KArLE : In-Situ Lunar Geochronology

NASA DALI Program for Instrument Maturation

Summer 2020 - Summer 2021

The Potassium-Argon Laser Experiment (KArLE) is an instrument being developed by Barbara Cohen at NASA's Goddard Space Flight Center for precise in-situ lunar geochronology (determining the age of rocks on the Moon). Dr. Cohen has partnered with Honeybee Robotics, and I am the project lead and systems engineer responsible for the design, build, and qualification of the sampling system to TRL6.

KArLE's sampling system during end-to-end functional tests in vacuum.

Background

An age-map of the Moon is critical to understanding its formation and history, and by extension the history of the solar system. However, the precise radiometric dating required to build such a map is currently only possible in Earth-based laboratories, and gathering samples from across Moon and returning them to Earth is prohibitively complex and expensive. KArLE addresses this problem by combining several instruments with flight heritage into a single package that can perform in-situ radiometric dating to +/-10%. KArLE also lowers the cost barrier to age-dating rocks from sites across the Moon with an architecture that can be scaled compatibility with everything from New Frontiers-class rovers to CLPS landers.

KArLE's sample handling system (SHS) must select rocks from the lunar surface, use a visual inspection ("triage") to help determine the scientific value of each rock, cache and index individual rocks in a rotating carousel, and finally deliver each rock to the instrument by hermetically sealing it inside an analysis chamber. The instrument determines the age of the rock by firing a laser to ablate a small sample; the potassium content in the rock is then measured via laser-induces breakdown spectroscopy (LIBS), and any liberated argon is measured via mass spectrometry. The ratio of potassium to argon is used to determine the age of the rock in a process analogous to carbon dating (but using different elements).

Dr. Cohen with the KArLE breadboard (source).

Concept of Operations

KArLE's sample handling system (SHS) is compatible with a variety of sample acquisition tools. Nominally, Honeybee's PlanetVac is used to deliver lunar surface material (regolith) to the SHS using a puff of high-pressure gas (more on PlanetVac here). The interface between the SHS and the sample acquisition tool is the triage station, a novel system for cleaning, isolating, inspecting, and handling rocks.

Front of triage station.

Back of triage station.

Once regolith is dumped into the triage station, a vibrating mesh chute and a puff of gas remove small rocks and dust, leaving only rocks of the desired size (5-20mm). A rotating brush at the bottom of the chute moves the rocks over a small chamber, sized to hold just one rock. A single rock falls in to the chamber, while the rest are swept over it by the rotating brush and returned to the top of the chute. This process has been demonstrated to be highly reliable and robust to jamming. A window allows for preliminary visual inspection ("triage") of the isolated rock: during a mission, a photograph would be transmitted to Earth for scientists to evaluate. A cylindrical diverting valve on the back of the chamber rotates to either deliver or reject the rock, depending on the triage result, which is ejected with a puff of gas. This sequence of operations can be repeated to iteratively examine each rock in a sample.

1/6

SHS carousel and elevator assembly.

Delivered rocks are transported through a tube to sample cups on the SHS carousel, which is based on a heritage design from Honeybee's Sample Manipulation System on the Curiosity Rover. KArLE's carousel is an actuated, hubless ring holding 20x sample cups (each of which can hold a single rock) that rides along four rollers to precisely move sample cups from the triage station interface to the instrument interface. Once a sample cup is positioned at the instrument interface, it can be lifted from the carousel into the analysis chamber by the elevator. The elevator can apply up to 3200N preload to deform the copper gasket brazed on each sample cup against a titanium knife-edge on the chamber and create a gas-tight seal at all operating temperatures (-40C to +70C). The elevator is also based on a heritage design from the Sample Manipulation System, but includes a few modifications for improved system characterization (such as a load cell under each of the two synchronous leadscrews that lift and lower the cups for high resolution force characterization). Once the sample cup is sealed to the analysis chamber, the instruments can analyze the rock.

Complete sample handling system assembled on GSE platform for end-to-end testing.

Testing and Performance

Three test campaigns have been completed to verify the performance of the SHS: leak characterization of the knife-edge seal, thermal-vacuum (TVAC) qualification of the carousel and elevator, and demonstration of the end-to-end sampling chain (including PlanetVac) in a vacuum environment.

Leak Rate Characterization

A tight seal between the sample cup and analysis chamber is critical to KArLE's science goals. Radiometric dating requires an accurate measurement of the argon content of the sample, and any argon that escapes from the chamber will not be seen by the mass spectrometer. To verify the performance of the system, we used a helium leak detector (HeLD) to characterize the leak rate across the knife-edge seal between the sample cup and analysis chamber as a function of seal force at the cold (-40C), nominal (23C), and hot (70C) operating temperatures of the SHS.

The science requirement a leak rate no greater than 2.3E-6 atm*cc/sec He STD; the SHS achieved leak rates <3E-9 atm *cc/sec He STD at all temperatures. Thermal control of the system posed some unique challenges due to the complex interface, and measuring leak rates this small (at the bottom end of the HeLD's range) required iterative development of a detailed test procedure.

Ready for leak testing.

Pat (left) and Caleb monitoring the system during leak testing.

TVAC Qualification

The purpose of TVAC qualification is to verify the functional performance of the SHS at the relevant environmental conditions, and that the system meets or exceeds performance requirements in those conditions. Telemetry data collected during the test campaign was used to assess the health of the SHS and establish a baseline for nominal operation.

Over the course of five days, we completed 5x thermal cycles from -40C to 70C (<2E-5 Torr) and a total of 19x functional tests (ambient temperature at the beginning and end, as well as at least one test at each saturation temperature). While some outgassing was observed during the hot cycles of the test, the source is materials primarily used in the GSE such as Kapton tape and thermal grease. At the end of the five-day test, the system was at 2E-7 Torr (a record for that chamber). Thermal control posed a challenge during this test campaign as well due to the complexity of the interfaces in the SHS. Thermal isolation strategies such as Mylar shrouding (for radiation) and G10 standoffs (for conduction) were necessary to meet the thermal gradient and stability requirements.

Three faults were observed over the course of the test campaign; each was able to be root-caused and addressed without breaking the chamber seal or reworking any of the EGSE. We determined that these three issues (one worksmanship issue, one software bug, and one poorly-tuned control loop) were not fundamental flaws of the system so did not invalidate the technology demonstration (TRL6 claim).

TVAC qualification environmental data.

Thermally-isolated SHS ready for TVAC.

End-to-End Demonstration

The carousel and elevator (and EGSE) were to be delivered to NASA GSFC for integration into the rest of the instrument, so development of the triage station beyond the concept level had been descoped early on to help prioritize in unpredictable times (COVID-19). After qualifying the carousel and elevator in TVAC, however, we were sufficiently ahead of schedule that we could revisit that part of the program. Over the course of two months, the team took the triage station from concept sketches to a functional prototype (including EGSE and software), integrated it into the SHS, and tested the end-to-end system (including PlanetVac) in a vacuum chamber at ambient temperature.

This end-to-end test demonstrated the full functionality of the sampling system by sequentially walking through the entire concept of operations, from acquiring regolith in PlanetVac to selectively delivering rocks to sample cups in the carousel to sealing a rock in the analysis chamber. We designed the test for maximum visibility, using separate "dirty" and "clean" vacuum chambers with clear acrylic fronts, and filmed the entire sequence of operations over the course of three days. The footage is being used to make a narrated walkthrough of the sampling system, which will be released on YouTube and used as content during conferences and future proposals.

The team after finishing end-to-end testing (from left): Nate Jensen, Luke Thompson, Jack Emery, me, Kris Zacny, Pat Corrigan, Bernice Yen, and Caleb Lang.

Acknowledgements

Working to advance the KArLE instrument under the DALI program has been a unique opportunity to experience a remarkable range of the technology development cycle in a relatively condensed timeframe, and I'm grateful to all those who contributed:

Barbara Cohen for her vision and leadership, and the rest of the NASA team for their support and enthusiasm.
Kris Zacny and Stephen Indyk for providing their critical guidance, and for doing so while allowing me to maintain a high degree of autonomy.
Caleb Lang (mechanical), Jack Emery (electrical), Luke Thompson (software), and Pat Corrigan (mechanical) for their hard work, invaluable insights, flexibility, and patience. You didn't just make this project possible; you made it fun.
And to the rest of the team at Honeybee: Nate Jensen (analysis), Bernice Yen (program management), Hunter Rideout (mechanical), and Mary Tirrell (contracts) for successfully juggling their support of this project with all of their other responsibilities.

Publications

Development and Testing of a Sample Handling System for In-Situ Lunar Geochronology with KArLE. Costa JT, Lang CT, Corrigan P, Emery JW, Thompson LA, Jensen NA, Rideout HT, Indyk S, Yen B, Zacny K, Mullin M, Cattani F, Frese E, Stysley P, Cohen BA. Proceedings of the 2022 IEEE Aerospace Conference, Big Sky, MT. March 2022.

Paper, Presentation