The attack on an Israeli airliner taking off from Mombasa, Kenya has generated a lot of concern about the safety of the air transport system. This is understandable, especially since terrorists are trying hard to get portable anti-aircraft missiles. Several men, reportedly linked to al Qaeda, were arrested in Hong Kong in early November when they tried to exchange heroin and hashish for stingers, the American version of such missiles.

But all the reporting and commentary on the recent attack has failed to mention an important fact. The attack in Kenya was not the first time that terrorists have used a shoulder fired surface-to-air missile against a commercial airliner. In the 1970s, such an attack brought down an airliner in Africa.

Ever since, those who study terrorism have wondered why more of these attacks have not occurred. As press reports have detailed, commercial airliners are extremely vulnerable to such missiles, especially as they takeoff and land.

Considering why more portable missile attacks have not occurred helps us understand how terrorists think about their business and provides some perspective on the recent attack. It reminds us that what terrorists attack is not decided just by our vulnerabilities. It is decided also by the cost to the terrorists of exploiting those vulnerabilities.

Perhaps the most obvious reason why more portable missile attacks have not occurred is that they do not always work. In fact, they seem to fail frequently. The missiles missed the Israeli plane. The successful strike in Africa thirty years ago was followed by at least a couple of failed attempts. (It may have been preceded by other failed attempts.)

Terrorists do not like to fail. Terrorism is how the weak can be strong and show the strong that they are not so strong after all. On this basis, the weak can pressure the strong into giving them what they want. Failure is weakness and defeats the purpose of the terrorists.

We cannot overestimate the importance of the need to succeed. Shortly before the Israelis killed him, Salah Sh’hadeh, the leader of Hamas’ military wing, told an interviewer that his organization carefully planned every suicide operation. A committee reviewed each proposed attack, he explained, even watching surveillance film of the intended target. The committee did not care how many people an attack might kill. That was up to Allah. The important thing was that the attack succeed.

Why have portable missile attacks failed? Accurately firing such a missile is a skill. To remain proficient, one must practice. Where does one practice such a skill when you are trying to keep your operation a secret? Training people is also an expense and when you send trained personnel out to conduct an attack, from which they may not return, you would like them to succeed. But missiles may not be reliable. Over time, certain of these weapons reportedly become stale. Also, they must be bought on the arms market. How does a terrorist know that what he buys has not been tampered with or is available only because it is damaged? Sh’hadeh complained bitterly about the scarcity of good quality weapons, including anti-aircraft and long-range missiles, and "bloodsucker arms dealers" who charge high prices.

Compare a suicide attack. Your highly skilled bomb maker is safe. You risk only untrained, disposable volunteers. The bomb is under your control. Your people made it. The technology is simple. Generations of terrorists have worked out simple designs and in many cases shared them. If for some reason, as has happened, the bomb does not go off, the person carrying it can walk away and no one recognizes a failed operation. In addition, a human being is the most subtle and responsive guidance system that an explosive could have. He or she can make numerous small adjustments to make sure the attack succeeds.

From the viewpoint of the terrorist organization, portable missile attacks on commercial aircraft are, compared with other kinds of attacks, a risky investment with scarce resources.

Unfortunately, this does not mean that such attacks will not occur. The terrorists may judge that the possible damage they could do to the air transport system justifies the risks of additional failed portable missile attacks. As our air security becomes better (its currently most effective element is probably the determination of passengers to stop suicide hijackings), the terrorists may believe they have no other way to attack the air transport system, which has always been a favorite target. They may even believe that heightening our concern about the safety of the system, and causing us to invest more to secure it, which a failed attack does, is enough of a gain for the investment they have made. Finally, al Qaeda (a possible, if not likely) organizer of the recent attack has shown a tendency to keep at something until it gets it right.

But it may also be the case that the failure in Kenya will discourage terrorists from making missile attacks on commercial aircraft a standard feature of their repertoire. Based on past experience and what we know about terrorists, we have some reason to believe that such attacks will not replace suicide bombings as a preferred method of attack.

David Tucker is a Member of the Board of Advisors at the Ashbrook Center for Public Affairs at Ashland University and an Associate Professor of Defense Analysis and Co-Director of the Center on Terrorism and Irregular Warfare at the Naval Postgraduate School. He is the author of Skirmishes at the Edge of Empire: The United States and International Terrorism. The views expressed here are his own and do not reflect the position of the Naval Postgraduate School, Navy Department, or Department of Defense.