SEIS’s deployment on Mars (III)
Deployment of the first seismometer on the Martian surface by the InSight lander (part III)
In the first two parts of this series of articles on the deployment of the SEIS seismometer on Mars, we saw first the instrument’s levelling, then the centring of the VBB pendulums, and finally the different steps needed to correctly deploy the ribbon cable (tether) that connects the seismometer to the InSight lander. This third and final part focuses on the critical positioning of the wind and thermal shield (WTS), which was successfully completed on 2 February 2019 during sol 66.
SEIS is a particularly eye-catching instrument due to its hexagonal shape and copper-coloured remote warm enclosure box (RWEB), which goes well with the ochre shades of Mars. The WTS would nonetheless be a serious contender if there were such a thing as “the most handsome instrument” contest! The tight budgets allocated to the development of spaceborne equipment obviously do not allow aesthetic considerations to be taken into account, but in the case of the WTS, the response to a series of practical requirements actually led to an object with its own kind of rugged beauty.
Weighing some 10 kg and comprising a white 70-cm-diameter dome made of honeycomb aluminium and three retractable legs, the WTS has an extendable skirt designed to cover stones and other natural hazards on the surface so as to completely isolate the seismometer from Mars’ hostile environment, especially temperature variations and wind.
It is this extendable skirt that makes the WTS’ appearance so unique: from top to bottom, it is formed of a folded strip of gold-coloured insulating material bordered by a narrower strip of chain mail just like that used by medieval knights. One part of the chain mail is designed like snake scales (lorica squamata), these “scales” actually being small iron plates that partially overlap. The other part is a strip of more conventional chain mail. Combined with the seismometer’s sundial, which transports us back to Babylonian times, this chain mail confers on the SEIS experiment a truly symbolic meaning. The SEIS project thus manages to clearly unite space technology and the legends of humanity’s past.
During sol 62, the IDC took some stereo images of SEIS and the workplace where the WTS was to be set down. The goal was to characterize the position in 3D space of these two objects so that these details could be used to model the movements of the robotic arm, which was working blind (it being impossible to change in real time any of the arm’s movements in response to camera images). Many pictures were also taken of the service loop throughout the day to detect the slightest movement in the tether.
That same sol, the gripper was freed for the second time since landing. It had been stowed along the length of the robotic arm during sol 50 so as to leave room for the bucket to pull on the tether. During this step, some dust particles dropped off the gripper or its support and fell onto the IDC lens, rather like snowflakes. The dust does not pose any particular problem, however, and the few particles will be blown off by the wind or vibrations due to the arm’s motion.
Operational once again, the gripper was then very precisely positioned during sol 63 above the handle of the WTS, which was still solidly attached to the lander deck by a Frangibolt. After detailed verification of the gripper position data, the gripper successfully grabbed the WTS handle on sol 65, thus green-lighting the most critical part of the deployment sequence: placing the WTS over SEIS.
The engineers’ main goal in this phase was to make sure that the WTS did not touch the SEIS seismometer. There had to be a gap of at least 5 cm on average between the instrument and its protective shield. Positioning errors (dependent upon the precision of the robotic arm’s motors, the uncertainty in relation to SEIS positioning data, etc.) were estimated at around 3 cm. If contact was unavoidable, it was best to occur on the LSA service loop side rather than the RWEB itself.
Carried out on sol 66 (Saturday 2 February 2019) fully automatically, with no real-time control from Earth, mission seismologists were greatly looking forward to WTS deployment. By protecting the seismometer from temperature variations and wind, the WTS plays a key role in reducing noise to a level at which the slightest shivers reaching the Martian surface can be recorded.
The WTS is the last barrier in a series of protective devices designed to reduce as much as possible disturbances due to the Martian environment: it is in addition to the warm enclosure box (RWEB), the sensors’ evacuated sphere and devices at VBB pendulum level. It is also an emblematic step. When the instrument deployment arm lifts it off the lander deck, the WTS will expose InSight’s ultra-sensitive auxiliary payload sensor system (APSS) to the Martian environment for the first time. A little later, it will definitively hide the SEIS seismometer from our eyes and rule out any further use of the solar compass on the top of the RWEB.
WTS deployment operations began on 1 February 2019 at around 22:30 with the dispatch of the command sequence. On the Red Planet, InSight was scheduled to begin following the instructions at 9:50 local Mars time (sol 66), the first operation being activation of the Frangibolt attaching the WTS to the lander deck. Deployment with the robotic arm (IDA) began at around 10:00 (2 February 2019 at 03:00 in California) and lasted around one hour.
For safety reasons, SEIS had been switched off prior to WTS positioning operations. However, unlike for its own deployment on 19 December 2018 (sol 22), it was switched back on immediately after the WTS had covered it. The goal was to quickly check any 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 shield. During the analysis of the shield’s thermal insulation performance, SEIS was programmed to operate only a few hours per day for at least two sols. This is because once in place, the WTS stopped the Sun’s rays heating up the RWEB as they had up until then, so the instrument took longer to warm up. On the other hand, temperature losses were greatly reduced at night. As the project engineers did not know when the seismometer would be fully thermally stabilized now that it was covered by the WTS, it was not switched on continuously straight away.
deployment schedule for Mars, the time at which the different relay satellites orbiting around the Red Planet flew over the lander’s position and the available bandwidth, the engineers in charge of InSight’s deployment system had to wait until Saturday 2 February 2019 at 11:35 in California to receive the first images.Given the
Unlike for the deployment of SEIS, the preliminary telemetry data of use to the engineers, such as Frangibolt temperature or the robotic arm movement counter, were not able to be downlinked. The orbiter that could have transmitted this information—Mars Reconnaissance Orbiter—was too low on the horizon to be able to play its role optimally (if the overhead pass had been rescheduled, InSight could have opened this communication window to indicate that the arm had entered safety mode following an anomaly).
When the long-awaited images finally arrived, they were immediately retransmitted to all the screens in the InSight Mission Control Centre, in the downlinked data room (where JPL engineers were riveted to their consoles) and the main Control Room where most daily meetings are held and where many mission participants had gathered for the event.
Numerous tests had been carried out with InSight’s twin (the ForeSight engineering model) here on Earth prior to WTS deployment, but despite all the images acquired during these simulations, the arrival of pictures from Mars still came as a kind of shock. Firstly, because WTS deployment involved major risks, including colliding with the SEIS instrument. But it was also the visual impact of the photos that struck everybody present. Unlike the SEIS seismometer, which was deployed during the early twilight hours of sol 22, the WTS was deployed in the middle of the morning and the ICC benefitted from good light and shadows. What is more, it was placed not only on the surface of Mars, but over another object already on that surface. Finally, the fact that the extendable skirt was compressed during deployment emphasized its “flying saucer” look (an effect that would have been lost had the skirt been fully extended).
As soon as the white dome appeared in the field of the first image from the ICC, project engineers realised that the skirt had not deployed as it had during ground simulations. This was not that surprising. Compressed under the shield for six and a half months during the cruise phase in space at glacial temperatures, then on Mars for over two months, the skirt had become more rigid. Martian gravity being three times weaker than Earth’s gravity, the weight of the chain mail around the bottom edge of the skirt (designed to follow the ground contours to seal off the seismometer) was not enough for the skirt to drop down as soon as the robotic arm raised the WTS off the deck.
ICC photo received on the morning of Saturday 2 February (sol 66), the chain mail can clearly be seen to be unfolding. After analysis of the first seismic data from the SP sensors, mission seismologists and engineers realised that the noise level measured was not compatible with the skirt’s position as seen on the images of sol 66. They therefore assumed that the skirt had continued to unfold, a hypothesis that was later revealed to be true.This hitch was quite quickly considered as minor by the engineers in charge of InSight payload deployment. On the last
The photos taken during sols 67 and 68 show the skirt correctly unfolded, warmed by the Sun and weighted by the chain mail around its bottom edge. It had been decided not to release the gripper from the WTS handle just in case further operations to help the skirt unfold completely were necessary (raising the shield and gently shaking it). The disadvantage of this temporary situation was that the robotic arm was blocked in this position so the IDC could not be used to take detailed images of the skirt from a better viewing angle.
An analysis of the positioning data (using the black and white markers on the RWEB and WTS, which serve as geometric reference points) revealed that the WTS had been placed almost perfectly above the SEIS seismometer and that it was not touching the instrument. There was a gap of at least 4 cm between the two. The exact position of the WTS with respect to the seismometer would be refined using new images taken by the robotic arm’s IDC once the gripper had been released.
The inclinometers of the instrument’s levelling cradle (and the seismic pendulums) showed that the seismometer had not moved during WTS deployment. The shield’s thermal performance was observed to be very good, and the impact on component temperature variations clearly visible. The reduction in noise level is quite simply spectacular. Satisfied with these technical audits, project engineers gave their go-ahead to free the gripper during sol 70. This step, which marks the end of the physical deployment of the SEIS experiment on the Martian surface, also marks the beginning of operations to deploy the HP3 heat flow sensor. One of the main objectives of SEIS is to record the vibrations generated by the HP3’s mole as it burrows downward in order to determine the subsoil’s structure.
Final configuration of the SEIS seismometer
Once the WTS had been deployed, important operations were planned on the VBB pendulums located at the very heart of the SEIS instrument inside an evacuated titanium sphere. The first advantage of WTS deployment was in being able to switch the sensors to science mode, which is better suited to seismic measurements than the engineering mode used until then. However, the delay in deploying the tether connecting SEIS to the lander led to the decision to switch to science mode on sol 58, when the WTS was still on the deck. The second operation entailed calibrating the pendulums for the first time using a coil in the feedback system. Finally, the thermal compensation device mechanism (TCDM) was to be operated for the first time to reduce the sensors’ sensitivity to residual temperature changes inside the evacuated sphere. The SP sensors do not need any particular set-up following WTS positioning.
Under its protective dome, the SEIS seismometer’s capabilities could then be fully exploited. Once the WTS was in position, all the seismic sensors were able to be switched on and left on night and day to send back an uninterrupted data stream without fearing the extreme Martian temperatures. Up to the time of writing, SEIS was switched off during the coldest 10 hours of the night, and heaters were regularly activated to protect the instrument from the glacial temperatures. SEIS’s seismic sensors have been operating continuously since sol 70. The wind activity, which had been fully characterized when the seismometer was still on the lander deck, has now been very effectively reduced.
Once the final commissioning phase has been completed, the SEIS scientific campaign can begin. Nothing moving on Mars will then be able to escape the attention of InSight’s seismometer, and the Red Planet’s tiniest vibrations will finally reveal its innermost secrets.
The wind and thermal shield, WTS.