The case for a fleet of Martian helicopters
by Ari Allyn-Feuer
|What could hundreds of Ingenuity-style helicopters accomplish on the Martian surface? A lot.
Ingenuity, which made 72 flights and traversed 17 kilometers prior to its recent retirement, weighed less than two kilograms. Ingenuity didn’t contain any rare or special materials, like plutonium radioisotopes or xenon gas, that would prevent manufacturing many such probes.
Suppose that NASA were to create a helicopter like this, but designed to be produced in large numbers and to enter, descend, and land on Mars independently, operating without a parent rover like Perseverance. They would instead communicate directly with satellites in low Mars orbit.
A swarm of such helicopters, packed into small individual Mars entry vehicles, would be launched on a carrier vehicle to the Mars vicinity. The carrier vehicle would aerobrake into a Mars orbit of some kind and then, prior to final atmospheric entry, release all the helicopter probes in such a way that they would reenter in a very broad range of locations across the Martian surface, perhaps approximately uniformly across the globe of Mars. This could be done by releasing them at low relative velocities in all directions, at the apoapsis of a highly elliptical, highly inclined Mars orbit whose periapsis dips into the Martian atmosphere. At the low orbital velocity of periapsis, dispersal of the probes could throw them onto very different orbital planes, reentering Mars at different times, resulting in their reentering and landing at very different points on the Martian surface.
How many such helicopters could such a mission send? A SpaceX Falcon 9 launch vehicle can throw about 4,000 kilograms into a trans-Mars injection, while the Falcon Heavy is able to throw about 16,800 kilograms. The Mars 2020 spacecraft, including the flight stage and entry, descent, and landing (EDL) system, weighed 3,649 kilograms and was able to place the Perseverance rover, weighing 1,025 kg, onto the Martian surface. This is a ratio of Martian landing mass to trans-Mars injection mass of 28%. If, hypothetically, the same ratio applied to a hypothetical helicopter swarm, and the helicopters each weighed two kilograms, like Ingenuity, then about eight kilograms of trans-Mars injection mass might be needed per helicopter. If this were true, then the Falcon 9 might be able to throw about 500 tiny helicopters to Mars, and Falcon Heavy might be able to throw about 2,000 such helicopters!
This could be an overestimate. The carrier vehicle would have to perform some functions that the Mars 2020 cruise stage did not, including aerobrake at Mars prior to terminal reentry, attain a precise orbit, and decouple many small probes in space. Reentry vehicles might be less mass-efficient at small size. In addition, this system would require a significant constellation of communication relay satellites in low Mars orbit, which would be its own mission or missions. Maybe it would be significantly less helicopters than the naive estimate above.
It’s theoretically possible that tiny helicopters could be more mass-efficient to land on Mars than other forms of probes, for two reasons. Smaller cargoes are generally easier to aerobrake through any atmosphere, because they have more surface area per unit mass and a lower terminal velocity. And while prior Mars probes have featured complex retrorockets, parachutes, and skycranes to brake themselves from a subsonic descent through landing, it’s theoretically possible that small helicopters could be uniquely mass-efficient for the terminal phase of landing. Perhaps they could be designed to land directly, by first feathering their propellers to slow themselves, then turning on their propellers and slowing to enter level flight, and finally landing under their own power. If this were possible, the EDL system for a small helicopter could be little more than a heat-tolerant balloon containing the helicopter, which would enter the Martian atmosphere, slow to a subsonic speed, and then pop itself to release the helicopter to land by itself.
|Handling hundreds, even thousands, of tiny probes at once would require a big change in how NASA thinks about and controls robotic probes.
It may not be 500 or 2,000 helicopters with a Falcon 9 or Falcon Heavy. If the carrier vehicle made up half of the trans-Mars injection mass, and the individual reentry vehicles were only half as mass-efficient as the cruise stage and EDL stage of Mars 2020, then, hypothetically, only 100 tiny helicopters could be carried on a Falcon 9, or 400 on a Falcon Heavy.
At any rate, this is not important, because even with pessimistic assumptions, hundreds of helicopters could be launched in one mission. Regardless of the precise numbers, it would be a large number.
What could hundreds of Ingenuity-style helicopters accomplish on the Martian surface? A lot. While the scientific payload of such probes could only be measured in the hundreds of grams, this is enough to contain basics like cameras, thermometers, barometers, anemometers, and so on. And they would, like Ingenuity, make dozens of flights over tens of kilometers of traverse, each.
Getting close-up photography, temperature, insolation, pressure, and wind data at tens of thousands of locations spread all over the Martian surface, over a period of years, would be transformative. The Mars 2020 mission team debated 60 different potential landing sites for Perseverance before choosing to send it to Jezero Crater. A swarm of tiny helicopters could visit them all, and hundreds more besides.
In addition, by making variant helicopters carrying only one or two instruments each, scientists might be able to fit a variety of more sophisticated instruments that have been developed for CubeSats, which have included, according to one document, “infrared spectrometers, a variety of radiometers, magnetometers, radar for bistatic radar studies (relatively) high-resolution optical imagers, highly sensitive photometers, Lidar, field and particle sensors, and neutron spectrometers.” It may be that NASA could make some kind of radar and/or geochemistry instruments small enough for Ingenuity-style helicopters.
Handling hundreds, even thousands, of tiny probes at once would require a big change in how NASA thinks about and controls robotic probes. Space probes have never been manufactured in such volume before. The idea of hundreds or thousands of similar probes would be a big change in how probes are designed and manufactured. Unit costs would undoubtedly plunge.
It would be a big shift in how the probes were controlled. Rather than choosing where on the surface the probes would land, NASA would have to accept hundreds of random locations, and plan the scientific operations of individual probes based on where they landed. Rather than controlling each one individually with dedicated teams making intensive effort, a team managing the entire fleet of probes would have to make systematic rules or algorithms for decisions about the tasking of a large number of helicopters based on their individual circumstances. This might also be an opportunity to allow, for example, individual university teams to control individual helicopters landing in areas of interest for their research.
It would also be a big shift in how communication with probes is handled. Command queues for the hundreds of helicopters would have to be uploaded to Mars in large batches, to be relayed to the individual helicopters as individual satellites orbit above them. The data coming back would, in turn, have to be relayed from hundreds of points on the surface to a number of satellites in low Mars orbit or to a higher-bandwidth communication platform in a higher orbit, a role filled by the Mars Reconnaissance Orbiter for the last two decades but which might require a new orbiter for this mission, possibly based on laser communication systems recently tested in deep space.
If this kind of approach worked, it could be an example for subsequent space probe missions. Many small, mass-produced, independent cubesat-style probes with a single larger carrier vehicle could disperse and visit other collections of related destinations elsewhere in the solar system, like asteroids in the main belt or Sun-Jupiter Lagrange points, Kuiper Belt objects, or the Jovian or Saturnian moons. Perhaps many robot helicopters could land on Titan. Perhaps many small balloon probes could enter the atmospheres of Venus, Titan, or the gas giants, and dwell at the altitudes most conducive to probe operation over long periods of time.
|If we manage to saturate Mars and other bodies with scientific data collection using large numbers of tiny probes, the legacy of this tiny, lovable helicopter probe will be mighty indeed.
In the field of satellite manufacturing, megaconstellations like Starlink and Kuiper have recently resulted in the development of several important enabling technologies for large probe swarms, including cheap, low-power ion thrusters and mass deployment mechanisms. Such mechanisms might translate well to mass produced deep space probes.
Ingenuity was damaged and forced to retire this January. But it proved something important: tiny probes weighing just a few pounds can function and return important data from distant planets. If we manage to saturate Mars and other bodies with scientific data collection using large numbers of tiny probes, the legacy of this tiny, lovable helicopter probe will be mighty indeed.
NASA should study the feasibility of this kind of approach to a Mars probe, and if it proves feasible, consider how to prioritize it in the next decadal survey.
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