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Jake Rosenthal

Mars 2020 and the Adaptive Caching Assembly: An Intern’s Perspective

Posted by Jake Rosenthal

12-10-2015 14:29 CDT

Topics: Mars 2020, personal stories, explaining technology

At dawn, the sun illuminates the mall of NASA’s Jet Propulsion Laboratory, welcoming the summer interns, who will work alongside some of the most brilliant minds in the world. To brush shoulders with engineers and scientists who have decades of experience is both intimidating and thrilling. However, it becomes apparent rather quickly that we are collaborators; the only competition among stargazers regards which of the stars we explore first.

My days at JPL, for the most part, began and ended in an unassuming lab, tucked away in the basement of an office building. My assignment was to test the materials and develop the technology necessary for the first planetary sample return mission in human history. The drastic and heart-wrenching difference between the success and failure of our mission depends upon the placement, durability, and strength of a piece of metal about the size of a U.S. nickel.

The proposed Mars 2020 rover mission will be the first human endeavor intended to store samples from the surface of Mars for potential return to Earth. The rover will host the Adaptive Caching Assembly (ACA), a subsystem responsible for retrieving and storing cylindrical sections of rock, or core samples, from the Martian surface.

Instruments selected for the Mars 2020 rover


Instruments selected for the Mars 2020 rover
On the mast are upgraded versions of instruments on Curiosity: Mastcam-Z (color, stereo, 3D, zoom-capable cameras); and SuperCam (upgraded version of ChemCam). On the arm are PIXL, an X-ray fluorescence spectrometer and imager, and SHERLOC, a Raman spectrometer and imager. RIMFAX is a ground-penetrating radar; MEDA is a meteorological package; and MOXIE will advance goals in in-situ resource utilization by producing oxygen from carbon dioxide.

The precursor of the ACA, the Sample Caching System, was designed to store all of the sample tubes in a single cache, or storage unit. However, after much deliberation, it was decided that in the context of a mission so vast in impact, the risk that the samples would be inaccessible by the subsequent mission would be far too great. With a sort-of ‘don’t put all your eggs in one basket’ mindset, the caching team designed the ACA to distribute the tubes in small piles across the surface of Mars. As a result, a secondary rover mission may be planned; it could trail its predecessor, picking up the samples left behind. Currently, the missions after Mars 2020 are spoke of in generalities.

For the first part of the caching sequence, the ACA works in tandem with the rover’s percussive drill. From the holding bay, within the body of the rover, an empty sample tube is extracted and inserted into the hollow drill bit. When the rover drills into the Martian surface, a core sample is forced into the tube. With a sample acquired, the tube is received by the ACA. A series of metal cylinders is then inserted into the tube, which is designed to clean the tube in preparation for sealing and hold the sample in place. Lastly, a seal is plunged into place, storing the sample for possibly the next twenty years. One distant day in the future, a scientist — perhaps not yet born — will receive a preserved piece of Mars, a perfect specimen of another world.

The seal is a shaped, metal token. Its size exceeds the inner diameter of the tube—just barely, but just enough—such that the tube expands. The seal must be unsurpassable by the science contained within the sample and exterior contaminants that may be encountered during interplanetary transportation. If a seal were to degrade after insertion or become askew in flight or be dislodged in Earth-bound freefall, the sample would lose much of its scientific value. Contamination would disallow scientists from distinguishing the native components of the sample and those portions that are Earth-born.

Throughout seal development and testing, helium atoms provide an analog for the chemical and geological constituents of a core sample. Helium atoms, as opposed to, say, nitrogen atoms, are used because they have relatively small atomic radii. If helium cannot seep through the seal, nothing can.

During its time on Mars, the coupled seal and tube will be vulnerable to the dramatic environmental events of the planet. Since the atmosphere of Mars is so thin (less than one percent the thickness of Earth’s atmosphere), little heat is trapped on the planet; most of it escapes before nightfall. When the temperature drops, the metal tubes and seals are susceptible to contraction, but not necessarily at the same rates, and the seal could be lost. To maintain the integrity of the sample, the seal must not weaken during the swings in temperature experienced between Martian days and nights. These changes only occur at the submillimeter level; however, even a tenth of a millimeter is an open field to an atom of helium—an avenue of contamination and means of science loss.

To test sealing in variable thermal environments, I fixed a sealed tube inside a thermal chamber. As the temperature rose to a preset maximum temperature and fell to a minimum temperature, I ejected helium onto the seal. A Helium Leak Detector, a device which counts helium atoms, then outputs the number of atoms which seep through the seal. Using Excel spreadsheets and MATLAB, I compiled plots which displayed how the leak rate varied with temperature. The data was often a subject of discussion at weekly team meetings.

Another complication involved in sealing is due to the drilling process. When the rover’s drill bit, with the sample tube held inside, is plunged into the Martian surface, the interior of the tube is abraded by the coarse rock and coated in dust. Within the walls of the tube, riddled with peaks and valleys, the seals must sit unhindered and secure.

The ability to circumvent abrasion and dust adhesion depends upon the geometry and composition of the circumferential ridge of the seal, referred to as the tooth. I performed tests with varying tooth specifications against dusted sealing surfaces, and used the Helium Leak Detector for assessment as before. For future testing, it is under consideration that a soft metal might conform to the jagged walls and fill in the gaps. However, such a metal may decrease the force between the seal and the tube, thus adversely affecting the seal, or it may destroy the seal altogether. When a tube is sealed, hundreds, perhaps thousands of pounds of force will be generated. Another test which I conducted involved applying a load to teeth of different geometry and analyzing deformation with the use of MATLAB. It is evident that the tooth must be exceptionally strong to withstand the load it must bear. Therefore, the seal could end up having a robust core to provide the sealing force, with a soft outer coating to counteract abrasion.

As each day came to a close, I prepared for the next day’s assortment of testing and data analysis. The old test pieces were exchanged for new ones, and the machinery was set up as to proceed without delay. While the sun set behind the mountains, I sensed my steps slowing with each passing second, and I fought the urge to turn back. Our journeys to understand the universe comprise an unfolding story. There is perhaps nothing more difficult than being left to wonder, even for a moment, what comes on the next page. There is no telling what we might find.

Over ten weeks, I took part in humanity’s next step toward potentially finding life on Mars or the signatures thereof. The task is challenging and complex, and will require much more deliberation, development, and testing. But every moment of the process is just as exciting as the last. Because the only thing more difficult than finding life on another planet is imagining how the world will change when we do.

My gratitude to those with whom I worked is inexpressible. It was a privilege to be an intern for NASA and to work on a project so immense in its significance. I am proud to be an interplanetary ambassador for the human species, going in peace for the joy of discovery.

See other posts from October 2015


Or read more blog entries about: Mars 2020, personal stories, explaining technology


Arbitrary: 10/13/2015 03:51 CDT

Well well well, mister intern, this concept does not make sense. A rover which drops its samples along its road to be picked up by another rover driving in its tracks. Then what's the point with the first rover? It is not as if MSL has drilled hundreds of times, is it? So that it has gotten heavy by all the drill samples it collected. So you mean to suggest that a second rover should follow the tracks of the first rover. That would take a few years of driving. Two rovers doing the same thing on the same place on Mars... There are so many things wrong with this concept that if I were your supervisor, I would have to tell you that you'd better look for another kind of job. The 2020 rover obviously should collect its drill cores in a cache hanging off its side easily collectible by a small fast simple fetch rover (from a stationary sample return vehicle) even if the 2020 rover itself is dead by then. It is a no-brainer, and this proposal is worse than that! That's a FAIL in my book. NASA should (and will) not spend billions of bad ideas. Your work on sealing Mars samples can be just fine, I think and hope so, but you should not publish texts which connect you with idiotic concepts I hope you did not conceive.

Paul McCarthy: 10/14/2015 03:28 CDT

Surely it is better odds (I've claimed before) to keep part of each sample on board and deposit half on the ground as currently planned, for the apparently quite reasonable possibility (judging from Opportunity and Curiosity) that Mars2020 can/will survive long enough to itself deliver samples to a sample retrieval device on the return vehicle? Only if Mars2020 couldn't do this, or retrieval failed, would the second very expensive (retrieval) rover be sent for the ground samples. Surely this roughly doubles the odds of success and potentially vastly reduces the cost, for little greater initial outlay. We don't need to get into quibbles about supposed difficulties splitting samples etc. Let's face it, EVERY sample, or tiny fraction thereof, will be one of the greatest revelations ever. So all that's needed to improve odds and potentially lower costs is, by hook or by crook, keep any sort of retrievable (sealed) samples on Mars2020 and ALSO dump any sort of retrievable (sealed) samples on the ground. Doesn't matter if they're anywhere near half and half, or even from the same samples or drill holes. Seems insane to wilfully write-off completely Mars2020's fractional potential as a sample transporter to the return vehicle.

Karen: 10/14/2015 06:51 CDT

Agreed, @Arbitrary. I can just imagine if NASA sent someone to the grocery store: "Hey, I want some things from the store. But I don't have the money right now to bring them home. So I'm going to send you to the store to pick up a bunch of items and scatter them in bags all over the store. Then later when I actually have money I'll send someone to go retrace all of your footsteps and pick up the bags and actually bring them home..." Seriously, what sort of stupid logic is this? It's just pretending to actually be doing something useful. "As a result, a secondary rover mission may be planned; it could trail its predecessor, picking up the samples left behind. Currently, the missions after Mars 2020 are spoke of in generalities." You Never, Ever Do This. This is otherwise known as "throwing away money". Either you have a plan or you don't, you don't just say "we'll design something that may or may not be useful for a mission we haven't even thought out yet, let alone found funding for"

Pete Waydo: 10/14/2015 03:18 CDT

Well Jake, welcome to the Internet and its resident genius experts! Excellent article distilling highly complex and nuanced topics into digestible bites, even if some aren't up to the task. Don't worry, as your supervisor I am NOT going to tell you to look for another kind of job! Keep up the good work. I'm looking forward to your next installment.

David Frankis: 10/14/2015 05:06 CDT

I see Pete Waydo beat me to it. I'm embarrassed to be a member of the Planetary Society by the condescending tone and ignorant arguments of the comments above. Van Kane did an excellent job on this blog on 8th May explaining why NASA adopted this strategy for Rover 2020's sample caching. Thank you for this article, Jake.

Karen: 10/14/2015 08:48 CDT

@David Frankis I read the previously given justifications for the Mars 2020 rover's caching strategy. I found it ridiculous then as well - turning the already questionable activity of caching samples for an undesigned and unfunded return mission into a farce, wherein you have to have the *actual* sample return mission include a rover nearly as complex as Mars 2020 (minus the science hardware) just to collect what Mars 2020 left behind (long-range roving capability, communications, cameras, robotic arm that can deliver to internal storage bays, etc). If you're doing that, it makes no sense whatsoever to not just have that robot take the samples with its arm in the first place. Heck, if a sample return mission ever does get funded, and they don't for some reason or another decide to scrap the idea of collecting the caches, they probably will have a way for the rover to take samples on its own anyway - even if under non-ideal conditions and as a last resort - as a backup in case cache collection goes awry. It would be stupid not to. Stop wasting money. If you want a sample return mission, plan out the whole sample return mission, fund it, build it and launch it. Don't bundle this "trust us, we're doing sample return!" theatre with a science mission.

Paul McCarthy: 10/15/2015 08:00 CDT

I don't agree with those above who harshly claim that depositing cache samples on the ground is stupid or a waste of time. And yes, Jake gives nice info about it. But Karen, in her second post, also raises a valid point about ensuring overall success: probability will certainly be higher if the sample return rover has sampling capability itself. But that just further confirms my argument above: this whole adventure is one where a "belts and braces" approach will add only minor cost, and is immensely justified. It is perfectly clear that overall odds of success are greatly increased by building two rovers with similar skills: sampling capability plus sample delivery. The first rover ground-caches some samples, and retains and seeks to deliver some other samples. Only if sample delivery fails is the second rover launched. This could save one entire, very expensive mission. And the additional cost of including both ground-caching and sample delivery skills on each rover would surely be minor, especially if both rovers are constructed very similarly. Surely some "belts and braces" approach like this (there is great room for variation) yields a much higher overall probability of success, at little more than the cost of the two currently envisaged missions?

Karen: 10/15/2015 09:37 CDT

If it were to be written into the mission plan that after the clear majority of the potentially useful science in the area is done, or when the rover's ability to rove is becoming degraded, that the rover will go into hibernation mode at a potential landing site with the best samples still on-hand and await the arrival of a (roverless) return rocket, with intent to then be woken up and load the rocket... if that was the plan then I could get behind the concept - sort of**. But I've seen no evidence so far that this is part of the plan. Without that being design intent, all this caching system is doing is taking up weight and space that could have been used by equipment conducting actual science. ** The dropping of sample caches still makes no sense, though. Because even if your initial rover becomes disabled and can no longer deliver the samples it has on hand to the future return vehicle, and thus you have to send a second "collector" rover, a collector rover that only has to move up to the disabled Mars 2020 rover and retrieve the samples from it would be a much simpler device than one that has to survive roaming dozens of kilometers in the footsteps of Mars 2020. Really, I just don't get the logic of dumping samples at all. The core samples are said to be about the size of a piece of chalk each. So they're certainly not going to bog down the rover weightwise if you have to carry them all. You have to ship in the canisters to hold the samples whether you dump them or not - and if you dump them you have to additionally ship in whatever bundling hardware holds them all together and protects them. So since you have to ship them anyway, it doesn't help you save mass or volume to have dumping as part of the plan.

Karen: 10/15/2015 09:41 CDT

The logic presented earlier for dumpable caches was basically "if you have room for 30 samples, and mission success is defined as 20, you'll be tempted to fill up the 20 quickly and be really stingy with the remaining 10 - whereas if you dump caches, you can hit your mission success, dump, and keep going and not feel the need to either rush or hold out." But there's a huge gap in that logic - if you have empty dumpable caches on-hand that can hold vastly more than the aforementioned 30 samples, then you're not comparing apples to oranges. If you have 200 dumpable cache slots onhand, then you should compare to a rover that has 200 non-dumpable cache slots onhand. Wherein that whole "30 sample limitation" problem vanishes.

fichtner: 10/19/2015 06:50 CDT

Awesome work Jake. I'm not agreeing with the above comments. This article was about your role as an intern. Congrats. Weather or not NASA is wasting their time not picking up samples with the same Rover or not is a discussion for NASA. I find it amazing that you are so young and filling these big shoes as a scientist. Testing leaks and working on the drill bit scenarios. I wonder if they happen to consider a pilot hole for the drilling or pre drilling with a smaller bit 1st. It's really interesting how you needed to consider the temperature variances and helium releases. What a great time you must have had filling your duties. I'm very proud of you Jake. I'm sure you're going to learn much about space and contribute to finding so many answers to so many questions. It's a limitless field you are entering. Congratulations. Shoot for the stars. Maybe next year you can suggest to NASA that they have a drone pick up the samples and save them an expensive trip in 20 years. lol Great job..

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