(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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