Those planes that look like specks in the stratosphere are flying so high because they are merely passing by the United States -- flying bananas to Germany, Canadians to Mexico and Europeans to Jamaica. But should that exempt such flights from the full security screening they would get if their destination were in this country?
As the Obama administration works to harden domestic defenses against terrorism, some experts point to a potential vulnerability from thousands of flights that pass over the United States each week.
Although the United States regulates overflights, the cargo aboard them is not screened to federal standards, and passenger lists are not matched to names on the terrorist watch list maintained by the Transportation Security Administration.
The TSA says that other countries "have their own cargo security protocols that apply to those aircraft." The TSA has not implemented the new Secure Flight program to scrutinize passengers boarding overflights. That behind-the-scenes operation is designed to ferret out potential terrorists through a process that begins with airlines collecting detailed information when someone buys a ticket.
Security experts are divided about the severity of the risk.
Scanning all the cargo that flies over the country "is totally unrealistic," said Yossi Sheffi, director of the Center for Transportation and Logistics at the Massachusetts Institute of Technology. "We have tens of millions of packages flying almost every night. We can't stop the huge flow of packages from all over the world. There has to be a balance between acceptable risk and the economy."
But Richard Bloom, a longtime U.S. intelligence operative who teaches counterterrorism courses at Embry-Riddle Aeronautical University in Arizona, said a terrorist could "explode a plane with a dirty bomb or a biological weapon or an actual nuclear weapon on board, and that material will spread wherever it crashes."
TSA spokesman Greg Soule said in a statement that the agency "continues to work with our international and industry partners to ensure the successful implementation of vetting overflights. Secure Flight is a phased-in program, and addressing routes that overfly the United States is the next phase in its implementation."
Some of the costs would fall on other nations and their air carriers. The TSA has authority to divert planes from U.S. airspace if it detects a security risk, and there have been occasions when planes have been turned away. The agency, however, declined to comment for security reasons.
The issue of cargo aboard international flights came to the fore in October when bombs packaged in printer cartridges were found aboard U.S.-bound cargo planes near London and in Dubai. According to U.S. and British officials, the packages sent from Yemen were addressed to Chicago-area synagogues and designed to detonate in flight.
The federal government estimates that 55 to 65 percent of cargo bound for the United States aboard passenger planes is screened. Many experts say that scanning the rest of it, and the vast volume flown in cargo planes, would be economically infeasible.
Most passenger plane overflights originate in or are bound for Canada, but the route over the North Pole is the most direct between northern Europe and the Americas. The FAA said that about 92 percent of overflights by cargo planes were headed to or from Germany.