Vacuum solution approach for the SEIS instrument on the InSight mission to Mars
Abstract
The SEIS seismometer on the NASA InSight mission to Mars is of paramount importance for the surface-based geophysical investigation of Mars. The instrument contains three Very-Broad-Band (VBB) sensors [1], which enable the determination of the origin and magnitude of earthquakes, and aid the understanding of the planet structure. Due to the low mass, which couples to the seismic waves, the instrument has an inherently high sensitivity to the ambient pressure, which causes viscous damping and increased noise through heat transfer. Vacuum of <0.01 mbar is required for optimum performance and 0.1 mbar is considered the end-of-life pressure limit. In 2015, the instrument encountered anomalies that prevented achieving such vacuum levels. An air leak was first discovered and, subsequently, the non-evaporative getters were found to have insufficient capacity to cope with the instrument outgassing rates. Due to the insufficient time to resolve these issues, the InSight launch was postponed for 2018 [2]. The Evacuated Container (EC), the housing of the SEIS instrument, was redesigned to address the vacuum anomalies. The vulnerability to physical leak was alleviated on component level. Mitigating the outgassing rate requirement necessitated the development of novel zeolite-loaded aerogel (ZLA) getters [3]. Such materials were devised, tested and implemented in place of the original non-evaporative getters. The getter adsorption requirements were derived from residual gas analysis (RGA) spectra, taken during the SEIS bakeout events in 2015, as well as from a VBB outgassing test conducted in 2016. At the 117 _C bakeout temperature, the spectrum was dominated by volatile organic materials, originating from an incompletely outgassed adhesive with a 105 °C glass transition temperature, whereas water was the overwhelmingly dominant component of the constituents of the RGA spectra at room temperature. Therefore, water became the primary adsorbent target for the new getters. The other measured components, H _{2}, CO and CO _{2}, can also be explained by byproducts of water breakup at the hot tungsten filaments. However, CO _{2} can also be a cross-linking byproduct of adhesives, and titanium and stainless steel alloys contribute H _{2}, albeit at extremely small rates, reduced through annealing. These adsorption needs were addressed as a precautionary measure. CO _{2} adsorption capability was also implemented in lieu of a redundancy mechanism for the mitigation of a potential Mars atmospheric leak during the mission. Water and CO _{2} adsorption capabilities were provided by custom-developed ZLA getters, made by incorporating Na ^{+} and Ca ^{+} ion-exchanged faujasite zeolite micron-sized particles into an aerogel matrix, which facilitated molecular transport of the adsorbent to the zeolite. The ZLA getters were prepared from a liquid precursor, solidified into six metal getter canisters, engineered into the space allowed for the six original SAES getters. The canisters were open to the EC volume through a 1 μum filter to eliminate particulate contamination form the getters. Validation and verification tests with assembled canisters were done to assess the level of water desorption from the getters at the bakeout temperatures, as well as to evaluate the getter adsorption capacity. Accelerated test conditions utilized water vapor as a proxy for outgassing at significantly higher rates. Due to the inability to complete the testing of Pd ^{+} exchanged zeolite on time for InSight to be added to the ZLA mixture for H _{2} adsorption, the latter was addressed by SAES Ti/Pd getter films, deposited on modified thermal shields, welded to the shells of the Evacuated Container. The capacity of the Ti/Pd getters was tested at the manufacturer. Details of the verification and validation program for ensuring a viable vacuum solution for InSight are presented. The SEIS crown, containing the three sensors and the six getter canisters was first baked to reduce the outgassing from fresh adhesives. After welding the shells, whose fabrication included a 320 °C anneal step to minimize H _{2} outgassing, the hermeticity of the complete SEIS instrument has been tested and verified to the state-of-the-art level (<1x10 ^{-10} mbar.L/s He) in the full temperature range from 105 °C to +60 °C. The final instrument bakeout was done at 100 °C for 10 days. A satisfactory initial pressure of 3x10 ^{-5} mbar, measured by the pressure rise method, was achieved at room temperature, and the instrument was separated from the ground pumping system by pinching off the connecting copper tube (queusot). After the pinch-off, there is no possibility for a direct pressure measurement inside the instrument for levels below ∼ 0.002 mbar; a heat-transfer method was developed by CNES to measure higher pressures. The most recent in situ pressure measurements have not yet detected a pressure increase. A worst-case SEIS pressure forecast, constructed on the basis of getters test data, exceeds the mission requirements. Given the SEIS thermal conditions during the mission, the expected vacuum operational VBB environment is, expectedly, <10 ^{-5} mbar for a duration far exceeding that of the primary mission. Furthermore, the ZLA getters have the capacity to deal with a significant atmospheric leak on Mars should such a necessity arise. [1] P. Lognonné et al., "InSight and Single- Station Broadband Seismology: From Signal and Noise to Interior Structure Determination", in Lunar and Planetary Science Conference, Lunar and Planetary Inst. Technical Report, 43, (2012) p. 1983. [2] W.B. Banerdt et al., "The InSight Mission for 2018", Lunar and Planetary Science XLVIII (2017), p 1896. [3] M.P. Petkov et al., "Zeolite-loaded aerogel getters as a primary vacuum sorption pump in planetary instruments", in preparation for publication.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E1292G