A megaton or megatonne is a unit of mass equal to 1,000,000 metric tons, i.e. 109 kg or 1 teragram (Tg). The official SI symbol for the megaton is Mt, but MT is also being used; beware that the latter is also (unofficially) used for the metric ton in some contexts. See 1 E9 kg for a comparison with similar masses.
The kiloton or megaton of TNT is used as a unit of energy, approximately equivalent to the energy released in the detonation of this amount of TNT. The megaton of TNT has traditionally been used to rate the energy output, and hence destructive power, of nuclear weapons. This unit is written into various arms control treaties, and gives a sense of destructiveness as compared with ordinary explosives, like TNT. More recently, it has been used to describe the energy released in other highly destructive events, such as asteroid impacts.
- A gram of TNT by definition for arms control purposes is 1000 thermochemical calories, which equals 4.184 kilojoules (KJ).
- A ton of TNT, (a metric ton = 1000 kg) is therefore 4.184 x 109 J = 4.184 gigajoules (GJ).
- A kiloton of TNT is therefore 4.184 x 1012 J = 4.184 terajoules (TJ).
- A megaton of TNT is 4.184 x 1015 joules = 4.184 petajoules (PJ).
(This definition is a conventional one. The actual measured output of a gram of TNT is a little less, 652 thermochemical calories = 2724 J) (http://muller.lbl.gov/teaching/Physics10/chapters_Jan_2005/Chapter01.htm)
The first nuclear bomb tested at the Alamagordo test site released 18.6 kilotons of TNT (Rhodes, page 677), or approximately 78 terajoules.
The Little Boy weapon dropped on Hiroshima had a yield of approximately 13 kilotons of TNT (54 TJ). Thus, a megaton of TNT is equivalent to roughly 77 Hiroshima bombs. The Nagasaki bomb, Fat Man, released 20 kilotons of TNT = 84 TJ.
The largest nuclear weapon ever detonated was the Tsar Bomba, which had a yield of 50 megatons of TNT (210 PJ). The most powerful nuclear weapon ever produced was a version of the Tsar Bomba that would have yielded some 100 megatons of TNT. Typical H-bombs today have a yield of around 1 megaton of TNT.
The impact of a roughly 15 kilometre-wide meteorite or comet with the Earth can yield upward of 100 million megatons of TNT = 4.184 x 1023 J. Such impacts have been hypothesized to be the cause of prehistoric extinction events. This is the Nemesis hypothesis of Richard Muller. (Muller website) (http://muller.lbl.gov/teaching/Physics10/chapters_Jan_2005/Chapter01.htm)
The 30 May 1998 magnitude 6.5 earthquake in Afghanistan had an energy release "equivalent to a 2000 kiloton nuclear explosion". (USGS) (http://earthquake.usgs.gov/faq/nuclear.html) Another example, the quake of magnitude 9.0 off west coast of northern Sumatra, Sunday, December 26, 2004 at 00:58:49 UTC, released an estimated 2 x 1018 joules (2 EJ), or "475,000 kilotons (475 megatons) of TNT, or the equivalent of 23,000 Nagasaki bombs." (USGS (http://earthquake.usgs.gov/eqinthenews/2004/usslav/neic_slav_faq.html))
Rhodes, Richard. The Making of the Atomic Bomb, New York: Simon and Schuster, 1986.