When NASA launches the Perseverance rover on its trip to Mars this week, it will have a companion tucked next to it in the Atlas V rocket’s nose cone: A helicopter called Ingenuity, which is set to become the first rotorcraft ever to fly on another planet. This experimental miniature helicopter could open a whole new field of Mars exploration as it surveys the planet from the air.
But if you think it’s tough to design a ground vehicle to maneuver around a planet hundreds of millions of miles away, imagine trying to design a helicopter which can fly in an atmosphere so thin it’s barely there, in freezing temperatures, while navigating autonomously.
We spoke to a lead engineer and a senior scientist on the Ingenuity project at NASA’s Jet Propulsion Lab to find out how they did it and what the future of Mars exploration might look like.
Building a helicopter that can fly on another planet comes with numerous challenges, the most pressing of which is how to make something stay in the air when the atmosphere is so thin. Mars’s atmosphere is only around 1% the density of the atmosphere on Earth, which is the equivalent of being at an altitude of 100,000 feet. To demonstrate how difficult that makes flight, the altitude record for helicopter flight on Earth is just over 40,000 feet.
Helicopters work by moving air very fast using rotating blades, which push the air downward and create lift. But on Mars, the thin air offers very little lift, even when moved with blades. Although the designers got some help from the fact the gravity is lower on Mars, at just over a third of the force of gravity on Earth, there was still the significant problem of making a craft that could support itself with only a thin atmosphere to work with.
“The solution to that problem is low mass,” Josh Ravich, mechanical engineering lead for Ingenuity, told Digital Trends, “which was the overall hardest challenge of the entire mission, to keep the mass low.” The entire helicopter needed to weigh under 4 pounds (1.8 kilograms) which necessitated using carefully selected materials, and the main chassis is very small, being a cube of 14cm (5.5 inches) in size.
And the weight issue put limitations on other aspects of the craft too: “We’ve got to balance between how much power you can carry in the form of batteries to run the vehicle, and how big your blades can be,” Ravich said. The batteries are needed as power is collected using a solar panel on top of the vehicle which allows it to charge autonomously.
The blades of the helicopter need to be large — they have a span of just under 4 feet (1.2 meters) — to provide sufficient lift for the vehicle to fly. To make blades that were both large enough and light enough, the team used new materials including composites similar to carbon fiber. There are four blades in total, arranged into two rotors, each of which spins at up to 2,400 rpm, much faster than the approximately 500 rpm speed typical of helicopter blades on Earth.
The problem of cold
Another issue that needed material innovations was the problem of the surface temperature, which can fall as low as minus 100 degrees Fahrenheit at night. When it’s that cold, electronic systems don’t work reliably and the vehicle needs to use precious power to stay warm. So the Ingenuity team came up with a solution using thin layers of insulation around the vehicle’s delicate electronic components.
“Normally you’d solve this by putting a lot of thick insulation in there, however, insulation is quite heavy,” Ravich said. “So we ended up using some of the atmosphere itself, just like a duck or a goose would have a layer of insulation under their feathers, we use the gas from the Martian atmosphere. If you use enough thin thermal blankets you can get a little bit of insulation.”
One final complicating issue caused by the cold is the problem of how dampening materials react to low temperatures. “Most helicopters on Earth have physical elastic dampers that lift weight coming into the center hub of the helicopter,” he said. These dampers absorb the considerable vibrations caused by the blades rotating at very high speeds. “But those don’t work as well at Mars temperatures, so we had to do a lot of design to make that work as a more rigid system.”
It’s not possible to directly fly the helicopter from Earth due to the communications lag of several minutes between here and Mars. Instead, Ingenuity will be mostly autonomous, using its sensors to detect the environment around it and moving accordingly.
For this task, it will use onboard instruments including a navigation camera, a laser altimeter, and an accelerometer gyroscope package called an inertial measurement unit (IMU). Using these tools, the craft can work out where it is headed and how far away it is from the ground. It can even do some hazard detection to avoid potential obstacles in its path.
That means that the technicians on the ground give the craft a flight plan, and then Ingenuity can execute it, as Ravich explained: “The way the helicopter is flown is that we input a flight plan, basically a flight path, saying ‘spin the blades for this long, fly over to here, turn around, fly over here’… and then Ingenuity does that sequence by itself.”
The helicopter needs to stay within communications range with the rover, which is approximately one kilometer, and ideally should have a direct line of sight. But beyond that, Ingenuity can operate independently and can charge, take off, and land without any support from the rover. The plan is for the helicopter to tackle one challenge at a time, to see just how capable it is of maneuvering around the planet.
“We’ll fly a series of increasingly complex missions,” Ravich said. “Nominally, the mission is one to three flights, but it could be as many as five flights depending on how things go… Each flight is going to be a little more complex. The first one, we’ll rise up, hover around, land. The second one might be rise up, turn around, maybe move a little, then come back and land. Towards the end, if things are going well, they might decide to rise up, fly off that way and find a new landing spot and keep that as the next base of operations.”
Proving the concept
Ingenuity isn’t intended as a science mission, so it won’t be collecting science data — although experts hope that they’ll be able to make use of some of the data it does collect. The aim of the mission is to demonstrate that it’s technologically feasible to fly a rotorcraft on another planet and to collect engineering data to help design future Mars helicopters.
That means that there’s some degree of flexibility in how the craft can move, as there’s no need for it to maneuver to an exact location on the surface. The craft will likely stay within a few hundred yards of the Perseverance rover, so it can position itself relative to that. “To some extent, I don’t think it matters too much how precise we are as we’re flying — the helicopter will know exactly where it thinks it is,” Ravich said. “From a higher level, it doesn’t matter too much if it’s 10 feet this way or 10 feet that way when it lands — as long as it lands safely.”
If the Ingenuity concept works in practice as anticipated, helicopters could provide invaluable assistance to future rover missions, taking images of the surface and making exploration faster and more accurate.
Matt Golombek, a veteran of Mars science missions who specializes in choosing landing sites on Mars and who was the principal investigator for the first proposal for the Mars helicopter, explained to Digital Trends how helicopters could be beneficial to future exploration operations.
Filling in the resolution gap
One of the most valuable tasks future helicopter missions could perform would be taking high-resolution photos to fill in what is known as the “resolution gap” of Mars surface images. This refers to “the difference between the highest resolution images we have from orbit, which are about 25 centimeters (about 10 inches)per pixel and are called HiRISE images, versus what you can see on the ground in previous rover missions, where our resolution is something closer to 3 centimeters per pixel,” Golombek said. “That’s about an order of magnitude.”
Although the high-definition images of the planet’s surface taken using the HiRISE instrument are incredibly detailed considering they are captured from orbit, they aren’t detailed enough to show structural features of the land like outcrops, or to identify areas of scientific interest such as particular rocks for rovers to visit. So rovers have to explore around the area in which they land to find rocks or other features that are scientifically interesting to investigate.
A helicopter could be used as a scout for rover missions, taking images that are more detailed than those possible from orbit. These images could be used in identifying areas of particular scientific interest, so the team could send the rover right to the most valuable targets for research.
Expanding rovers’ areas of coverage
One thing you might not realize about Mars rover missions is how small an area each rover covers, as they have limited power to operate on, and every move they make needs to be carefully planned. Perseverance, for example, will cover between 3 and 12 miles (5 and 20 kilometers) over its main mission. And the planet’s furthest-ranging rover, Opportunity, covered an incredible 28 miles (45 kilometers) over its 14-year lifespan. As impressive as this is for a rover exploring a distant planet, these distances represent just a fraction of the total surface area of Mars.
A rover might take weeks to travel a kilometer, for example. Whereas Ingenuity could travel up to one kilometer in just 90 seconds, although the team doesn’t plan to run the helicopter at such fast speeds on its first mission. But future helicopters could explore a much larger area of the planet, and images they captured would be invaluable for putting rover findings into a broader context. Such images would help scientists to understand the global geology of the planet and tell them whether the areas studied by the rover are representative of the larger martian environment.
The helicopter could also help to extend the area of investigation by substantially reducing the amount of time it takes for rovers to navigate around the surface. Currently, rover driving routes are determined using the highest-resolution images available, but these images don’t always show obstacles or dangers so that drivers have to navigate slowly and carefully.
“Typically, the maximum rovers drive in a day is 60 to 100 meters,” Golombek said. “But if you had this high-resolution information, that would tell you specifically where the safe drive paths were, you could double or triple that easily, and thus get to your destination much more quickly.”
Finding a landing spot
Before a rover can explore, though, it needs to land. And the process of selecting a landing site could also benefit from aerial support.
“Landing site selection is a combination of characterizing how safe the surface is to land with the spacecraft you’ve designed and built — landers don’t like to have big rocks underneath them which could impale them or tip them over, steep slopes are generally not a good thing, and areas that are very fluffy that you might sink into are bad choices — so there’s a whole suite of what we call engineering constraints,” Golombek said.
These engineering constraints are also complicated by Mars’s thin atmosphere, as this makes it harder for vehicles to slow themselves down using parachutes as they come in for landing. So the team needs to consider the elevation of a landing site as well, to ensure the vehicle can land there safely.
“And then you have science objectives, which are based on the payload that you’re carrying and the science objectives of the mission — the things that you want to learn and find out about Mars,” he went on. “And you have to weigh all of those together to come up with a place [to land] that’s both safe and also scientifically interesting for that particular mission.”
“There’s always ambiguity in the orbital data that you use to infer what’s really down on the surface”
The people who select landing sites, like Golombek, are relying largely on images taken from orbit to puzzle out which sites will meet these criteria. And the tiniest of deviations from what is expected can cause problems, such as those being experienced by the InSight lander which landed on Mars in 2018. The InSight team managed to find a location that was appropriately flat and free of rocks, and their predictions about the materials that make up the surface were entirely accurate. However, the soil beneath the surface of where the lander sits turned out to be slightly different than expected, having been compacted into a tougher material called duracrust. And this has caused many problems in trying to bury the lander’s heat probe beneath the surface.
“There’s always ambiguity in the orbital data that you use to infer what’s really down on the surface,” Golombek said. “In general, for landing site selection we’ve been very good at measuring and characterizing the engineering constraints — the rock abundance and the slopes, and so on — mostly because the HiRISE images are high enough resolution to see big rocks and measure slopes. But we’ve been a little bit less accurate at understanding what I’d call the geologic setting. That is, how that area came to be, what were the main geologic forces that shaped it. That’s been tougher.”
As the images obtained from orbit have a limited resolution, it’s difficult to see the kinds of details that are needed to most accurately identify targets of scientific interest, such as particular sedimentary rocks. Having much higher resolution images like those which could be captured by a helicopter would be invaluable in picking landing sites that were both safe for the vehicles and maximized the chances of making important scientific findings.
Helicopters could even carry different sorts of instruments such as ground-penetrating radar which could tell scientists directly about what lurks beneath the Martian surface.
Helicopters could be used for more than just support of other missions, though. Such a machine could potentially be outfitted with any type of camera, such as radar, infrared, or thermal imaging instruments which can reveal the composition and mineralogy of the martian ground.
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This is important as these tools can identify certain minerals, such as clay, which form when water is present. Areas with high densities of these clay minerals are key targets for research into whether there may once have been life on Mars.
Some of the most interesting targets for scientists to research are escarpments, or steep cliffs formed by erosion, because these reveal the layers of rock which were laid down over time. Looking at these layers is like looking back in Martian history. However, because they are steep and rocky, these areas are difficult for rovers to explore and they have to proceed very carefully. The rover Opportunity, for example, spent an entire year carefully driving around the edge of one such escarpment to image it, while “those sorts of images could have been acquired in a couple of days by a helicopter,” Golombek said.
When asked if there was a particular location on Mars he’d personally like to explore with helicopters, Golombek laughed. “There’s hundreds — thousands!” he said. “The surface area of Mars is similar to the exposed, surface area above water of Earth. Think about the differences between the Grand Canyon and the Himalayas, between the coastal zones and the interiors. There are so many different places that would tell you interesting things.”
Both experts agreed that the future of Mars exploration was not a question of either helicopters or rovers, but rather of using both as required for different tasks.
“I’m an engineer at heart, so to me, they’re all tools in the toolbox,” Ravich said. “For atmospheric bodies like Mars, there’s going to be a strong case that an aerial vehicle is the answer for whatever you want to do. If you want to go down into a big hole like a canyon, or if you want to climb a mountain, that’s going to be the best answer. But there’s always a limit on what we can carry — that’s why birds are so light and elephants aren’t — so you’re always going to be able to do more science and carry more with a [ground] vehicle.”
The need for multiple types of vehicles becomes even more clear when humans enter the picture, when planning for future manned missions to Mars. “We’ll probably need both too,” Ravich said. “If you look at people today, we interact with ground vehicles and air vehicles, and I don’t see that changing.”
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