Two out of three advanced U.S. Sparrow missiles missed their targets during the air clash with Libyan fighter jets over the Mediterranean last Wednesday, and several weapons analysts say they are not surprised.

The one time a U.S. pilot fired a much less sophisticated Sidewinder missile, it hit the Libyan plane dead-on, and that was not much of a surprise, either.Each Sparrow costs $115,000. A Sidewinder goes for $34,000. Both missiles are made by Raytheon.

Sidewinders have consistently outperformed Sparrows in combat. During the Vietnam War, according to official data, 27 percent of all Sidewinders fired hit their targets. Only 9 percent of Sparrows hit theirs.

The main difference between the two weapons, and the key source of their varying success rates, lies in how they seek their targets.

The Sidewinder is equipped with a heat-seeking sensor; if it is aimed toward a hot object, such as an airplane, it moves in that direction the way a magnet moves toward steel.

There are limitations to these sorts of missiles. They do not work at long range - the Sidewinder is useless beyond 10 miles - and they can lose their way if clouds, fog or other hot objects block their sensors.

The Sparrow is guided to its target by radar. The airplane that launches a Sparrow, in the case this week, the U.S. F-14, bathes the target with a thick radar beam, which bounces off the target and returns to the area of the plane. The Sparrow missile contains a sensor that homes in on the energy from this reflected beam.

The problem, according to several weapons analysts, is that the F-14 must continue to train radar on the target for as long as it takes the missile to get there. If the F-14 maneuvers during this process, the missile's track can disappear.

Chuck Myers, a former director of air warfare in the Pentagon who consults on the subject, said in a telephone interview Friday that the launching plane "can turn a certain amount, and it will continue to bathe that target and reflect radar energy back to the missile that's seeking it. But if you go beyond that limit, the missile is without guidance. There's nothing for it to home in on anymore."