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The more information that accumulates on the crash of the Siberian Airlines aircraft on Thursday, the more the assessments tend to lean toward the possibility that a Ukrainian surface-to-air missile was the culprit. What adds a great deal of weight to this possibility is the report, leaked from the Pentagon a short while following the crash of the aircraft, claiming that a missile was indeed the cause of the accident.

An American spy satellite, similar to those that warned of the launches of Iraqi Scuds during the Gulf War and equiped with infra-red sensors, is said to have captured the launch and explosion of the missile.

According to the information that continues to flow from the Ukraine and Russia, military exercises involving the firing of surface-to-air missiles were being carried out in the Black Sea region at the time of the crash. The immediate question is, of course: How can a civilian aircraft, flying along international flight paths, fall victim to a military exercise, the likes of which are frequently conducted in the area? The launching of an air-defense missile occurs only after a series of commands are passed on from the headquarters controlling a number of missile batteries to the command center that oversees the actual missile being fired, and then to the person responsible for the launch itself.

Routine safety measures and checks are carried out to prevent any accidental strikes against civilian aircraft. For example, the airspace in the area in which the military exercise is being conducted is closed to civilian air traffic; all airlines and air traffic control stations receive information on the closing of the airspace in advance; and airlines usually divert their aircraft away from the military exercises, along other air paths.

Another manner in which civilian aircraft are protected is based on the technology of a transponder with which they are equipped and which emits signals defining them as friendly airplanes. The "friend or foe" signal is made up of a four-digit code that is received by the radar stations of military air traffic controllers and indicates whether the aircraft in the area is part of the exercise or a civilian airliner.

What then are the mistakes that could result in the launching of an air-defense missile against a civilian airliner?

One of the possibilities is that an error occured with the computer that was analyzing the data received from the control and command headquarters of the missile batteries. In a past incident involving a highly-sophisticated, U.S. air-defense naval vessel in the Persian Gulf, an Iranian airliner was wrongly identified as a military aircraft carrying out threatening maneuvers against the ship. In spite of the sophisticated technology on board the vessel, an air-defense missile shot down the Iranian airliner. Such an error could have occured with the computers of the Ukrainian missile battery.

Another possibility is that the missile was launched in another direction, but targeted the airliner by mistake, either because of faulty orders given to the missile by its controllers, or as the result of a technical problem in the missile itself. The head of the missile is equipped with an independent radar system that allows the weapon to fix on its target toward the latter stages of its flight. It is possible that the missile was not originally aimed at the aircraft, but that the missile's radar locked on to the airliner on its own. Perhaps, not being aware of an airliner in the area, the missile's controller did not use the self-destruct mechanism built into the device.

A BBC report released the day after the accident said that two air-defense missiles had gone out of control during the exercise and had strayed from their planned paths. One of these missiles may have struck the Siberian airliner. It is also possible that the airplane was flying in a no-fly zone, as a result of a navigation error; or perhaps the pilot was not aware that the area was closed off for the military exercise.

The Ukrainian claim that it was impossible that one of their missiles had struck the airliner because the distance between the missile batteries and the aircraft had been more than 200 kilometers is baseless. The Ukrainians are in possession of air-defense missiles with ranges in excess of 300 kilometers, such as the S-200.

Another missile used by the Ukrainians, the S-300, has a shorter range but also comes in a naval version. This means that the missile could have been launched from one of the many ships participating in the exercise at the time. In any event, the exact location of the hit is still not clear because a plane plunging to earth (or into the sea, as in this case), from 33,000 feet may crash as much as 300 kilometers from the area in which it was struck.

What will lend further clarification to this incident will be the willingness of the United States to share the information captured by its spy satellite. A further boost to a more accurate analysis of what took place will be the recovery of the two black boxes that were on the aircraft and that recorded the final moments of the pilots and the fateful plane.