This strange mod to the F-35 kills its stealth near Russian defenses - and there's good reason for that

Lockheed Martin/Chad Bellay

A+B: An F-35A and F-35B during weapons delivery accuracy (WDA) testing.

When the F-35 flies over friendly countries for overseas deployments, you may have noticed some strange tags on the body of the otherwise sleek jet.

Check out the photo below.

Lockheed Martin

What are those?

Every single angle and surface of the F-35 has been precisely machined to baffle radar waves, so little notches like the picture above would totally defeat the purpose behind the weapons system that cost about $400 billion so far.

Here's a photo of the Marine Corps' F-35B flying clean.

Lockheed Martin

An F-35Bs takes off from the USS America beginning Oct. 28 until mid-November.

The notches, which are actually called Luneberg reflectors, serve a purpose. The reflectors increase the F-35's radar signature several hundred times over, so that a plane that would normally be nearly impossible for civilian air traffic controllers to spot will now give off a big, safe blip.

Perhaps the F-35B above didn't need the reflectors because it took off from sea, away from potential spotters.

In addition to helping friendly nations spot the stealth jet, the markers on the F-35 may serve another, more military purpose.

In October of 2015, just days after Russia began its air campaign to bolster Syrian President Bashar Assad, national security writer Dave Majumdar wrote in War Is Boring that Russia may be using its anti-air systems to gather intelligence on the F-22, another US stealth aircraft operating in Syria.

"While it appears the Russians are following their standard doctrine with regard to the deployment/employment of their ground and air assets, it's certainly not out of the question to use their newer air-to-air assets as a form of 'operational testing' in the real world environment," one senior US Air Force intelligence official told Majumdar at the time. "In a sense, we're doing the same thing with our F-22s."

REUTERS / Business Insider

Russia operates the very same ground and air assets in Syria as it does in Eastern Europe near Estonia, where the F-35 recently appeared wearing the radar reflectors.

With the reflectors throwing off and exaggerating the real radar cross section of the F-35, the US could be preventing Russia from testing its defenses against the US's newest weapons system.