The Pentagon wants another crack at shooting down ballistic missiles with laser beams. According to The War Zone, the U.S. Missile Defense Agency is looking to put lasers on high-flying drones. Their mission: to intercept enemy missiles during the so-called "boost phase."

This is the second attempt to build such a thing, following an aborted project that involved mounting a giant laser on a Boeing 747. As reported at The War Zone, the Pentagon's planned drone would fly at upwards of 63,000 feet of altitude and would loiter near enemy territory for a day or more, ready to shoot down ballistic missiles rising from below with a concentrated pulse of laser light.

Ballistic missiles are very difficult to shoot down. Long-range missiles lob their warheads into a trajectory that typically involves a flight through low earth orbit before descending upon their targets at speeds of up to Mach 10. Missile flight is divided into three phases: boost phase (launch), midcourse (coasting through space) and terminal (the minutes or seconds before impact).

YAL Airborne Laser in 2004, shortly before test. Missile Defense Agency.

The U.S. has built defenses for various types of ballistic missiles that involve intercepts at the midcourse and terminal phases. For example, the Ground-Based Midcourse Defense and SM-3 missiles are designed to hit longer-range missiles in the midcourse phase. Meanwhile, THAAD and the ballistic missile defense (BMD) version of Patriot, PAC-3, are designed to hit warheads in the terminal phase when they are closer to their targets.

But the Pentagon currently has no defense that engages missiles in the boost phase, and as The War Zone points out, shooting down missiles during these early moments is the "holy grail" of ballistic missile defense. Ballistic missiles are slowest in the boost phase, making them easier to hit. During the boost phase, the actual warheads, decoys, and other penetrating aids designed to fool enemy defenses are still attached to the missile. So, if you shoot down a ballistic missile in the boost phase, then you could take out multiple warheads and so-called "pen aids" with a single shot.

The problem with boost phase is that your defenses must react very quickly, and for that to happen, your defensive weapon must be as close to enemy territory as possible. Most modern ballistic missiles are launched from mobile launching pads, making launch locations difficult to predict.

Iranian Shahab-3 missile, 2009. AFP photo. Getty Images

In the 1990s and 2000s, the U.S. spent $5 billion developing laser weapons for this kind of purpose. The effort culminated in the YAL-1 , a 747 jumbo jet outfitted with a chemical oxygen iodine laser (COIL). The onboard laser required 110 pounds of chemicals and equipment for every kilowatt of laser, and the laser was approximately in the megawatt class. That required a lot of airplane, hence the use of a 747. The YAL-1 was cancelled by Secretary of Defense Robert Gates after he came to the conclusion the system "didn't work" and would cost billions per year even if it did.

Fifteen years later, the Missile Defense Agency wants to try again. What's changed? Two big things: the rise of solid state lasers, and drones. While the COIL laser needed a 747 to store the chemicals and electronics involved in generating a one-megawatt laser beam, new solid state lasers are much smaller. The weight goal of General Atomics' 150 kilowatt HELLADS laser weapon , currently under development, is approximately 1,650 pounds. Scale that upward to a 1 megawatt laser and you have a weight requirement of 11,550 pounds, which is right in the Missile Defense Agency's drone payload requirement.

Pilotless drones have other advantages. They are cheaper to fly and can remain in flight for much longer periods of time than manned aircraft. In fact, the MDA wants the new laser drone to be capable of loitering for up to 36 hours at altitudes greater than 63,000 feet. Flying higher than the YAL-1, this laser drone would have more time to engage the target. It would also be more difficult for enemy forces to shoot down—and not need its own fleet of flying bodyguards to ensure its safety.

The Airborne Laser as it was supposed to work. Getty Images

So would the laser drone actually work? Here's one possible stumbling block. At the time of canceling the YAL-1, Gates said that to be truly effective, the Airborne Laser had a range in the tens of kilometers, but it needed "something like 20 to 30 times more (power) than the chemical laser in the plane right now to be able to get any distance from the launch site to fire." Otherwise, a YAL-1 would've been forced to fly over Iran to intercept an Iranian missile. That's an act of war in and of itself.

A similarly armed, high altitude laser drone would be a different story. In 2015, the head of the Missile Defense Agency claimed that a drone flying at 63,000 feet or higher would have a much longer range, in the hundreds of kilometers, due to the lack of laser-degrading particles in the atmosphere at very high altitudes. A scaled-down technology demonstration is scheduled for 2021, although North Korea's progress toward fielding an intercontinental ballistic missile capable of reaching the continental United States may lend a sense of urgency to the program.

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