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IPGP’s final tests on the seismic sensors of the SEIS Martian seismometer on the InSight mission

On October 5th, 2017, the Institut de Physique du Globe de Paris (IPGP) reached another key milestone for the InSight mission, delivering the spare model of the SEIS seismometer sphere (protective enclosure and its three associated broadband sensors) to CNES, five months after the delivery of the flight model.

IPGP’s final tests on the seismic sensors of the SEIS Martian seismometer on the InSight mission

Opening of the martian sphere

Publication date: 23/10/2017

General public, Observatories, Press, Research

Related observatories : InSight Observatory

Highly supervised opening of the Mars chamber at the IPGP observatory in Saint-Maur-des-Fossés on September 27th, 2017. This test facility reproduces conditions of the surface of Mars on Earth (© IPGP/Philippe Labrot)

The SEIS seismometer is the main instrument of the InSight mission, which will be launched on May 5th, 2018 from the Vandenberg spaceport in California (USA). Placed under the banner of NASA’s Discovery programme, InSight’s objective is to use Martian seismic activity to accurately characterise the Red Planet’s internal structure for the first time.

The heart of the SEIS instrument is made up of three VBB (Very Broad Band) pendulums, sensitive to frequencies lower than 5 Hz, for which the Planetology and Space Sciences team at the IPGP is responsible.

Financed by the CNES, they were assembled by SODERN and then extensively tested using a wide range of technical resources, including those of the IPGP’s Saint-Maur-des-Fossés Observatory. Regular inspections and contamination checks were also carried out in the clean rooms of the Paris Diderot Space Campus.

“Some of the tests, for example, involved exposing the sensors to a high vacuum and temperatures ranging from -105°C to +45°C, in a chamber simulating the drastic conditions on the surface of Mars,” explains Taoufik Gabsi, CNRS research engineer. One of the objectives was to select, on the basis of a large number of technical criteria, the three best pendulums for the seismometer’s flight model, and then three other candidates for the spare model.

“The VBB pendulums in the SEIS seismometer can measure almost imperceptible displacements, smaller than the size of a hydrogen atom, which makes the instrument extremely sensitive,” explains Olivier Robert, CNRS research engineer and designer of the sensor displacement detector.

The spare SEIS seismometer chamber, housing a set of 3 ultra-sensitive seismic sensors, inside the Mars caisson. The precisely controlled inclination of the instrument reduces gravity along the measurement axis of a given pendulum, and thus simulates Martian gravity for this sensor on Earth (© IPGP/Philippe Labrot).

For both the flight model and the spare model, the seismic sensors were installed in a titanium containment vessel built by NASA’s Jet Propulsion Laboratory. These enclosures were then evacuated and sealed, before being delivered to CNES in Toulouse. In the clean rooms of the French space centre in Toulouse, the VBB enclosure was then integrated into the motorised levelling system (supplied by the Max Planck Laboratory in Göttingen, Germany), to which short-period seismic sensors (developed by Imperial College in England) and thermal protection are also attached. This set of sensors and its acquisition electronics (developed by the Swiss Federal Institute of Technology in Zürich) make up the SEIS instrument.

The flight unit of the SEIS seismometer being tested in Denver (USA) with its carrier vessel, the InSight lander. The vacuum enclosure that protects the seismic sensors (visible in the previous photo) is now installed on a levelling cradle (whose feet can be made out), and covered by an orange thermal shield. Here again, the tilt of the instrument simulates Martian gravity (© NASA/JPL/Lockheed martin)

The spare model will now continue its test sequences (environmental, functional and performance) under the responsibility of CNES in Toulouse, before being shipped to the clean rooms of Lockheed Martin in Denver, USA. There it will join the flight model already delivered in July 2017, which has since been connected to the lander to undergo extensive testing in different operating configurations.

“It’s obviously a relief to have been able to cross the finishing line after years of effort to develop, test and validate these clockwork devices that are the VBB clocks,” says Sébastien de Raucourt, research engineer at Université Paris Diderot. “A large part of the technical challenges that had to be overcome stemmed from a paradox: developing an ultra-sensitive instrument capable of withstanding the violence of a launch and landing on Mars, and then carrying out measurements in conditions that are by their very nature incompatible with precision seismic studies”, emphasises Sébastien de Raucourt.

For the IPGP’s technical staff, however, the work is far from over. The engineers will continue to be called upon for all the tests carried out on Earth, before taking part in operations on Mars, once the InSight probe has landed on the Elysium plain of Mars in November 2018.

The IPGP-Université Paris Diderot-CNRS instrumental technical team responsible for the VBB pendulums is made up of Sébastien de Raucourt, Taoufik Gabsi, Tanguy Nebut, Sylvain Tillier, Olivier Robert , Michel Parise and Mélanie Drilleau. It was reinforced by several engineers on fixed-term contracts or as technical support, as well as by engineers from JPL and CNES during the test campaigns at the IPGP.

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