THERE鈥橲 a suspicious flash over a remote part of the world. It looks like a rogue nuclear test and international panic breaks out. Yet it could be a harmless meteor going pop in the upper atmosphere.
Now researchers say they can tell the two apart using openly available infrasound data. If they are right, it鈥檚 good news for the Comprehensive Test Ban Treaty (CTBT), which is currently languishing as the US refuses to sign, partly on the grounds that it is not enforceable.
American military satellites are almost certainly able to tell the difference between exploding meteors, known as bolides, and nukes by looking at the radiation they give off. But the Pentagon doesn鈥檛 release data from these satellites until weeks or months after the event.
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One alternative is to listen in to low-frequency infrasound shock waves, which travel up to 1000 kilometres from the site of a nuclear explosion. Dozens of infrasound stations are preparing to join the CTBT鈥檚 enforcement network, so there is a pressing need to be able to interpret the signals in real time (快猫短视频, 8 June, p 12).
Now Elisabeth Blanc and colleagues at the French Atomic Energy Commission in Paris say they鈥檝e solved the problem. They analysed data on a bolide that landed in Tahiti in 2000, releasing as much energy as a nuclear bomb, equivalent to around 3 kilotons of TNT. In work accepted at The Journal of the Acoustical Society of America they say they can reconstruct the path taken by an exploding meteor by combining infrasound from several nearby stations. 鈥淎n explosion is a point source while a meteorite is a moving source, producing infrasound for a few seconds,鈥� says Blanc.
Meanwhile Doug ReVelle and colleagues at Los Alamos National Laboratory in New Mexico have shown that infrasound can be more reliable than satellite images. They point out that porous meteorites can release up to ten times as much energy as hard rocks, making them look far brighter to satellites than expected for their size (Geophysical Research Letters vol 29, p 14).
Terry Wallace, a seismologist at the University of Arizona who has testified to the US government on the possibility of verifying the CTBT, says the work is important for the success of the treaty. It doesn鈥檛 rely on high technology, so other countries don鈥檛 have to depend on the US, he says. 鈥淚nfrasound is a very open technology.鈥�
But Hein Haak, who runs a database on bolide events at the Royal Netherlands Meteorological Institute, is not convinced that these techniques will tell bolides and nukes apart. He points out that Blanc鈥檚 and Brown鈥檚 groups both knew in advance that bolides were the source of the data they analysed. Brown鈥檚 group had obtained US satellite data, and Blanc spoke to an airline pilot flying from New Zealand to Tahiti who鈥檇 had a clear view of the incoming meteor.
If researchers rely on infrasound data alone, wind patterns could make it almost impossible to tell what the source was, he says.
Close call
A bolide with as much energy as the Hiroshima bomb burnt up over the Mediterranean last month. An American general told the US Senate last week that we should be thankful the event didn鈥檛 happen over India or Pakistan, which are currently in conflict and have nuclear weapons.
In this case military experts soon established that the event was moving westwards and disappeared quickly, indicating that it probably wasn鈥檛 a nuclear explosion. But several such bolides enter the atmosphere each year and they don鈥檛 respect geographical boundaries. 鈥淲hen military people see bright things in the sky there isn鈥檛 much time for them to make a decision,鈥� says bolide expert Doug ReVelle at Los Alamos National Laboratory in New Mexico.