Wilbur and Orville Wright made the world’s first heavier-than-air powered flight in December 1903. Little did they imagine that, 118-years later, a small piece of the fabric they used for the wings of their Wright Flyer would find its way onto the surface of another planet. Yet, in February of this year, that is exactly what happened when NASA’s Perseverance rover made a spectacular landing at Jezero Crater on the Martian surface. While Perseverance is the centrepiece of the mission, hitching a ride with it was Ingenuity, a small solar powered 2 kg helicopter drone. Two days ago, on 19th. April 2021, Ingenuity, or Ginny as it has been nicknamed, made the first powered heavier than air flight on another planet. The fabric sample of Wright Flyer’s wing is nestled underneath the solar panel on the top of the drone.
The complexities of getting to the surface of Mars aside, the Martian environment presents numerous engineering challenges to the use of aircraft. To sustain helicopter flight one must generate sufficient lift with the rotor blades to overcome the force of gravity. Acceleration due to gravity at the surface of Mars is 3.7 ms-2, which is approximately one third that experienced on the surface of Earth. Thus, all objects on the surface of Mars weigh approximately one third of what they weigh on Earth. Whilst this is certainly a positive for flight, atmospheric pressure at the surface of Mars is about 1/100th of that at Earth. In fact, it is so rarefied that one would need to ascend to some 35 km above the Earth, well into the stratosphere, to encounter the same air pressure on our own planet. Only high altitude balloons and the occasional rocket powered flight, or spacecraft during re-entry, have operated at these altitudes on Earth. The maximum service altitude for the famous Lockheed U-2 spy plane is some 10 km lower.
The amount of generated lift is a function of the surface area of the rotor blade, the air speed over that surface, and the air density. Air speed can be increased by increasing the spin rate of the rotor. However, the size of the rotors are restricted by what can fit inside the aeroshell – the heatshielded capsule which protects the aircraft during the fiery high speed entry into the atmosphere from space. The low air density at the surface also greatly reduces the efficiency of a given lift generating wing or rotor blade, which increases the power requirements needed to spin the blades. More power requirements typically mean a heavier on board power source, thus reducing the mass of the scientific payload that any such rotorcraft may carry.
Generating sufficient lift in the thin air of Jezero Crater requires that Ingenuity’s rotors spin at 2400 rpm. When fully charged from the on board solar panel, the batteries on Ingenuity provide enough power for a flight time of roughly 90 seconds. All flights are conducted fully autonomously as the time taken for any commands transmitted by radio to traverse the distance between Earth and Mars precludes real time control by humans.
Further complications arise due to the dust in the Martian atmosphere. Helicopter blades spinning in dust laden air can generate high levels of static electricity which discharge through arcing, not unlike the sparks generated when combing ones hair or handling woollen fabrics. Martian dust also has a tendency to settle onto surfaces over time, including solar panels, which gradually reduces their power generating efficiency.
Given these challenges, it is then quite an achievement to successfully have performed the first powered heavier than air flight on Mars. Ingenuity has 5 scheduled test flights, with each steadily increasing both the distances flown and the altitude reached by the drone. Ingenuity is really a technology demonstrator; its instrument payload consists of two on board cameras.However it paves the way for future more capable vehicles, which will widen the scope of the science that can be done on Mars with aircraft. For example, drones can rapidly survey an area around the rover and scout out potential driving routes which include interesting locations and the avoidance of hazards. The latter is no trivial difficulty; in 2009 a previous NASA Mars rover, Spirit, drove into a deep sand dune and became permanently stuck, eventually resulting in the ending of the mission. Whilst the landing zone of Perseverance is well surveyed from orbiting spacecraft, the cameras on Ingenuity can still produce images of the locality at much higher resolution. Aerial drones can also reach locations that are inaccessible to a rover, such as the steeper slopes of canyon walls or the interior of deeper craters, and they can rapidly collect and return geological samples from a wide area to a rover for analysis. Designs are already in progress to realise these goals. The first flight of Ingenuity therefore begins a new and intriguing chapter in our exploration of Mars.