(meteorobs) 100 impacts on the Moon ... and counting

Bruce McCurdy bmccurdy at telusplanet.net
Thu May 29 16:49:11 EDT 2008


    I keep waiting for somebody else to post this link to MeteorObs but 
nobody has, so I guess I should. List members may be interested in the below 
release from NASA's Meteoroid Environment Office:

http://science.nasa.gov/headlines/y2008/21may_100explosions.htm

    ... noting the milestone of 100 observed impacts on the Moon over just 
two years of an automated program to that end. One paragraph that caught my 
eye:

                        "During meteor showers such as the Quadrantids or 
Perseids, when the
                        Moon passes through dense streams of cometary 
debris, the rate of lunar
                        flashes can go as high as one per hour. Impacts 
subside when the Moon
                        exits the stream, but curiously the rate never goes 
to zero."

    "Curiously"? Not to this veteran of numerous sporadic meteors, an 
explanation which is indeed mentioned in the text a couple of paragraphs 
later. I wonder however, if the annual variations in sporadic rates might 
possibly be divined from lunar observations? Given the size of the 
"particles" required to generate 7th-10th magnitude flashes on the lunar 
dark side, perhaps a better association could be made with fireball rates 
than sporadic rates.

    Of course such nuances might be overwhelmed by observational bias. From 
the linked PDF containing detailed info of the 103 impact candidates, it is 
apparent that the large majority of non-shower observations occur when the 
Moon is 4-7 days old. Indeed, this is obvious from the distribution on the 
map. Such observations may have an inherent seasonal bias given the a much 
wider observing window in the spring when the first quarter Moon is of nigh 
northern declination and more accessible to (northern hemisphere based) 
observing programs. The preferred phase might also benefit from some 
physical manifestations due to the dark limb being the *leading* limb of the 
Moon's "true" motion (w.r.t. the Sun) around First Quarter; and further that 
the Moon's orbital velocity (w.r.t. Earth) of ~1 km/s would be in roughly 
the same direction of motion as Earth's, leading to slightly more violent 
(and brighter) impacts. Whether any of these effects could be filtered out 
of the statistical noise might prove problematic. Indeed, rather than my own 
idle speculations, I would be interested to read the opinions of one or more 
of the experts on this list as to what the lunar impact study might teach us 
about the impacting meteoroids and their distribution.

    When this news item was recently raised on the RASCals discussion list, 
veteran Ontario-based observer Geoff Gaherty mentioned that he was involved 
in a major program dating back to the dawn of the Space Age to visually 
observe for such flashes. Visual observers on this list might find this 
exchange interesting:

        Geoff:
        The ALPO had a visual lunar meteor project back in the '50s which I 
participated in,
        and we only observed in the waxing and waning crescent phases,
        concentrating on the dark side.  Interestingly, we never had a 
single
        confirmed lunar meteor despite many hours of observations logged by
        multiple observers at different locations (necessary to rule out
        terrestrial meteors).  After 15 years of negative results, we 
concluded
        lunar meteors were too faint to be detected, and closed down the 
project.

        Bruce:
        >     Geoff, given that hard-won experience, are you surprised with 
both the
        > number of recorded impacts and especially the brightness thereof? 
Surely you
        > would have seen a 7th or 8th magnitude flash? Did you see any that 
were
        > unconfirmed by a second observer?

        Geoff:
        I was just an observer in the trenches for the ALPO Lunar Meteor
        project, and wasn't involved in the analysis process, nor the 
decision
        to shut it down.  Unfortunately, the fellow who was in charge at the
        end, Kenneth Chalk from the Montreal Centre, seems to have vanished 
off
        the face of the Earth.  I will say that I _am_ surprised at the 
number
        and brightness of the recently imaged flashes; I'd have thought some 
of
        us would have seen something that bright.  We were largely quite
        experienced lunar observers, using 3" to 8" scopes.

    Judging from the closing paragraph of the linked article up top, there 
will be opportunities for amateur contributions to the current program as 
well.

    An alternate story can be found here:

http://www.astronomynow.com/080521Over100lunarimpactexplosionscaughtintheact.html

    Bruce
    *****
 




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