The latest test, the third in a fivetest series, evaluated the engine at elevated firing temperatures, and a new hybrid throat nozzle. The Stage 2 solid-rocket motor concept will undergo four tests. Both stages should fly this year in a flight test of the booster system.

When finished, the booster rockets will accelerate the missile, which measures about 12-m high and 1-meter wide, to about 6 km/sec, all the while adjusting its trajectory to bring it nearer the ICBM. The missile will use ground-based telemetry from a mobile-launch system. (The launch system, including rockets and launchers, fits in a C-17 transport aircraft.) The rocket then ejects the Exoatmospheric Kill Vehicle, which homes in on the ICBM using an IR seeker and rams into it, using kinetic energy, not explosives, to destroy the missile.

When completed, which will likely take five to eight years or more, a battery of 10 missiles based in Italy should protect all of Western Europe against Middle East missile attacks. Similarly, a battery in Norfolk, Va., could protect the East coast of the U.S. from launches 300 to 15,000 km off the coast.

Other companies involved in the development include Raytheon, which is building the Kill Vehicle, and ATK, Orbital Sciences, Honeywell, Aerojet, Ball Aerospace, and Kuchera.