Interactive VBB pendulum

On the following page you may access an interactive 3D model of the very broadband (VBB) pendulum. Simply download the PDF file. You can then easily find out all about the seismic sensors at the heart of the InSight probe’s SEIS seismometer:

• The interactive PDF file contains a simple but realistic interactive 3D model of the InSight VBB pendulums. Use your mouse for rotations (left click), translations (click on both left and right buttons at the same time) or for zooming in or out (turn the wheel).

• Use the right-hand menu to discover the different components of the pendulum. By clicking on a given component, not only will it be highlighted (the non-essential parts become transparent), but it will be described by a short text in a beige rectangle at the bottom of the page. It is also possible to run animated sequences showing the pendulum’s balancing mechanism (BM) or the thermal compensation device mechanism (TCDM).

• Use the left-hand menu to run general animated sequences that will help you understand the role of the balancing motor and thermal compensation device mechanisms in the VBB pendulum and see how the pendulum vibrates when a seismic wave is detected. There are also short descriptive texts.

 Interactively explore the structure of a VBB pendulum and find out how it works (interactive PDF file) (© IPGP/David Ducros).

 To get the most out of this interactive PDF file, you need to :

• Download and install the most recent version of Adobe Acrobat Reader DC (no other PDF file viewers are recommended or able to be used).

• Download the PDF file "InSight SEIS interactive VBB pendulum EN v1.3.pdf" by clicking here (current version: 11-06-2018 / June 2018 ).

• Open the file with Adobe Acrobat Reader DC. A security message is displayed due to the presence of 3D content in the file (an option deactivated by default). Click on the "Options" menu in the yellow security bar and choose "Always approve this document".

• If possible, use the "Display"/"Full-screen Mode" (keyboard short-cut CTRL-L) to have a full-screen display.

• You are now in charge of one of the first very broadband seismic sensors ever sent toward Mars !

Last updated: 12 june 2018

An ultrasensitive and robust robotically deployed instrument

A VBB pendulum beside the evacuated sphere that houses it (© IPGP/SODERN/CNES).

The Very Broad Band seismometer aboard the InSight space probe is an extremely sophisticated instrument with two decades of engineering behind it. It has benefited in particular from the legacy of Mars 96 and NetLander, missions that were unfortunately never completed.

At the heart of the instrument lie three exquisitely sensitive pendulums that will be able to detect the tiniest movements of the Martian surface. The displacement of the moving part of the pendulum will be very precisely measured electronically. A feedback mechanism will be continuously applied to the moving part with respect to its equilibrium position so as to further increase the measurement precision.

At this point we already have a perfectly functional pendulum. However, several tweaks were necessary to make the whole assembly suitable for operation under Martian conditions.

To fine-tune the pendulums for Martian gravity, mission engineers designed a motor-driven equilibrating system. In addition, each pendulum is fitted with a special thermal compensation mechanism to guard against the rapid and very wide temperature variations on Mars.

The most hostile factor on Mars for seismometers is doubtless the huge temperature variations that occur between day and night and over the course of the seasons. The SEIS seismometer therefore has several thermal barriers.

This cut-through view of the SEIS seismometer shows the dome-shaped wind and thermal shield (WTS), the remote warm box (RWEB), the levelling platform, and the inside of the evacuated sphere protecting the VBB pendulums (©IPGP/David Ducros).

Like a Russian doll, the three pendulums nestle under extreme vacuum conditions within a titanium sphere which is itself enclosed by a protective honeycomb-structured cover that uses the Martian atmosphere as an additional thermal insulator. Finally, the whole assembly is placed under the WTS, a heavy wind and thermal shield designed to minimize thermal contrasts and offer some protection against gusts of wind.

In contrast to the Viking missions, where the seismometers remained physically attached to the lander, InSight has the ability to place the instrument on the ground using a robotic arm with a gripper. In the first instance, the arm will grasp the seismometer in order to place it on the ground. A semi-rigid umbilical tether connects the instrument to an electronics unit (or “eBOX”) inside the InSight lander; this provides electrical power, digitizes signals and transfers digital data to and from the lander's onboard computer.

The sphere containing the pendulums is mounted on a metal cradle fitted with three motor-driven legs to provide a very accurate level with respect to the horizontal. The quality of the set-up on the ground is absolutely essential to guarantee the seismometer’s optimal operation.

Bruce Banerdt, the InSight mission’s Principal Investigator, points at a terrestrial Wielandt-Streckeisen STS2 seismometer, installed immediately under the SEIS Martian seismometer (© NASA/JPL-Caltech/Lockheed Martin).

Only when the instrument is perfectly positioned can it be covered by the wind and thermal shield. All the conditions will then be met to be able to start the measurements, which will last two Earth years.

Last updated : 18 september 2017

A miniature seismometer etched in a silicon chip

In addition to the Very Broad Band (VBB) pendulums, the SEIS seismometer will be supported in its task by a set of three short-period (SP) sensors that are sensitive to seismic waves whose frequency exceeds 1 Hz (1 cycle per second).

The SEIS instrument’s short-period sensor (© Imperial College London).The role of the short-period seismometer, which is not one of the mission-critical instruments, is to provide partial redundancy should the very broad band (VBB) seismometer fail. Sensitive to a frequency range (what geophysicists call “bandwidth”) from 0.1 to 40 Hz, it partially covers the band encompassed by the VBB pendulums.

Designed at the outset to register frequencies (up to 50 Hz) higher than those to which the VBB seismometer is geared, the short-period seismometer significantly extends the bandwidth within which the InSight mission can investigate seismic signals.

Put another way, the SP sensors can be seen, metaphorically speaking, as a second pair of ears. They will come to the aid of the InSight mission if the main pair of ears (the VBB) is struck down with deafness, and be able to hear strident signals that would otherwise be inaudible.

Miniaturization: small but tough

In contrast to the VBB pendulums, which have a significant mass and volume, the short-period seismic sensors have been considerably miniaturized. Ion-etched in a silicon wafer, they are lightweight and no bigger than a 1 euro coin. Each sensor’s mobile mass is of just 1 g, as against 190 g for the VBB pendulums.

The development of the SP microseismometer and its three associated sensors presented project engineers with several challenges, including that of being able to withstand the violent shocks associated with space missions during events such as lift-off and landing. During these brief but severe events, the InSight spacecraft will have to absorb considerable accelerations of a few hundred g or more.

3D representation of the SEIS seismometer short-period sensor
(© Imperial College London).Seismometers being truly sensitive instruments designed to measure very tiny accelerations of just a few nano-g, it is not hard to understand that they will respond badly to violent shaking in all directions.

Nevertheless, the SP seismometer has been designed to withstand such mechanical affronts. Like the VBB pendulums, the mobile mass does not need to be locked during the flight (from launch to landing) since its propensity to move from side to side is restricted by stops.

Despite its small size and apparent simplicity, the SP seismometer can detect vibrations in all three spatial directions, i.e. one vertical and two horizontal axes (though in contrast to the VBB seismometer, the pendulums are not inclined). An arrangement of electrodes enables the displacement of the mobile mass to be measured. The system has been designed so that it can continue to function even if the horizontal levelling system has failed to achieve optimal positioning.

Under British responsibility, the SP seismometer has been designed and built by Oxford University and Imperial College London.

Last updated: 25 October 2016

A very complex yet modular mechanism developed by the Institut de Physique du Globe, Paris

An ingenious thermal protective barrier using Martian air

Geophysicists on Earth always try to install seismometers in places where the temperature is stable, ideally in disused mine galleries or shafts.

Cut-away of the SEIS instrument, showing the structure of the Remote Warm Electronic Box (© IPGP/David Ducros).

On Mars, the daily extremes of temperature are a major obstacle for seismic measurements. On the InSight landing site, the Elysium Planitia, meteorologists estimate that the difference in temperature between day and night could be as much as 70 to 80°C. This represents a very uncomfortable situation for a seismometer as sensitive as SEIS, especially when attempting to measure long-period (i.e. low frequency) seismic signals. Higher frequency signals are less influenced by thermal fluctuations.

The engineers responsible for developing the InSight seismometer therefore wanted to improve the degree of thermal protection. This is why the pendulums, which form the heart of the instrument, are enclosed in a highly evacuated sphere whose inner surface is coated with a thermal screen. However, these precautions are not sufficient and the sphere itself is placed inside a sophisticated thermally-protected enclosure known as the RWEB (Remote Warm Enclosure Box).

Tiny cells for trapping Martian CO₂

With a mass of 0,75 kg and measuring roughly 42 cm across, this hexagonal cover makes good use of the exceptional properties of an unexpected ally: the Martian atmosphere itself!

The CO₂ making up the Martian atmosphere persists in any interstice provided the gap between hot and cold walls is less than 2 cm, regardless of the temperature difference between the inside and the outside.

Successive layers of Mylar®— another material known for its low thermal conductivity and used in survival blankets— surrounds the sphere. Each layer is spaced less than 2 cm from its neighbour so as to form an effective trap for CO₂. The inside of each layer is silver-coated while the outside is gold-coated in order to further improve the thermal insulation.

At the top of the RWEB is a small rod terminated by a sphere forming a kind of handle which can be grasped by the five fingers of the grabber on the InSight lander's robotic arm.

Sundial

Around the handle is a small sundial, which enables geophysicists to estimate the azimuth— i.e. the orientation of the seismometer with respect to the Red Planet's north pole—using their knowledge of the seismometer's location on Mars and the time at which the direction of the shadow is measured.

However, the sundial will not work for long. Once on the ground, the RWEB enclosing the titanium sphere which itself houses the seismometer and its three pendulums will be quickly covered over by another protective dome known as the WTS (wind and thermal shield).

Last updated : 18 septembre 2017

SEIS’s electronic brain

Electronic feedback card in the eBOX (© IPGP).

While a small part of the electronics controlling the SEIS seismometer has been installed close to the sphere containing the pendulums  (proximity electronics designed to pre-amplify the signals), most are some distance away on the InSight lander, inside an enclosure known as the eBOX.

With a mass of 5 kg and kept warm inside a specially designed compartment where it can function protected from the rigours of the Martian climate, the eBOX hosts nine electronic boards, all absolutely vital for the operation of SEIS.

DC card

The first electronic (DC) board controls the SEIS instrument power supply. The SEIS instrument receives from the lander electrical power at 28 V that it converts into the different voltages needed to operate the seismometer’s various components. Given its importance, there are two identical boards for purposes of redundancy. If the first one malfunctions, the second one can be used as a backup.

The InSight lander will be in sleep mode for most of the time, and it is expected that it will be woken up for only a few hours a day, in particular for transmitting data acquired by the instruments and activating the sensors. During these sleep phases, SEIS will be completely autonomous, and the power supply board, connected directly to the lander batteries, must therefore be extremely reliable.

AC card

External view of the eBox electronics unit (© ETHZ).

The second (AC) board is responsible for data acquisition and instrument control. It interfaces with the electronic feedback boards, those dedicated to the levelling system and the SP (short-period) micro-sensors.

This board provides the commands to initiate a VBB pendulum or choose between the SEIS instrument's two operating modes: engineering mode, for configuring the instrument, or science mode, for acquiring seismic signals.

The AC board also offers other functions such as short-circuit detection or the acquisition of data from environmental sensors such as temperature probes and various supply voltages, including power drawn from the lander for SEIS. It is connected to the InSight lander's Command and Data Handling system (C&DH) and, like the previous board, it is also redundant.

Feedback boards

The eBOX houses three “feedback” boards, one per VBB pendulum.

Coupled to the feedback device (comprising a magnet and a set of 3 coils) fitted to each pendulum, these boards are responsible for managing the feedback loop, which ensures that at every instant the slightest movement of the mobile mass is counterbalanced by an exactly equal and opposite force, ensuring that the pendulums continuously return to their equilibrium position.

The feedback mechanism is essential, since it is from this that the seismic measurements come. What is actually read by the boards and sent back to the scientists on Earth is just the restoring force that serves at any instant to bring the mobile mass back to its equilibrium position. Another advantage of the feedback device is that it optimizes the performance of the VBB pendulums, reducing by a factor of 100 the effect of temperature variations on their mechanical properties.

SP & LVL card

Detail of the feedback loop control board (© IPGP).

The last two boards housed in the eBOX are responsible for managing the short-period (SP) seismometer (which also has a feedback loop) and the levelling mechanism.

The three long-period VBB pendulums inside the sphere are supported by three micro-seismometers sensitive to short periods. Being slightly less critical than the VBB pendulums, it was possible to use off-the shelf electronic components, though naturally they had to undergo a drastic qualification cycle. Taking up much less room than the VBB pendulums, the three SP micro-seismometers only need one electronics board for all three feedback circuits.

There is no redundancy for either the VBB sensors or the SP sensors. However, they both share the same range of measurements, between 0.02 and 100 seconds. The SP seismometers perform better for periods less than 0.2 seconds, while the VBB sensors perform better for periods longer than 0.2 seconds. If one of the six axes should be lost due to failure it will be replaced by the remaining ones, and the triaxial measurement of the acceleration of the Martian ground will thereby be maintained.

The levelling mechanism (LVL) has its own control board. Using the three motor-driven legs this system can very precisely align the SEIS instrument to the horizontal, and hence Martian gravity. Given that this mechanism will only be implemented at the very beginning of the mission, once the instrument has been placed on the ground by the robotic arm, the electronic control board is not redundant.

Low electronic noise

One of the characteristics of the eBOX is that it can acquire signals from the seismometer at very low noise levels. Given the effect that electronic noise could have on the performance of the sensors, the project’s electronics engineers have addressed the issue of noise generated by the boards. The feedback boards thus integrate space-qualified capacitors with very low thermal sensitivity that were designed and manufactured specifically for the purpose.

The eBOX is the responsibility of the Swiss Federal Institute of Technology (ETHZ), which also designed and built the data acquisition (AC) and power supply (DC) boards, in addition to the electronics controlling the temperature sensors. The feedback boards were designed by the various teams responsible for the seismic sensors. The boards that manage the VBB pendulums were developed in France by the Institut de Physique du Globe de Paris (IPGP) and built by EREMS in Toulouse, France. The SP seismometer control board was supplied by Great Britain, while the levelling mechanism control board was developed in Germany.

Last updated: 25 October 2016

SEIS's lifeline

3D representation showing the SEIS seismometer and the end of the tether, with the service loop and the opening device (Load Shunt Assembly) (© NASA/JPL-Caltech).

Once on the ground, the SEIS seismometer will remain linked to the InSight lander through a sophisticated umbilical tether in the form of a semi-rigid flat cable.

This tether, housed in a tether storage box (TSB), will automatically unroll when the seismometer is grabbed by the robotic arm.

Three metres long and 4.5 cm wide, the cable links the SEIS instrument to the eBOX, an electronics unit placed in a thermally-controlled compartment inside the lander. This cable not only provides electrical power to the seismometer but also allows the passage of electrical signals, measurements obtained by the instrument, and command signals.

To avoid any vibrations transmitted through the tether from the lander (perhaps due to movement of the solar arrays when blown by the wind), damping devices have been installed.

One of the most important of these is a “relaxation” or “service” loop. A mechanical part attached to the seismometer forces the cable into a loop, which greatly reduces the potential propagation of waves from the spacecraft. This clever idea is commonly used on Earth when geophysicists deploy seismometers. Once the instrument is on the ground, it is generally recommended that the cable carrying power and data should be wound once around the seismometer’s protective cover.

The spooling mechanism of the SEIS seismometer tether (© NASA/JPL-Caltech/Lockheed Martin).

Other subtle enhancements have gradually been added according to the project engineers’ inspiration. The tether, for instance, has been fitted with a small rod terminated by a small sphere allowing it to be grasped by the gripper at the end of the robotic arm. If the situation warrants it, it will therefore be possible to grab the cable on the ground and change its position.

A metal plate of roughly 300 g and fitted with prongs is also attached to the lower part of the cable; this tether prong mass will help to improve contact with the ground and prevent possible micro-movements.

The tether and its various accessories have all been developed by the Jet Propulsion Laboratory (JPL).

Last updated: 25 October 2016

A (very) sophisticated spirit level

A pictorial 3D representation of the levelling system. This consists of a ring to which are attached three telescopic legs, three proximity electronics units for the VBB pendulums, the three short-period sensors and finally the devices that allow the assembly to be handled by the robotic arm on the InSight probe (© NASA).

In order to be able to operate under the most favourable conditions, the SEIS seismometer should ideally be placed on as flat and as horizontal a surface as possible.

On Earth, a slab of concrete would have been poured and SEIS would have been levelled using knurled screws. On the Moon, and as with the Apollo programme, an astronaut would have manually levelled the instrument using a spirit level after having oriented one of its axes towards Earth. On Mars, obviously none of this will be possible.

Project engineers have therefore designed an ingenious mechanism in the form of a cradle, within which is placed the sphere containing the VBB pendulums. This structure has three electrically motorized legs.

Once the instrument has been placed on the ground by the InSight lander's robotic arm, inclinometers placed on the structure of the cradle will measure its angle.

For maximum performance, the pendulums inside the sphere must be positioned at a certain angle with respect to the direction of Martian gravity (the gravity vector being parallel to the vertical). Inside the sphere the pendulums have been mounted at a slant such that they form an angle of 30.5° with the horizontal. This angle must be complied with once on Mars, and the only way to do this is to ensure that the sphere is positioned perfectly horizontally.

It is highly probable that, after having been deployed by the robotic arm, the SEIS instrument will find itself on a slightly sloping surface or that one of its legs will be resting on a stone. In this particular instance the project team will undoubtedly choose to reposition the seismometer, but regardless of the efforts made to ensure that the instrument ends up on the flattest possible area, the situation will never be perfect.

Cradle levelling test for the SEIS seismometer on the Aeolian island of Vulcano, Italy, in June 2016. The metal disc visible on top of the platform will not be present on the flight model. Here it serves as a support for a terrestrial seismometer used during the tests in the absence of the Martian SEIS seismometer (© Brigitte Knapmeyer-Endrun).

The motorized legs of the cradle come into play at this point. With a maximum vertical displacement of 6 cm, the legs are able to retract or extend to ensure the sphere is perfectly horizontal to within 0.1°. This mechanism will allow the SEIS instrument to accommodate slopes up to 15°.

The second job of the cradle is to ensure optimum contact with the ground, given that it will not be possible to register any worthwhile seismic signal unless there is good coupling with the Martian surface that remains stable over time.

The motorized legs therefore end in a conical point to facilitate penetration of the ground and increase instrument stability. A metal ring slightly higher up the leg will prevent too deep a penetration and improve contact with the ground. Finally, small collars made of insulating material will protect the motors from the Martian dust and insulate the legs against variations in temperature.

Finally, a special calibration mechanism will allow a very subtle and known inclination to be applied to the cradle; this will serve to calibrate the pendulums so that they will then be able to measure the tides on Mars' moon, Phobos.

The levelling mechanism was developed by the Max Planck Institute in Germany.

Last updated: 25 October 2016

A protection against wind and temperature

Seismometers are so sensitive that they should ideally be located in very quiet locations. On Earth, they are often placed underground in places such as abandoned mineshafts. Temperatures are astonishingly stable in such places and there is almost complete silence. Any disturbances related to atmospheric activity such as wind are automatically eliminated.

The wind and thermal shield (WTS) (© CNES).

It will not be possible on Mars to dig a hole and bury the seismometer, even if the robotic arm had a bucket and could act as a mini-digger.

Engineers at the Jet Propulsion Laboratory have therefore developed a protective dome to be lowered onto the seismometer once it is on the ground. Under its high-tech shield, SEIS will be well protected against the very marked temperature contrasts between night and day that exist on Mars. Gusts of wind will also be greatly reduced.

The WTS consists of an aerodynamically shaped aluminium cover with a honeycomb structure to which is attached a gold-coated thermal skirt. The whole assembly rests on three legs which will deploy automatically once the robotic arm lifts the dome off the lander's platform. It will be brought to above the seismometer (previously deployed on the ground) before being slowly lowered.

Artist concept showing the protective role of the WTS at the martian surface (© IPGP/David Ducros).

The extendable skirt is bordered around its circumference by a kind of chain-mail, not unlike that worn as armour by mediaeval knights. Its weight alone allows the skirt to descend. Its platelet structure also confers a second advantage, namely its ability to effectively cover obstacles such as pebbles, enveloping their surfaces and hence sealing off the WTS.

The dome measures 69 cm in diameter (35 cm high) and has a mass of 9.5 kg, which is greater than that of the seismometer it is protecting. Ideally it should cover the seismometer (which is 36 cm along its major axis) as symmetrically as possible and without touching it. There should be a space of at least 6 cm between the instrument and the WTS.

Despite the great care taken in its design, it is not impossible that violent gusts of wind or a dust devil might dislodge or even lift the dome, causing it to fly away. The shield has nonetheless been developed to withstand squalls of 60 m/s and should even be able to survive winds of 100 m/s.

The WTS has been developed by NASA's Jet Propulsion Laboratory. Its effectiveness has been tested in the field using a prototype first in the Piton de la Fournaise crater on Reunion Island, and then in California's Mojave desert.

The extendable skirt of the Winds and Thermal Shield (WTS), with the chain mail at the bottom (© NASA/JPL-Caltech/IPGP/Philippe Labrot).

Last updated: 2 january 2019

Studying Mars without contaminating it

Right from the very start of space exploration, scientists have been aware of the risks inherent in sending unsterilized spacecraft to other planets. Signed in 1967 by numerous organizations involved in the conquest of space, the Outer Space Treaty drawn up by the United Nations stipulated that precautions must be taken to avoid contamination of bodies in the solar system by terrestrial germs. Developing in a favourable medium, these could potentially eliminate possible extraterrestrial microorganisms or destroy chemical markers of inestimable scientific value in understanding the origins of life.

Swabbing the sphere flight model (© Hervé Piraud/IPGP/SODERN/CNES).

Under the United Nations umbrella of planetary protection, space agencies must comply with a set of rules and regulations drawn up by the Committee on Space Research (COSPAR).

The precautions that have to be taken depend on the target celestial body and the type of mission. In the case of a simple fly-by of the Moon, planetary protection constraints are minimal: the risk of crashing into the surface is limited by the type of mission (fly-by), and what is more, the lunar surface is deemed to be extremely hostile to all forms of life. However, there is a major risk of contaminating Mars, especially when a craft lands on its surface.

A IVa category mission

According to the COSPAR rules of planetary protection, the InSight mission belongs to category IVa, reserved for missions whose aim is not to detect signs of life but which are intended to land on planetary surfaces with significant potential for researching traces of life and studying the origins of life. The constraints imposed to prevent contamination at all costs are drastic, and have a significant impact on the mission budget.

Decontaminating the seismometer

Bagging the sphere flight model before carrying out a leak test (© Thierry Cantalupo/IPGP).

Since the SEIS seismometer has to come into contact with Martian soil, the instrument must be as clean as possible.

During construction, the various components of the instrument will have been regularly disinfected, e.g. by cleaning the surface with antiseptic products such as isopropyl alcohol.

Any activity around the instruments is obligatorily in a cleanroom meeting the ISO 7 standard or higher.

In these ultra-clean working environments, there must be fewer than 10,000 particles per cubic metre of air. The rooms are under positive pressure to prevent pollutants from being carried in from the outside, and technicians must wear one-piece antistatic suits with integral masks. It is mandatory to disinfect all tools brought in from outside.

Once assembled, the seismometer must again be sterilized, either by baking (at 110°C for 50 hours) or by exposure to a gaseous plasma of hydrogen peroxide. Furthermore, throughout the construction process, samples are periodically taken off different components to check the degree of surface contamination.

The SEIS seismometer must harbour no more than 20,000 microbes in all. To avoid any recontamination after cleaning or sterilization, e.g. during transport or handling, team members use special Tyvek ® bags that allow connectors to pass through.

Launcher avoidance manoeuvre

The SEIS seismometer is not the only system that has to submit to the rules of planetary protection. These apply to the entire mission, including the space probe and launcher.

Effectivement, la sonde InSight n'est pas la seule à voyager vers Mars. L'étage supérieur Centaur de la fusée Atlas-V, qui fournit la poussée nécessaire pour arracher la sonde à l'emprise de l'attraction terrestre, part également vers Mars. Etant donné qu'il n'est pas envisageable de stériliser un engin aussi volumineux qu'un étage de lanceur, il faut trouver un moyen pour s'assurer que la trajectoire suivie par l'étage Centaur ne croise pas celle de Mars.

The InSight probe is not the only spacecraft heading to Mars. The Centaur upper stage of the Atlas-V rocket (which provides the thrust necessary to free the probe from Earth's gravity) is also leaving for Mars. Given that it is not feasible to sterilize something as big as a launcher stage, some means must be found to ensure that the course followed by the Centaur stage does not coincide with that of Mars as it orbits the Sun.

The rules of planetary protection stipulate that the probability of Centaur crashing into Mars must be less than 10^-4 over a period of 50 years. Hence, during launch and—contrary to what one might think—InSight and its launcher are not pointed directly at the Red Planet. In fact, everything is done to make the rocket miss its target! It is only by using manoeuvres to correct the trajectory during the flight cruise phase that the spacecraft is progressively brought back on course.

Furthermore, and by way of an additional precaution, immediately after the InSight probe separates close to Earth, the Centaur stage carries out a contamination and collision avoidance manoeuvre (CCAM) to further reduce the risk of contaminating the Red Planet.

Finally, even the risk of the InSight spacecraft crashing into Mars following some critical failure during the journey from Earth to Mars is taken into account. Interstellar navigators must demonstrate that the probability of a collision with the planet is minimal, and numerous simulations have been carried out to convince the regulators responsible for making sure that the rules of planetary protection are satisfied.

Clearly, the principles of planetary protection have significant consequences for space missions.

Decontamination and sterilization of the space probe

Permitted sampling zones on the SEIS sphere flight model (© rights reserved).

Like the seismometer, the InSight probe has been assembled and tested in a cleanroom. Its components have been regularly cleaned and/or sterilized and many precautions have been taken to avoid recontamination.

The level of contamination has been quantified by samples taken at regular intervals. There must be fewer than 500,000 spores (which is how germs are counted) over the whole of the probe, including the cruise stage. Being destined to land on Mars, the acceptable number for the lander is even lower, with no more than 300,000 spores allowed. The mean number of spores on the surface of the spacecraft must be less than 300/m2. On the inside, the acceptable quantity of spores is slightly greater, given that only a crash would lead to these spores being released into the Martian environment (whether the surface or atmosphere).

All the organic materials used in the construction of InSight have been subject to review. Depending on the mass sent to Mars, each material is either simply documented or inventoried along with a sample which is then kept.

Even though InSight is not conducting any experiments to detect life, scientists have to be extremely careful about sending carbon-containing molecules to Mars. The risk that these may one day compromise the chemical analyses carried out by other missions in the future is by no means negligible, hence the importance of maintaining an up-to-date register of exactly what has been deposited on Mars.

Finally, it is worth mentioning that the InSight landing site is not in what planetary protection rules describe as a “special region” i.e. an area of the planet Mars where forms of terrestrial life might have a strong chance of propagating as a result of ice or a film of liquid water close to the surface.

Located at the equator, the soil on the Elysium Planitia is completely dry; it was for this reason that the penetrator on the HP³ instrument, which has to bore down to a depth of 5 metres, received the approval of the planetary protection committee. If specks of ice had been present on the surface, the situation would have been radically different because of the heat liberated by the drilling process.

Last updated: 26 October 2016

External noise and sources of interference

The SEIS seismometer on the InSight probe is extremely sensitive. It is actually a thousand times more sensitive than those on the 1976 Viking probes, which means that it can register displacements of less than the width of a hydrogen atom. Such an amazing technical feat is essential since Mars is a lot quieter than our own planet. The greatest source of seismic interference on Earth is the crashing of ocean surf and pounding of waves on the shoreline. This generates such a continuous seismic noise that terrestrial seismic stations can detect storms in nearby oceans.

Of course, human activity also produces a great deal of noise contamination at the surface. It is not hard to imagine that the passing of an underground train or the battering of a jack-hammer can be picked up by a seismic station. Finally, the continual motion of Earth's atmosphere also leaves its mark. Wind and variations in temperature and pressure can seriously upset seismic measurements, which is why seismometers tend to be installed in mineshafts whenever possible.

With neither sprawling cities nor expanses of water in constant motion, the Red Planet is a very quiet world. However, the weather can affect seismic observations, just as it does on Earth, so it should be measured independently using a sophisticated weather station, and then decorrelated from the signals provided by the instrument.

Wind

A SEIS qualification model was tested under the exceptional conditions afforded by the quietest seismic site in Europe, a former mine under the Black Forest in Germany (© rights reserved).

While it is obviously not possible to bury the SEIS seismometer under the Martian surface or place it in disused mine workings, project engineers have sought as far as possible to isolate the instrument from atmospheric disturbances. In the long-period domain, which is of particular scientific interest, atmospheric changes and fluctuations due to wind and pressure will be a major source of ground movement, independent of any seismic activity.

To overcome this kind of disturbance, the instrument is protected from the wind by a wind and thermal shield (WTS), but the protection is not absolutely perfect. In addition, buffeting due to the wind can be picked up by the seismometer through the ground.

Depending on the pressure, the layer of air around the landing site will tend to push on the ground, causing it to deform slightly; this movement will be registered by the seismometer.

The vibration of the lander's solar arrays—which are very thin and therefore tend to vibrate easily in the wind rather like the wings of an insect—will also be a source of noise interference, as will the soil being compacted under the three legs of the WTS.

As an additional precaution, pressure variations will be constantly monitored by an ultrasensitive barometer. However, when gusts exceed 3 to 3.5 m/s, the seismometer will start to pick up an appreciable noise level resulting from the deformation of the ground due to wind-generated pressure fluctuations. Although this noise will of course be an obstacle in detecting very small tremors, it will also be a new source of data for project seismologists and atmospheric specialists.

Temperature

More than wind, temperature is the SEIS seismometer’s number one enemy on Mars. Temperature contrasts on the Red Planet are so great that it is absolutely essential that any change in the temperature of the pendulums be kept to a minimum. Despite the precautions taken, some components will always be sensitive to temperature, altering the centre of gravity or the stiffness of the springs and pivots.

To counteract temperature variations, the pendulums are placed inside a sphere under strict vacuum conditions. This sphere is then protected by multiple insulating barriers such as the RWEB (Remote Warm Enclosure Box) or the dome-shaped wind and thermal shield (WTS). The TCDM is a special device designed to minimize the pendulums’ sensitivity to changes in temperature.

Any changes in the outside temperature will be carefully monitored by the InSight lander’s weather station, as well as by temperature sensors inside the sphere, on the levelling system and inside the eBOX electronics unit. The data collected will then be used to correct as far as possible the long-period signals (collected mainly by the VBB seismometer) which are transmitted over very long periods.

The huge contrast in temperature between day and night (a night-time drop of around 60 to 80°C) will also generate indirect disturbances. The expansion and contraction of lander materials will furthermore give rise to cracking sounds, like those made by a house in winter when the temperature plummets and the roof beams shift.

Internal noise and sources of interference

Internal noise in space instruments is measured in a controlled environment such as a cleanroom (© Lucile Fayon).There are celestial bodies in the solar system even quieter than Mars. The Moon, for example, has no oceans, atmosphere or noisy towns. Hardly anything has moved there for billions of years, and small tremors that would be totally inaudible on Earth can be heard and analysed. However, in an almost perfect silence, another source of noise more insidious than the external noise of oceans, the atmosphere or human activity, makes its presence felt.

If we were to take a terrestrial seismometer to the Moon and switch it on during the long lunar night when the temperature is very stable, not varying by more than a few thousandths of a degree Celsius, we would see on the recordings interference signals. On Earth, these would be completely drowned out by the planet’s background noise, but on the Moon they would be only too visible. So where does this parasitic noise come from?

A seismometer placed in an environment totally isolated from the slightest disturbance does not give a perfectly flat signal, but shows a subtle but nonetheless real activity; its origin lies in the heart of the instrument itself, generated through various phenomena such as thermal agitation, mechanical drift or electromagnetic interference.

Thermal agitation

If we observe an aqueous suspension of oil through a microscope, we might be surprised to see that the oil droplets that should be completely motionless are in fact vibrating and moving about randomly in all directions. This erratic motion, known as Brownian motion after its discoverer, the botanist Robert Brown, is due to the thermal agitation of the particles. Because of its immense sensitivity, the moving part of the pendulum in the SEIS seismometer is affected by thermal agitation, tending to oscillate randomly and independently of ground displacement; this occurs under the effect of impacts by the rare molecules of gas still present in the evacuated sphere.

Brownian motion can never be eliminated by isolating the seismometer from the external environment: the only way to reduce it is to build massive seismometers, reduce friction in the moving parts or lower the temperature. A similar form of Brownian noise also exists in the circuits of electronic boards, especially when high-value resistors are used. Thermal agitation combined with electronic thermal noise together constitute the main source of parasitic noise for seismic waves having periods of a few seconds; it is precisely these waves that are of most interest to scientists on account of their potential for discovery.

Electromagnetic disturbance

The SEIS seismometer will bathe on Mars in the electromagnetic field generated by the lander and the planet's ionosphere, that part of the upper atmosphere where certain atoms or molecules are ionized.

As we have seen previously, most of SEIS' sensitivity to the magnetic field comes from the springs on the VBB pendulums that are machined from an alloy of iron and nickel, compensating for temperature variations by altering their magnetic properties. The short-period (or “SP”) seismometer is constructed entirely from silicon, and is not sensitive to magnetic fields.

Again, just as for temperature, magnetic disturbances can be recorded independently using the Insight fluxgate magnetometer (IFG) and then subtracted from the seismometer signals.

Other internal sources of interference

Other sources of disturbance, such as electronic noise, can interfere with SEIS measurements. This type of noise is particularly found in the differential capacitive sensor (DCS), which measures the displacement of the moving part of the pendulum relative to the fixed part, but it also concerns the circuits in the feedback system and the analogue to digital converters to name but a few. Parasitic electrostatic forces may also reduce the instrument's sensitivity. Mechanical parts can drift, though this can be corrected by the feedback loop and compensated for by recentring the pendulums using the equilibrating mechanism.

Characterizing the internal noise level

The sources of internal noise in the SEIS seismometer are fairly well understood, and have been characterized by the engineers who developed the instrument.

The sources and intensity of internal noise have been predicted theoretically by analytical models and validated over several generations of seismometers. Although it is impossible to directly measure the noise levels of the seismometer pendulums, engineers were able to make an indirect (but accurate) estimate by comparing the measurements of terrestrial noise made by SEIS with data obtained at the same time by terrestrial seismometers in various observatories. Although more accurate, these are heavier and cannot be transported to Mars.

Last updated: 08 february 2017

Temperature, impacts and vibration, vacuum, radiation and electromagnetic fields, everything has to be tested!

Inspection of the SEIS seismometer sphere (¬© Hervé Piraud/IPGP/SODERN/CNES).

Testing is of absolutely vital importance in the space sector. On this depends the correct operation of an instrument in space, and hence the success of the mission.

Test models

The battery of tests starts right from the instrument’s design phase. The structural and thermal model (STM) is used to validate two key aspects of an instrument: its structural strength, and its behaviour under thermal stress.

Engineers then develop a functional qualification model (QM) on which an entire series of test is conducted, including electrical tests. If these are satisfactory as a whole, a flight model intended to be launched into space can then be built, at the same time as a spare that can be used in the event that the flight model develops a failure. Flight and spare models are, of course, tested in the same way as the qualification model.

The number of tests to which a space instrument is subjected is remarkable, and reflects the challenges encountered during its development. Generally speaking, tests are split into two major groups: functional tests and environmental tests.

Functional tests: does the instrument work properly?

Measuring instruments in the seismic cave at St-Maur (© Thierry Cantalupo/IPGP).

The first thing to check is that the instrument functions properly in the environment in which it is intended to work.

Developing a Martian seismometer on Earth is not necessarily a great deal of fun. Not only does the instrument have to behave correctly on Earth but it also has to operate perfectly under Martian conditions, i.e. at glacial temperatures (down to 65°C below zero) and in an atmosphere that is mostly CO₂.

The seismometer is therefore regularly put into gas-tight chambers that simulate the Martian atmosphere. However, some aspects are impossible to replicate, such as the Martian gravity, which is one third of terrestrial gravity.

Project engineers have used some clever tricks to get as close as possible to reality. In the case of the SEIS seismometer, a small mass is added to the pendulum at the pivot, or it is inclined at a particular angle perpendicular to the inclined plane to obtain the same value of acceleration as that on Mars. However, this test is not without consequences, since terrestrial gravity nonetheless continues to exert its influence on the pendulum pivots.

Performance

The functional tests also encompass notions of performance. Not only must the instrument work, but it must also satisfy a minimum level of performance, e.g. in terms of sensitivity, so that it can meet the scientific problems for which it was designed. Many tests address the question of performance, and the data collected are analysed in minute detail to detect any unusual or disappointing behaviour.

Despite the efforts made to check the correct operation and performance level of the SEIS Martian seismometer, questions necessarily remain that will be identified and resolved only once SEIS is on Mars.

SEIS is an instrument designed to be ultra-sensitive since it is to be used on a planet that is seismically very quiet. Without the incessant noise from the pounding of ocean surf and the tumult of human activity, Mars is silent when compared to the Earth.

When powered up on Earth, the SEIS seismometer is immediately disturbed by a level of background noise that makes it impossible to determine its real characteristics with any exactitude. The engineers responsible for its design have tried placing the prototypes in environments cut off from the world, such as at the bottom of an abandoned mineshaft deep in the Black Forest, but in vain. Even down there the Earth is too noisy for periods of less than 20 seconds, and only a comparison between the signals recorded simultaneously both by SEIS and terrestrial seismometers was able to reveal the instrument’s own noise.

Environmental tests: impacts and vibration

Removing the SEIS sphere from the gas-tight chamber (¬© Hervé Piraud/IPGP/SODERN/CNES).

Space is an unforgiving place, and makes life complicated for SEIS.

The seismometer must not only continue to function but must also maintain its level of performance, i.e. its great sensitivity, after being subjected to assaults that would destroy any sophisticated seismometer designed solely for terrestrial use.

On lift-off, the seismometer will be have to endure short but extremely violent shocks or impacts, and will be jolted in all directions. The slightest fragility in the most delicate mechanisms could have disastrous consequences. On Earth, the ability to resist shocks and vibration (generally caused by the detonation of pyrotechnic devices over the course of the mission) is tested on instruments of torture known as shakers.

Compared with other instruments, the seismometer presents a sizeable challenge since, by definition, it comprises a moving part that has to move freely along a fixed part under the slightest motion. During critical phases such as lift-off or landing, there might be some advantage in locking the moving part, unlocking it only once it is on the ground.

Viking 1's inability in 1976 to disengage the seismometer pin means that this system is frowned on by engineers, and no locking system has therefore been used on the SEIS seismometer. The ability of the pendulums to withstand impacts and vibration relies on the fact that the moving part has very little play. Its displacement is limited to just a few tens of microns, though this does not prevent its being able to measure large magnitude tremors.

Temperatures and vacuum

The second feature of the space environment is the enormous temperature fluctuations. During the voyage from Earth to Mars, and once on the surface of the Red Planet, the SEIS seismometer will experience extreme changes in temperature. To check its thermal behaviour on Earth, the instrument is progressively taken to high temperatures in an oven (up to 60°C) before being placed in chambers at glacial temperatures down to -75°C. Some tests on the first models even went down below -100°C.

By definition, space is (almost) empty, and tests have therefore been carried out to check that the seismometer operates correctly in the complete absence of air. Using "Martian" chambers, project engineers check that the instrument is capable of withstanding the Martian atmosphere, i.e. CO₂ at just a few mbar. This atmosphere—very different from that on Earth—can have some surprising effects on electronic systems, being conducive, for example, to electrical arcing.

Radiation and electromagnetic fields

Testing a VBB pendulum (© Patrice Latron/IPGP).

In the absence of an atmosphere and a deflecting magnetic field, space is bathed in harmful radiation from the Sun or galactic space. This radiation consists of highly energetic particles that can cause immense damage to electronic components.

The computer on which you are reading these words would almost immediately cease to function in space due to corruption of the RAM or registers in the central processor by cosmic rays or the solar wind. Batteries of tests must therefore be carried out to check that the SEIS seismometer is sufficiently hardened against radiation, whether on the flight from Earth to Mars or once on the Red Planet (where it will, however, be relatively protected by the planet's atmosphere).

Finally, other tests aim to validate the operation of the seismometer when exposed to surrounding magnetic fields. Once on the Martian surface, the InSight lander's activity will generate its own magnetic fields. Here, for example, the most significant source is without a doubt the radio antennas, especially the UHF antenna which maintains communication with the US Mars Reconnaissance Orbiter. Tests have been conducted on Earth at the Toulouse Space Centre belonging to the French space agency, CNES, to check whether or not the seismometer is disturbed by the antenna in question when the latter is transmitting or receiving a radio signal.

Faced with this dizzying list, it is not hard to imagine the incredible amount of work the engineering teams who designed and developed the SEIS seismometer have had to do. Entire rooms could easily be filled with just the raw data, test reports, technical anomaly listings, reviews and summaries.

This is a price worth paying, however, for scientists to benefit from an instrument that can register and characterize the tiniest tremors of an extraterrestrial world hundreds of millions of kilometres from Earth.

Last updated: 26 October 2016

Martian seismology left on the sidelines

Mars is, without a doubt, the most studied planet in the solar system. Since the start of the conquest of space, a little fewer than 50 probes have been launched from Earth towards our cosmic neighbour. Most have failed to arrive at their destination or have not worked as intended, but over the decades humankind has nevertheless been able to unveil many aspects of this rust-red desert world.

The first flybys by the American Mariner probes in the 1960s gave the first close-up views of the Martian surface, as well as essential data about the pressure and composition of the atmosphere, which turned out to be very thin and unbreathable. In 1971 Mariner 9 was inserted into orbit, a good position from which to make a global survey of the planet. The revelation of gigantic volcanoes, huge canyons and ancient riverbeds held the promise of even more extraordinary discoveries, and the Viking mission was born. Its stated objective was to find traces of life.

The Viking spacecraft, comprising two orbiters and two landers, reached Mars in 1976. The Viking mission remains to this day one of the most ambitious explorations ever undertaken by humankind. This mission revolutionized our understanding of Mars, but paradoxically also put an end to the Martian obsession, or temporarily at least: the biological experiments on board the landers returned no conclusive result.

A unique feature in the history of Martian exploration was that each of the two Viking landers included a seismometer. The instrument on Viking 1 could not be unlocked and remained unusable, but the one on Viking 2 was able to record data. Unfortunately, geophysicists were disappointed as winds disturbed the instrument to such an extent that despite months of observation, no true seismic signal ever reliably emerged from the data collected.

Martian exploration started again properly in 1996, 20 years after the Viking mission. It was a particularly glorious year for NASA, which launched two outstandingly successful missions: Mars Global Surveyor, which orbited the planet, and Mars Pathfinder, which landed a miniature rover for the first time.

Releasing capsules for a network mission (©rights reserved).

The Pathfinder mission in particular was the focus of an unprecedented media frenzy that took even NASA by surprise. Thanks to the Internet, the whole world was able to follow the first perambulations of the mission's star, the Sojourner rover. Few people knew, however, that the Pathfinder mission was just the tip of a far more ambitious iceberg, MESUR. This gigantic project would have landed no fewer than 16 geophysical stations on Mars, all fitted with seismic sensors. Pathfinder had been designed from the outset as a technology demonstrator for MESUR, and its primary objective had been not to land a small rover, but... a seismometer.

At that time the Optimism seismometer developed at the Institut de Physique du Globe de Paris (IPGP) had been recommended by a review panel of American seismologists. However, this recommendation came to nothing because of technical integration difficulties.

The Russians—also in the race in 1996—were unluckier. The ambitious Mars 96 mission, involving several European laboratories and consisting of an orbiter, two ground stations and two surface penetrators, crashed into the Pacific Ocean shortly after launch.

This was another very serious setback for Martian seismology since the small autonomous stations included in particular the broad band Optimism instrument developed at IPGP, on which geophysicists were counting heavily to help pierce the mysteries of Mars' internal structure.

Several projects were initiated over the years in an attempt to send seismometers to Mars. The French-American NetLander project, designed to land four observation stations on the Red Planet, was shelved in the 2000s even though work was well advanced and the project was at the end of phase B.

The European Space Agency envisaged landing a seismometer on Mars under the ExoMARS programme. Revised several times for technical and budgetary reasons, its objectives became less ambitious and once again, planetary scientists had to reconcile themselves with the cancellation of the Humboldt geophysical package.

However, all the work carried out over the course of the various projects brought to a halt partway through, or failed missions such as Mars 96, was not wasted. The SEIS seismometer on board the InSight probe due to be launched in 2018 represents a legacy going back several decades.

Forty years after the first attempts of the Viking landers and 20 years after the launch failure of Mars 96, InSight will take up the baton once again and may finally give geophysicists the opportunity to pursue their quest to unveil the mysteries of the Red Planet that have lain buried in its impenetrable depths for billions of years. If we look back into the past, it can be seen that Martian seismology has roughly one chance every 20 years. It is both much, and very little. This is not an opportunity to be missed.

Last updated : 26 october 2016

The Mars 96 Optimism experiment: a seismometer on Mars 20 years after Viking

On the 16th November 1996, Russia re-engaged with one of its old obsessions—the exploration of Mars—by launching a particularly ambitious probe known as Mars 96. Consisting of an orbiter carrying an impressive list of scientific instruments, the mission also included two small autonomous stations capable of landing gently on the surface in addition to two penetrators which, launched like darts from orbit, would embed themselves in the rocky surface to study the subsoil.

The Mars 96 capsule landing sequence (© David Ducros).

The stakes were high for Mars 96, and the size of the space probe was probably only matched by the breadth of international cooperation.

This major project involved scientists from many countries, including France. Among the instruments proposed by the French laboratories was the Optimism seismometer mounted on small ground stations.

The Mars 96 broad band seismometer

Optimism was actually a French acronym for Observatory of PlaneTary scIences: MagnetIsm and Seismology on Mars, though it humorously reminded everyone at the same time of the need to be optimistic when trying to listen out for seismic tremors on a planet as quiet as Mars.

Before Mars 96, the only seismometer ever to have worked on Mars was the one on the Viking 2 lander, which had left scientists imagining that seismic tremors were rare events on the Red Planet. Over a 19-month period of listening, admittedly under difficult conditions, just one quake was possibly detected. A certain dose of optimism seemed necessary for those scientists seeking to detect Martian tremors, given that the small ground stations aboard Mars 96 had to be fully automated, being incapable of receiving commands from Earth. This challenge—which is still pertinent today, as proven by the InSight mission—was taken up by a French team, who developed an instrument much more sensitive than that launched in 1976 on the US Viking mission.

Designed by the Institut du Globe de Physique de Paris (IPGP/CNRS) in collaboration with the Institut National des Sciences de l'Univers (INSU/CNRS) and the SODERN (EADS) company, development of the Optimism seismometer posed many challenges. As with all space seismometers, the instrument had to be small, light (mass being a major source of cost for space activities), power-efficient and very sensitive, while still being able to withstand the bumps and bangs of lift-off and landing.

The Optimism seismometer for the Mars 96 mission (© Hervé Piraud/IPGP).

The challenge was taken up by the technical teams. With a mass of just 450 g the instrument fitted into a roughly 9 cm cube and consumed very little electrical power (a mere 67.5 mW).

The Optimism seismometer was sensitive to long-period seismic waves (from 0.5 to 10 seconds), but on the vertical axis only.

The displacements of the mobile mass, attached to a spring, were registered by two sensors, one measuring the position of the pendulum, the other its speed. The sensitivity was already very good since Optimism could react to ground movements in the order of nanometres over periods of 2 seconds. The seismometer was powered by a battery with a lifetime of one month, though it was also connected to the power supply of the station that housed it.

The pendulum was enclosed within a relatively highly evacuated titanium hemisphere roughly 1 litre in volume. This was particularly important since it allowed any temperature variations inside the probe to be effectively reduced.

Optimism was not deployed on the Martian surface like SEIS, but was designed to operate inside the stations once on the ground after having jettisoned their airbags. This setup no doubt lacked the performance advantages of InSight but was nonetheless a great improvement on the Viking landers. This progress was made possible in large part by the use of a rigid carbon fibre structure integrated within the small station and supplied to the Babakin Space Centre by DT-INSU and the IPGP. This structure improved the seismic coupling for the Optimism experiment.

The mobile mass could be recentred by a small motor that could also adjust the equilibrium position of the pendulum as a function of the uncertainties related to the magnitude of gravity at the landing site. Once the stations had landed, the seismometer was automatically levelled by gravity to an accuracy of 1°. To obtain the most reliable measurements possible, Optimism was also supported by temperature sensors and inclinometers.

Other geophysics experiments apart from seismology had been included in the Mars 96 mission programme. This is why the ground stations included a weather station and a magnetometer. The penetrators were also fitted with a short-period seismometer, an accelerometer, a temperature sensor and a magnetometer, all constructed by the University of Braunschweig in Germany, the data being acquired by Optimism's electronics package.

Rotative view of the Mars 96 Optimism seismometer (© IPGP).

The Mars 96 orbiter releases two capsules containing the small autonomous ground stations (© Manchu/Ciel & Espace).

The loss of Mars 96

The exploration of Mars is very risky, and the fate of the Optimism seismometer bears witness to this fact. Despite the care, effort and resources that went into its development, what remains of the instrument now lies at the bottom of the Pacific Ocean.

On the 16th November 1996, roughly one hour after lift-off, the fourth stage of the Proton launcher experienced a major malfunction, ultimately condemning the Mars 96 probe to a low Earth orbit. Under the effect of atmospheric friction, the spacecraft’s orbit deteriorated rapidly and it plummeted into the depths of the Pacific Ocean the following day, taking with it the hopes and dreams of an entire army of scientists and engineers. The internal structure of the Red Planet would remain shrouded in mystery for a while longer.

Now it falls to InSight to walk in the footsteps of the Viking probes, 40 years after their touchdown on the surface of the Red Planet and 20 years after the dramatic launch failure of Mars 96. Will Mars this time reveal its most intimate secrets? Just as in the past, optimism is still in the air today among planetary seismologists.

Characteristics of the Optimism seismometer

• Vertical-axis seismometer (BRBZ, Broad Band Z-axis).

• Measures the displacement of the proof mass: capacitive sensor for position and inductive sensor for speed.

• Bandwidth: 0.02 to 2 Hz.

• Sensitivity: in the order of 10-9 g between 0.1 Hz and 1 Hz, i.e. a gain of 100 compared to the Viking landers.

• Shock resistance: 200 g for 10 ms (to be compared with the sensitivity of 10^-9 g).

• Sampling rate: between 1 every 4 s and 4 per s.

• Data rate: 1 Mbit/day (0.5 Mbits per telemetry opportunity).

• Thermal expansion: mobile mass displaces 300 nm per degree Celsius.

The Optimism team

The logo of the Optimism seismometer (© IPGP).The Optimism instrument was financed by French space agency CNES. The Institut de Physique du Globe de Paris (IPGP) was responsible for scientific aspects, while the technical directorate of the Institut National des Sciences de l'Univers (DT/INSU) was responsible for technical aspects. The SODERN (EADS) company participated in Optimism's design and was selected to construct it. A company called SOREP was chosen for the electronics part. Finally, many other partners worked on the project, including AETA, Martin-Pfeil, VERELEC, the Paris Observatory, the Laboratoire de Physique du Solide in Meudon, and TU-Braunschweig-IGM.

Last updated : 26 october 2016

NetLander, the first real attempt to deploy a network of seismometers on Mars

Background

Artist’s view of the Mars Premier Orbiter carrying the Netlander modules. This Franco-American mission aimed to return Martian soil samples to Earth (© CNES/David Ducros).The aim of the international NetLander project was to deploy a network of 4 identical stations on Mars. Each small station was equipped with a suite of measuring instruments, including 2 seismometers.

Developed in the late 1990s, the NetLander mission was characterized by a strong international contribution, since it involved not just France but also the United States, Finland, Germany, Belgium, Italy, the United Kingdom and Switzerland. The NetLander stations would leave for Mars on an Ariane 5 launch vehicle, under the umbrella of a vast Franco-American programme to return samples from the planet, initiated just after the announcement of the potential discovery of fossilized microorganisms in Martian meteorite ALH84001.

The sample return mission required two landers to be sent to Mars in 2003 and 2005, each equipped with a rover and a rocket-like Mars Ascent Vehicle (MAV). After having taken several soil, rock and atmospheric samples, the rovers would place their precious harvest on board the MAV, which would then boost the sample canisters into orbit around Mars.

In 2005, an Ariane 5 launch vehicle was planned to launch an orbiter and lander. After having been placed in orbit around the Red Planet using a novel but risky aerocapture technique, the main objective of the orbiter was to recover the sample canisters and bring them back to Earth in a return module. Its secondary mission was to deploy the NetLander capsules, which did not have their own means of transport to Mars.

Artist’s view of the Premier Orbiter releasing the Netlanders (© CNES/David Ducros).

The mission that we have briefly described to return samples from Mars was very ambitious, and technical difficulties threatening the project became apparent from the very start. In 1999, NASA's loss in quick succession of Mars Climate Orbiter, followed three months later by Mars Polar Lander, a prototype of the spacecraft intended to return the samples, put an end to the project.

Without the support of NASA, the French space agency—CNES—decided that just sending the orbiter to release the NetLanders and test the aerocapture technique was financially unrealistic, not to mention growing political tensions with the United States.

As a consequence, the NetLander project, which had already entered phase B, was shelved in the early 2000s, dashing all hopes of being able to finally place seismometers on Mars.

The NetLanders

Transported to Mars as passengers on board the French sample return orbiter, the NetLanders shared many features with the disc-shaped MESUR modules, which is hardly surprising given that both missions had the same objective, i.e. landing a network of geophysical observatories on the Red Planet.

Landing sequence of the NetLanders (© CNES/David Ducros).

The NetLanders had a thermal shield to allow them to survive the journey through the atmosphere as well a parachute to slow the package down sufficiently to survive the impact with the Martian surface. Two airbags would cushion the final shock and, after having bounced several times, the 47 cm diameter capsules would finally came to rest before opening like a flower. The stations would be powered by solar arrays and have communications antennas.

From an instrument point of view, the capsules were particularly well equipped: each one carried a mast fitted with a camera, a complete weather station and a geophysical package comprising two seismometers, a magnetometer, a radar set, an electric field sensor and temperature probes. The telecoms system was installed to make geodesic measurements, and there was also an onboard microphone provided by the Planetary Society for a teaching initiative.

The SEISM seismometer

One of the main objectives of the NetLander mission was to determine the internal structure of Mars. To this end, the 4 stations would be directed to precise points on the Martian surface to establish a sensory network to triangulate any quakes. Three stations would be deployed roughly 1,000 km apart in a triangular configuration, with the fourth being sent to the antipodes.

The sphere of the SEISM seismometer for the NetLander mission. Click to see inside (© Hervé Piraud/IPGP).

The identical stations were fitted with two seismometers, one sensitive to short-period seismic signals (100 mHz to 50 Hz), the other able to measure long-period signals (10 mHz to 10 Hz), with a very-long-period output for measuring tides. It is this feature that is of particular interest here, given that SEIS, the InSight seismometer, has inherited its characteristics.

Known as SEISM, this seismometer only covered 2 Very Broad Band axes (one horizontal, one vertical) placed head to toe, while SEIS has three. The missing broad band axis was replaced by one of the short-period seismometer axes, enabling it to locate tremors in 3 dimensions.

The pendulums were enclosed in an evacuated sphere 18 cm in diameter which provided some protection against the particularly large temperature variations that occur on the surface of Mars. Temperature, pressure and inclination sensors were included in the package.

The sensitivity of the SEISM pendulums was already high and comparable to InSight, but the conditions of installation would have lowered the precision compared to InSight. The NetLanders had no deployment mechanism capable of placing the seismometer on the surface of Mars, and it was therefore deployed on the ground via 3 apertures in the floor of the probe, through which the legs could descend. The instrument would then be detached from the structure of the lander capsule. If this sophisticated system were to fail, the seismometer would then have been compelled to function as and where it was—i.e. inside the lander capsule—which would have been unsatisfactory due to ineffective contact with the ground (the famous coupling by which geophysicists set so much store).

SEISM was supported in its mission by a miniaturized short-period 3-axis seismometer. Etched in a silicon wafer, this microseismometer was sensitive to ground motion characterized by fairly high frequencies (0.1 Hz to 100 Hz). This frequency band is not optimal for surface waves generated by distant quakes, but it is for regional and local seismic activity.

As SEISM was to SEIS, so the NetLander short-period seismometer was a precursor to the short-period (SP) seismometer that is now on the InSight probe.

Last updated : 26 October 2016

An ambitious Martian geophysical network project

Releasing capsules as part of a planetary network mission (©rights reserved).

During the 1990s, NASA investigated a mission concept known as MESUR (Mars Environmental SURvey).

Initially put forward by the Ames Research Center (commonly known as NASA Ames), the scientific objectives of this ambitious project, costing between 750 million and 1 billion dollars, were many and varied: atmospheric research, geological investigations, and the search for signs of life. It was well received by the scientific community, who saw it as a unique opportunity to continue their study of Mars after the Viking probes in 1976.

MESUR involved the deployment of multiple science stations (16 were originally planned) at numerous points on the surface of the Red Planet, including the poles. These would have formed an observational network encompassing the planet. Disciplines such as meteorology or seismology that require the acquisition of data from multiple locations would have been undertaken under ideal conditions. By its very nature, a network can tolerate the loss of an element, which is a sizeable advantage in a domain as risky as that of space exploration.

The MESUR stations

The probes involved in the MESUR project were broadly disc-shaped. With a mass of roughly 160 kg, they were fitted with airbags to cushion their impact with the Martian surface during the final landing phase. Bristling with numerous instruments, they were all designed on the same model. They carried sensors to study the structure of the atmosphere as the stations plunged towards the ground, a weather station, a descent and/or surface camera, a spectrometer to analyse the elemental composition of the rocks and soil, a three-axis seismometer and an analyser for volatile material. They would have been powered by the decay of radioactive elements.

Seismometer

Among the key instruments of the MESUR stations was a seismometer that absolutely had to be deployed on the ground in such a way as to be isolated from the inevitable vibrations of the lander (a requirement not met during the Viking mission, with unfortunate consequences: often buffeted by wind on the lander platform, Viking 2's seismometer failed to send back any usable data).

Engineers then applied themselves to studying possible mechanisms for deploying the seismometer: ejection by a spring or placement using an arm or flexible mast, for instance. However, none of the solutions envisaged was adopted by the designers, mainly because they were too rough for the sensitive instrument to be deployed. The very relative agility of the arms on the Viking landers (the solution adopted by the InSight mission) militated against this device at the time. Attention then turned to an attractive alternative, namely a microrover.

At that time another NASA centre—the Jet Propulsion Laboratory—was very interested in the potential of mobile robots or “rovers” for exploring the planet Mars. The immobile nature of the MESUR science stations seemed inconsistent with the characteristics of such vehicles. In view of the small size of the envisaged stations, if mobile robots were to be carried on board they would also have to be small, whereas the rovers envisaged were heavy and bulky. Nevertheless, some engineers at JPL separately started to consider the possibility of developing a microrover.

A mission as ambitious as it was complex

Diagram of the MESUR project’s geophysical observation stations (© NASA).

To reach its final size, the MESUR network had to rely on several launches, each spaced two years apart (consistent with the constraints of celestial mechanics). Spreading the launches out over time also allowed costs to be reduced.

The first stations therefore had to have a long life so that they would still be operating when the last stations became active. They had to have a lifetime of at least 8 years: 6 years waiting and 2 additional years for data acquisition with a network operating at full capacity.

It was then necessary to determine the landing sites. The stations focused on the weather would have to be distributed all around Mars, from the equator to the poles through the middle latitudes, while the seismic stations were to be placed in a triad formation.

Despite that, not all the scientists were happy with the choice of sites, geologists' requirements being different from those of meteorologists, geophysicists and exobiologists.

Finally, there was no probe orbiting Mars at that time and capable of relaying data back to Earth, and the issue of getting scientific data back to Earth posed a major problem for the project designers. To address the lack of communications infrastructures in orbit around Mars, a telecoms orbiter was then added to the mission, significantly increasing its cost.

The birth of Pathfinder and the end of MESUR

The increasing complexity of the project and budgetary constraints led to a drastic revision of the MESUR project’s initial ambitions. Contrary to the initial schedule, the 1996 launch window was assigned to a technology demonstrator known as SLIM (Surface Lander Investigation of Mars). This single station was intended to demonstrate the technological merits of MESUR and convince the political powers that be of the project's merit.

Those scientists who were counting on a network to allow them to study Mars in its globality did not hide their disappointment at this solitary station that left practically no room for measuring instruments. To save the project, JPL—which supported a mobile solution for exploring Mars—promoted the idea of using a microrover to deploy instruments such as a seismometer or X-ray spectrometer to collect chemical information about the Martian rocks. The objective was also to rekindle public interest in the mission, since the general public was still hungry for technological novelty.

Initially run by NASA Ames, responsibility for the MESUR project was then shifted to JPL. At that stage it still involved deploying a network of 16 stations around Mars, making use of the launch windows in 1998/1999 (4 stations), 2001 (4 stations and a telecoms orbiter) and 2003 (the last 8 stations using 2 launchers).

Artist's impression of the Pathfinder probe and the Sojourner microrover (© Manchu/Ciel & Espace).

The SLIM demonstrator was also a good opportunity for JPL to test a new mission concept then being studied, called Discovery.

Less expensive, carried out within a tight schedule and oriented towards a precise theme, these projects presented an alternative to the extremely complex and costly missions to explore the solar system that were NASA's daily bread.

The SLIM demonstrator, whose lift-off was still slated for 1996, was then renamed MESUR Pathfinder. The seismometer was removed from the mission on cost grounds. Even though offered free of charge by another country, integrating it in the lander would nevertheless have increased the costs. Since its priority was deemed less than either the microrover or the camera, it was cancelled.

By that stage, Pathfinder was drifting further and further way from MESUR. Compared with the initial stations, its life was no more than 30 days (as against several Martian years of operation), solar arrays had replaced the nuclear-powered generator, the number of scientific instruments continued to fall, and the probe no longer looked like a hollow saucer, but a tetrahedral. Only the idea of landing using airbags had been kept. Compared to the ambitious Viking mission in the 1970s, the whole thing was pretty paltry and only the Sojourner microrover seemed to be of any real interest.

MESUR was finally abandoned for budgetary reasons. After the dramatic loss of the Mars Observer probe in 1993, NASA was no longer willing to invest in missions costing over a billion dollars. The mission proposed by JPL was no longer a technology demonstrator for the MESUR project, but was transformed into a mission that would usher in the era of "faster, better, cheaper", and which would initiate the Discovery programme to which InSight now belongs.

On the 4th of July 1997, 20 years after Viking, Pathfinder landed on the rust-red surface of Mars. For NASA, this was a triumphant return to the Red Planet. The mission was not only a technical success, validating the airbag system, but was very popular with the media. The meanderings of the Sojourner robot were followed on the Internet every day by millions, fascinated by the adventures of the little explorer as well as by the magnificent colour images of the Martian landscapes in Ares Vallis.

However, this should not distract us from the fact that the initial objective of the small Pathfinder station was to put a seismometer on Mars. For geophysicists, MESUR's demise was a great disappointment; however, these same geophysicists did not yet know that the road to Mars would be even longer than expected...

Last updated : 08 february 2017

ExoMARS, the European Space Agency's ambitious programme

On 16th October 2016, the Trace Gas Orbiter spacecraft released the Schiaparelli module towards Mars (© ESA/David Ducros).

At the start of the 21st century, the European Space Agency (ESA) initiated a very ambitious programme to explore the solar system, carried out first by automated probes and then, in the longer term, by humans.

The Red Planet plays a major role in this project, being the target of several missions under an umbrella programme known as Aurora. The dawning of this new programme underlined ESA’s overriding motivation to seek signs of life on Mars, whether living or fossilized, before embarking on a mission to return samples to Earth, and ending up with the final stage of putting people on the Red Planet.

As the years have gone by, ESA has been forced to gradually lower its expectations, the Aurora programme constantly coming up against budgetary constraints and technical hurdles that require frequent revisions.

In the still uncharted domain of Martian planetary seismology, one option in particular still excited geophysicists. The idea was to deploy on Mars a network of small modules similar to the Beagle 2 capsule, jettisoned towards Mars in 2003 by the first ESA mission to Mars, Mars Express. Sadly, Beagle 2 never reached the planet’s surface. Each module of the BeagleNet mission was planned to include a seismometer inherited from that on board the NetLander modules.

ExoMARS

In the end, the ExoMARS mission was given priority. Though oriented more towards exobiology investigations, it did not, however, ignore other disciplines such as seismology.

It was in this context that geophysicists proposed to provide a lander module with a full array of geophysical instruments. Expected to operate for at least two years, the ground station would be launched from space by a satellite designed to closely study the Martian atmosphere while orbiting around the planet. The launch was scheduled for 2013 on board an Ariane 5 launcher, enabling planetary scientists to finally achieve their old dream of putting a seismometer on Mars.

Humboldt, a suite of 11 instruments

Previously known as the Geophysical and Environmental Payload (GEP), the static Humboldt package comprised 11 instruments: besides the seismometer (which we shall mention in a moment), there was also a device for measuring heat flux (inherited from Beagle 2 and very similar to InSight's HP³ instrument), a magnetometer, a complete weather station (measuring temperature, pressure, wind and humidity) and a geodesic experiment using a radio telecommunications system.

This payload, identical to that on the InSight mission, also contained sensors to measure radiation and ultraviolet rays, dust detectors, an instrument to measure the electrical charge of the atmosphere, and finally a ground-penetrating radar to study the subsurface.

Landing sequence of the Schiaparelli module (© ESA).

The SEIS seismometer

Already known as SEIS at the time, the ExoMARS seismometer benefited from the work carried out by the teams at the Institut de Physique du Globe de Paris on the NetLander mission, and even before then on the Mars 96 mission.

It consisted of 2 very broad band (VBB) oblique inverted pendulums in an evacuated sphere, similar to those on the InSight probe. A short-period broad band microsensor stood in for the third axis to form a hybrid 3-axis system (2 VBB axes and 1 SP axis). 

With a mass of 2.2 kg, this seismometer had a locking system to protect against violent jolts during lift-off and landing.

Cancellation of Humboldt

The Schiaparelli demonstrator, ExoMARS 2016 mission (© ESA).

As far as ESA was concerned, the ExoMARS roadmap was becoming more and more difficult to follow, and it became necessary to clip the project’s wings. In 2009, at the Preliminary Design Review (PDR) that concluded phase B, the ExoMARS mission was postponed until 2016, and the Humboldt payload was abandoned for technical (mass) and budgetary reasons.

The vast array of instruments planned at the start was whittled down to just one small meteorological station, together with a sensor for measuring the electric charge of the Martian atmosphere. These 2 devices were placed on board a small capsule called Schiaparelli (in honour of the great Italian astronomer Giovanni Schiaparelli), and lifted off on a Proton rocket at the same time as the TGO satellite in March 2016. Unfortunately, the capsule crashed on Mars on the 19th October 2016 due to a critical malfunction in the guidance software.

From the network of 16 stations bristling with instruments that was originally planned, the American MESUR project was reduced to sending a technology demonstrator practically devoid of any science due to insufficient payload capacity. Similarly, the initial ambitions of the Aurora/ExoMARS programme have had to be significantly downsized.

The missions lucky enough to leave for Mars have behind them a legacy spanning decades but laced with much bitterness and shattered hopes. Initially planned for the short term, objectives have been shunted several decades into the future, nothing being able to be done about it.

When the InSight probe lifts off for Mars with its seismometer on board, it will finally realise the long-anticipated dream of geophysicists involved in the shelved MESUR and NetLander projects, the Mars 96 mission (that failed on launch) and ExoMARS (divested of its seismometer prior to flight, which ended in a crash landing). The road that leads to the Red Planet is tortuous, full of pitfalls and obstacles, so the first seismic data to be received from way up there will be all the more precious.

Last updated: 26 October 2016

A mechanism of unprecedented precision

The SEIS seismometer consists of three sophisticated pendulums sensitive to a broad range of vibrations and firmly attached inside a titanium sphere in strict vacuum conditions. They are thus qualified to be called Very Broad Band (VBB) sensors. Designed from scratch for the Martian environment (which is far more hostile than that of Earth), SEIS leaves little to be desired when compared to the best terrestrial seismometers. Although its performance is inferior to the best instrument on the market, that instrument is ten times heavier, and very power-hungry.

A two-part pendulum joined by a pivot

Pictorial representation of a VBB pendulum (© IPGP).

As with all terrestrial seismometers, VBB pendulums have a moving part that oscillates with respect to a fixed part. This oscillation is driven by the seismic disturbances transmitted to the instrument through the ground. A lamellar, strip-type pivot with a quite astonishing operating principle allows the moving part to articulate with respect to the fixed part and is effectively the pendulum's axis of rotation. It was designed to allow extremely small movements (of very low amplitude) to occur with complete freedom in the absence of any intervening friction.

Inverted pendulum: sensitivity and instability

The SEIS seismometer exploits the inverted pendulum principle. Unlike a conventional pendulum, where the mobile mass is suspended vertically by a spring from a fixed base, the inverted pendulum is a far more unstable system.

Fixed (in red) and mobile part (in green) of a VBB pendulum (© IPGP/david Ducros).

Imagine a flexible rod firmly anchored to a fixed base with a mass attached at the top. It is easy to understand that such a precariously balanced device is sensitive to the slightest vibration. It is also clear that, at the slightest nudge, the mass will tend to flop irreversibly over to the right or left unless the system has been designed and balanced with sufficient care.

The inherent instability of the inverted pendulum makes it more sensitive than the conventional pendulum, even with a lightweight mobile mass, which is a huge advantage in the space sector. The SEIS mobile mass is just 190 grams, as opposed to the far heavier masses of terrestrial seismometers, which are not far off 1 kilogram for the best instruments.

A specially designed spring

Although unstable by their very nature, the inverted pendulums in the SEIS seismometer maintain their equilibrium using a lamellar spring formed into a semicircle; this continuously applies a restoring force to the mobile mass, thereby preventing it from ultimately conceding to the force of gravity.

Each spring is unique, and none is interchangeable among the three SEIS pendulums. They are individually manufactured and take into account the characteristics of the pivots, which are also unique.

None of the three pendulums is therefore identical either; each has its own personality. The tailoring performance of the engineers responsible for designing and manufacturing these technological gems matches that of any top fashion house.

Side view of a Very Broad Band (VBB) pendulum (© IPGP).

Displacement sensors

Using a set of electrodes attached to the moving and fixed parts, each pendulum can measure very precisely at each instant the motion of the ground through the motion of the moving part.

A feedback system, which uses an electromagnetic coil to continuously feed back the location of the pendulum relative to its equilibrium position, further increases the performance of the seismometer, in particular for very slow displacements of the ground.

Three axes for three spatial directions

For any given pendulum, the moving part can be displaced in only one spatial direction, which is defined by the pivot’s axis of rotation. The InSight seismometer is a three-axis instrument, i.e. it consists of one pendulum for each direction in space, making three in all.

It would be logical to think that one of the pendulums would be positioned vertically, the other two being disposed horizontally. In reality, although the three pendulums are mutually at right angles, they are actually inclined obliquely at 32.5° to the horizontal. Although it might appear strange, this arrangement distributes the noise over all three axes and consequently increases the signal to noise ratio.

With an oblique geometry, any jolt or vibration will cause the three axes to react with varying intensities. Let us take the case of a purely vertical impulse acting on a three-axis seismometer whose sensors are aligned exactly along the three spatial directions: one vertical and two horizontal pendulums. In this case, we see that only the vertical sensor is excited; the impulse, having only a vertical component, is effectively invisible to the horizontal pendulums.

3-D model of a VBB pendulum (© IPGP / David Ducros).

On the other hand, if the group of three sensors are inclined slightly, a purely vertical impulse will induce a signal in each of them. The strongest signal will be recorded by the sensor that is closest to the vertical, but the other two will also register something. In a sense, we can say that an oblique configuration distributes (and therefore divides) the noise level over all three sensors, rather than being concentrated on just one. Although this configuration requires all three sensors to register vertical motions, it has been adopted by InSight quite simply because it allows a 3-axis instrument to be built using the same type of sensor.

A Martian instrument

While the pendulums of the SEIS seismometer hinge around systems found in all terrestrial seismometers (pivot, spring, displacement sensors, feedback), other systems have been designed specifically to meet the requirements of the mission, making SEIS absolutely unique, even among the best terrestrial seismometers.

A sophisticated equilibrating mechanism enables the pendulum to adapt to Martian conditions, such as the reduced gravity or an inclination due to the surface on which the seismometer rests (unless corrected by the levelling system). A thermal compensation device and supporting mechanism (TCDM) will enable the pendulums to adjust to daily and seasonal temperature variations.

All the electronics in the instrument are also designed to withstand the radiation to which they will be subjected on their trip from Earth to Mars (cruise phase), as well as the operating temperatures on Mars, which are significantly lower than those encountered in terrestrial geophysical observatories (except for those deployed in the Antarctic).

Pendulum details (© Hervé Piraud / IPGP / SODERN).

Because of the huge temperature contrasts that are a feature of the Martian weather, project engineers have had to isolate the pendulums from the external environment as far as possible by an extraordinary number of insulating layers. This is the case, for example, of the evacuated container which contains the pendulums in a vacuum, the RWEB (Remote Warm Enclosure Box) which contains the sphere, plus the dome-shaped WTS (Wind and Thermal Shield) which covers the whole assembly.

However, despite these precautions, the temperature within the sphere will vary slowly and inevitably with the passage of time, by about 20 degrees in summer and 10 degrees or so in winter. The TCDM will finely adjust the pendulums’ centre of gravity, needed to compensate for any sudden temperature fluctuations and obtain the best seismic sensor performance.

Last updated: 19 january 2018

A gas-tight evacuated titanium sphere

Seismometers are very sensitive to the slightest variation in temperature. On Earth, they are generally installed in environments where the temperature is very stable, such as in caves or mineshafts. It is impossible on Mars to dig a hole, and the instrument will therefore be subjected to particularly wide daily and seasonal temperature variations. Even on the InSight landing site, the Elysium Planitia, the difference in temperature between day and night will be 70°C on average.

Sphere opened up, showing the two hemispherical shells as well as the Very Broad Band pendulums (© IPGP/SODERN).

In order to reduce temperature contrasts as far as possible, the seismometer's very broad band pendulums are placed in a highly evacuated titanium sphere.

With a volume of approximately 3 litres (i.e. roughly the size of a watermelon), this enclosure—constructed by the Jet Propulsion Laboratory in Pasadena—comprises two very light titanium hemispheres, laser-welded around a circular ring on which three pendulums (one for each spatial direction) are solidly attached.

The ring is also fitted with a series of vacuum feed-through connectors allowing electrical cables to pass through and a small copper pumping tube used to evacuate the sphere, i.e. to remove the air so the sphere’s contents are kept under vacuum conditions.

During the crucial stage of evacuation, the sphere is first baked to maximize vaporization of compounds that might condense on the inside. The air is then removed using very powerful pumps. Once all the gaseous contents have been evacuated, the pumping tube is crushed using a specially designed crimper to finally hermetically seal the sphere.

Temperature sensors are added for monitoring and diagnostic purposes.

Protecting the pendulums

One of the roles of the evacuated sphere is to allow the pendulums to operate in as clean an environment as possible. Slipping into certain places such as the pivot or motion sensing electrodes, a micrometre-sized particle or a sticky film of organic molecules could arrest the delicate mechanisms of a pendulum and paralyse the seismometer.

3D representation of the protective sphere around the SEIS seismometer (© IPGP).The vacuum within the sphere eliminates the Brownian motion that would otherwise seriously interfere with seismic measurements. When heated, the innumerable particles (atoms and molecules) that make up the atmosphere jostle randomly and incessantly, rather like dancers in a night club. This is what physicists call Brownian motion.

Some particles would inevitably strike the mobile mass of the pendulums, generating a seismometer signal. This is why it is vital to remove all the gas from the sphere, because evacuation is the only way of eliminating as much of this source of interference as possible.

Although the pressure of the Martian atmosphere is 1/100 that on Earth (less than 10 mbar on Mars as against 1 bar on Earth), it is still too dense for the pendulums. One possible technique would have been to fit the sphere with a filter valve (to prevent the ingress of dust) so as to balance out the pressures inside the sphere and outside in the Martian atmosphere. However, this would not have been sufficient.

Even more importantly, the absence of air in the titanium sphere also provides the pendulums with the best possible isolation from variations in the outside temperature, which is absolutely fundamental for the quality of seismic measurements, especially when it comes to recording long-period waves. This also explains why so many means of insulation are used to protect the SEIS seismometer from fluctuating temperatures.

Flight model of the SEIS sphere (© IPGP/SODERN/CNES/Piraud).

Not only does the titanium sphere provide an evacuated environment for the pendulums, but its internal surface is coated with a very thin layer of thermal insulation. The sphere itself is enclosed in an external insulated shell (RWEB) and, once on the ground this assembly will itself be covered by a substantial wind and thermal shield, or “WTS”.

Ensuring a low pressure

Over the course of time, the pressure within the sphere will inevitably rise. To keep a check on the pressure inside the sphere, in the absence of internal barometers, there are temperature probes on the pendulums that will allow engineers to assess the quality of the thermal insulation at any given instant, and hence deduce the level of the vacuum within the sphere.

At the start of the InSight mission, just after landing, the pressure in the sphere will be roughly 0.01 mbar. Two Earth years later, at the end of the mission and provided no major leak has occurred in the meantime, the pressure will have risen to around 0.1 mbar, a level nonetheless sufficient to continue making seismic measurements should the mission be extended.

The three VBB pendulums inside the titanium evacuated container (© SODERN).

In order to guarantee that very low pressures in the sphere are maintained, it is also fitted with what are effectively "gas sponges". The purpose of these devices is to trap any gases that might still be present in the sphere after baking and evacuating. They will also absorb any Martian CO2 should a micro-leak appear while on Mars.

Last updated: 26 february 2018