David Keir, London
There can be few people who have not by now seen the spectacular video images of the meteor blazing across the Russian sky in daylight on 15 February this year. The light output was startling in its brilliance, casting intense shadows of buildings and vehicles. Some of the footage also captured the sound of the event. But how many realise that this provided a further proof of the capability of the Comprehensive Nuclear-Test-Ban Treaty international monitoring system?
In fact, a lot of the sound produced by this object as it entered the earth’s atmosphere could not be heard by human ears, as it was of a lower frequency than we can detect. This, so-called infrasound, with a frequency of less than 20 Hertz, is one of the signatures of a nuclear explosion too and is a phenomenon that the CTBT International Monitoring System network has been set up to detect. Infrasound detection is one of the four key monitoring systems of the CTBT; the others being seismic signal detection, hydro-acoustic sensing and radionuclide detection in the atmosphere.
In an excellent piece on the CTBTO website (www.ctbto.org
), released from Vienna on 18 February, the organisation reports that the CTBT infrasound network did indeed detect the signals from the Russian meteor event. The infrasound signal was detected by 17 infrasound stations in the CTBTO’s network and the furthest station from the meteor path to record the sub-audible sound was some 15,000 kilometers away in Antarctica. Actual data from the station in Kazakhstan is also shown in the article.
Sound is, of course, a sequence of pressure waves propagated in air (and other materials too) and is just one component of the impact that this type of meteor has as it burns up rapidly in the atmosphere. This type of object spends its life in space —as a so-called meteoroid— in an environment free of oxygen and essentially friction-free. It is only when it enters the earth’s atmosphere and heats rapidly, due to friction with the air, and ignites due to the oxygen present. This rapid oxidation of the material in the meteoroid means that it burns with the bright light seen in the videos and emits the accompanying sound waves.
As well as the pressure wave caused by the meteor breaking the sound barrier in air (which caused the damage to windows and other structures), judging by the data recorded, there was a longer period of infrasound output as the object travelled across the sky.
Pierrick Mialle, an acoustic scientist at CTBTO, is quoted as saying that the 15 February meteor signal picked up by the detectors was not from a single explosion, but from the object burning in air, traveling faster than the speed of sound. He further stated that this was one of the ways it could be distinguished from mining explosions or volcanic eruptions.
Mialle also indicated that scientists around the world will be using the CTBTO's data, (some of the strongest signals the infrasound network has ever detected) to better gauge the object's breakup and discern more about the object's final altitude, energy released and how the meteor disintegrated.
The particular property of infrasound waves, that they are attenuated less than higher-frequency (audible) sound, is a fortuitous feature that the CTBTO detector network makes good use of. Once again this shows the value of such a network, in operation around the clock, for the detection of events, expected or otherwise, and provides confidence in the overall International Monitoring System for detecting nuclear tests, anywhere in the world.
Last changed: Feb 21 2013 at 5:21 PM