At 2:42 pm Eastern on Sunday, February 12, mere hours before , a pilot flying an F-16 fighter jet . The pilot did so at the direction of President Biden and the Secretary of Defense, and the event marked the fourth shoot-down over U.S. airspace in eight days.

“Based on its flight path and data we can reasonably connect this object to the radar signal picked up over Montana, which flew in proximity to sensitive DOD sites,” reads . “We did not assess it to be a kinetic military threat to anything on the ground, but assess it was a safety flight hazard and a threat due to its potential surveillance capabilities.”

While militaries have used balloons for war dating back to , using balloons for covert peacetime surveillance is a relatively more recent phenomena. In , the United States sent balloons with film cameras across the Soviet Union and China, hoping to capture evidence of massive military industrial undertakings. Project Genetrix, a program that launched over 400 balloons in early 1956, ended weeks after it started, with only 40 balloons recovered and a very incomplete picture derived from what they could see. Wind, as propulsion, can certainly carry lighter-than-air objects across the sky, but steering becomes difficult. The advent of high-altitude spy planes and then largely superseded the role of untethered surveillance balloons for military intelligence.

That better, more controllable options exist for military surveillance doesn’t change the reality that the U.S. is now apparently detecting and shooting down balloon-like objects in territorial skies.

During an on-the-record call with the press that occurred while the Super Bowl was on TV, General Glen VanHerck, Commander of North American Aerospace Defense Command (NORAD) and U.S. Northern Command (NORCOM), offered one explanation.

“[R]adars essentially filter out information based on speed. So you can set various gates, we call them velocity gates, that allow us to filter out low-speed clutter. So if you have radars on all the time [...] looking at anything from zero speed up to, say, 100, you would see a lot more information,” said VenHerck.

“We have adjusted some of those gates to give us better fidelity on seeing smaller objects. You can also filter out by altitude. And so, with some adjustments, we’ve been able to get a better a categorization of radar tracks now. And that’s why I think you’re seeing these overall. Plus, there’s a heightened alert to look for this information.”

One reason for the greater frequency of sightings is a deliberate recalibration of sensors to pick up more information. Radar systems, like those on jets or the ground based systems used by NORAD, are calibrated to detect the most relevant threats. NORAD, which was founded to watch the polar skies for or missiles, is designed to look for large, fast-moving objects, especially ones capable of carrying nuclear warheads. Adjusting the calibration on sensors means picking up other objects.

For a lower-stakes example, consider what happened on April 16, 2015, when political activist and postman Doug Hughes landed a gyrocopter on the . The man flew his small, ultralight vehicle through secure airspace, in violation of a No Fly Zone, and while he was only delivering letters, his flight path revealed gaps in what existing air defense radar can see. At a Congressional hearing afterwards, a Navy admiral noted that under existing filters, the Gyrocopter flight registered the same as either a .

Changing the parameters of what can be seen means expanding what can and will be seen as a threat. It also calls into question whether such objects have transited above the U.S. in the past, evading detection because sensors were not calibrated to see them. After , the Department of Defense announced such balloons had been observed a few times before over the United States.

“I’m not currently tracking any other objects at this point. That doesn’t mean there couldn’t be more at some point in the future, but right now, we’re not seeing anything,” said VanHerck. “As far as, you know, previously [...] I don’t know if there was more. We do know after the fact that there were high-altitude balloons because we went back and we were able to reconstruct them.”

Cross-continental balloon flight is slow, so if there has been a pause in launches, it will take some time for the already-launched balloons to pass. While balloons may not offer all the advantages of a satellite, there is one possible reason to use them, even if control and communication with balloons is limited.

“They can linger longer and get much more concentrated images much faster than a satellite,” . “Two other advantages – they are so cheap that they’re expendable, and it turns out the U.S. couldn’t readily detect them for years. Beijing’s balloons were undetected or, at least, uncomprehended by the Americans for years.”

Still, before the balloon sightings and shootdowns end up as so much fuel for a new Cold War, it’s worth noting that, so far, the the observed objects were not even sending communication signals. If the objects were designed to probe defenses, it’s a strange omission to have them avoid communicating what, exactly, they discovered with their probes.