A decade ago yesterday, the US carried out one of the most famous and surprising military actions of the 21st century: the covert raid on Osama bin Laden’s compound in Abbottabad, Pakistan on May 2, 2011.

To get to that location from Jalalabad, Afghanistan, the SEAL team (and a dog) flew in two helicopters, a trip of some 90 minutes. Those choppers, as former president Barack Obama describes in his new memoir, A Promised Land, were “two Black Hawk helicopters that had been modified for stealth.”

That the US employed stealthy helicopters that hadn’t been previously acknowledged to exist until that day was fascinating even for people who don’t consider themselves to be aviation geeks. Even more thrilling was that the wreckage of one of them was left behind after it crashed.

“When I think back to those 10 years, it’s a remarkable peak behind that black curtain of US defense capability,” says Tony Osborne, the Aviation Week bureau chief in London.

Ten years on, here’s a look back at what we know about those helicopters—and the science of stealth aircraft in general.

What is a stealthy aircraft?

Modern-day planes like the F-35 are commonly called “stealthy,” but more technically, they’re referred to as “low observable.”

The phrase refers to several different goals. Aircraft designers want a low-observable aircraft to be invisible to an adversary’s radar. Plus, they’d like it not to create an infrared signature or too much acoustic noise. They’ll also consider electronic warfare. Low-observable aircraft should have the ability to jam an enemy, and also not actively send out signals that allow it to be tracked.

So while there are various ways to think about what stealth means with different aircraft, “usually radar is the highest priority,” says Peter Westwick, author of the book Stealth and an adjunct professor of history at the University of Southern California.

For its part, radar works by bouncing radio waves off the object; then a signal boomerangs back to an antenna. Ideally, a low-observable aircraft won’t return a useful signature.

After radar, a common secondary goal would be to try to mitigate the infrared signal emanating from that aircraft. “Stealth aircraft, the focus is on the radar, but they will also have features that will reduce the heat signature from the engine exhaust,” Westwick says. Perhaps an aircraft’s exhaust could exit above its body, where IR cameras couldn’t see it, for example.

And while it seems like a distinctly modern idea, Westwick notes that the roots date back to the 1960s and 70s, and an awareness of US aircraft susceptibility. “Soviet radar systems definitely put American aircraft at a major disadvantage—and this was made clear both by the American experience in Vietnam, and then by the Israeli experience in the [1973] Arab-Israeli war.”

Development of stealth aircraft in the US truly began in the 1970s, famously resulting in Lockheed’s F-117 Nighthawk attack aircraft and then later, Northrop’s B-2 bomber. Along the way, Lockheed created a prototype of the F-117 called “Have Blue,” and Northrop made a bizarre-looking aircraft called “Tacit Blue.”

When it comes to low-observable aircraft, shape is key. As Westwick examines in Stealth, the defining feature of the angular F-117 was its facets, or flat panels. Northrop was known for leaning into a curvier approach for flying machines.

Either way, the differing design mentalities had the same goal: to keep the radar returns low. (Radar-absorbent materials play a key role, too.) Plus, curves have the benefit of being more aerodynamic than facets like the highly angular Nighthawk had. “You’re still trying to send [the radar waves] off in another direction, but then you’re also trying to make the airplane more flyable,” Westwick says. “Flat plates are very inefficient aerodynamically—there’s a reason why birds’ wings are curved.”

Low-observable helicopters are hard to make
The Nighthawk and then the B-2 Spirit were the first low-observable production aircraft, and today, the US military flies fighters like F-22s and F-35, both of which have bodies that appear curvy; so does a rendering of the forthcoming B-21 Raider bomber from Northrop Grumman. Facets seem passé.

But helicopters, with their big sides, huge swinging top rotor, and smaller tail rotor also twirling around in another orientation, pose an even greater challenge. Whirlybirds are loud, in-your-face machines, and attempting to retrofit one to be low-observable would be hard. “It’s very difficult to make an aircraft stealthy—in fact, it’s very difficult to do it after you’ve designed it,” says Osborne, of Aviation Week. “To make a helicopter is probably doubly so, because helicopters are inherently non-stealthy.”

Todd Harrison, who directs the aerospace security project at the Center for Strategic and International studies, sees it the same way. “Helicopters, fundamentally, are never going to be as stealthy as something like the B-2 bomber,” he says.

But it could be helpful to just be less observable, or less loud, than usual. “Stealth is not a binary thing—it’s not that you’re either stealthy or not,” Harrison says. “In reality, it’s degrees of stealth.”

Photographic evidence provided a tease of the design of the helicopters used in the raid, and people dug into it right away in 2011. Because one of the Black Hawks crashed—the team blew it up before they departed—a piece of the tail remained.

Looking at that photograph in the present day, Osborne comments that a few modifications from a standard Black Hawk are clear. One is that it appears to have had more tail rotor blades than a Black Hawk’s typical four. An additional blade or blades would let that rotor spin more slowly, but still achieve the performance the helicopter needs. “But if you slow it down, that means it’s quieter,” he says.

He also notes the presence of additional smooth, cover-like material. “They’re hiding all those intricate components of the tail rotor gearbox,” he says. “All those odd little rivets—where the tail wheel would come down on the Black Hawk—all those areas where radar energy could get stuck, and then bounce back out, they’re all disappeared.”

“It’s a different construction method,” he adds. “It’s not rivets—it’s smooth, slab-sided surfaces.” That, plus radar absorbent material, could help dial down the radar return, and tweaks like the extra tail rotor blades could have made it acoustically quieter.

Ultimately, all these changes—and it’s impossible to know what the rest of the bird looked like—were likely designed to help it dodge radar and be as quiet as possible. (A tradeoff may have been that it was heavier.) It may not have been a completely stealth helicopter, but it may have been a stealthier one.

Black Hawks down to be sneaky

Black Hawk helicopters are made by Sikorsky, which is part of Lockheed Martin, and Doug Birkey, the executive director of the Mitchell Institute for Aerospace Studies, sees a possible connection between a canceled Boeing-Sikorsky helicopter program called Comanche and the retrofitted helicopters of the raid. “The thought with the Black Hawk was that they took elements of that knowledge, and they applied what they reasonably could to a Black Hawk—in terms of shaping and coatings—to try to reduce its signature.”

That, plus following a careful route into Pakistan that they had figured out in advance, would have helped. Ultimately, considering shapes, coating materials, and other changes would have helped make the helicopter more low-observable, but it wouldn’t have been truly stealthy. “They were just buying down the risk as much as possible,” Birkey says. “And I think a lot of it probably came out of Comanche.”

Reached for comment, a Sikorsky representative referred Popular Science to the US military. A public affairs officer for US Special Operations Command told PopSci via email that “we do not [have] any releasable information about those aircraft.”

Last year, The Drive—a sibling website to Popular Science—published what it calls “the first photo ever of a stealthy Black Hawk helicopter.”

It may have looked like a Transformer
The helicopters probably weren’t the only flying machines making use of low-observable tech. As Mark Bowden says in his 2012 work The Finish—a thorough journalistic account of the mission and events preceding it—a drone called the RQ-170 Sentinel played a key role, too. It had a “high-powered lens, which would provide a live video feed of the assault,” he notes in the book. That drone is less of a secretive entity than the helicopters, to be sure, and the Sentinel was known for flying out of Kandahar, Afghanistan.

In the decade since the raid, other written accounts of the mission have appeared beyond Obama’s account and Bowden’s reporting. In No Easy Day, Mark Owen (his real name is Matthew Bissonnette) writes about his background as a SEAL and taking part in the mission. He notes that in his helicopter, they “sat on the floor or on small camp chairs purchased at a local sporting goods store before we left.” Taking the proper seats out was a way to make the bird lighter. And Esquire has a long, sad piece about the man who shot bin Laden—in it, the SEAL refers to the helicopter as resembling a “Decepticon.”

Finally, there’s a twist on the use of low-observable aircraft in the raid. The typical understanding of stealth is that it allows one country to avoid an adversary’s detection, but something different happened with this incursion. “Why did we use stealthy helicopters?” asks Harrison, of CSIS. “It wasn’t to hide from bin Laden—he didn’t have radar. It was to hide from our partners, the Pakistanis.”

“Sometimes, you use stealth to hide from your partners,” he continues, “because you may not be able to trust that they’re not going to leak what you’re doing, or they may not be happy with what you’re doing if they knew about it in advance.”