SESAME - Surface Electric Sounding and Acoustic Monitoring Experiment

 Lander Philae with SESAME sensors
zum Bild Lander Philae with SESAME sensors

SESAME is a complex of a dozen sensors, which are accommodated at different places on the lander, and a dedicated data processing unit. It will measure physical properties of the comet.

SESAME comprises the instruments Comet Acoustic Surface Sounding Experiment (CASSE), Dust Impact Monitor (DIM), and Permittivity Probe (PP).

The integration of SESAME components as well as the development of the CASSE instrument were coordinated by the former Institute of Space Simulation of the German Aerospace Center (DLR) in Cologne in collaboration with the engineering company of von Hoerner & Sulger GmbH. The flight software of SESAME was developed at the department of Nuclear Chemistry of Cologne University.

The operation of the instruments and the interpretation of the data are planned and executed by the DLR Institute of Planetary Research in close cooperation with scientists from several European countries.

CASSE - Comet Acoustic Surface Sounding Experiment

 CASSE Transmitter
zum Bild CASSE Transmitter

The CASSE instrument shall investigate the cometary surface by acoustic sounding methods. To this end, the soles of the Lander feet will be used as transmitters and receivers of elastic waves. The landing gear of Philae is a tripod, where each foot has two soles that can move vertically with respect to the leg and the other sole. This ensures good ground contact of both even on a rough surface.

The acoustic transmitter and receiver system of CASSE has two principal operation modes:

  • Passive listening: listen to acoustic emissions of the comet itself like a seismometer. Such emissions may comprise release of thermal stresses, particle impacts, and quakes. Sounds form Philae experiments may also be recorded. The hammering mechanism of the MUPUS experiment is of special interest, since it represents a strong source at a known position.
  • Perform actively, like a sonar, a sounding of the cometary subsurface structure and to determine elastic constants. An analysis of echoes from the underground might allow to deduce the subsurface structure, e.g. any layering.
 CASSE Receiver
zum Bild CASSE Receiver

One of the soles of each foot contains a piezo-electric transmitter, which excites acoustic vibrations of the sole. These vibrations can be recorded by piezo-electric accelerometers. In each of the other three soles of Philae. Temperature-dependent resistors (PT1000 type) will measure the sensor temperatures.

CASSE was developed at the former DLR Institute of Space Simulation and was built in collaboration with the engineering company of von Hoerner & Sulger GmbH (electronics). The transmitters were developed by the Fraunhofer Institute for Nondestructive Testing (IZFP), Saarbrücken. The CASSE sensors are commercial tri-axial accelerometers, type 4506, from the Brüel & Kjaer company. The CASSE instrument is operated by the DLR Institute of Planetary Research.

Properties of CASSE:  
Sounding frequency 0.1 to 6 kHz
Receiving frequency < 100 kHz
Temperature range (up to 12 channels simultaneously) -160 to +100°C
Mass 550 g (without soles)
Power 1250 mW



DIM - Dust Impact Monitor

The Dust Impact Monitor does not investigate the cometary surface directly; rather it registers the signals of impacting cometary particles on the sensor cube.

 DIM Sensor: The three active sides with piezo plates are visible
zum Bild DIM Sensor: The three active sides with piezo plates are visible

The sublimation of the volatile constituents of the cometary surface - mainly water ice – carries away dust and ice particles with sizes ranging from micrometer to centimeter. If their initial velocity is too small to leave the cometary gravitational field, they fall back to the cometary surface.

The DIM sensor cube, which is mounted at the upper edge of the balcony of Philae, registers impacting particles from three directions with piezo plates and the electronics analyses the signals with respect to their wave-form (amplitude and contact duration). From this data and plausible assumptions of the particle density one can deduce the masses of the particles, their velocities, number, directional distribution and the time dependence of these parameters. From these data one can deduce the starting velocities of the particles that are correlated with the outgassing velocity and the flux of the cometary gases.

DIM was developed and built at the Atomic Energy Research Institute (EK) of the Hungarian Academy of Sciences (MTA) in Budapest. Now the Max-Planck-Institute of Solar System Research (Göttingen, Germany) takes care of the DIM instrument.

Properties of DIM  
Measuring limits:  
    Particle radii Teilchenradien 0,2 to 5 mm
    Particle massesTeilchenmassen 3 • 10-8 to 5 • 10-4 kg
    Particle velocitiesTeilchengeschwindigkeiten 0,25 to 2 ms-1
Mass 405 g
Power ≤ 170 mW

Dust Impact Monitor (MPS)
Dust Impact Monitor (MTA-EK)


PP - Permittivity Probe

The Permittivity Probe shall determine the water ice content in the cometary surface layer and its variation with time.

 PP electrode (brown mesh) below the hammering mechanism of the MUPUS-Pen experiment
zum Bild PP electrode (brown mesh) below the hammering mechanism of the MUPUS-Pen experiment

To this end PP employs the quadrupole technique, where two electrodes, connected with an AC generator, induce a variable current into the cometary matter. Two additional electrodes detect the resulting voltage variation and phase shift with respect to the injected current. There are three transmitter electrodes on the Philae cometary lander (one in a lander foot and the other two close to be integrated into the sensors of the experiments MUPUS-Pen and APXS). The two receiving electrodes are mounted into the soles of the remaining two feet (see CASSE page). Using the variable geometry of different transmitter electrode-combinations, one can infer the complex permittivity down to a depth of about 2 m.

PP electrode (brown mesh) below the hammering mechanism of the MUPUS-Pen experiment

Thus one can determine the electrical conductivity and the relative permittivity. These material parameters react quite sensitively to the amount of polar molecules, especially water, and their temperature. Using these data one can calculate the water content of the surface, which can vary during one comet day and with the distance of the comet to the sun, and possibly also with the water-vapor production rate. From this it is possible to describe the outgassing behavior and to infer the composition of the comet.

Operating in passive mode, the PP instrument can detect plasma waves with frequencies up to 10 kHz, which originate from the interaction of the solar wind with the emitted cometary particles and gas and are a measure of the cometary activity.

PP was developed and built at the Finnish Meteorological Institute (FMI), Helsinki, in collaboration with the Finnish engineering company Environics Oy and the ESA technology center ESTEC. FMI operates the PP instrument now.

Properties of PP  
Sounding frequency 0,01 to 10 kHz
Receiving frequency 20 up to 40 kHz
Mass 270 g
Power ≤ 320 mW

Permittivity Probe (FMI – Finnish Meteorological Institute)

Klaus Seidensticker
German Aerospace Center

Institute of Planetary Research
, Asteroids and Comets
Tel: +49 2203 601-3104

Fax: +49 2203 61768

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