What is the Buk surface-to-air missile system, and why is it important?
The key issue at play here is altitude. Today's flight, MH17, the Malaysian airlines passenger jet that crashed in Ukraine, may have been shot down. But so far it seems that it was flying at approximately 10,000 meters when it went missing. That is far above the maximum intercept height for surface-to-air missiles which have previously been recorded in the hands of separatists, namely the Igla shoulder-fired rocket, and the Strela-10 vehicle (which, by the way, evidence suggests was supplied to the separatists by Russia on July 2-3).
Several weapons, including the Buk, are capable of hitting aircraft at this height. But so far, none of those weapons have been documented as being in the hands of separatists. Not only had it never been filmed or photographer, there were never any reports of the Ukrainian military losing such a weapon.
However, on July 14th we ran a story that one news agency, TV Zvezda, had run a report that a Buk was captured by separatists who stole it from the Ukrainian military. The problem, however, is that TV Zvezda is the official news agency for the Russian Ministry of Defense, and they are the only source for this story. In other words, if the Russian military were going to give the separatists a Buk, they would need to plant a story about how the separatists got the Buk in the first place, and they'd likely use TV Zvezda to do it.
Today, AP reporters did see a Buk in eastern Ukraine -- the first time one has been seen. And now we have this:
But an intrepid microblogger noticed something odd a few days ago -- the press officer for the separatists actually tweeted a picture of a Buk that was reportedly under their control:
Sure enough, that page has been deleted: