According to the Washington Post, “[f]our men accused of planting bombs outside synagogues in the Bronx and plotting to fire missiles at military planes were convicted on Monday, in a case that was widely seen as an important test of the entrapment defense.”

During the trial, “[p]rosecutors said the men, who all lived in Newburgh, N.Y., willingly cooperated with an informer working for the Federal Bureau of Investigation who posed as a terrorist and supplied the men with inert bombs and Stinger missile tubes.” The prosecutors also noted that the men “planned to travel to Stewart International Airport in Newburgh, north of New York City, to fire missiles at military transport planes.”

Odds are, even if these terrorists had gotten hold of real ground-to-air shoulder-fired missiles, the attack would have failed. Typically, trained shooters have about a 50 percent chance of hitting a plane in flight. In November 2002, terrorists failed in an attempt to shoot down an Israeli jetliner with two SA-7 shoulder-fired missiles after takeoff from the airport at Mombasa, Kenya. Poor marksmanship saved the lives of over 260 passengers and crew.

These most recent convictions, however, are a reminder that, someday, this could happen. We can’t always rely on incompetent terrorists. The problem is that, unlike 9/11, there will be little practical measures that can be done after the fact to mitigate the threat. A successful attack will likely stun the global airline industry for months.

While the Department of Homeland Security has spent years and millions of dollars on science experiments studying how to deal with this threat, there has been little tangible action. Land-based directed-energy defenses could be used to protect selected airports and other critical infrastructure. The technology is mature enough, and the costs would reasonable. These systems could be made sufficiently mobile to move around to protect different targets. They would also have the added bonus of having applications for military use in protecting infrastructure and aviation in combat zones.

When a successful attack on commercial airliner happens, and it will someday, it will be called the “black swan” that no one could have predicted. We will spend billions more after the attack than what we could have reasonably spent before hand to mitigate the threat.

Stopping attacks, as the recent convictions in N.Y. demonstrate, remains the best and most cost-effective means for dealing with terrorism. In some cases, however, and this is one of them, it is worthwhile to have a back-up plan.