NASA engineers expected their Mars helicopter to crash after 5 liftoffs. It just landed its 15th flight.

• NASA sent its Ingenuity helicopter to Mars with no guarantee that it would successfully fly.
• But it has flown higher, farther, and faster than engineers had hoped.

NASA employees had a moonshot idea as they were building the Perseverance rover: What if it carried a little helicopter to Mars?

Engineers didn't know if a helicopter would work on Mars. It would be hard to lift off in air with 1% the density of Earth's atmosphere. That's the equivalent of flying at three times the height of Mount Everest. But NASA hoped to prove it was possible through a technology demonstration.

NASA engineers built a little rotorcraft called Ingenuity, then packed it into the last available space in Perseverance's belly. The engineers weren't sure that Ingenuity would even survive its first night on the cold Martian surface. They also feared it wouldn't fly when they gave the command, or that it would crash during one of its five planned flights.

But Ingenuity dispelled those fears time and again. On Saturday, the tissue-box-sized drone completed its 15th flight.

NASA is still processing data from the latest aerial escapade. But if the flight went according to plan, that would mean Ingenuity rose nearly 40 feet into the air, then zipped over 1,332 feet of Martian ground in just under 129 seconds.

NASA has been waiting for its Mars helicopter to crash

On February 18, Ingenuity and Perseverance landed in Jezero Crater, the dried-up bed of an ancient lake. Scientists think it's possible that Lake Jezero hosted an ecosystem of Martian microbes more than 3.5 billion years ago. If such alien microbes once existed, they could be fossilized in the mineral deposits that fell to the bottom of the lake and became rock.

At first, engineers didn't expect Ingenuity to fly higher than 15 feet above Jezero Crater. Project manager MiMi Aung said in early April that by the fifth flight, the helicopter "would be unlikely to land safely, because we'll start going into unsurveyed areas."

"If we do have a bad landing, that will be the end of mission," she added. "The lifetime will be determined by how well it lands, pretty much."

But Ingenuity has stuck the landing at least 14 times (and likely 15, pending NASA's confirmation of the details of the latest flight).

After its first five flights, Ingenuity performed so well that NASA gave it a 30-day extended mission. On its first bonus flight, flight six, the helicopter flew over previously unsurveyed terrain.

Since then, it has visited several unsurveyed locations, over rocky terrain and rippled sand.

During those flights, Ingenuity took up-close photos of outcroppings and rock fields where the Perseverance team was considering sending the rover. NASA then used the images to identify safe, flat routes the rover could follow. Ingenuity's pictures also helped scientists determine what kinds of rocks were in the area - information that could offer clues about Jezero Crater's past if Perseverance were to take samples from them.

"The ability to fly the helicopter out into terrain that the rover cannot possibly traverse, and bring back scientific data - this is extremely important for future missions that could combine a rover with a reconnaissance helicopter," Perseverance scientist Ken Farley said in an April briefing.

When the helicopter outlasted its 30-day extension, NASA gave it another one. The agency decided to keep flying Ingenuity until the helicopter crashed or interfered with Perseverance's operations. Neither has happened yet.

In fact, Ingenuity has operated flawlessly for almost every flight. It had one rough ride in May, when a technical glitch caused it to roll and pitch mid-flight. But the helicopter still landed safely.

For its last two flights, NASA pushed Ingenuity even harder by increasing the helicopter's rotor-spinning speed.

To generate enough lift in the thin Martian atmosphere, Ingenuity must spin its two pairs of blades in opposite directions at 2,400 rotations per minute (rpm). But the air is even thinner now that summer is in full swing at Jezero Crater. That's because winter conditions at the planet's south pole turn carbon dioxide into snow and ice, removing some of the gas from the atmosphere. So the blades have recently sped up to 2,700 rpm.

Ingenuity has now flown like that twice.

NASA is planning another four to seven flights to bring the helicopter back to the site where Perseverance first landed on Mars. Beyond that, the agency hasn't disclosed its plans.

Eventually, NASA hopes to send more ambitious helicopters to Mars - and possibly to other worlds. Space drones similar to Ingenuity could someday survey difficult terrain from above, study large regions faster than a rover can, and even do reconnaissance for astronauts.

NASA is already developing one such helicopter mission: A rotorcraft called Dragonfly is set to launch toward Saturn's moon Titan in 2027. It aims to investigate whether that methane-rich world could host alien life.