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