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Lunar Impact!

May 29, 2013

Three decades ago, I was part of a company's attempts to align itself with the ill-fated goal of bringing the US into the metric system, a process called metrication. At that time, the US had half-heartedly decided to "go metric," and there were even road signs posted in both miles and kilometers. I even used proper metric units of magnetism in a few of my papers, much to everyone's confusion. The SI unit for magnetic field strength is amperes per meter, but everyone prefers the oersted.

To add to the confusion, in a vacuum (or in air, if you're not too particular), the oersted is the same as a gauss, the unit of magnetic induction or magnetic flux density, so we can write the conversion equation,
1 A/m = 4πx10-3 gauss (in a vacuum)
If we keep everything in the meter-kilogram-second system, as we should, we have
1 A/m = μo T
where T is the field strength in tesla (= 104 gauss), and μo is the permeability of free space
μo = 4πx10-7 henrys per meter.
People will persist in using customary units, since these often make things easier to visualize. I still have a carpenter's rule in my workshop marked in feet and inches. Likewise, when we're talking about big explosions, we don't talk in joules; rather, we talk about tons. The tons, of course, are tons of TNT, 2,4,6-trinitrotoluene (C6H2(NO2)3CH3), a high explosive.

Just as octane is the standard for motor fuels, the TNT equivalent is used to quantify things that go boom. A ton of TNT (a metric ton, of course) is defined as having an energy equal to 4.184 gigajoules.

If the number, 4.184, seems familiar, it's the conversion factor between calories and joules (1 calorie ≡ 4.184 J). In theory, the energy release of a gram of TNT is 4853 joules, and experiments have given a value of 4686, but convenience outweighs accuracy in this case.

There was a rather large meteor strike on the Moon on March 17 of this year. The impact was so intense that you would have noticed the flash of light from it with your unaided eye. It was at stellar magnitude 4, which made it nearly as bright as the Andromeda Galaxy. NASA has characterized this meteor strike, and they've created the following scorecard of this explosion. The energy, of course, is listed in tons of TNT.[3]

Figure caption

Scorecard for the March 17, 2013, meteor impact at Mare Imbrium.

(Still image from a NASA YouTube Video.)

The impact was in the Mare Imbrium region of the moon, as shown in the photograph, below. NASA has been monitoring meteor impacts on the Moon for the past eight years, and the flash of light from this impact was nearly an order of magnitude greater than anything seen to date.[2] This however, would pale in comparison with the theorized formation of the Moon's Giordano Bruno crater.[3]

Five monks from the Canterbury Abbey reported a massive flare from the Moon's direction on June 18, 1178. The geologist, Jack B. Hartung, theorized in 1976 that this was the meteor impact that formed the Giordano Bruno crater.[4] Such an impact, however, would have had notable side-effects, including a meteor storm on Earth, which were not observed. It's possible that the monks, instead, saw a meteor exploding in Earth's atmosphere right above them.[3] Figure caption

The Mare Imbrium region of the Moon.

NASA Image.)

(Click on the image for a larger version with registered impacts from 2005 to present, with the recent impact highlighted in red.)[2)]

The experience of the lunar meteor monitoring program is that meteor strikes on the Moon are quite common. The March 17, 2013, Mare Imbrium impact appears to be associated with a meteor stream that also dropped some fireballs into Earth's atmosphere. More than half the lunar impact events come from the known meteor showers, such as the Leonids and Perseids. The other half are from random events, which are called sporadic meteors.[2]

The motivation for such research is to determine the safety of extended lunar exploration, and what times of year may be most inhospitable for moonwalks. The NASA Lunar Reconnaissance Orbiter may be able to image the crater, supposedly about twenty meters wide, resulting from the March impact. Such imaging will validate lunar impact models.[2]

Artist's conception of a major lunar meteor strike.

An artist's conception of a major lunar meteor strike. The meteor's kinetic energy is converted into the heat of molten rock and their associated vapors. (Still image from a NASA YouTube Video.)


  1. Tony Phillips, "Bright Explosion on the Moon," Science@NASA Web Site, May 17, 2013.
  2. NASA ScienceCast: Bright Explosion on the Moon, YouTube Video, May 16, 2013.
  3. The Mysterious Case of Crater Giordano Bruno, Science@NASA Web Site, April 26, 2001.
  4. J. B. Hartung, "Was the formation of a 20-km-diameter impact crater on the moon observed on June 18, 1178?" Meteoritics, vol. 11 (September 30, 1976), pp. 187-194.

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