the
Defense Department.
In terms of cost, developing deflection probes and enhancing
already existing detection mechanisms would be relatively inexpensive
compared to some other space efforts. The importance of this safety
net is priceless if a large planetoid approaches Earth.
Planetoid impacts have helped to shape the history of our planet
and its life forms. As the second millennium approaches, Earth should
finally be able to avoid this catastrophic phenomena which it has
endured for over four billion years.
Annotated Bibliography -
1. William K. Hartmann and Ron Miller, A HISTORY OF EARTH, Workman
Publishing Company, Inc., New York, pages 44-57, 1991. An
excellent resource for understanding the scope of planetoid
impacts on Earth history.
2. Ibid., pages 90-93.
3. Ibid., pages 158-174. Hartmann and Miller give a fascinating
and beautifully illustrated description of the impact. There is
a tremendous amount of information on this subject, including the
September 17, 1993 issue of SCIENCE magazine, which states that the
size of the planetoid was even larger than previously thought.
4. Ibid., pages 175-177.
5. Hartmann and Miller give an account of the Tunguska impact on pages
207-209. There is also a very informative article in the December,
1993 issue of ASTRONOMY magazine.
6. Shannon Brownlee, "How to Prevent the Extinctions", page 30,
DISCOVER, 5:5, May, 1984.
7. Brownlee, op. cit, page 31.
8. Brownlee, op. cit, page 31.
9. Statement of Dr. Wesley T. Huntress, Jr., before the Subcommittee
on the Space Committee on Science, Space, and Technology, House of
Representatives, March 24, 1993. Obtained through NASA Spacelink
via Internet. A very substantive description of NASA's role in
detecting Near Earth Objects.
About the Author -
Nicholas M. Burk is a Northampton High School student in
Massachusetts seeking a career in the administration of space public
policy. Nicholas has written a great deal on the economic and
environmental benefits of an expanded space program and is seeking
to have some of this work published. His related interests include
cosmology and the effects of space travel on human psychology.
Nicholas may be reached on the Internet at: nburk@smith.smith.edu
HUNTING FOR COMETS:
ONE OBSERVER'S SUCCESS STORY
by Michael Janes
Courtesy of Paul Dickson (p.dickson@asu.edu), Editor of the
Saguaro Astronomy Club's newsletter, SACNews, in Phoenix, Arizona.
The September, 1992 Labor Day weekend reached a high point for
some valley amateur astronomers on Sunday. Leon Knott, who recently
moved to the valley and became a new member with SAC, hosted a small
get-together in Mesa, Arizona. Among the people there was a friend of
Leon's visiting for the weekend from New Mexico, Howard Brewington.
Howard and his wife live in Cloudcroft, New Mexico, at an elevation
of 2,200 meters (7,400 feet). Out in front of their home is Howard's
observatory, which houses a forty-centimeter (sixteen-inch) f/4.5
reflector on an Alt./Az. mounting. The primary mirror was figured by
Howard and the telescope design was done by Leon. Piggybacked on that
telescope is a twenty-centimeter (eight-inch) f/4.3 reflector. The
design of the observatory does not allow for good viewing to the north.
However, it does provide good views of both the western and eastern
horizons.
In the winter of 1987, Howard was actively photographing comet
Bradfield. By the end of its apparition, he was "bit by the comet
hunting bug." The first half of 1988 was spent conducting a
photographic search with a twenty-centimeter f/1.5 Schmidt camera.
This type of search was not effective, considering the time involved
to take the photographs in relation to the amount of sky covered plus
the expense. So in the summer of 1988, Howard converted to a visual
search method.
Don Machholz of California searches for comets by dividing the sky
into about forty quadrants, examining each for an interloper. David
Levy will sweep up and down slowly across the sky. Howard takes a
different approach from the methods used by these accomplished
astronomical observers.
When asked about his search methods, Howard replied: "I have only
four quadrants, two in the evening and two in the morning. I just
make sweeps in azimuth sixty degrees long and I just wait for the
object to come into the eyepiece." This motion in azimuth is coupled
with the rotation of our Earth, allowing for a shift of one field in
altitude after the sixty-degree sweep. The skies over Cloudcroft, New
Mexico, seem to be similar to our own here in Arizona during 1992.
Says Howard: "When the skies are good I spend anywhere from twenty to
twenty-five hours a month at the eyepiece. But this year I've been
lucky for ten hours."
In November of 1989, after ninety-three sessions, fourteen months,
and 230 search hours, Comet 1989a1, Aarseth-Brewington, was discovered.
Howard's first comet rose to third magnitude by December and is reviewed
by David Levy in his Star Trails column in the April, 1990 issue of SKY
& TELESCOPE magazine. Many photographs are also included in that issue.
The searches continued for over one year until January 7, 1991.
This next object turned out to be comet Metcalf, which had been lost
after its discovery in the winter of 1906-1907. Periodic comet
Metcalf-Brewington, 1991a, has a period of eight years, though its
last orbit took it towards Jupiter, which increased its distance by
one astronomical unit (AU), making any future returns unlikely.
Comet 1991a exhibited an outburst of ten magnitudes over a period
of thirty hours. Two nights before Christmas of that same year, Comet
1991g1 was co-discovered by Mauro Zanotta of Italy just twelve hours
prior to Brewington's observation.
The date of August 29, 1992 brought Howard's fourth discovery and
his first morning comet. At magnitude 11.5, it was outward bound at a
distance of two AU. This comet is now too faint for amateur telescopes.
Although there have been a handful of good comets in recent years,
I feel that we are due for a *great* comet. According to Howard
Brewington: "I plan to find the next great comet. It'll be the
biggest disappointment of my life if I don't." When asked if there
was one comet observation that stands out from the rest, Howard
replied: "Well, the best comet I've seen in my life was mine."
Related EJASA Articles -
"Cometary Conundrums", by M. Leon Knott - June 1993
"In Pursuit of Comet Swift-Tuttle", by Harry Taylor - September 1993
"A Personal Adventure in Home Computing: The Origin of Comet
Shoemaker-Levy 9", by Andrew J. LePage - March 1994 (this issue)
About the Author -
Michael Janes has been a serious amateur astronomer for seven
years and currently observes with a forty-centimeter reflector.
Michael's observing programs include the Herschel 400 list, cata-
clysmic variables, and the planet Mars. In addition to being a
member of the Saguaro Astronomy Club (SAC), Michael is also a member
of the American Association of Variable Star Observers (AAVSO).
THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC
March 1994 - Vol. 5, No. 8
Copyright (c) 1994 - ASA