Removal of wind and heat shield
On Mars, deployment operations for the WTS wind and heat shield began just after the SEIS seismometer service loop was fully opened on sol 61.
Publication date: 02/02/2019
General public, Press, Research
Related observatories : InSight Observatory
Related themes : Earth and Planetary Interiors
During Sol 62, the IDC camera collected a number of stereo images of SEIS, as well as of the WTS drop zone. The aim was to be able to characterise in detail the position in space of the two objects and to enable precise 3D modelling of the movements of the robotic arm, which is operating blind here (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 the slightest movement of the cable.
During the same sol, the grapple, which had been stowed alongside the robotic arm during sol 50 to free the shovel and enable it to be used to pull the cable, was released for the second time since landing. Now operational again, the grapple was then precisely positioned during sol 63 above the capture handle of the wind and heat shield, located on the lander deck and securely attached by a frangibolt. After a thorough check of the grapple location data, the actual capture of the shield took place successfully on sol 65, enabling the most critical stage of the deployment sequence to be undertaken, the actual removal of the shield from above SEIS.
During this phase, the most important objective for the engineers was to ensure that the WTS did not touch the SEIS seismometer at all. An average gap of at least 5 centimetres had to be maintained between the instrument and its protective dome. Positioning errors (due to the accuracy of the arm motors, uncertainties in the SEIS location data, etc.) have been estimated at around 3 cm. If contact were to occur with the instrument, it would be preferable for it to be on the side of the service loop (LSA), rather than at the level of the RWEB thermal enclosure.
Removal of the WTS
The removal of the WTS was carried out fully automatically on Sol 66 (Saturday February 2nd, 2019), without any real-time control from Earth, and was eagerly awaited by the seismologists working on the mission. By protecting the seismometer from variations in temperature and wind, the shield is essential for achieving a noise level compatible with recording the tiniest vibrations of the Martian surface.
The WTS is the final barrier in a series of protections designed to neutralise disturbances from the Martian environment as effectively as possible: it joins the RWEB thermal protection, the spherical vacuum enclosure for the VBB sensors and other devices located on the pendulums themselves. It is also an emblematic step. By being lifted from the lander’s deck by the robotic arm, the WTS will expose the ultra-sensitive pressure sensor of InSight’s APSS weather station to the open air for the first time. It will also permanently remove the SEIS seismometer and prevent any further use of the solar compass located at the top of the RWEB.
For safety reasons, SEIS was completely switched off for the removal of the WTS. However, unlike when it was deployed on December 19th, 2018 (sol 22), it was switched back on almost immediately after the WTS was in place. The aim was to be able to check changes in the inclination and position of the seismometer very quickly, and to acquire seismic data with the SP and VBB sensors to characterise the noise level under the protective bell. During the phase of analysing the shield’s thermal insulation performance, SEIS was programmed to operate for only a few hours a day, for at least two floors. The presence of the WTS had the effect of preventing the sun’s rays from heating the RWEB, as they had done until now, so the instrument took longer to warm up. On the other hand, heat loss became much less significant at night.
Analysis of the location data (using the black & white markers on the RWEB and the WTS as a geometric reference) has shown that the WTS has been positioned 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 using new images acquired by the IDC camera on the robotic arm, once it has regained its freedom following the release of the grapple.
The inclinometers on the instrument levelling cradle (as well as the seismic pendulums) showed no displacement of the seismometer during the WTS removal manoeuvre. Thermal performance is very good, and the impact on temperature variations of the various components of the seismometer is clearly visible. Finally, the reduction in noise levels is considerable. Satisfied with these technical assessments, the engineers gave the go-ahead for the grapple to be released during Sol 70. This step, which marks the end of the physical deployment of the SEIS experiment on the surface of Mars, also marked the start of operations to remove the HP3 heat flux sensor. One of the first objectives of SEIS will be to record the vibrations generated by the penetration of the HP3 mole in order to determine the structure of the subsurface.
Final configuration of the SEIS seismometer
Following the removal of the WTS, major operations are scheduled on the VBB seismic sensors, located at the heart of SEIS and enclosed inside a vacuum titanium sphere. The first benefit of the presence of the WTS is to be able to switch these sensors to scientific mode, which is much better suited to seismic measurements than the engineering mode used until now. However, following a delay in the installation of the seismometer cable, scientific mode was in fact successfully engaged as early as ground 58, while the WTS was still located on deck. The second operation will consist of calibrating the pendulums for the first time using a coil forming part of the feedback system. Finally, the TCDM device will be commissioned to reduce the sensitivity of the sensors to residual thermal variations affecting the inside of the vacuum sphere. As far as the SP short-period sensors are concerned, no specific settings are required following installation of the WTS.
Under its protective dome, the SEIS seismometer can be used to its full potential. Thanks to it, all the seismic sensors have finally been able to be switched on continuously, day and night, to send back continuous data, without fear of periods when temperatures become too extreme. Until now, SEIS was effectively switched off during the 10 icy hours of the night, and heaters were regularly activated to protect the instrument from the cold. Since sol 70, SEIS’s seismic sensors have been operating continuously. The winds, whose activity was very well characterised when the seismometer was still on the lander’s deck, are also now being countered very effectively.
Once the final commissioning phase has been completed, the SEIS scientific campaign can begin.
