[HN Gopher] How NASA Designed a Helicopter That Could Fly Autono...
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How NASA Designed a Helicopter That Could Fly Autonomously on Mars
Author : hacksilver
Score : 215 points
Date : 2021-02-18 09:09 UTC (13 hours ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| 4gotunameagain wrote:
| I wish we could have more information on the terrain relative
| navigation implementation..
| dataflow wrote:
| Obligatory Veritasium video link:
| https://www.youtube.com/watch?v=GhsZUZmJvaM
| jshier wrote:
| I had the opportunity to speak with one of Ingenuity's engineers
| in the NASA pavilion at the EAA Fly In in Oshkosh, WI in 2017 (or
| perhaps 2016). This was before it had been accepted for the
| Rover2020 mission but just after they had proven the concept
| flight capable. I asked him about flight conditions on Mars and
| what they had to do to get their design airborne in such a thin
| atmosphere. He made a few points.
|
| 1. Given the thinness of the atmosphere, the counter rotating
| blades are balanced among thickness, length, and rotation speed.
| They're essentially as big as they can spin with the power they
| have at a speed where the tips are just under the Martian speed
| of sound.
|
| 2. I asked if wind was an issue. He said that it wasn't an issue
| for the stability of the craft (Martian wind moving much of
| anything is a movie trope) but the turbulence it caused could be
| a major issue for the blades, as they're essentially small wings,
| like a helicopter. The turbulence of a cross breeze could disrupt
| the flow of air over the blades and cause them to lose lift.
|
| 3. The design of the craft was essentially locked once they
| confirmed it could fly. They would keep most of their original
| hardware and software, rather than rebuild anything, which I
| found surprising. Given this was before they were accepted on the
| mission I'm not sure if anything changed. It does appear to be
| pretty equivalent to what we saw there.
| DAlperin wrote:
| Here's one of the referenced software frameworks (if I understand
| correctly) used for the ingenuity control systems:
| https://github.com/nasa/fprime
| f6v wrote:
| Imagine how far the technology could go had we sent 10 times the
| missions we do now.
| ryan_j_naughton wrote:
| Can someone who understands aeronautical engineering explain why
| they went with a helicopter over a winged aircraft?
|
| My guess is that takeoff would be too difficult for a traditional
| winged aircraft and VTOL was way too complicated.
|
| But given the thin nature of the atmosphere, a helicopter seems
| even more difficult to get lift than a winged aircraft.
|
| Can someone explain or link to the science between lift and air
| density and it's relation to helicopters and winged aircraft?
| Thanks!
| interestica wrote:
| > But given the thin nature of the atmosphere, a helicopter
| seems even more difficult to get lift than a winged aircraft.
|
| Easier to speed up a helicopter's wings (rotors) than speeding
| up an aircraft which would need more and more runway.
| NikolaeVarius wrote:
| it needs to fit inside the fairing. also what do winged
| aircraft use to fly?
| tkinom wrote:
| Wonder the same thing, I have the impression that Quad or Hex
| copter would be more stable/reliable with more redundancy build
| in.
| carabiner wrote:
| Quad/hex would be less efficient and heavier for the same
| performance (range, speed, service ceiling), but yes, more
| stable. In this mission, I suspect minimizing weight (as a
| rocket payload to be sent through space) and maximizing range
| (hop flights on Mars on a limited power source) were a
| priority. This is also why full size helicopters are almost
| all single main rotors with a TR.
| aero-glide2 wrote:
| No runways on Mars, so takeoff could be difficult. Maybe we
| could do that once we have well prepared surfaces. Helicopters
| can also hover and are much more manoeuvrable than fixed-wing.
| Lift is proportional to density. L = Cl x .5 x rho x v^2 x S (L
| is lift, Cl a non-dimensional coefficient which depends on wing
| shape, rho is density, v is speed and S is surface area of the
| wing). For fixed wing aircraft, v is speed of aircraft. For
| helicopters its the speed of the blade (when its in hover. When
| its moving forward its a bit more complex).
|
| Haven't done the math but the aircraft has to be going pretty
| fast ('v' in the equation) to generate the required lift. This
| also means longer runway lengths.
| inetstarrrrrr wrote:
| What's the point of flying a helicopter on Mars? What
| scientifically relevant data do you get? Its fun and cool but
| why?
| pengaru wrote:
| I find it hard to believe you genuinely asked yourself this
| question and came up empty before posting this vacuous comment.
| hoseja wrote:
| Well, you may be able to get places a rover can't. But as they
| say in the article, this is just a demonstrator to show if they
| can fly on Mars at all.
| fudged71 wrote:
| The article covers this.
|
| In the future maybe they can fly with LiDAR to get more
| accurate surface details for the rovers to navigate with, and
| finding interesting features
| shatnersbassoon wrote:
| I would have thought atmospheric data that you can't get
| elsewhere - you'd be able to learn things about pressure and
| wind at altitudes that rovers can't get to.
| inetstarrrrrr wrote:
| I don't think it will fly that high to make a measurably
| difference to ground level. It only flies for 90 seconds. How
| high can you get in that time?
| aero-glide2 wrote:
| This is a tech demo. This will help design future Martian
| drones.
| simonh wrote:
| * Scouting possible destinations or routes for the rover.
|
| * Imaging areas from above with higher fidelity than possible
| from orbit, and from angles not possible from a rover.
|
| * Retrieving samples from areas inaccessible for a rover.
| BelenusMordred wrote:
| It's just a demonstrator but can travel 100m in each 90 second
| flight.
|
| For comparison, the Curiosity rover has travelled 23 km over 8
| years.
| arethuza wrote:
| Perhaps we could eventually get things like a video of a drone
| flying over the 4km high cliffs of Echus Chasma:
|
| https://apod.nasa.gov/apod/ap080723.html
|
| There is a whole new world out there!
| virtue3 wrote:
| main thing I've seen on mark rober's youtube is that it allows
| you to really scout out locations with a speed unheard of for
| rovers.
|
| Also, you can't really(easily) get a helicoper stuck on rocks
| :)
| Daniel_sk wrote:
| To test the technology for next missions. This is just a proof
| of concept.
| holoduke wrote:
| Because exploring is our nature. And because it will be the
| first test or flying something in an alien atmosphere. Tons of
| data alone from the first 5 seconds is very interesting. It's
| the first step to commercial airliners on Mars :)
| BorisTheBrave wrote:
| Can someone explain the phyics of flying a helicopter in such low
| density atmosphere? It must be less efficient than on earth, but
| evidently efficiency isn't proportional to density or else it
| could not fly at all.
| lmm wrote:
| > It must be less efficient than on earth
|
| No it mustn't. You'd expect efficiency to be higher, all else
| being equal, since there's less drag to overcome - aircraft can
| generally fly more efficiently at high altitude on Earth (up to
| a point). The limiting factor is that helicopter flight only
| works while the rotor tips are subsonic, so as the air gets
| thinner the "IAS" that a pitot tube on the rotor (that's going
| as fast as you safely can) would experience gets lower and
| lower, and so the flight dynamics will be like a helicopter
| with a slow rotor, but speeded up. But for a small helicopter
| the square-cube law is on your side and so a low "IAS" is quite
| doable.
| aero-glide2 wrote:
| The blades spin much faster to generate the required lift (lift
| proportional to density). Mars having lower gravity also helps.
| ldargin wrote:
| See https://www.wired.com/story/a-helicopter-ride-over-mars-
| nasa....
| drone_aero wrote:
| Using actuator disc theory, accounting for 38% gravity and 1.2%
| air density, Ingenuity would need about 2.1x more power to
| hover on Mars.
|
| But it's not that simple. The reynolds number which Ingenuity's
| blades are operating on Mars is extremely low, 10,000 to
| 20,000. This makes the airfoils less efficient compared to what
| you might design on Earth.
|
| Compare to a Trex 450 (30% heavier weight equivalent)
| helicopter on Earth, it's got 10+ minutes of endurance and has
| a smaller diameter rotor, whereas Ingenuity has a 90 second
| stated endurance, although that is probably limited by
| navigation too.
| zyxzevn wrote:
| Thanks for your answer. It is hard to find good information
| about the aerodynamics.
| amelius wrote:
| If you just want images from a different angle, why not tether it
| to the rover?
|
| Also can't you use a kind of balloon instead to save power?
| [deleted]
| coldcode wrote:
| I would think the atmosphere is too light to support a balloon.
| gambiting wrote:
| I do remember reading about this - a balloon would work, but
| it would be absolutely enormous, far larger than anything we
| can bring into Mars currently. Maybe some day.
| amelius wrote:
| Another thing I'm wondering: the drones can probably carry
| only a small (light) camera, whereas a satellite can carry
| much more and thus also a much better camera. Wouldn't a
| satellite+camera be a better option?
| rtkwe wrote:
| For a given aperture there's a limit to the angular
| resolution you can get which means you can only see
| objects above a certain size from orbit. The highest
| resolution we had in 2013 had a resolution of ~.3m/pixel.
| That's too large for anything beyond rough mapping for
| things like landing or a vague plan for driving.
|
| TL;DR a decent camera at <1km is hard to beat from >100km
| no matter how good the satellite camera is.
|
| https://space.stackexchange.com/questions/1140/whats-the-
| hig...
| zokier wrote:
| You mean like HiRISE:
|
| https://en.wikipedia.org/wiki/HiRISE
|
| or HRSC:
|
| https://en.wikipedia.org/wiki/High_Resolution_Stereo_Came
| ra
| nharada wrote:
| There actually have been airship proposals for Mars, but as
| you'd expect because of the tiny atmosphere they're...
| different:
| https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_...
| sdoering wrote:
| I find, besides all the extremely exciting, technical challenges,
| the following sentence incredibly fascinating:
|
| > "This [is] the first time we'll be flying Linux on Mars."
|
| But regardless of that, I find it so incredibly cool and
| inspiring that during my lifetime, a human flying vehicle will
| fly on Mars. I was a space nut in my childhood and youth and love
| to see this.
| neurostimulant wrote:
| Linux flying on Mars and Electron regularly launched to LEO.
| What a time to be alive :)
| Animats wrote:
| I'm amazed they were able to pack enough energy in the thing to
| get the props to go fast enough to work in Mars's barely there
| atmosphere. Keeping it upright is standard equipment in even low-
| end quadcopter drones.
| secondcoming wrote:
| Could they use it to try blow dust off the Opportunity rover's
| solar panels to see if it'll boot again?
| kam wrote:
| No, it's not going to land anywhere near Opportunity.
|
| https://mars.nasa.gov/resources/24729/map-of-nasas-mars-land...
| MayeulC wrote:
| I guess the atmosphere would be too tenuous to try to blow dust
| away with it (max pressure should be a bit more than
| Ingenuity's mass/rotor surface, so not much).
|
| How long before a commercial autonomous helicopter-based solar
| panel dusting service on Mars, I wonder?
| MayeulC wrote:
| s/mass/weight/
| happy-go-lucky wrote:
| > It's kind of an open-source victory, because we're flying an
| open-source operating system ...
|
| That is in reference to Linux. I think it can be a proud moment
| for Linus Torvalds and the team.
| londons_explore wrote:
| Seems shortsighted to end this mission after 30 days.
|
| I imagine it's purely for mission planning complexity reasons.
| But a more sensible approach seems to be to say "after 30 days
| you have to limit comms to 100 bytes/sec back to earth and stay
| 100 meters from the rover".
|
| In the future I could imagine the more modern processor on the
| helicopter might become handy... Some neural network based
| navigation system might run on it but not the rover main cpu...
| Also the helicopter might be able to get very good aerial photos
| that could be turned into 3d models to figure out the best spots
| to do science. It should be able to see much better than orbital
| radars.
| pridkett wrote:
| > Seems shortsighted to end this mission after 30 days.
|
| At this point, NASA is getting rather famous for this.
| Opportunity had a 90 day mission that survived for nearly
| fifteen years. Curiosity, a much bigger and more complicated
| machine, is still doing science 8.5 years later.
| throwaway0a5e wrote:
| The official mission length is just how long they have to wait
| before calling it a success. They expect it to last much
| longer. Since this is primarily a technology demonstrator once
| it's a "success" more options open up for actually
| experimenting with tactics and the limits of the device. It's
| like how you wait until you've left the parking lot to do a big
| smokey burnout and see how fast a rental goes in 1st gear.
| retSava wrote:
| So many interesting details in this article. Snapdragon 801-based
| hw running Linux, sensor fusion from three sensors whereof some
| bought COTS from sparkfun (who make breakout boards for sensors,
| basically the sensor datasheet recommended design). 3 flights
| planned, but potentially more which will be planned after the
| three first. 30 day lifetime window, due to dependecy on the
| rover, which needs to conserve resources.
|
| I would looooove to know more more more about this. What did dev
| of this look like (eg simulators)? What's the flight envelope?
| What are the most important risks and how are they mitigated?
| What internal discussions took place (do this, prio that, don't
| do X etc).
|
| Just so much interesting stuff! Can't wait :)
|
| edit: from wikipedia: "Each flight is planned to be at altitudes
| ranging from 3-5 metres (10-16 ft) above the ground.[1] In up to
| 90 seconds per flight, it could travel as far as 50 metres (160
| ft) downrange and then back to the starting area".
|
| "The helicopter uses counter-rotating coaxial rotors about 1.2
| metres (4 ft) in diameter".
|
| Etc. Recommend the wiki page on it:
| https://en.wikipedia.org/wiki/Mars_Helicopter_Ingenuity
| BelenusMordred wrote:
| I went down this rabbithole not long ago with the rovers on-
| board computer. The writeup from the people who designed the
| system was really approachable for someone without any
| aerospace experience. There's a lot of challenges and tradeoffs
| involved.
|
| Some takeaways:
|
| > Constrained CPU: 133MHz PowerPC, 128MB RAM, 4GB storage
|
| > Software written in C, OS is VxWorks
|
| > Only get contact a few times a day, rover must be charged and
| ready each time regardless of uncompleted tasks. Scheduling is
| hard.
|
| > Some tasks require parts preheated for X time, which depends
| on ambient temp, they use a lookup table for the time of day
| instead of measuring the temp directly. (Assume this is for
| reliability purposes)
|
| > High level activities are constructed in a GUI by the
| operations team, low-level tasks are written in XML, both are
| uploaded to the rover as a binary plan file.
|
| It's a good read, do recommend it.
|
| https://ai.jpl.nasa.gov/public/documents/papers/rabideau_iwp...
| milchek wrote:
| > Some tasks require parts preheated for X time, which
| depends on ambient temp, they use a lookup table for the time
| of day instead of measuring the temp directly. (Assume this
| is for reliability purposes)
|
| That's an interesting one. Guessing that there also must not
| be that much variation of temp and weather conditions?
| nuccy wrote:
| Actually the computing power of Ingenuity surpasses one of the
| Perseverance rover (and probably by a lot). The rover has a
| radiation hardened RAD 750 CPU (based on PowerPC 750
| architecture, which was introduced in 1997 to compete with
| Intel's Pentium II [1,2]) with 250-150nm technology process
| operating at 200Mhz[3]. Ingenuity's Snapdragon 801 can reach up
| to 2.5 GHz and is based on more energy-efficient 28nm modern
| (released in 2014) ARM architecture [4]. This CPU even has Wi-
| Fi and GPS, though probably not much of a use on Mars :)
|
| 1. https://arstechnica.com/science/2019/11/space-grade-cpus-
| how...
|
| 2. https://en.wikipedia.org/wiki/RAD750
|
| 3. https://mars.nasa.gov/mars2020/spacecraft/rover/brains/
|
| 4. https://www.qualcomm.com/products/snapdragon-processors-801
| interestica wrote:
| I haven't seen much on the 'sky crane' portion. It seems that
| it will just use whatever fuel it has left after the landing
| procedure to just get as far away as possible. It seems like
| the perfect place to stick some experimental imagery gear:
| film everything you can all the way down and transmit it back
| to Perseverance. And there's the possibility that it would
| survive its crash too with all hardware intact.
| mzs wrote:
| There are also two redundant TI MCUs on the copter
|
| >TMS570LC43x high-reliability automotive processor operating
| at 300 MHz, with 512 K RAM, 4 MB flash memory, UART, SPI,
| GPIO
|
| https://rotorcraft.arc.nasa.gov/Publications/files/Balaram_A.
| ..
| donquichotte wrote:
| Do you know why radiation hardening is not needed for the
| Ingenuity? Is there a mechanical shield? Or is it just the
| shorter runtime that makes the use of an off-the-shelf
| processor acceptable?
| jhurliman wrote:
| I had the opportunity to go down to JPL and speak with team
| members about this design decision. The space hardened
| processors are not fast enough to do real time sensor
| fusion and flight control, so they were forced to move to
| the faster snapdragon. This processor will have not flips
| on Mars, possibly up to every few minutes. Their solution
| is to hold two copies of memory and double check operations
| as much as possible, and if any difference is detected they
| simply reboot. Ingenuity will start to fall out of the sky,
| but it can go through a full reboot and come back online in
| a few hundred milliseconds to continue flying.
|
| In the far future where robots are exploring distant
| planets, our best tech troubleshooting tool is to turn it
| off and turn it on again.
| nynx wrote:
| I'm a little surprised they didn't go for three separate
| computers and compare them for every operation, or
| something like that, but I'm sure they have their
| reasons.
| teraflop wrote:
| I've never seen an off-the-shelf processor that has
| hardware support for doing that kind of cross-checking on
| every instruction. And doing it in software would
| probably add so much overhead that the error-checking
| would be much more likely to fail than the application
| code.
|
| If you're willing to relax your real-time constraints a
| bit, and risk a brief period of incorrect behavior before
| the error is caught, the problem becomes vastly easier
| and cheaper to solve.
| trhway wrote:
| >off-the-shelf processor that has hardware support for
| doing that kind of cross-checking on every instruction.
|
| it is usually done with COTS CPU by either running the
| CPUs in lockstep (the simpler early generations of CPU)
| or by inserting hardware checkpoints at various points
| like branches, by number of instructions, etc. A recent
| such commercial system was the triple Itanium from
| Tandem/NonStop(HP).
| jack_h wrote:
| There are the ARM Cortex-R series of processors which
| have two cores running in lockstep for fault tolerance.
| alfla wrote:
| Do you have a source for those fast reboots? It's running
| Linux after all
| chasd00 wrote:
| is Ingenuity running Linux? All of the flight controller
| software i've seen for autonomous drones don't use an
| operating system.
| lights0123 wrote:
| Yep, from the article:
|
| "This the first time we'll be flying Linux on Mars. We're
| actually running on a Linux operating system. The
| software framework that we're using is one that we
| developed at JPL for cubesats and instruments, and we
| open-sourced[0] it a few years ago. So, you can get the
| software framework that's flying on the Mars helicopter,
| and use it on your own project. It's kind of an open-
| source victory, because we're flying an open-source
| operating system and an open-source flight software
| framework and flying commercial parts that you can buy
| off the shelf if you wanted to do this yourself someday.
| This is a new thing for JPL because they tend to like
| what's very safe and proven, but a lot of people are very
| excited about it, and we're really looking forward to
| doing it."
|
| [0]: https://github.com/nasa/fprime
| nynx wrote:
| A few hundred milliseconds seems easily doable with a
| custom linux distro.
| yason wrote:
| Unless your /dev/sda wants a fsck. :-)
| RapidFire wrote:
| This is the coolest part of the mission IMO.
|
| My big question is where does it land? Does it dock with the
| rover? It seems to power itself via a solar array. About 100
| days into the mission they plan on launching the helicopter for
| the tiny lifespan window.
|
| I think the limit on its lifespan is really, "How long until
| destroyed by wind/dust". And then how long until it cannot
| charge its own batteries/sustain itself.
| foobarbecue wrote:
| I agree, this is the coolest part, but the most useful part
| for humanity is MOXIE.
|
| It lands on the ground. And yes, dust on the panels is a big
| concern. (I work at JPL but not on Perseverance / Ingenuity)
| noselasd wrote:
| There's a bit of info here too :
| https://www.youtube.com/watch?v=mQu9m4MG5Gc
| manicdee wrote:
| Veritasium did an episode 'Mars Helicopter' containing some
| interesting discussions with the team that built it.
|
| https://www.youtube.com/watch?v=GhsZUZmJvaM
|
| nm it was already linked elsewhere
| protoman3000 wrote:
| Unfortunately the article does not talk about how this helicopter
| would fare in a sandstorm. Are the winds strong enough to tip it
| over?
| SubMachineGhost wrote:
| Mars has 1% of earth's atmosphere, so it will not affect the
| vehicle, The answer comes from an interview with the engineers
| here : https://youtu.be/GhsZUZmJvaM?t=534
| londons_explore wrote:
| There is practically no wind on Mars. The wind speed is super
| high, but the air density is so low it won't impact anything
| more than dust.
| spaetzleesser wrote:
| This is really interesting to think about. On Venus it's the
| opposite. Any kind of wind speed will have a huge impact.
| zeristor wrote:
| One thing that I have been wondering, if the atmosphere is so
| thin, then how does the RTG cool? I seem to have nerd-sniped
| myself.
|
| It needs a temperature differential to generate electric current
| and it looks to have fins for convective cooling but the
| atmospheric pressure is a few % of that of sea level Earth.
| gbrown wrote:
| I believe the fins are primarily radiators. Conduction,
| convection, radiation.
| pengaru wrote:
| mars is freezing cold, it's like dust covered ice ball.
| iso1631 wrote:
| But the heat still needs to leak into the environment, and a
| thin atmosphere means little convection, so it's still just
| radiation - same problem as in a vacuum.
| nkrisc wrote:
| Space is cold and yet getting rid of waste heat is a
| challenge for any spacecraft. The ISS has giant radiators to
| dissipate heat. It doesn't matter how cold Mars is if it's
| difficult to transfer the heat away, such as in a very thin
| atmosphere which has less capability to carry away heat than
| Earth's atmosphere. There needs to be some kind of medium for
| the heat to transfer away (aside from radiation), regardless
| of ambient temperature. It's why a vacuum-walled thermos can
| keep things warm even in a cold environment. That's what they
| were asking about.
|
| Heat can be conducted away by contact with another object,
| probably not what you want if you're using an RTG to power
| your spacecraft, you don't want your RTG to heat it too much.
| It can be convected away, which is more difficult in a thin
| atmosphere like Mars has. Or it can be radiated away by
| blackbody radiation, which some certainly is, but that is
| limited and isn't much.
| trhway wrote:
| > blackbody radiation, which some certainly is, but that is
| limited and isn't much.
|
| Boltzman equation has temperature in the 4th power, so it
| is quickly becomes a lot.
|
| https://en.wikipedia.org/wiki/Thermal_radiation
|
| "Using the formulas below shows a human, having roughly 2
| square meter in surface area, and a temperature of about
| 307 K, continuously radiates approximately 1000 watts."
| joshvm wrote:
| You can read a bit about this in quite some detail in this pre-
| print from the 80s: https://www.osti.gov/servlets/purl/1033420
|
| ".. radiation, diurnal and seasonal temperature".
|
| "Convective cooling by the Martian atmopshere, even on a cold
| windy day makes only a relatively minor contribution to heat
| rejection"
| tomas789 wrote:
| I implemented a visual-inertial navigation based on the algorithm
| called MSCKF. It would be really interesting to know what kind of
| algorithm they are using. They mentioned state propagation in the
| article. This might indicate they are using an algorithm based on
| extended kalman filter.
|
| It takes quite some care to make those algorithms robust. Even
| when you run in an environment covered with diverse visual
| features. The state divergence is a real thing which essentially
| means the copter is falling from the sky (Do they call it like
| that on Mars?).
| blt wrote:
| Probably MSCKF.
| tpmx wrote:
| > With all this in mind, getting Ingenuity to Mars in one piece
| and having it take off and land even once is a definite victory
| for NASA
|
| Sorry, slightly off-topic: Not a native English speaker. Isn't
| this sloppy writing? Seems like declaring victory ahead of time?
| Nixinova wrote:
| The sentence is fine; they're saying the fact that it took off
| and landed successfully is a victory in itself.
| tpmx wrote:
| The probe containing the helicopter hasn't landed on Mars
| yet, though. It's landing in 44 minutes.
| neolog wrote:
| Then yeah it's bad or misleading.
| tpmx wrote:
| Thanks.
| nemof wrote:
| techpod did an episode recently with the engineering camera
| payload uplink lead from nasa jpl doug ellison. it was very cool.
|
| from the episode summary:
|
| > "Friend of the podcast Doug Ellison from NASA's Jet Propulsion
| Lab stops by to give us the lowdown on the newest Mars rover
| Perseverance, which will be landing on the red planet in just a
| few weeks, plus all kinds of fun info about Lagrange
| transceivers, making oxygen out of thin air, flying helicopters
| on other planets, and recording home movies at mach 25."
|
| https://techpod.content.town/episodes/71-curiosity-and-perse...
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(page generated 2021-02-18 23:01 UTC)