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At an estimated 6 to 7 kilotons (a kiloton being the explosive equivalent of 1,000 tons of TNT) this 2013 test represents North Korea's second probably successful nuclear test. The first (2006) test of less than one kiloton, is widely considered to have been a "fizzle". Fizzle means not a full or correct fission of explosive material producing a lower yield than expected (in this case by North Korea).
The second (2009) test of a Plutonium device is considered North Korea's first successful test. The third (2013) test is believed to have the added degree of difficulty of being a miniaturized (warhead size) device. This third test may also have added significance if it was North Korea's first test primarily using Highly Enriched Uranium (HEU eg. 80%+ U235).
The South Korean Defense Ministry estimated a yield of 6 to 7 kilotons (based on US-South Korean seismic monitoring) http://abcnews.go.com/International/north-korea-nuclear-test-highly-provocative-obama/story?id=18474237.
This article indicates the difficulty of measuring yield http://lewis.armscontrolwonk.com/archive/6288/making-yield-estimates#more-3242 . There needs to be considerable discussion and compromise between various yield estimates, from different locations, using differing sensors-sources to arrive at an agreed and always approximate yield range.
More complete data on yield, Plutonium or HEU and possibly other nuclear device characteristics might be available from the US - based on various collection methods, including WC-135 Constant Phoenix (see aircraft and basing details below). One or more WC-135 Constant Phoenix aircraft might need to fly from Nebraska, collect air samples in North Korea's region and assess the air samples back in Nebraska to provide more publicized data.
Although the test was underground minute amounts of explosive gases and dust (from vents or blast doors and seals when opened) will enter the atmosphere permitting collection and analysis. Data from US (and perhaps Chinese, Russian and South Korean) satellite, drone and other aircraft data would be crossed checked with ground and ship based data to reach estimates as to the explosive yield, in kilotons, for the 3rd nuclear test. Also type of chemical explosive (such as Plutonium and/or Highly Enriched Uranium, Tritium?) used and perhaps physical size of the nuclear device (bomb).
WHY HAS NORTH KOREA HELD NUCLEAR WEAPON TESTS?
Most nuclear powers carry out several weapons tests to evolve devices into reliable weapons, see http://www.lowyinterpreter.org/post/2013/02/13/North-Koreas-third-nuclear-test-What-does-it-mean.aspx under different environmental conditions, using differing explosive types (Plutonium, HEU, boosted fission, on the way to fusion (thermonuclear) weapons). Testing is also to explore different weapon delivery methods requiring differing warhead characteristics. For North Korea (NK) warheads would differ for ballistic missiles, free-fall jet aircraft delivered bombs, cruise missiles, maybe artillery shells, perhaps bombs wired into submarines or torpedoes or in shipping containers,
Reasons why NK is going nuclear at all may include:
- prestige/pride for NK as a nation, regime and God-leader under otherwise barely legitimate rule. Illegitimacy (no right to rule) includes NK being unable to look after (e.g.. feed) its own people and not allowing non-Kims to choose a leader outside of the Kim dynasty
- deterrence against all the countries that have traditionally invaded it over past millennia-decades, including Russia, Japan, China and more recently the US and South Korea.
- as a counterweight to the conventional superiority of the militaries in the above countries.
NK is clearly odd that it is not one part of a symmetrical, Mutually Assured Destruction (MAD) relationship unlike what the US-Soviet Union used to be and perhaps the relationship of India-Pakistan now.
Given NK's massive poverty as a nation there is also puzzling irrationality as to how it relies on nuclear blackmail as its main foreign policy - to coerce aid and diplomatic backdowns from surrounding countries and Great Powers?
CONTEXT ON THE EARLY SEISMIC ESTIMATES OF YIELD
There are many classified sources and methods to estimate yield but seismic measures http://en.wikipedia.org/wiki/Seismometer are usually the methods that are publicly cited. One reason is that these measurements of vibration appear to be intuitive. Another reason is that seismic measuring equipment is usually commonly available civilian equipment used for measuring earthquakes, volcanic activity and other natural phenomena. See http://www.examiner.com/article/north-korea-tests-nuclear-weapon-underground-recorded-on-usgs-seismographs which indicates:
"The United States Geological Survey (USGS) recorded a 5.1 tremor on the Richter Scale in North Korea early Tuesday in what was first believed to be, and later confirmed, an underground nuclear weapon test by the North Korean government. The country's previous tests, carried out in 2006 and 2009, were substantially smaller."
"The explosion was recorded by USGS seismographs about 24km (15mi) ENE of Sungjibaegam, North Korea at a very shallow depth of less than 1 km. [next bit very significant] The USGS later changed the classification of this event to a nuclear explosion based on Ambassador Rice's statements at the United Nations Security Council Stakeout."
Some details on a USGS website here http://earthquake.usgs.gov/earthquakes/eventpage/usc000f5t0#summary
[My Commeents] Complicating the public release of info is that the USGS would probably send the CIA/State Department/White House more information than is publicly available.
The USGS seismic data (and more seismic data from CIA's and US military's own seismic sensors) would be mixed with information from other air/sea/land/space sensors and data passed by other countries (eg. China, Russia, Japan) before public statements by US and South Korea are made.
Then, I imagine, additional data will arrive and then meetings held (in the US and between Six Party Talks as well as between UN Security Council members) on how much information to release and when. These ongoing processes will generate new or altered public announcements over the next few days, even weeks.
Judging by minimal mention of North Korea in the State of the Union Address it seems the US doesn't want to glorify this new North Korean "achievement". So much information may be long delayed.
TWO OF THE AERIAL SENSORS- US RQ-170 Sentinel (stealth) drones
Much depends on atmospheric and other nuclear data collected by:
Much depends on atmospheric and other nuclear data collected by:
which are reportedly working in or for the 5th Reconnaissance Squadron at Osan Air Base. This base is a USAF facility located in the Songtan section of Pyeongtaek City, South Korea, 64 km (40 mi) south of Seoul. Such drones would fly just outside of North Korean airspace to collect radioactive cloud (and other chemical reaction) air samples.
See http://www.nytimes.com/2011/12/08/world/middleeast/drone-crash-in-iran-reveals-secret-us-surveillance-bid.html?_r=0 [In relation to Iran and also North Korea] "American satellites have been trained on...nuclear facilities. But the RQ-170 Sentinel, which can fly at an altitude of 50,000 feet, is considered vital to the effort. While an orbiting surveillance satellite can observe a location for only a few minutes at a time, a drone can loiter for hours, sending a video feed as people move about the site. Such a “pattern of life,” as it is called, can give crucial clues to the nature of the work being done, the equipment used and the size of the work force. …In addition to video cameras, independent experts say the [RQ-170 Sentinel] drone almost certainly carries communications intercept equipment and sensors that can detect tiny amounts of radioactive isotopes and other chemicals that can give away nuclear research.
News reports in South Korea in 2009 said the United States planned to base the RQ-170 drone there to fly surveillance missions over North Korea, whose nuclear and missile programs are a top American intelligence target."
- the WC-135 Constant PhoenixBackground - The US Boeing 707 sized WC-135 Constant Phoenixs are all in the USAF's 55th Strategic Reconnaissance Wing based at Offutt Air Force Base, Nebraska, USA. The WC-135 collects samples from the atmosphere for the purpose of detecting and identifying nuclear explosions. It is also informally referred to as the "weather bird" or "the sniffer" by workers on the program….The Constant Phoenix’s modifications are primarily related to the aircraft's on-board atmospheric collection suite, which allows the mission crew to detect radioactive debris "clouds" in real time."