By The Metric Maven

Bulldog Edition

I very much like to check-out Phil Plait’s Bad Astronomy blog. His measurement presentation causes me to generally wince, but its topics are often rather interesting. On 2016-02-20 he wrote about the Largest Fireball Since Chelyabinsk falling over the Atlantic Ocean. The object appears to have possibly exploded (or just burned up) about 30 Kilometers above the ocean surface. Plait assures us there is no cause for alarm; this was a small meteorite. He states:

For comparison, the Chelyabinsk explosion, which was strong enough to shatter windows and injure more than 1,000 people (due to flying glass), had an equivalent yield of 500,000 tons of TNT, 40 times the energy of this more recent impact.

Plait provides a link to the NASA/JPL Near-Earth Object Fireball page, and presents a cropped graphic from the site:

I was quite pleased to see NASA using international dating for the entries. It all is nice and ISO 8601. The total radiated energy from the meteor is given in joules. It is presented in scientific notation with an exponent to the tenth power. Here is a larger section of the NASA table with the data for the new meteorite in the first row at the top:

The total energy radiated from the meteor as it cruises through Earth’s atmosphere is about 685.3 x 10^{10} joules. Why an exponent of ten? We can see as we go down the list that all of the examples have this same multiplier. When presenting data, scientific notation is not good for numerical comparison. One could choose a metric prefix and have 6.85 Terajoules instead. As the values are typically smaller than this, one could switch to Gigajoules and write it as 6853 GJ. In my view presenting comparison data with scientific notation hinders intuitive understanding. I know that some computer languages have a setting that parses the exponents so they are presented in engineering notation by steps of 1000.^{[1]} The exponent for all of the values given in the table can be expressed in Gigajoules. Below is a table with the recent meteorite and the Chelyabinsk radiated energies:

When Expressed in Gigajoules it is apparent just how much larger the radiated energy of the 2013 Chelyabinsk meteor was when compared to the recent one thought to have have exploded over the Atlantic Ocean.

When presenting the calculated total impact energy, NASA gets their Dr. Strangelove on and uses kilotonnes of TNT. As I’ve pointed, out the tonne is nothing more than the introduction of medieval measures into the metric system and should be eschewed. Worst of all, a tonne is a Megagram, and when you use a Kilo- prefix it produces a KiloMegagram. The last column is KiloMegagrams of TNT when actually expressed without hidden metric notation. It should be Gigagrams of TNT, but then metric is not NASA’s strong suit. The metric system has a well-defined unit of energy, the joule; the same one that was used with radiated energy.

One kilotonne of explosive is equal to 4.814 Terajoules. It appears that using Gigajoules is probably still a viable choice for presentation of the Calculated Explosive Energy:

In my view the change from joules to kilotonnes of TNT was just an anemic attempt at a “gee whiz” expression without making an *actual comparison*. An atomic bomb using nuclear fission ranges from about 4 000 Gigajoules to about 80 000 Gigajoules. The 2016-02-06 meteorite is in this range. The largest Hydrogen bomb exploded released an energy of about 210 000 000 Gigajoules. One can see the Chelyabinsk explosion of 2013 is about 23 times larger than a typical fission bomb, but is overwhelmed when compared to the amount of energy released by the largest hydrogen bomb.

How data is presented matters. I wish someone in a position to enforce change within NASA would create a position called a “numerical editor,” something like what in book publishing is called a copy editor, but concerned specifically with the presentation of numeric data for consistency and clarity. I leave you with a graph I’ve presented before. It shows that sometimes NASA presents data in an effective manner, they just need to make it consistent:

[1] Fortran 90/95 for Scientists and Engineers Stephen J. Chapman McGraw Hill 1998 pp 534-535

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