The InSight instrument placement area has been reproduced on Earth | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  The InSight instrument placement area has been reproduced on Earth

InSight's twin brother, named ForeSight, on the Jet Propulsion Laboratory test bench, with the SEIS seismometer deployed on the ground, in sol 18. The placement area was divided into three compartments. The small black pointer on the right indicates the future location of the HP3 heat probe (© NASA/JPL-Caltech/IPGP/Philippe Labrot)

Shortly after landing on 26 November, the InSight probe began to map in detail the few square metres of south-facing terrain just in front of the robotic arm, which will be used to deploy its two main instruments. Using the robotic arm's IDC camera, the lander methodically scanned the area dedicated to the SEIS seismometer and HP3 heat flow sensor, returning a wealth of data to engineers and geologists.

 

Inserted into various visualisation software, the information collected made it possible to build an extremely accurate digital terrain model, reproducing the smallest details of the surface. Every stone has been identified and numbered, the slopes measured, and the roughness determined, so that the location where the SEIS seismometer will be installed can be chosen as optimally as possible.

 

However, the computer study of the placement area was only part of the work. At the Jet Propulsion Laboratory (JPL) in California, a team of engineers have set up a complete test bench to reconstruct the Martian environment on Earth on the Elysium plain. A life-size replica of InSight, called ForeSight, equipped with the entire deployment system (IDA robotic arm, ICC/IDC technical cameras, etc.) dominates an area in which mineral material (crushed garnet) simulating Martian regolith has been dumped to a thickness of about 10 centimetres.

 

As soon as the characterisation of the landing site was completed on Mars, engineers worked to ‘terraform’ the sandbox. To reproduce the angle of the lander (whose deck tilts forward by about 4°), one of the probe's feet was raised. More or less sophisticated techniques were then used to position the largest stones (all those whose size exceeds 2 cm), and to model the surface.

 

The installation area was divided into three parts, using pieces of wood. The area just in front of the robotic arm is reserved for the SEIS instrument and its wind and thermal shield (WTS). A slightly smaller part, delimited to the right, corresponds to the location of the HP3 heat flow sensor. The two surfaces are separated by a small passage that allows engineers to move around the landing areas and reach the ForeSight landing gear.

 

While engineers sometimes use a simple tape measure to take measurements, they have also relied on the VICON system, widely used in the film and video game industries to capture motion. Sensors mounted on tripods frame the sandbox. Their role is to provide very precise information on the position, in the three dimensions of space, of small reflective spheres that engineers place when necessary on instruments or rocks.

 

Marleen Sundgaard (system engineer responsible for the InSight test bench at JPL) and Tom Hoffman (InSight project manager at JPL) monitoring the reconstruction operations of the SEIS and HP3 instrument placement site on the ForeSight test bench, in sol 15 (© NASA/JPL-Caltech/IPGP/Philippe Labrot)

However, the InSight instrument deployment team uses even more advanced technology. To sculpt the terrain so that it corresponds exactly to its Martian counterpart, engineers are equipped with Microsoft HoloLens augmented reality headsets. With this device, holographic information is projected onto the sandbox, with a 3D model of the probe then perfectly overlapping the real structure of the ForeSight landing gear.

 

Controlled by both hand gestures and voice, the headsets reveal information too subtle to be properly perceived by the naked eye. Through the visors, the slopes, hollows and bumps appear clearly in the form of coloured flat areas or multicoloured grids. By virtually projecting a digital terrain model onto the sandbox, engineers know where to remove equipment, add new equipment, or flatten a surface.

 

Once the ForeSight Mars rock garden has been properly modelled, engineers load the lander with the control sequences to be sent to Mars. To simulate Martian gravity, the IDA robotic arm is connected to a hoist, which allows it to be artificially supported. The entire placement sequence is then performed, usually in front of a very large audience of engineers and scientists involved in the mission and invited to come and watch the event through a large bay window overlooking the test bench.

 

The SEIS seismometer of the test bench topped with VICON spheres (sol 18). Note the small sphere on the handle (© NASA/JPL-Caltech/IPGP/Philippe Labrot)

With slow movements, sometimes graceful and sometimes jerky, the IDA robotic arm starts by positioning its grabber above the SEIS instrument. It then closes on the small semi-spherical handle mounted on the hexagonal copper thermal protection box (RWEB) that surrounds the sensors. Once the arm captures SEIS, the instrument is first lifted vertically, before the arm goes backwards and then makes a complete U-turn on itself. The arm then reaches out to its maximum extension while lowering down to slowly drop SEIS to the ground. All that remains for the grabber to do is to open and completely release the seismometer, which remains connected to the landing gear by a sophisticated cable composed of 5 very rigid and independent tethers. Once approved, the control sequence tested on Earth under the realest conditions possible described above will be transmitted to the Red Planet, and then executed fully automatically by InSight.

 

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