History of Lunar Impacts

The last Leonid meteor shower produced the first independently confirmed 
recordings of meteor impacts on the Moon. There have been many previous 
efforts to observe lunar meteor impacts, with some success, although none 
provided confirmed observations from separate locations. This 
article attempts to document some of the previous work.

In 1998, Martin Beech and Simona Nikolova, of the University of Regina, made 
specific predictions about lunar Leonids, published in their paper 
"Leonid flashers-meteoroid impacts on the Moon," in Il Nuovo Cimento, 
Note Brevi, Vol. 21C, N.5 Sept-Oct. 1998, pp.577-581.  The abstract 
reads:

We examine the conditions under which optical impact flashes might be 
observable on the Moon's disk during the times of annual meteor shower activity.
Our attention is primarily directed towards the Leonid shower given the high 
probability that it will undergo repeated outburst activity during the next 
several years.  The Leonid stream to Moon encounter geometry is discussed, and 
we find that the best probable times to perform optical surveys will be in 1990
and 2002.  We estimate that a one kilogram Leonid meteoroid might produce a 
magnitude-2 optical transient on the Moon's disk.  
                                     
Furthermore, Noah Brosch passed on this message from Bill 
Cook, who even on September 27, 1999 predicted that the Moon would be 
closer to the center of the 1899 Leonid stream than the Earth.  Most 
of us did not realize this until after Nov. 18.    

In June 1997, the Association of Lunar and Planetary Observers' (ALPO's) John 
Westfall gave a paper about the Lunar Meteor Search program of 1955 
to 1965 at the ALPO's 48th convention in Los Cruces, New Mexico.  
The full text, from ALPO Monograph No. 7, provided by Westfall, can be found 
here. Coordinated by Robert M. Adams, the project 
would eventually publish six "Progress Reports."  Unfortunately, the project 
never achieved its goal of simultaneous, independent observations.  While it 
was known that many impacts occurred each year from the Apollo lunar seismic 
network that operated during the early 1970's, it was still unclear how the 
impacts would appear visibly.  The possibilities included bright streaks in the
lunar atmosphere, clouds of ejecta being thrown out, newly formed craters, or 
bright point flashes.  Westfall concluded that the Lunar Meteor Search program 
should be resurrected using video equipment that is now relatively inexpensive.
In this way, two or more observers, reasonably far apart, could record the same
flash at the same time.  The paper gives much information, including estimates 
of impact visual magnitudes and frequencies as a function of impactor mass, and
contains many references to past work and observations. 

Lunar Transient Phenomena (LTP) are documented in NASA Technical Report R-227 
"Catalog of Reported Lunar Events."  This report can be found at 
http:www.mufor.org/tlp/lunar.html  
Some more recent LTP information can be found at 
http://www.evansville.net/~slk/ltppage.htm.  
These sites describe many temporary changes on the lunar surface, 
almost all controversial and unconfirmed, however, several of them could be 
meteor impacts.

Tom Dobbins, Acting Coordinator of the ALPO Historical Section, also gives the 
following information:

In 1959, Lincoln LaPaz calculated that a 10 pound meteorite hitting the moon 
would produce a visible flash, and that 10 of these should occur a year.  At 
the time, no visible flashes had ever been confirmed, and a competing theory 
states that meteorites impacting the moon would penetrate the porous surface 
and the dust and debris would obscure the flash this impact would throw out.  

Several probable (but unconfirmed!) sightings have occurred in the past.  In 
October of 1945, F.H. Thornton detected a point of light on the floor of 
Plato.  This occurred during the Orionid meteor stream.  He compared the flash 
to an "[anti-aircraft] shell exploding in the air at a distance of about 10 
miles."  In April of 1948, A.W. Vince saw a flash on the earth lit portion of 
the moon near the crater Grimaldi.  In August of that year, A.J. Woodward saw 
a flash "like a bright sparkle of frost" that lasted three seconds.  He claimed 
"it had the appearance of an object striking the moon."  Finally, in May of 
1951, H.P. Wilkins observed a bright point of light in the crater Gassendi 
that lasted for only one second.   These sightings were believed to be impacts 
on the moon, but no confirmation was obtained, and their validity remains in 
doubt.  

Some spacecraft impacts, or their craters, have been observed.  
Several years ago, a Japanese satellite called Hiten was crashed into the moon 
and an image of the impact was recorded with the 3.6 m Anglo-
Australian telescope. Since the mass and speed of this 
satellite are known, it may be possible to use this image to calibrate the 
images of the Leonid meteors hitting the moon.  Also, the craters 
caused by the early Apollo Command modules were identified by 
comparing Apollo and Lunar Orbiter photographs-click here for Tom 
Dobbins' message about them.  


Sources:
V.A. Firsoff, Strange World of the Moon, Basic Books, New York,
1959. p. 53.

F.H. Thornton, Journal of the British Astronomical Association,
57:143, 1947.

H.P. Wilkins, Our Moon, Frederick Muller, London, 1954. p.128.

A.J. Woodward, "An Unusual Observation of the Moon" Journal of the
Royal Astronomical Society of Canada, 42:194, 1948.

Patrick Moore, A Guide to the Moon, Norton, New York, 1953. p. 118.