Flying on Mars getting tougher as Ingenuity helicopter gears up for 14th hop

NASA's Ingenuity helicopter is getting ready for its 14th Red Planet flight, but the thinning Martian air is making such sorties more and more challenging.

The coming sortie, which could occur any day now, is a straightforward hop compared to some of the more daring scouting flights that Ingenuity has been making to aid NASA's Perseverance rover, mission team members said in a recent update. There's a good reason for the simplicity: The 4-pound (1.8 kilograms) chopper will test higher rotor spin speeds to see if it can keep flying in rapidly changing seasonal atmospheric conditions on Mars.

The flight plan calls for Ingenuity to take off, climb up to 16 feet (5 meters) and make a sideways maneuver before landing. The flight was originally supposed to take place no earlier than Sept. 17, but that was dependent on the mission team being ready for the opportunity. Updates will be published on Perserverance's official Twitter feed as more developments can be reported.

Related: Watch NASA's Ingenuity helicopter explore Mars' intriguing Raised Ridges

The short test flight, whenever it occurs, is expected to include a rotor speed of roughly 2,700 revolutions per minute (RPM), assuming that a planned ground test of a 2,800 RPM spin goes to plan. (By comparison, prior Mars experience had Ingenuity flying at about 2,537 RPM.) The greater rotation rate will, engineers hope, allow the drone to fly despite a thinning atmospheric density.

"It is actually getting more difficult [to fly] every day: I'm talking about the atmospheric density, which was already extremely low and is now dropping further due to seasonal variations on Mars," Ingenuity chief pilot Håvard Grip, of NASA's Jet Propulsion Laboratory in Southern California, wrote in the update. 

Grip explained that Ingenuity's flight campaign was designed to last just a few months after the Perseverance mission landed inside Mars' Jezero Crater in February. Ingenuity far exceeded expectations and is still flying, testing how rotorcraft could act as scouts for rovers or perhaps even human missions.

But Ingenuity wasn't designed for changing seasonal conditions. Originally, the atmospheric density in Jezero Crater was equivalent to about 1.2% to 1.5% that of Earth. But now the densities are approaching 1% during the afternoon hours preferred for flight, when currents off the ground cause less instability for the low-flying drone.

"The [atmospheric] difference may seem small, but it has a significant impact on Ingenuity’s ability to fly," Grip explained. Ingenuity's thrust margin, or the excess thrust the drone produces above what it requires to hover, has been decreasing as the Mars atmosphere thins. If the atmospheric density drops too far, Ingenuity could perhaps come close to a stall in mid-air.

"Thankfully, there is a way to tackle this issue — but it involves spinning the rotors even faster than we have been doing up to now," Grip continued. "In fact, they will have to spin faster than we have ever attempted with Ingenuity or any of our test helicopters on Earth. This is not something we take lightly, which is why our next operations on Mars will be focused on carefully testing out higher rotor speeds in preparation for future flights."

The Ingenuity team will be looking for a few potential issues. One is that the higher RPM, coupled with wind and helicopter movements, could make the rotor blades hit the atmosphere at roughly 0.8 Mach, or 80% the speed of sound. (The speed of sound on Mars is only three-quarters that on Earth, due to the Red Planet's much lower atmospheric density.)

"If the blade tips get sufficiently close to the speed of sound, they will experience a very large increase in aerodynamic drag that would be prohibitive for flight," Grip said. "For Ingenuity’s rotor we do not expect to encounter this phenomenon until even higher Mach numbers, but this has never been confirmed in testing on Earth."

Engineers will also watch out for potential resonances that could cause the helicopter to vibrate at particular frequencies, which at worst could "cause damage to hardware and lead to a deterioration in sensor readings needed by the flight control system," Grip said. Other considerations will include more power needed from the electrical system and higher loads required by the rotor system.

"It all adds up to a significant challenge, but by approaching the issue slowly and methodically, we hope to fully check out the system at higher rotor speeds and enable Ingenuity to keep flying in the months ahead," Grip said. "Stay tuned for updates."

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom or Facebook.