Explosion on the Moon??

Over the past 8 years, NASA scientists have documented
many small impacts on the Moon. The biggest, shown
as the red square in Mare Imbrium, was found on
a March 17, 2013 image. NASA image.

NASA and University of Western Ontario scientists have discovered the largest impact observed to date on the Moon in an 8-year old monitoring program aimed at identifying streams of space debris that pose a potential threat to the Earth-Moon system.  They estimate that the crater formed could be up to 20 meters in diameter. The impact generated a cloud of gas and pool of hot melted rock that gave off a flash detected on a monitoring video.

     NASA scientists estimated the impact velocity as 25 km/s (the number that appears in the press is 56,000 mph), and that the impactor was 0.3-0.4 meters in diameter weighing about 40 kg.  They haven't given the basis for these numbers. My "guess" is that they assumed that the diameter of the crater was related to, or equal to, the diameter of the luminous spot and that they back-calculated an impact energy for this. Then, assuming a most probable impact velocity of 25 km/s, the could back out a mass for the object.

     Taking these calculations forward, 1/2*m*v*v gives an energy of 12.5 x 10^9 joules.  By definition, a ton of TNT is defined as having an energy of 4.184 x 10^9 joules, and so dividing these two numbers gives a TNT equivalent of 3 tonnes equivalent TNT. Somehow the rounded number that came out in the press was 5 tons of TNT. Note that the press, and possibly NASA, did not distinguish between tonnes (so-called 'metric ton' which is 2240 lb or 1016 kg)) and tons (the so-called 'short ton' which is 2000 lb or 907 kg.)

    My thesis advisor, the late Gene Shoemaker, would be having fits about this whole discussion because he drilled into us students that meteorite impacts are not at all like chemical or nuclear explosions. Rather, they involve a process in which shock waves generate by the impact propagate both downward into the target material and backward into the impactor.  The shock in the target material travels down into the ground and outward away from the impact site. Meanwhile, the shock in the meteorite is reflected off of its back surface as a rarefaction, or expansion, wave. The expansion wave eventually overtakes the shock wave propagating down into the ground. Before it does, however, the meteorite has pretty much been vaporized and melted, as has a portion of the ground under and around the impact site. This material, generating the bright flash seen on the records, is ejected from the crater, some probably escaping into space, some returning to the surface around the newly-formed crater. Less highly shocked rock is also ejected from the crater onto the lunar surface, forming an ejecta blanket and perhaps some rays.  NASA has passed on word to the controllers of the Lunar Reconnaissance Orbiter who may be able to image the crater the next time the spacecraft passes over the impact site. This would provide a valuable test of the assumptions that went into interpreting the flash in terms of the impact dynamics and the crater properties.