According to an Australian terrorism expert, the threat of a surface-to-air missile attack on a commercial aircraft is much greater than a Sept. 11-type hijacking, and BangkokÂ’s Don Muang International Airport is probably the most vulnerable in Southeast Asia because of the volume of weapons that flow through Thailand and the likely presence of people with links to terror groups. Airlines the world over, including Qantas and British Airways, are studying ways to equip their fleets with anti-missile defenses. Thailand will face its greatest security challenge Oct. 19 when presidents and prime ministers from 21 nations meet for the Asia-Pacific Economic Cooperation summit. Every square inch of Bangkok is being checked, according to ThailandÂ’s police chief. Security will be tight and movement around the capital could be disrupted.