Deployment of the wind and thermal protection shield, February 2, 2019 | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  Deployment of the wind and thermal protection shield, February 2, 2019

On March, the deployment of the WTS wind and thermal protection shield began just after the SEIS seismometer service loop was fully opened during sol 61.

 

The last seconds of visibility of the SEIS seismometer before its definitive disappearance under the protective bell of the WTS during a deployment test at the Jet Propulsion Laboratory with InSight's twin brother, the ForeSight landing gear (© NASA/JPL-Caltech/IPGP/Philippe Labrot).

During sol 62, the IDC camera collected a number of stereo images of SEIS, as well as the WTS drop zone. The objective is to be able to characterize in detail the position in space of the two objects and allow the precise and 3D modeling of the movements of the robotic arm, which operates here blindly (no real-time adaptation of the arm's movements in relation to the camera images can take place). A large number of images of the service loop were also acquired throughout the day to detect any cable movement.

 

During the same sol, the grapple, which had been stowed along the robotic arm during sol 50 to release the shovel and allow it to be used to pull the cable, was released for the second time since landing. Now operational again, the grapple was then accurately positioned during sol 63 above the wind and heat shield capture handle, located on the landing gear deck and securely attached by a frangibolt. After extensive verification of the grapple location data, the actual capture of the shield was successfully completed on sol 65, allowing the most critical step in the deployment sequence, the actual deployment of the shield over SEIS, to be initiated.

 

During this phase, the most important objective for engineers was to ensure that the WTS did not touch the SEIS seismometer at all. An average space of at least 5 centimetres must be maintained between the instrument and its protective dome. Positioning errors (due to the accuracy of the arm motors, uncertainties in SEIS location data, etc.) were estimated at about 3 cm. If contact were to occur with the instrument, it would be preferable for it to be located on the service loop side (LSA), rather than on the RWEB thermal enclosure.

 

Images of the IDC camera showing the WTS deployment operation during the sol 66

(© NASA/JPL-Caltech).

 

Deployment of the WTS

 

Carried out during sol 66 (Saturday, February 2, 2019) in a fully automatic manner, without any real-time control from Earth, the installation of the WTS was awaited with great anticipation by the seismologists working on the mission. By protecting the seismometer from temperature variations and winds, the shield is essential to achieve a noise level compatible with recording the smallest vibrations of the Martian surface.

 

The WTS is the last barrier in a series of protections designed to best neutralize disturbances in the Martian environment: it is in addition to the RWEB thermal protection, the spherical vacuum enclosure of the VBB sensors and other devices located at the clocks themselves. It is also an emblematic step. If, by being lifted from the landing gear deck by the robotic arm, the WTS will expose the ultra-sensitive pressure sensor of the InSight APSS weather station to the open air for the first time, it will also permanently make the SEIS seismometer disappear and prevent any further use of the solar compass located atop the RWEB.

 

For safety reasons, SEIS has been completely switched off for the installation of the WTS. However, unlike what happened during its deployment on 19 December 2018 (sol 22), it was reactivated almost immediately after the WTS was installed. The objective is to be able to check very quickly changes in the seismometer's inclination and position, and to acquire seismic data with the SP and VBB sensors to characterize the noise level under the protective bell. During the analysis phase of the shield's thermal insulation performance, SEIS was programmed to operate for only a few hours a day, for at least two sols. The presence of WTS has the effect of preventing the sun's rays from heating the RWEB, as it has done until now, so the instrument has taken longer to warm up. On the other hand, heat losses became much lower at night.

 

Analysis of the location data (using the black & white markers on the RWEB and WTS as a geometric reference) showed that the WTS was placed almost perfectly above the SEIS seismometer and that its inner wall does not touch the instrument. The space between the two objects is at least 4 cm. The exact position of the WTS in relation to the seismometer will be refined with new images acquired by the robotic arm's IDC camera, once its freedom has been regained following the grapple stall.

 

Part of the SEIS team, just after the arrival of images showing the deployment of the WTS in the InSight operations room at JPL. From left to right: Tom Pike (Imperial College), head of the SP short-lived sensors, Ken Hurst (JPL), systems engineer and SEIS technical expert, Philippe Lognonné (IPGP), scientific manager (PI) of the instrument and Eric Beucler (University of Nantes), seismologist. (© NASA/JPL-Caltech/IPGP/Philippe Labrot).

The inclinometers of the instrument leveling cradle (as well as the seismic pendulums) showed no movement of the seismometer during the WTS removal operation. The thermal performance is very good and the impact on temperature variations of the different seismometer components is clearly visible. Finally, the reduction in noise level is considerable. Satisfied with these technical assessments, the engineers gave the green light for the release of the grapple during the 70th sol. This step, which marks the end of the physical deployment of the SEIS experiment on the surface of Mars, also marked the start of the deployment operations for the HP3 heat flow sensor. One of SEIS' first objectives will be to record the vibrations generated by the penetration of the HP3 mole to determine the structure of the subsoil.

 

Final setting of the SEIS seismometer

 

Following the removal of the WTS, major operations are scheduled at the VBB seismic sensors, located in the heart of SEIS and enclosed within a titanium vacuum sphere. The first benefit of the presence of the WTS is the ability to switch these sensors to scientific mode, which is much more suitable for seismic measurements than the engineering mode used until now. However, due to the delay in installing the seismometer cable, the scientific mode was actually successfully activated since sol 58, while the WTS was still located on deck. The second operation will consist in calibrating the pendulums for the first time using a coil that is part of the feedback device. Finally, the TCDM device will be put into service to reduce the sensitivity of the sensors to residual thermal variations that affect the inside of the vacuum sphere. For short period SP sensors, no specific parameter setting is required following the placement of the WTS.

 

Under its protective dome, the SEIS seismometer will be able to be used to the maximum of its potential. Thanks to it, all the seismic sensors could finally be switched on permanently during day and night, to send back continuous data, without fear of periods when temperatures become too extreme. Until now SEIS was effectively turned off during the coldest 10 hours of the night, and heaters were regularly activated to protect the instrument from the cold. Since sol 70, SEIS seismic sensors are in permanent operation. Winds, whose activity was very well characterized when the seismometer was still on the landing gear deck, are now also very effectively controlled.

 

Once the final commissioning phase is completed, the SEIS science campaign can begin.

 

Part of the Jet Propulsion Laboratory (JPL) InSight Deployment System team, during a WTS removal test on the ground 18. Several members of the mission attend this full-scale rehearsal from the gallery overlooking the test bench (© NASA/JPL-Caltech/IPGP/Philippe Labrot).