InSight’s instrument drop-off area has been reproduced on Earth
Shortly after landing on November 26th, 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.
Publication date: 15/12/2018
General public, Observatories, Press, Research
Related observatories : InSight Observatory
Related themes : Earth and Planetary Interiors
Using the IDC camera on the robotic arm, the lander methodically scanned the sector dedicated to the SEIS seismometer and the HP3 heat flux sensor, sending a wealth of data back to the engineers and geologists.
Injected into various visualisation software programs, the information collected was used 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 best possible choice can be made as to where the SEIS seismometer will be installed.
However, the computer study of the placement sector was only part of the job. At the Jet Propulsion Laboratory (JPL) in California, a team of engineers has set up a complete test bench to recreate the Martian environment of the Elysium plain on Earth. A full-scale replica of InSight, called ForeSight, equipped with the entire deployment system (IDA robotic arm, ICC/IDC technical cameras, etc.) dominates an area in which a mineral material (crushed garnet) simulating Martian regolith has been dumped to a thickness of around 10 centimetres.
Once the characterisation of the landing site on Mars had been completed, the engineers set about ‘terraforming’ the sandbox. To reproduce the inclination of the lander (whose deck leans forward by around 4°), one of the probe’s feet was raised. More or less sophisticated techniques were then used to position the most important pebbles (all those larger than 2 cm) and shape the surface.
The installation area has been compartmentalised 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 heat shield (WTS). A slightly smaller area, marked off to the right, is the location of the HP3 heat flux sensor. Finally, the two surfaces are separated by a small passageway that allows engineers to move around the removal areas and reach the ForeSight lander.
While the engineers sometimes use a simple tape measure to take measurements, they also relied on the VICON system, widely used in the film and video game industries to capture movement. Sensors mounted on tripods surround the sandbox. Their role is to provide very precise information on the position, in all three dimensions of space, of small reflecting spheres that the engineers place on the instruments or rocks when necessary.
However, the team responsible for deploying InSight’s instruments is using even more advanced technology. To sculpt the terrain so that it corresponds exactly to its Martian counterpart, the engineers are using Microsoft HoloLens augmented reality headsets. Thanks to this device, holographic information is projected onto the sandbox, with a 3D model of the probe superimposed perfectly on the real structure of the ForeSight lander.
Controlled both by hand gestures and by voice, the helmets reveal information that is too subtle to be perceived correctly by the naked eye. Through the visors, slopes, hollows and bumps appear clearly in the form of coloured flat areas or multicoloured grids. By virtually projecting a digital model of the terrain onto the sandbox, the engineers know where they need to remove material, add material or smooth out a surface.
Once ForeSight’s Martian rock garden has been correctly modelled, the engineers load the command sequences onto the lander to be sent to Mars. To simulate Martian gravity, the IDA robotic arm is connected to a hoist, enabling it to be artificially supported.
The entire removal sequence is then performed, usually in front of a large audience of engineers and scientists involved in the mission, who are invited to come and watch the event through a large picture window overlooking the test bench.
With slow, sometimes graceful and sometimes jerky movements, the IDA robotic arm begins by positioning its grapple above the SEIS instrument. Its jaws then close on the small semi-spherical handle that sits on top of the hexagonal copper-coated thermal protection block (RWEB) that encases the sensors. Once SEIS has been captured by the arm, it lifts the instrument vertically, before moving backwards and then performing a complete half-turn on itself. The arm then extends to its maximum extension while lowering itself to slowly deposit SEIS on the ground. All that remains is for the grapple to open to release the seismometer completely, which remains attached to the lander by a sophisticated cable made up of 5 very rigid and independent ribbons. Once validated, the command sequence tested on Earth in the most realistic conditions just described will be transmitted to the Red Planet, to be executed fully automatically by InSight on site.
