## 2014-04-12

### March 2013 Moonstrike

A nice real-world example of a strike against the lunar surface . . .

The notable details are provided in a quick graphic . . . they list the rock as 40 kilograms, 0.3 to 0.4 meters wide, traveling at 56,000 miles per hour (because it was totally helpful to suddenly convert to the imperial system after talking in SI kilograms and meters), and the explosion is said to be worth about 5 tons of energy.

The approximate volume of the impactor given the size provided (and assuming an average of 0.35 meters and a sphere) would be about 0.18 cubic meters.

V= 4/3 π r^3
V= 4/3 π (0.35^3)
V= 4/3 π (0.042875)
V= 1.33333333333333333333*pi*0.042875
V= 0.17959438003021651346

The approximate density of the impactor given the mass would thus be 220 kg./m^3.

That's really quite surprisingly low.  That's about a fifth the density of water . . . the impactor would've floated!

56,000mph is about 25000 meters per second.   The kinetic energy of the impactor would have been:

E = 0.5 m v^2
E = 0.5 (40) (25000^2)
E = 12500000000 J
E = 12.5 GJ
E = 2.98 tons TNT

This is somewhat different from their figure of 5 tons but within the ballpark.

The crater is believed to be about 20 meters in size.

I'm still weirded out by their density figure, though.

In any case, as seen in the video, the flash lasts about a second, and was sufficiently bright to have been visible from Earth.  I wouldn't trust the apparent size of the flash given the camera's intent of detecting dim flashes, but I could be wrong.