Superconducting Gravity Gradiometer for Planetary Dynamics

Proposed Research. We propose to mature a superconducting gravity gradiometer (SGG) that measures all six components of the gravity gradient tensor for future planetary missions. A new and innovative design, based on three decades of development (mainly funded by NASA), gives a potential sensitivity of 0.1 mE Hz^(−1/2) in the frequency band of 0.1 mHz to 0.1 Hz (1 E = 10^(-9) s^(-2)). The measurement band and sensitivity can be optimally tuned in-flight during the mission by changing the resonance frequency of the SGG, which allows measurements of both high-resolution static gravity field for geological and geophysical investigations and time-variable fields for atmospheric and climate studies from the same mission. Relevance to NASA. The present study would help advance several goals and questions stated in the Planetary Science Decadal Survey 2013-2022, including (1) Characterization of planetary interiors to understand how they differentiate and dynamically evolve from their initial state, (2) Determination of the nature and evolution of the geologic processes that have created and modified the Martian crust over time. Key parameters of atmospheric mass transport dynamics on Mars and Venus can be constrained via accurate monitoring of temporal changes in their gravity fields. The internal structure and dynamics of Mars and Venus can also be better characterized by measuring static gravity fields much more accurately than at present. In sample return missions to small bodies (like OSIRIS-Rex and CAESAR), the body's gravity field must be surveyed from a relatively high orbit to enable a surface landing. Even from a high orbit, the SGG could greatly reduce gravity errors near the surface of the body. The SGG is a viable solution to reducing the cost, size and complexity of future planetary gravity missions, and thus accomplishing major programmatic goals of the Planetary Science Division. The SGG has a precision two orders of magnitude better than the electrostatic gravity gradiometer that was used on the GOCE mission and the atom-interferometer gravity gradiometers under development. The SGG requires only a single spacecraft and measures all components of the gravity gradient tensor and is more sensitive to short-wavelength gravity signatures compared to satellite-to-satellite tracking (SST). With its resonance frequency tuned to a lower frequency, the SGG is also competitive with the SST for long-wavelength signals. Proposed Methodology. The new SGG will employ three pairs of superconducting test masses, with their sensitive axes oriented orthogonal to each other. Each such pair is levitated on two precisely aligned superconducting tubes. By measuring differences in the motions of these test masses, using superconducting circuits, all six components of the gravity gradient tensor are simultaneously measured. Under PICASSO, a prototype single-axis SGG was constructed and demonstrated. Under MatISSE, we will develop a full-tensor SGG, and implement an in-situ axis alignment system by using persistent currents, which will enable common-mode rejection to 10^(−8). The SGG requires cooling to <6 K, and the goal for a space mission is to use a cryocooler to enable a mission lifetime of 5-10 years. The most significant issue for such a mission has been the potential for vibration from the cryocooler to couple into the SGG. There are two candidate cryocoolers for a future SGG mission that have successfully mitigated vibration: Northrop-Grumman's MIRI cryocooler and Creare's turbo-Brayton cryocooler. TRL. In PICASSO, we constructed and tested a prototype single-axis SGG in the laboratory. The proposed tensor SGG for MatISSE is a combination of three single-axis SGGs with in-situ sensitive axis alignment capability. The entry TRL of this instrument for MatISSE is 4. We will advance the tensor SGG to TRL 5 by the end of the project, by demonstrating adequate performance while testing it under simulated cryocooler vibrations.

Improved measurements of internal structures for future planetary science missions

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