If you could fly a drone in the skies of Mars, you could cover a lot more territory far more quickly than with a rover. But designing one is an enormous challenge.

On 19 April 2021, a tiny experimental helicopter named Ingenuity lifted off the Martian ground and into the history books. The autonomous machine’s rotors spun furiously in the thin atmosphere to produce enough lift, propelling the craft to the height of a single-storey building. Ingenuity hovered and then landed safely, delivering humanity’s first controlled flight on another planet. The site where it landed was named Wright Brothers Field, after the aviation pioneers.

In the mid-2030s, a rotorcraft the size of a small car, called Dragonfly, is scheduled to take the next step. It will land on Saturn’s largest moon, Titan, to begin humanity’s first mission to explore it. In one hour, Dragonfly will fly further than any surface-based rover has ever travelled on another planet. The multi-rotor drone-like vehicle will fly across the surface of Titan, landing for one Titan-day (16 Earth days) to carry out experiments before flying on to its next destination.

But the greatest challenge – and maybe the greatest opportunity – for extraterrestrial aviation is the hellishly hot planet Venus, with its extreme heat, pressure and acidic atmosphere. No lander has survived for more than 127 minutes on its cracked, slate-like surface.

Instead, scientists are proposing to send two aircraft to Venus. One is a solar-powered glider-like aircraft which can fly indefinitely through the planet’s more benign upper atmosphere, the other a flying wing design that will fly through the hostile conditions close to the surface.

"Developing the technology to be able to land on Venus is difficult," says Dr Eldar Noe Dobrea, a senior scientist at the Planetary Science Institute, California, who is developing the mission concepts for Venus. "The only alternative is to fly through the atmosphere."

Teddy Tzanetos, a robotics technologist in the Aerial Mobility Group and team lead for the Ingenuity Mars Helicopter, is already working on the designs for the next generation of Martian helicopters. “We know what the Wright brothers’ first flight did for humanity here on Earth, and I think we’ll follow that same model on other planets,” he says.

"I hadn’t thought of an analogue comparison like that, but the Dragonfly is the next step after Ingenuity’s first flight," says Elizabeth "Zibi" Turtle, the principal investigator at Johns Hopkins Applied Physics Laboratory. “It will be the first [aerial] vehicle to carry its entire scientific payload from place to place."

Like the early polar aviation pioneers, Nasa engineers realised how aerial vehicles could revolutionise the exploration of new worlds. Iconic machines like the Martian landers Viking and Curiosity and orbiters like Titan’s Cassini will continue to play key roles in exploration where there is a suitable atmosphere, but there might be other options. Robotic and controlled dirigibles, helicopters, drones and even inflatable propeller planes (all proposals by Nasa scientists) could quickly gather high-quality data about large areas of a planet’s surface, avoid hazardous terrain, collect up-close imagery impossible from a rover or orbit, and see mission targets from different perspectives. Aerial vehicles like these can also go where rovers can’t – mountains, peaks, and even the inhospitable surface of Venus.

The problem for Nasa’s engineers is that the environment on each planet imposes a different set of constraints on the type of aircraft, its payload and capabilities. The technology available to the engineers poses similar constraints.

On Mars, the atmosphere is less than 1% as thick as that on Earth, which makes it very hard for an aircraft to produce lift
Saturn V rocket designer Wernher von Braun envisioned landing on Mars in a hypersonic glider. Science-fiction author Philip K Dick imagined human colonists on Mars in helicopters. Nasa engineers started looking at concepts for a Mars aircraft after the Viking landers in the 1970s, features of which ended up in today’s Predator drone used by the US military.

On Mars, the atmosphere is less than 1% as thick as that on Earth, which makes it very hard for an aircraft to produce lift. This in turn means that a Martian helicopter must be very lightweight, but still be able to lift its lithium-ion batteries, sensors and cameras, as well as the heating and insulation to keep it alive through the cold Martian nights. "If you can solve all these challenges and build an aircraft that weighs less then 1.8kg (4lb) then you have yourself Ingenuity," says Tzanetos.

"Our chief engineer and members of the team first started looking into the idea for a Martian helicopter in the 1990s, but the technology just wasn’t there," he says. "Fast-forward to the 2010s and it was, for a technology demonstrator."

The team also looked at fixed-wing aircraft, but on Mars a rotorcraft made more sense because it would be operating without an airfield.

Nasa has nine different technology readiness levels (TRL) which range from TRL1 for when "basic principles have been observed and reported", through to TRL9 or "flight proven" through mission operations.

In the 1990s, the type of batteries needed to power Ingenuity had only recently been developed and few had realised the potential of materials such as carbon fibre. Likewise, the sensors, lightweight computing muscle and algorithms to fly the machine weren’t mature enough. Nor were the human skills of building and flying them.

The main goal was to prove that we could fly on Mars, and we did that over 30 flights – Teddy Tzanetos
More than 20 years on, it is a different matter. Today, on Earth, drones deliver parcels and vaccines and are used for surveying crops and archaeological sites. “It was really the confluence of all these technologies coming together at the right time to enable Ingenuity,” says Tzanetos.

Ingenuity completed its test flights and is still flying. "The main goal was to prove that we could fly on Mars, and we did that over 30 flights," says Tzanetos. "The biggest impact we can have on the future is to continue to fly Ingenuity.

"Every flight that we successfully complete provides a treasure trove of engineering data which will be crucial for future generations to use."

Tzanetos says the team is also working on designs of rotorcraft that can carry far heavier payloads over much longer distances. "We want to have the answers for when Nasa asks the questions."

Titan is the opposite extreme to Mars. Saturn’s planet-sized moon has an ice-covered surface crust, under which there is an ocean covering the whole planet. It is punishingly cold and rains methane. It has been suggested that boats could explore the surface of the moon, submarines the subsurface sea, and airships the atmosphere.

"Titan’s environment is really uniquely suited to exploring with heavier-than-air craft," says Melissa G Trainer, deputy principal investigator for the Dragonfly mission. It has low gravity and a dense atmosphere, and this means that airplanes and helicopters can be bigger in size, carry heavier payloads, and have greater capabilities than on a planet like Mars.

Dragonfly is a confluence of all of the great development that’s happened here – Melissa G Trainer
Titan’s environment means that a rotorcraft such as Dragonfly can carry Nasa’s powerful nuclear battery, which is needed for the mission’s scientific goals, as well as the experiments themselves, the computing hardware, and the tough landing skis needed to cope with the rough surface.

The existing maps aren’t detailed enough, but the rotorcraft will fly over a potential landing site and fly on if it’s not safe to land. "Dragonfly will make its own maps of Titan as it flies," says Trainer. "This leapfrog approach is the least risky option."

Mars, though, has the advantage over Titan in one aspect. "The whole suite of orbiters around Mars that have been there for decades can do the scouting for Ingenuity and function as relay," says Turtle. "Dragonfly has to do the direct Earth communication and the local scouting itself."

It takes less than a day for data to reach Earth from Mars, to be analysed, and orders for Ingenuity to be sent back. On Titan, it will take far longer.

The next aerial expedition after that may be to Earth’s sister planet, Venus. The planet’s atmosphere is 90 times denser than Earth’s. Its temperature is around 475C (900F), and pressure is 93 bar (1,350 psi), equivalent to a mile under an Earth ocean.

"The Venus atmosphere is horrible but also great," says Dobrea. "There is a huge, thick deck of clouds 20km (12 miles) thick that starts at 50 km (30 miles) above the surface and goes up to 70km (45 miles) – that is denser than Earth’s atmosphere and easier to fly through. It should be possible to fly a solar-powered aeroplane at this altitude pretty much indefinitely, and it is possible to do so with existing technology."

But there might be another option – balloons
His second concept aircraft will flyer close to the surface. It is a "tremendous challenge", he adds, owing to the extreme heat, the lack of sunlight for solar power, and the pressure.

This aircraft uses an engine like a Stirling engine to convert the extreme heat close to the surface into energy to power the aircraft at cooler, higher altitudes. It would be one of just a few planes ever powered by such an engine.

But there might be another option – balloons.

It was a balloon which flew humanity’s first flight on an alien world. In June 1985, the Soviet-European Vega mission dropped two huge spherical balloons into the atmosphere of Venus. Their instruments hung in a gondola underneath.

"We knew that the two balloons had been released, but we didn’t know if they were still alive," says Robert Preston, leader of the US project to track the balloons. "All we saw on the oscilloscope screen was noise, and nothing but noise. Then there was a faint signal.

"I remember leaving the control room and seeing Venus bright in the early morning sky and thinking: ‘I’m there.’"

The Vega balloons went on to float at an altitude of around 54km (33 miles) to collect 46 hours of atmospheric data. "When considering the success of the Vega balloons, the correct response is that they were ‘extremely’ successful," says Jay Gallentine, space historian and author of Ambassadors from Earth: Pioneering Explorations with Unmanned Spacecraft.

"I know we’ll have aircraft on Mars again in the future," says Tzanetos, "and with Ingenuity we’re adding a new tool to the toolbox. Everything we’ve learned will help other generations explore not just Mars, but planets in other solar systems."

But that may be even more of a challenge, warns Nasa scientist Jonathan Sauder from the JPL Technology Infusion Group. "If you start looking at planets outside of our Solar System then it starts to get really crazy out there. There are planets made from ice or that have metal in the atmosphere. There are some that we couldn’t send anything we know of today without it getting completely destroyed, but there are other planets more like Earth."

Whatever the different environments, the physics will be the same whichever solar system humanity is exploring. "The lessons we have learned from operating aircraft autonomously on other planets in our solar system are the foundational building blocks of how humanity will fly in the future," says Tzanetos.

Sauder is designing a lander that can survive on Venus. The mechanisms created for what he initially called the Automaton Rover for Extreme Environments (Aree) may one day be found in landers exploring Mercury and probes floating deep within the gas giants, as well as machines exploring the Earth’s interior.

"When it comes to building a lander for Venus, the extreme environment means that a lot of the traditional components we put on spacecraft will not work," he says.

The pressure pushes the acid in the atmosphere into the components, meaning they must be made from stainless steel or titanium. The high temperatures melt electronics.

I am confident we will one day have rovers on the surface of Venus – Jonathan Sauder
Sauder’s solution? "Let’s make an entirely mechanical robot, an automaton, a steampunk rover." The initial design even had legs rather than wheels, inspired by the huge wind-powered mechanical sculptures, or strandbeests, of Dutch artist Theo Jansen.

For obstacle detection and avoidance, the lander uses a system of rollers and bumpers that, like a children’s toy, cause the lander to back up when it hits an obstacle and drive forward again in a slightly different direction.

"It might not be the most efficient, but it is robust and reliable, and will work in that environment."

However, it proved too difficult to do away with all the electronics. Instead, basic electronics that can work in high temperatures are used to measure temperatures and chemical compositions and transmit data to the orbiter, and consequently, the rover had to be renamed Hybrid Automaton Rover-Venus (Har-V, or Har-vee)

Then there is power. Solar isn’t an option because Venus has thick clouds and a 60-day night. Instead, Nasa engineers turned to the wind to directly drive the mechanical systems of the rover. The camera and chemical sensors are trickier still and are yet to be developed.

The chance that Har-V’s wheels will land on Venus may be slim, but there is every chance that its design will have influenced the rover that does.

"I am confident we will one day have rovers on the surface of Venus, and that the lessons learned from the HAR-V architecture will influence those designs," Sauder says.

BBC

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