The newest Sidewinders have become 'the weapons of choice' against unidentified objects over the US, top general says

• US jets have downed a surveillance balloon and three other objects over the US in recent days.
• To shoot them down, the jets have used the newest version of the Sidewinder missile, the AIM-9X.
• The high-tech AIM-9X is the best suited to take down the low-tech objects, a top US general says.

During World War I, militaries lofted observers in balloons to spot targets for artillery at ranges far beyond what troops on the ground could see. Naturally, this made them prime targets for fighter pilots like America's Frank Luke, who shot down 14 balloons during the war.

Those old "balloon buster" biplanes used machine guns — frequently armed with incendiary bullets — to destroy their explosive hydrogen-filled prey. But the US Air Force F-22 and F-16 fighters that destroyed a Chinese spy balloon and three other unidentified objects didn't use their 20 mm cannon. Instead, they used heat-seeking AIM-9X Sidewinder air-to-air missiles.

Why use a guided missile that costs nearly $500,000 apiece to shoot down a slow-moving object that could probably be dispatched with a few cannon shells? The answer is that balloon-busting is more dangerous than it appears.

World War I balloon hunters faced targets heavily protected by flak, that could be booby-trapped to explode, or could engulf an attacking fighter in a fiery plume after exploding. World War II barrage balloons, which were designed for low-altitude air defense, could snag aircraft on their steel tethering cables.

Many of the same risks and challenges still apply today. The US Air Force did consider downing the balloon and other objects with cannon fire, Gen. Glen VanHerck, head of US Northern Command and North American Aerospace Defense Command, told reporters on January 12.

"And the pilots in each situation felt that that was really unachievable because of the size, especially yesterday in that altitude, and also because of the challenge to acquire it visually because it's so small," VanHerck said. "It's also potentially a safety-of-flight issue because you have to get so close to the object before you see it that you potentially could fly into the debris or the actual object."

The F-22 is armed with the AIM-120 advanced medium-range air-to-air missile. With a range of more than 20 miles and an on-board radar to home in on its target, the AMRAAM is a formidable weapon against aircraft — but not against balloons. (The latest version, the AIM-120D, has an estimated range of nearly 100 miles.)

"Maintaining a radar track on an object this small is very hard," VanHerck said of the object shot down over Lake Huron on Sunday. "So taking a radar shot such as AIM-120 would be a lower probability of success."

That left the AIM-9X, the latest version of the venerable Sidewinder that has been destroying jets since the 1950s. "In each situation, the AIM-9X, a heat-seeking missile or infrared missile that sees contrast, has been the weapon of choice against the objects that we've been seeing," VanHerck said.

Though early Sidewinders had a range of a couple of miles, some sources estimate the range of the AIM-9X at more than 20 miles.

Earlier Sidewinders were designed to home in on the heat of a jet engine. Yet because their infrared guidance systems sought the hottest objects in their field of vision, they were also vulnerable to spoofing by decoy flares or to mistaking the sun for a target and flying toward it.

The latest Sidewinder is much more capable at discriminating between the temperatures of different aerial objects as well as background temperatures.

"What you have is a contrast between the environment and the objects themselves, which often gives [infrared] contrast, which allows the missile to track," VanHerck said. "And that's been very, very effective for the AIM-9X."

Shooting down a balloon is much easier than destroying a supersonic fighter. While the Chinese balloon was the F-22's first air-to-air kill, it's probably not the one that Raptor pilots were hoping for. Nonetheless, balloon-busting is not danger-free. For Frank Luke then or an F-22 pilot now, a kill is a kill.

Michael Peck is a defense writer whose work has appeared in Forbes, Defense News, Foreign Policy magazine, and other publications. He holds a master's in political science. Follow him on Twitter and LinkedIn.