NSSDCA ID: 2004-006C-06
Mission Name: PhilaeThe Surface Electrical, Seismic and Acoustic Monitoring Experiment (SESAME) is a complex of three instruments sharing common electronics. The three instruments are the Cometary Acoustic Sounding Surface Experiment (CASSE), the Permittivity Probe (PP), and the Dust Impact Monitor (DIM). The scientific objective of CASSE is to measure mechanical properties of the surface of the comet nucleus, PP is designed to study the surface electrical properties, and DIM determines the dust impact and production rate. The results from SESAME will give information which will help constrain models of comet and solar system formation.
The CASSE experiment is designed to use acoustic sounding to determine the mechanical properties of the upper layers of the comet nucleus. It consists of piezo-electric transducers (transmitters) and accelerometers (receivers) mounted in the feet of the lander. The transmitters work by having an alternating current pass through a piezo-electric material which induces vibrations. The vibrations are transmitted to the surface through the bottoms of the lander feet. The receivers are tri-axial piezo-elctric accelerometers which act as microphones to detect the acoustic waves in the surface. CASSE has two modes of operation. The active mode involves operation of the transmitters and subsequent measurement of the generated acoustic (seismic) waves after they have passed through the surface and subsurface layers using a phased-array configuration. This will allow determination of the P- and S- wave velocities and elastic properties of the surface and subsurface material, near-surface layering, structure within the layers, grain size estimates, and possible detection of a transition zone between the disturbed surface and the pristine interior of the nucleus. The passive mode involves use of the use of the receivers only to monitor seismic activity in the comet nucleus. Pt 1000 thermistors on each sensor will measure the temperature. The sounding frequency is 0.1 to 6 kHz, the receivers can detect frequencies below 100 kHz in 12 channels simultaneously. The mass of the CASSE instrumentation is .55 kg and it uses a maximum of 1.25 W. The primary investigator for CASSE is D. Mohlmann.
The PP experiment will use electrodes to examine the electrical properties of the comet's surface layer, specifically it will measure the complex permittivity (electrical conductivity and dielectric polarizability) at the Philae landing site. The primary scientific objectives are to estimate the water ice content near the surface of the comet and to study how the quantity changes from day to night and with distance from the Sun. This will aid in understanding the water sublimation, temperature, and outgassing behavior and the general composition of the comet. The instrument consists of five electrodes. Three are for transmitting the electric pulses, one located in one of the lander feet, one located near the APXS and one located on the MUPUS-Pen. Two receiver electrodes are mounted in the feet of the lander. The instrument employs the quadrupole technique, wherein a variable current is fed to two of the electrodes by an AC generator and the receiver electrodes measure the resulting voltage and phase. Using the variable geometry of different transmitter/electrode combinations the electrical conductivity and relative permeability of the surface can be measured down to a depth of two meters. The PP also has a passive mode in which no transmisions are made and the receivers can be used to detect plasma waves resulting from the interaction of the solar wind with emitted cometary material. The sounding frequency varies from 0.01 to 10 kHz and the receiving frequency is up to 20 kHz. The total mass of the PP is 0.27 kg and it uses up to 0.32 W of power. The primary investigator for the PP is H. Laakso.
The DIM experiment detects impacts of dust particles and measures their momentum. The objective is to profile the size distribution of cometary material falling back onto the nucleus after being ejected from the surface by sublimating volatiles. The DIM sensor cube is mounted on the upper edge of the balcony on Philae. It consists of a set of piezo-electric plates and the associated electronics to register the electric pulses generated by the impact of particles from three directions. The signals are analyzed with respect to their waveform (amplitude and contact duration). The masses of the particles, their velocites, number, directional distribution, and flux over time can then be estimated. The DIM can measure particles with radii from 5 x 10^-5 to 6 x 10^-3 m and masses from 5 x 10^-10 to 9 x 10^-4 kg. The instrument mass is 0.405 kg and it uses up to 0.17 W of power. The primary investigator is I. Apathy.
Mass: 1.225 kg
Power (avg): 1.7 W
Questions and comments about this experiment can be directed to: Dr. David R. Williams
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. Diedrich T. Mohlmann | Co-Investigator | dirk.moehlmann@dlr.de | |
Mr. Istvan Apathy | Co-Investigator | apathy@sunserv.kfki.hu | |
Mr. Walter Schmidt | Co-Investigator | Finnish Meteorological Institute | walter.schmidt@fmi.fi |
Dr. Harri Laakso | Co-Investigator | Finnish Meteorological Institute | harri.laakso@fmi.fi |
Dr. Klaus J. Seidensticker | Principal Investigator | seidensticker@europa.rs.kp.dlr.de |