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(meteorobs) Lunar Cratering
Thought folks on meteorobs might enjoy this.....Mark
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From: "Roger D. Curry" <rcurry@southeastdot net>
Date: Sun, 12 Jan 1997 07:34:06 -0500
Subject: Lunar cratering
There has been a recent thread about meteorite cratering on the Moon. In
April 1994, I wrote an article for the NEFAS Skywatch which discusses this
very issue. I had intended to post it earlier but a thorough search of my
hard drive and
archive media failed to find the article. As a last resort, I scanned the
article from a printed issue and used OCR software to convert it to a text
file and corrected the inevitable errors using a spell checker in Word for
Windows.
My apologies if I failed to catch some of the errors. By and large it is
correct.
I apologize for the length of this post but hope enough readers will find it
of interest to have made it worthwhile to post it.
Anyone wishing to use this material is welcome to do so, provided credit is
given to the author and the NEFAS Skywatch.
-----------------------------
Meteor Impacting on the Moon: Some New Thoughts
by Roger Curry
NEFAS Skywatch, April 1994
In the October 1993 Skywatch, I wrote an article entitled Astronomers
Benefitting from Star Wars, which highlighted several areas of research
that are the direct results of, or spin-offs from, the Strategic Defense
Initiative research.
One reported item was that the Air Force Space Command had imaged the
explosions of large meteors entering the Earth's atmosphere from a satellite
that is used to detect rocket exhaust plumes. There have been aver 130 of
these images
captured to date that show meteor explosions with kinetic energies
equivalent of up to 20 kilotons of TNT (the Hiroshima bomb was 13 kt). This
information was reported in August/September issue of Smithsonian Air &
Space magazine and later it was also written as a feature article for Sky &
Telescope).
A conversation with NEFAS member Wallace Baldwin resulted in a curiosity
which eventually led to a correspondence with Dr. John E. Westfall,
Executive Director of the Association of Lunar and Planetary Observers
(A.L.P.O.) and ultimately
to this article. Wallace pointed out that whereas the Earth's atmosphere
prevented these large meteors (of up to 200 feet in diameter) from reaching
the surface intact, the Moon has no such buffer, and the meteoroids that
enter the Moon's environs are only stopped at the lunar surface.
Wallace, in fact, has good reason to be interested in such lunar impacts:
while taking slides of the lunar region of Plato, Wallace captured a
mysterious red glow, that upon magnification, looks like an explosion near
the surface of the Moon. The slide has been examined by several scientists
and the ultimate conclusion was that it was an internal reflection caused by
the arrangement of the lens and tele-extender. This conclusion has never
been confirmed, however.
because other rolls of film exposed using the same arrangement of lenses has
thus far failed to duplicate the suspicious glow in question. In fact,
Ernest Rowland, of Rowlab in Jacksonville, after examining the slide under a
stereo microscope, disputes that the flash is anything other than an
explosion on the Moon.
Wallace thought that a multi-kiloton explosion on the Moon should leave a
crater visible from Earth. In order to get a more informed opinion, I
contacted Dr. Westfall via CompuServe with a number of questions and he was
kind enough
to reply, almost completely, to every question I asked him. The following
"interview" is directly quoted or accurately summarized from the correspondence.
An Interview with Dr. Westfall
Q. What size crater would be expected from a meteor of diameter of 150 to
200 feet should it strike the Moon?
A. From a table in Ralph B. Baldwin's book The Measure of the Moon
(University of Chicago Press, 1963), Baldwin has a 10,000 foot crater
created by a 215-ft nickel-iron meteor impacting at 25 km/sec. This would be
a rare event, though.
The February, 1994 S&T article talked about l-kiloton events and a single
kiloton event [the Smithsonian Air & Space article is the one that mentioned
a 20 kt event- ed.]. According to Baldwin , p. 164, a l-kt crater would be
287 feet across, and a 5-kt one would be about 465 feet.
Q. Does the Earth's gravity accelerate such an incoming body? or does most
of the velocity derive from the orbital velocity of the meteor? Would
meteors striking the Moon have a similar velocity to ones striking the Earth?
A: Meteors are accelerated only slightly by the Earth's gravity because they
are close to the Earth for only a short time. Thus, lunar impact velocities
would be only slightly lower than the terrestrial ones. [Don E. Wilhelms
(1987), The Geologic History of the Moon. U.S. Geological Survey
Professional Paper 1348. (See especially pp. 271-273 and the references
cited there.)]
Q. What aperture of telescope do you think would be required to image a
crater caused by a kt or multi-kt event such as those explosions seen in the
Earth's atmosphere?
A. To be reliably identified, a crater needs to have a diameter of twice the
telescope's resolving power. This implies that a 10,000 foot crater could be
spotted with a 5.6 inch telescope (assuming excellent optics and excellent
seeing). Again, I want to stress that the creation of a 10,000-foot crater
on the Moon would be a very rare event; for one thing, the Moon's near side
has only 3.7 percent the area of the Earth (although about 12 percent of the
Earth's land area). There would be no chance of detecting the 287 and
465-foot craters mentioned earlier. My experience is that the smallest fresh
crater that can be reliably identified from the Earth with an excellent
telescope and excellent seeing is about 1 km diameter (ca. 3300 feet).
Q. To your knowledge, have any new impact craters ever been discovered on
the Moon that can be positively identified as not being on previous
photographs of the same region taken with similar equipment under similar
conditions?
A. To the best of my knowledge. I know of no confirmed new craters on the
Moon observed from Earth. The Lunar Orbiters did photograph some new
man-made craters caused by impacts of probes and spent rocket stages!
Q. Should a search for new impacts by started, what would be the
difficulties? Several I can foresee would be the affects of libration,
phase, and distance from the Earth. How often do the same conditions exist
that would permit blink or stereographic comparison of regions of the Moon?
Does technology exist that would allow an image to be scanned into a
computer and have the computer correct for the effects of libration, phase,
and distance? Of even having the computer
do the comparisons?
A. The chief difficulty would be the extreme rarity of the creation of
craters large enough to be seen. My calculations, based on Wilhelms (1987,
Fig. 13.13A on p. 273), are that a new I-km crater would appear on the near
side of the Moon on an average of once every 66,000 years. Even if we
allowed the cratering rate to be 1,000 times higher, we'd still have to be
very patient! (Were the probabilities higher, your idea of a computer search
sounds interesting. I see no way to compare different phases, so the
photographs/images to be compared would have to be taken under similar
lighting. The computer should be able to adjust for varying distance.
Libration would be more of a problem because local topography would cause
position shifts; the search would probably have to be
confined to zones near the apparent disk center.)
Q. Would such a survey for new impacts help to better determine the
frequency that such objects strike the Earth? Would a survey have to include
a large area of the visible surface of the Moon, or would a survey of a
representative percentage of the lunar surface be enough to draw conclusions
for the rest?
A. Again, I think the new craters would occur too infrequently for a
significant sample to be observed. However, impacts might better be detected
by their flashes than by noticing the new craters [additional comments on
this later - ed.].
Q. Would the fact that the Moon presents basically only one hemis phere for
our examination and that this is the hemisphere that always faces Earth
cause the near side to take fewer meteorite strikes than the far side (from an
umbrella effect)?
A. Although I haven't calculated this, I feel that the impact rate on the
lunar near side would be only slightly less than that for the far side
(again, the references that Wilhelm cites would answer this question better).
Q. Has any such survey ever been proposed? Do you think such a proposal
should be considered seriously by the NSF or by an observatory committee?
Any ideas on the minimum time, equipment, and manpower that would be involved?
A. I don't know of any searches like yours that have been proposed. I doubt
if you could get it funded because the review committee would make
calculations similar to mine and conclude the likelihood of success would be
too small. Even if you had enough manpower and equipment to study the entire
near side to I-km crater diameter, the chance of success would be very low.
I might mention that, were the project conducted, there would be no need to
continuously monitor the Moon=97a thorough survey every few years would be
sufficient
Q. With the growing number of large-aperture amateur scopes and the CCD
imaging capability that is now in the hands of amateurs, is such a project
feasible for the community of amateur astronomers?
A. Assuming that the project were conducted, I think the amateurs could do
the job just as well as professionals. Some of the highest-resolution lunar
photography/images I have ever seen has been done by amateurs (e.g., Jean
Dragesco, Don Parker). To summarize, the problem is that, by any reasonable
calculation, the formation of new craters large enough to detect would be so
rare that there would be almost no chance of spotting one in a human
lifetime. This conclusion affected all of my answers above.
On the other hand, we might be able to detect the impact of much smaller
meteoroids by watch for their flashes on the unilluminated hemisphere of the
Moon. The problem is that I know of no data that tell us how much of an
object's kinetic energy would be converted to visible light (this is not to
say that no such data exist). I would be surprised if kiloton-size impacts
were not visible in a good telescope, though.
From 1955-1965, the A.L.P.O. conducted a "Lunar Meteor Search." When it
started, the idea was to look for light streaks in the lunar atmosphere; by
the end it was known that the Moon's atmosphere was far too thin for meteors
to heat significantly. On the other hand, a number of pinpoint flashes were
reported, and photographed at least twice (although the photographic
evidence is debated). The trouble was that, to the best of my knowledge, the
sightings were unconfirmed.
What I recommend is that a group of amateurs make videotapes in order to
monitor the dark side of the Moon to detect transient flashes. At least two
amateurs should be taping at all possible times from different locations.
Showing the
same flash would be good evidence for its reality. Over time, the project
might collect enough flashes to derive a reliable impact frequency.
Any such monitoring program should be coordinated with the A.L.P.O. Lunar
Transient Phenomenon Program; its Recorder is: David O. Darling 416 W.
Wilson St., Sun Prairie, WI 53590-2114.
My comments
I am very grateful to Dr. Westfall for the time he has taken to respond to
my questions. I would also like to point out that there have been several
significant " needle-in-a-haystack" searches that have been successfully
done by observatories and amateurs, including the search that led to the
discovery of Pluto by Clyde Tombaugh, the search for and discovery of
proto-planets or Kuiper Belt objects beyond the orbit of Neptune, and the
Nova Patrol (in which amateurs memorize the position of 20,000 to 30,000
stars to magnitude 8-or-so within the Milky Way and alert observatories
should a "new" 8th-magnitude or brighter star appear). Amateur searches
still find most of the comets detected; the availability of large-aperture
instruments have allowed amateur astronomers to detect a number of recent
supernovae. Most variable star observations are handled by amateurs. Lunar
Transient Phenomenae (lights on the Moon) are almost exclusively in the
domain of the amateur.
Should a project Like the Lunar Survey for New Impact Sites ever be
undertaken, it will take the dedication and time that, perhaps, it is only
an amateur's to give.
----------------------
Roger D. Curry <rcurry@southeastdot net> 30.2N/81.4W
Visit the Northeast Florida Astronomical Society Home Page at=20
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