“Recent claims that the meteor impact in Northern Troms may have had an explosive force comparable to the Hiroshima atomic bomb are vehemently rejected in this comment by Kaare Aksnes, professor of astronomy at the Institute of Theoretical Astrophysics, University of Oslo.

It is regrettable that claims have been made suggesting that the meteor impact in Northern Troms may have had an explosive force comparable to the Hiroshima atomic bomb. These claims are extremely exaggerated.

It is likewise regrettable that it may appear as if our Institute is behind these claims.

The atomic bomb detonated over Hiroshima had an explosive force determined as being equal to about 40,000 tons of TNT. This is approximately equal to the kinetic energy of a rock with a radius of 3 meters, a density of 3.5 grams per cubic cm, and a velocity of 30 km/s, as it enters the upper parts of the Earth’s atmosphere.

Several such impacts occur each year, and are observed by satellites set up to monitor the ban on atmospheric nuclear detonations.

However, such a rock would be almost entirely incinerated in the atmosphere due to heat caused by friction, and any remains that might make it to the ground would impact with a velocity comparable to that of a bullet shot from a rifle.

Almost all of the original energy would have been lost on the way to the ground, partly as a bright flash of light and thunderous noise in the seconds before the impact in the Reis Valley in Northern Troms. My estimate would be that the power of the impact would at most have been that of a somewhat powerful conventional bomb.

The last documented impact with an energy comparable to the Hiroshima bomb occurred in Tunguska, Siberia, in 1908. Calculations have shown that this may have been due to an object with a radius of 50 meters, exploding several kilometers above Tunguska, flattening trees in an area with a radius of 25 km.

I can also inform the public that the Institute of Astrophysics is taking part in a program involving the Nordic Optical Telescope at La Palma, Canary Islands, intended to assess the danger of collisions with near-earth asteroids.

So far, no objects have been discovered that will pose a threat in the foreseeable future.”

FIREBALL UPDATE: 300 tons of TNT. That’s the kinetic energy of the meteoritic fireball sighted June 7th in Norway, according to Prof. Peter Brown of the University of Western Ontario. Brown’s doctoral student, Wayne Edwards, arrived at this figure by analyzing infrasound and seismic data. Early reports of a Hiroshima-like event (12,000+ tons of TNT) were exaggerated.

“An object like this hits Earth about once a month,” notes Bill Cooke of the Marshall Space Flight Center. For comparison, “the Norwegian event was similar to, but somewhat less energetic than the Park Forest meteorite impact of 2003.”

OK, BA, erm …. how many ergs or joules in a ton of TNT? I tried to do a web search on the Gibbs energy of the reaction of TNT (from which, along with the molecular weight of trinitrotoluene that I can calculate, would give me the liberated energy per unit weight), but I couldn’t find a page that actually associated Gibbs energy with the oxidation of TNT (well, not insofar as providing numbers).

Strange, that.

Over-estimated explosive force of meteorite impact

Statements regarding that the recent meteorite-impact in Nord-Troms had the same explosive force as a Hiroshima bomb are rejected in this commentary by astronomy professor Kaare Aksnes, from the Institute for theoretic astrophysics at the University of Oslo.

It is unfortunate that it has been claimed that the meteorite-impact in Nord-Troms could have had an explosive force comparable to a Hiroshima bomb. This is a large exaggeration.

It is also unfortunate that it can be perceived as if our institute is behind this statement.

The Hiroshima bomb had an explosive force which is equivalent with approximate 4*10e7 kilos of TNT. This corresponds to the kinetic energy of a rock 3 m in radius, with a density of 3,5 g/cm3 and a velocity of 30 km/s as it enters the Earths upper atmosphere.

Such impacts occur several times every year and are registered by satellites which monitor the atmospheric nuclear testing ban.

The point of the matter is that this rock is almost completely combusted [vaporised] by the heat of friction, and what ever survives down to the ground will hit with a velocity probably smaller than a bullet.

Almost all of the original energy is dissipated, partly as light and sound during the seconds of the impact in Reisa valley in Nord-Troms. I believe that the impact energy was equivalent to a large conventional bomb.

The latest documented impact with an energy comparable to the Hiroshima bomb occurred in Tunguska in the Siberian in 1908. It has been calculated that it could have been an object with approximately 50 m radius which exploded several kilometres over Tunguska and flattened the forest in an area with 25 km radius.

I can also inform that the Astrophysical Institute participates in a observation programme with Nordic Optical Telescope in Las Palmas, the Canaries to survey the risk of impact with close-earth asteroids.

As of present there has been no discovery of any object which presents any danger for the foreseeable future.

Various numbers pop up all the time for the yield of the Hiroshima bomb, but a kiloton is a standard unit meaning 1000 tons of TNT (4×10¹⁹ ergs = 4×10¹² Joules).

I’m from Denmark, and danish and norwegian are very much alike, so I think I can clarify a bit.

Professor Kaare Aksnes works at the Institute for Theoretical Astrophysics at the University of Oslo.

Kaare Aksnes isn’t apologizing in his norwegian commentary. He is simply sorry that the power of the impact has been overestimated to be like a Hiroshima-bomb and that the estimate seemed to come from his institute.

Kaare Aksnes actually says the energy has been “very strongly overdone” (norwegian: “… meget sterkt overdrevet”).

The first part of the english article is pretty well the essens of the norwegian article. The second part of the english article (after the lines where he compares the impact to a conventional bomb) has nothing to do with the norwegian article by Kaare Aksnes, but is (I think) from the original (and overestimating) reports.

Best Regards

Leif B. Nielsen