VIENNA, Austria -- They're light but lethal -- and capable of bringing down a commercial jetliner within seconds.
Shoulder-fired, surface-to-air missiles similar to the ones involved in a sting operation this week are relatively inexpensive and chillingly easy to buy, said Jonathan Stevenson, an arms control analyst with the London-based International Institute of Strategic Studies.
"There are thousands of them out there, and they're fairly easy to procure -- at least the cruder versions," Stevenson said.
"There are 4,000 to 5,000 of these missiles around Iraq. Africa is full of unregulated weapons left over from Cold War sponsorships, and the Eastern Europeans are major league gun merchants. They're probably not a lot harder than buying a shotgun in places where there effectively is no government."
In fact, experts say it's amazing the world hasn't seen more attempts to down airplanes.
One reason may be that the rockets are still rather conspicuous when being fired.
They also aren't very accurate, and they were designed to take out helicopters and smaller military aircraft, not jumbo jets with powerful engines. Egyptian forces fired hundreds of SA-7s at Israeli bombers during the 1973 Arab-Israeli war, and most of the bombers survived the assaults, which experts say may have discouraged their wider use by terrorists.
In the past, the main threat to commercial flights was hijackings -- long, drawn-out affairs that guaranteed terrorists days of publicity without risking the kind of universal condemnation that mass murder would inspire.
But because airport security was tightened considerably after Sept. 11, making hijackings harder, some experts think terrorists may be rethinking the missiles.
The rocket at the center of Tuesday's arrest of British suspect Hemant Lakhani was a deactivated Russian SA-18 Igla missile. Weapons experts say the Igla is the most sophisticated, accurate and hard-to-obtain portable rocket.
But there are other choices for terrorists, including the American-made, Stinger shoulder-fired missile.
Intelligence officials say hundreds of Stingers are in Afghanistan, sent there by the U.S. government in the 1980s to help the Afghan resistance to Soviet occupation. Stingers also are said to be commonplace in the former Yugoslavia.
There are also thousands of a class of missile known as the SA-7 Grail, which is also called the Strela, or Arrow, in Russia. They have been produced in Russia, Eastern Europe, China, the former Yugoslavia, Egypt and other countries. Some have sold for as little as $500, according to U.S. intelligence.
"There's a considerable number of these sloshing about on the market. You can get the latest version or a 20-year-old rocket," said Wing Cmdr. Andrew Brookes, a weapons expert and a former reconnaissance pilot for Britain's Royal Air Force.
"The big question is how many of them have been bought by the bad guys. They wouldn't need many. A dozen would do the trick for terrorists trying to achieve their main objective: to strike fear into the general population."
Last November, terrorists fired two surface-to-air missiles that barely missed an Israeli charter airliner taking off from the airport in Mombasa, Kenya, with tourists returning to Israel. Al-Qaida claimed responsibility for the attempt.
In 1998, rebels in Congo used a shoulder-fired SA-7 missile to shoot down a jetliner carrying 40 civilians. There were no survivors.
"Clearly, there's an interest in bringing down a commercial airliner. It's probably inevitable that it will be tried again," Stevenson said. "There are a lot of people in the intelligence community who have been worried about this for a while."
Shoulder-fired, surface-to-air missiles chase the heat produced by an airplane engine and explode on impact. They are effective only while the target plane is flying low and slow, usually during takeoff and landing.
It wouldn't necessarily be easy to take out a jumbo jet with such rockets, which are designed to target one engine. The larger commercial airliners have four engines and can fly safely if one is knocked out.
But defense analysts say the weapons -- known in military jargon as MANPADs, or Man Portable Air Defense Systems -- have numerous tactical advantages that make them attractive to terrorists.
They weigh up to 40 pounds and their 5-foot tubes are compact enough to be concealed in a large duffel bag.
Although their performance varies depending on the type, the missiles tend to have a minimum range of 600 yards and a maximum of roughly 3 miles. They can hit airborne targets ranging from 50 feet to 10,000 feet.
"It doesn't take a rocket scientist to fire them," Brookes said. "They're not sophisticated. Someone with a bit of military experience could fire one off quite easily."
There is little that can be done to counter the threat, apart from equipping commercial airliners with anti-missile systems that fire decoy flares. Such systems, however, would cost up to $2 million per plane.
"Strictly speaking, there's nothing we can do in a democracy where people want the freedom to fly," Brookes said. "The most robust tactic is simply to resist the urge to stop flying and stay in bed. That's what the terrorists want."