For about a week in April, scientists at NASA's Jet Propulsion Laboratory searched anxiously for signs of life on Mars.
Lost somewhere in the undulating terrain of a Martian riverbed was Ingenuity, the pint-size, astonishingly sturdy helicopter that had just completed its 49th flight on the Red Planet. The team searched each day for a radio signal that could confirm the aircraft was okay.
On April 2, Ingenuity soared 52 feet up into the Martian sky, a record height for the drone, to take a suborbital photo of Mars's landscape.
After landing, it disappeared. When scientists attempted to upload instructions for a subsequent flight, Ingenuity's radio sign was gone.
Scientists eventually located Ingenuity after six days of searching as the helicopter's companion on Mars, the Perseverance rover, crested a ridge and drove closer to where the helicopter had landed.
NASA engineer Travis Brown described the episode in a recent blog post, offering a dramatic look into the agency's exploration of Mars and the incredible resilience of the Ingenuity helicopter. Its hardiness continues to surprise NASA two years after scientists expected the tiny craft to break down.
The helicopter is flying again, Ingenuity team lead Teddy Tzanetos said, and its longevity has inspired the team to include helicopters modeled on it in a future Mars mission.
"It's absolutely ridiculous," Tzanetos said. "This is a once-in-a-lifetime kind of thing."
Ingenuity defied the odds the day it first lifted off from Martian soil. The four-pound aircraft stands about 19 inches tall and is little more than a box of avionics with four spindly legs on one end and two rotor blades and a solar panel on the other. But it performed the first powered flight by an aircraft on another planet -- what NASA billed a "Wright brothers moment" -- after arriving on Mars in April 2021.
Still, Ingenuity was never meant to be more than an $80 million proof-of-concept. It hitched a ride to Mars with Perseverance, an SUV-size rover that would carry out NASA's intended mission to study Martian soil.
Ingenuity, controlled via radio signals relayed from Perseverance, completed its five-flight mission, a simple series to prove that the helicopter's design would work in the thin Martian atmosphere, in May 2021. Then, Tzanetos' team received approval to keep flying.
"At that point, we're on borrowed time," Tzanetos said. "None of the mechanisms were designed to survive longer than that."
Somehow, they did, for months and months and dozens more flights. By May 2022, it seemed as if Ingenuity's miraculous story would finally plummet down to (Martian) earth. Winter was setting in, and NASA feared the lower temperatures would cause Ingenuity's solar-charged batteries to fail, or even freeze overnight.
The helicopter entered a low-power state after its 28th flight in late April 2022, and scientists said they weren't sure if it would fly again.
Incredibly, Ingenuity's delicate parts stood up to the Martian cold. But NASA still faced the challenge of reconnecting with the helicopter every time its components froze, Tzanetos said. The Ingenuity team adjusted by using data on Martian sunrises to calculate when the helicopter would thaw out each morning and regain enough charge to power on.
The result? A game of hide-and-seek of sorts, where NASA sent Ingenuity on flights, then used its model to calculate when the helicopter would be back online to receive its next instructions. It was enough to get Ingenuity and its wily mission team through the Martian winter.
"We still need to play some of these games every once in a while, depending on how cold it is or windy it is overnight, " Tzanetos said. "But the team's gotten very good at that."
NASA entered its most nerve-racking game of hide-and-seek with Ingenuity in April after Flight 49, as the helicopter accompanied Perseverance onto bumpy terrain thought to be an ages-old river delta.
Members of the team weren't concerned when they failed to connect with the helicopter over the first few days after the flight, Brown wrote; their process sometimes required several days to find Ingenuity. But their fears grew as almost a week passed.
Tzanetos wondered if the plucky helicopter's luck had finally run out.
"Every [day] is a blessing" for Ingenuity, Tzanetos said. "You're always prepared for the end of the mission."
Finally, six Martian days after losing contact with Ingenuity, the team detected a "single, lonely" radio signal, Brown wrote. The next day, another signal appeared -- confirmation that Ingenuity lived. The team eventually concluded that a ridge had blocked the helicopter's signals from reaching the rover.
Ingenuity flew again for the 50th time on April 13, soaring about 59 feet high to break its altitude record once more.
Tzanetos said the team will continue pushing Ingenuity's limits. While Perseverance continues with its assignment gathering Martian soil samples, Ingenuity is free to roam the skies ahead of the rover as a scout, gathering troves of valuable data on the Red Planet and its own performance as Mars's first aircraft.
And Ingenuity probably won't be the last. In 2028, NASA plans to send a lander to Mars to retrieve the samples Perseverance gathered. The craft would then launch off Mars in another astronomical first for the agency and return the samples to Earth for study.
That mission has been redesigned in light of Ingenuity's success, Tzanetos said. NASA now plans to send two helicopters of a nearly identical design with the lander as backup to retrieve Perseverance's samples in case the rover breaks down by the time the lander arrives in 2030.
It's unlikely Ingenuity will still be flying by then. But for now, the plucky helicopter refuses to die.
"Two years ago, if you asked me what do I hope will become of Ingenuity, I would have said, 'Well, I hope that our children or our grandchildren can build off this,'" Tzanetos said. " ... Here we are, Ingenuity's still flying, and we're designing the second generation."