If there’s a U.S. security mission, there’s a drone for it: killing terrorists, seeing over the next ridge, hunting illegal immigrants. Now that Iran claims to have captured one of the United States’ most secretive flying robots, chances are there’s another use for them: nuclear inspector.

Why use drones to spy on nuke sites when the United States has giant, expensive spy satellites in space? The same capabilities that make them handy for spotting insurgents in Pakistan also make them helpful at sussing out clandestine nuclear weapons work. They provide persistent, detailed imagery and video of nuclear facilities — something that other spy systems aren’t quite as good at.

Drones are very good at parking themselves in the air, loitering over a target of interest for hours on end. Stealthy drones like the RQ-170 Sentinel, which allegedly crashed in Iran, add an element of surprise. Those are important advantages, given the limitations of image-capturing tools like satellites. Satellites’ orbital mechanics can give a heads-up to espionage targets and make glimpses of facilities very brief.

“Reconnaissance satellites fly four miles a second,” explains Tim Brown, an imagery analyst at Globalsecurity.org. “They’re going to be orbiting from North Pole to South Pole every 90 minutes and they’re going to be over Tehran or Isfahan or Bushehr at exactly the same time once every three days, more or less.”

That makes the time when satellites are spying overhead predictable and short.  ”It’s roughly a minute to a minute-and-a-half window, and after that you can go back to what you were doing,” says Brown.

India’s 1998 nuclear tests shows how countries can leverage these limitations to keep some of their secrets hidden from America’s eyes in space. India made sure to have little activity going on at its test site while U.S. satellites were overhead in the run up to the explosions. As a result, the eventual tests ended up catching the U.S. intelligence community by surprise.

With the unpredictability of drone surveillance, however, it’s harder for an adversary to practice such deception. Manned spy planes can also help keep collection unpredictable, but they can’t linger for long periods of time over a target without risk to a pilot. (Or without stressing that pilot’s human physiology.)

That extra time in the air collecting intelligence helps peel back the layers of a nuclear program in ways that shorter stakeouts can’t. Lengthy video surveillance is particularly useful. It can reveal unusual spikes in activity around a facility and the kinds of personnel entering and exiting — facts that might indicate whether it’s connected to nuclear weapons development.

Drones can also fly closer to their targets, allowing their cameras and sensors to capture better imagery of the ground below. The extra resolution can yield important clues for intelligence analysts. ”You can get a much closer look at some of the tunneling activity to see if there’s routine mining operations or installation of a factory inside a mountain,” says Joe Circincione, president of the nonproliferation Ploughshares Fund. That’s particularly helpful given Iran’s penchant for burrowing its nuclear sites inside mountains.

Not all nuclear clues are revealed in images of the ground, though. Some are floating in the air above clandestine facilities. Drones can be equipped with sensor packages that have “the ability to sniff the atmosphere for telltale signs of radionuclides that could indicate processing of nuclear materials,” says Cirincione.

It’s worth noting that the U.S. drone flights over Iran wouldn’t be the first time America has used an unmanned spy plane on a nuclear program. In the mid-1960s, Lockheed built a super-fast drone, the D-21, that could be launched from both B-52s and the stealthy mach-3  M-21 Blackbird. Its four missions over China’s Lop Nor nuclear testing range were troubled, leading the fatal crash of a launch plane and difficulties in collecting ejected cameras. And much like what’s reportedly the case with the RQ-170, wreckage of the D-21 ultimately ended up in the hands of its target, China.

But since then, the development of smaller, lighter and more advanced sensor packages combined with the ability of drones to loiter and autonomously take off and land has made them a much more reliable nuclear surveillance platform.

On Thursday, CNN cited unnamed military sources confirming that the RQ-170 was indeed conducting nuclear surveillance over Iran. Expect to see much more of that in the years to come. “We’re still at just the beginning of the exploitation of this technology,” says Cirincione.