Figure 4.2.1.2: The Equator-S experiment electronics box.
Four flight and one spare models of the EFW and ASPOC instruments were developed for the Cluster mission: EFW in collaboration with the Swedish Institute of Space Physics and other laboratories in Finland, France, Norway, Russia, UK and USA (Gustafsson et al., 1997); and ASPOC with the Space Research Institute in Graz and other laboratories in Austria and Norway (Riedler et al., 1997). Most of the central electronics unit and the electric sensor pre-amplifiers of EFW and the mechanical parts of ASPOC were designed and fabricated in the Division. The flight models integrated on the four Cluster spacecraft were destroyed as a result of the Ariane 501 launch failure.
Following approval of the Phoenix integration, the fifth model of EFW has been slightly modified, implementing recent experience gained on the nearly identical instrumentation on Polar. This model was assembled in France with other modules of the Wave Experiment Consortium (WEC) in late 1996. The ASPOC and WEC instruments will be integrated on the Phoenix spacecraft in early 1997.
The potential fabrication of additional EFW and ASPOC models is awaiting a decision on the Cluster II recovery mission.
References
Gustafsson, G. et al. (1997). Space Sci. Rev.
79, 137.
Riedler, W. et al. (1997). Space Sci. Rev.
79, 271.
The Division is cooperating, together with the same consortium as for the Wind mission (see 4.2.3.3), in the development of the energetic particle instrument for the Equator-S satellite. Equator-S is a small satellite being developed by the Max-Planck- Institut für Extraterrestrische Physik (D), and is due to be launched in late 1997 into a highly eccentric Earth orbit with an apogee of 64 000 km.
The instrument is very similar to the 3-D Energetic Particle Experiment flown on Wind. The Division was responsible for the design and manufacture of the analogue electronics (Fig. 4.2.1.2), led by T. Sanderson; the project engineer was J. Henrion.
Figure 4.2.1.2: The Equator-S experiment electronics box.
The HASI experiment on the Cassini/Huygens Probe will record the pressure and temperature profiles during the descent through the atmosphere of Titan, the largest satellite of Saturn, in November 2004 (Fulchignoni et al., 1997).
A subset of HASI, the Permittivity, Waves and Altimetry (PWA) analyser, will additionally measure the conductivity and permittivity of the environment, search for electromagnetic and acoustic waves, and extract information about the satellite surface topography from the return signal of a radar altimeter. The PWA electronics and sensors were developed by this Division, in cooperation with the Instituto de Astrofisica de Andalucia (E), the Space Research Institute of the Austrian Academy of Sciences, and the Laboratoire de Physique et Chimie de l'Environnement (F). The flight model of this instrument has been integrated on the Huygens Probe and submitted to environmental testing.
A balloon flight to test HASI's performance in an atmospheric environment was organised by our Spanish colleagues and launched from Leon (E) on 1 December 1995. A development model of the complete HASI instrument and sensors was integrated on a full scale mockup of the Huygens Probe. Measurements were performed at altitudes of up to 30 km. This campaign was named Comas Solá, in honour of the Spanish astronomer who discovered the atmosphere of Titan in 1908. A. Butler, R. Grard, H. Svedhem and A. Toni were the Divisional staff involved in the balloon campaign.
One important parameter, atmospheric ion conductivity, was measured with two relaxation probes. The potential of each electrode was temporarily maintained at +5 V or -5 V and then allowed to return to its equilibrium level; the conductivity is inversely proportional to the time constant of the signal's exponential decay. The altitude profile of the ion conductivity measured during the balloon ascent and the parachute descent is shown in Fig. 4.2.1.3. The conductivity is of the order of 1.5x10-13 mho/m at the Earth's surface and increases exponentially as a function of altitude, with a scale height of nearly 10 km. These results are in excellent agreement with previous measurements.
Figure 4.2.1.3: Altitude profile of ion conductivity in
Earth's atmosphere, obtained by HASI during the Comas Solá
balloon campaign. The straight line represents the best fit to
the measurements.
The analysis of the PWA data from this campaign is also leading to decisive progress in our understanding of the electrostatic charging phenomena frequently observed during balloon flights. A mutual impedance probe designed to measure the electron conductivity in Titan's atmosphere did not return any results as there are no free electrons in Earth's atmosphere. The sensitive wave analyser, however, detected the presence of a 50 Hz signal radiated from the ground by power lines during a major part of the flight.
Finally, an extensive analysis of the radar data revealed critical information that has resulted in a significant improvement of the performance of the instrument that will fly on Huygens.
Reference
Fulchignoni, M., Grard, R. et al. (1997). ESA SP-1177.
The SSP instrument will measure a large number of physical parameters during the last phase of the descent to Titan and on the surface. The instrument is, in particular, designed to operate in a possible liquid hydrocarbon ocean and will characterise this liquid. The Division's contribution is an acoustic subsystem consisting of a sounder and a sound velocity measuring device. The sounder will detect rain or hail during the last 30 km and provide information on surface topography and altitude, to complement the radar altimeter data, for the last 1 km. In the event of a liquid landing, the depth of this liquid will be measured down to 1 km. The second part will measure the sound velocity every second, beginning at 50 km altitude, by timing an ultrasonic pulse propagating over a fixed distance. In addition, the sound velocity will be measured in the liquid if landing occurs in an ocean.
The acoustic subsystem was designed, built and tested by the Division (H. Svedhem, Co-I) and was delivered to the PI group (J. Zarnecki, University of Kent, Canterbury), where it was integrated into the SSP instrument. Integration on the Huygens Probe and system tests have been carried out (Fig. 4.2.1.4). Final calibration of the sensors will be performed shortly before the Probe is shipped to the launch site.
Figure 4.2.1.4: Huygens Surface Science Package (SSP) sensors
integrated on the Flight Model Probe.
Reference Zarnecki, J., Svedhem, H. et al. (1997). ESA SP-1177.
An instrument for the detection and characterisation of Martian and interplanetary dust is being developed by the Division together with the Technical University of Munich. This Mars Dust Counter (MDC) will be launched on Japan's Planet-B Mars orbiter in August 1998. The primary objective is to search for the proposed, but not yet observed, dust rings around Mars. In addition, other populations of cosmic dust will be monitored during the cruise and orbital phases. The Martian rings should be generated and maintained by the interaction of meteoroids with the planet's satellites Deimos and Phobos. Planet-B is intended to last in orbit for at least one Martian year.
The instrument, based on the two dust experiments flown on Hiten and BremSat, is of the impact ionisation type and will detect particles with masses down to 10-15g. This type of detector is at present the most sensitive for detecting small dust particles. It uses the signals from the plasma generated by the hypervelocity impacts to derive the mass, velocity and flight direction. Extensive work has been done to further improve sensitivity and data confidence. Different configurations have been tested in the dust accelerator of MPI für, Kernphysik, Heidelberg, to optimise the detector and the electronics design. Small iron dust particles have been fired at the detector at 1-60 km/s.
The Division's contribution covers the design and manufacture of the full electronics for the instrument, and assistance in the operations and data analysis. On our side, D. Klinge, J.-M. Castro, H. Svedhem and G. Schwehm are involved.
Building on the micro-technology experience accumulated during the hardware development of the ion emitters for spacecraft potential control (e.g. ASPOC on Cluster), an Atomic Force Microscope (AFM) for the Rosetta mission was proposed. MIDAS (Micro-Imaging Dust Analysing System), will image cometary dust particles ranging in size from a few tens of nanometres up to about 4-5 µm. From experience gained with Giotto and Vega, it appears that there is an abundance of particles in this size range, which also covers the building blocks of pristine interplanetary dust particles, i.e. the silicate core particles (0.1-0.2 µm diameter) and the ~0.01 µm refractory organic mantle. The microscope will resolve surface features on the dust particles with a resolution of up to 4 nm. The instrument will collect and image particles by atomic force microscopy and provide morphological and statistical information on the dust population, including texture, shape, size and flux. Textural information is indispensable in clarifying the factual relationship among the dust constituents. Inter-growth texture is a fundamental property of natural rocks and particles, and MIDAS will provide the necessary image information. The major anticipated results are the size and texture of individual cometary particles and the building blocks of 4 nm to 5 µm.
MIDAS was pre-selected in February 1996 for flight on the Rosetta Orbiter. It will be provided by a European consortium under the leadership of W. Riedler (Space Research Institute, Graz, Austria). The Division's responsibility is to develop and build the AFM and nearly all the mechanisms required to collect, transport and analyse the cometary particles. The required hardware will fit into a volume corresponding to a standard shoe box. At the end of 1996, the hardware development was in an advanced state and a laboratory model was under assembly (Fig. 4.2.1.6). Initial results prove that the 4 nm resolution can be achieved. The design requirements call for a mechanical decoupling of the microscope from the spacecraft structure in order to minimise the disturbing influence of mechanical noise generated by spacecraft mechanisms on the measurements near the comet. ESTEC experts are in the final stage of their finite- element analysis of the mechanical design to provide critical inputs to the design of a damping mechanism. The Divisional lead scientist is R. Schmidt, supported by H. Arends (electromechanical design) and other colleagues.
Figure 4.2.1.6: Laboratory model of the Atomic Force
Microscope. A defined area of the wheel is exposed to the
cometary dust. The wheel can be displaced and rotated in order
to select one of the cantilevers for scanning the exposed area.
The cantilever array will be mounted on top of the small cubic
piezo in the centre of the larger rectangular piezoelement. The
elements together move the cantilevers in three dimensions.
To ensure that the data of this novel instrument can be understood in scientific terms and to guarantee that the instrument will meet its scientific goals, a dust analysis programme has been initiated at SSD. An expert mineralogist took up duty as a Research Fellow (J. Romstedt) in October 1996 and will establish an AFM image and interpretation data base of interplanetary dust particles.
OSIRIS is a versatile imaging system designed to address key problems of the cosmogeny of comets by investigating the physical and chemical processes that occur in and near the nucleus. The investigation of the nucleus itself requires high spatial resolution over a wide wavelength range but with modest spectral resolution (lambda/Delta~10). The investigation of the innermost dust and gas comae requires a wide field of view with a selection of narrow band interferences filters (lambda/Delta lambda greater than or equal to 100) to image the 2-D gas distribution. OSIRIS consists of two cameras: a narrow angle camera (NAC) with a 2.4 2.4° FOV and a wide angle camera (WAC) with a 12 12° FOV. Two identical full-frame, 2048x2048 pixel CCDs with 14 µm pixels will equip both the NAC and the WAC.
OSIRIS was pre-selected as a PI instrument in April 1996, to be provided by a European consortium under the leadership of U. Keller (MPAe, Lindau, D). Following funding difficulties for the original consortium, ESA was asked to participate in the development of the Data Processing Unit (DPU). The Division accepted this challenge because its participation in OSIRIS will be an important step for future scientific expertise in planetary imaging.
The request to provide the OSIRIS DPU is being implemented by a joint venture between the Technical Directorate (Spacecraft Control & Data Systems Division) and the Science Directorate (Solar System Division). This partnership will provide the opportunity to direct the development of an onboard payload data processor within the ESA Technological Research Programme (TRP) to a specific, real application, thus providing a cost-effective contribution to the Rosetta instrumentation.
The DPU consists of two Digital Signal Processors (DSP) modules, an image buffer (mass memory) and relevant interfaces. The first phase of its development started in late 1996 and will last through 1998. Its objective is to design and develop a prototype and prepare a technical baseline for the implementation of the flight model in a second phase. The detailed share of work and funding between the two Directorates for that phase is open at this stage.
U. Telljohann is the project engineer in the Division.
Reference
Thomas, N. et al. (1997). Adv. Space Res., in press.
The scientific payload pre-selected for the Rosetta Orbiter includes an integrated package of six instruments, RPC, for measuring plasma particles, electric and magnetic fields and waves. The Division is collaborating in the development of two of these modules: the Langmuir Probe (LAP) and the Mutual Impedance Probe (MIP), with the Laboratoire de Physique et Chimie de l'Environnement (F), the Swedish Institute of Space Physics and the Finnish Meteorological Institute. R. Grard and J.-P. Lebreton are RPC Co-Is; U. Telljohann provides engineering support.
The LAP and MIP sensors will analyse the plasma that populates the diamagnetic cavity in the close environment (10-100 km) of the cometary nucleus. This cavity contains a mixture of hot and cold electrons. The cold population (10-2eV) is in thermal equilibrium with the atmosphere; the hot population (1 eV) consists of electrons freshly generated by the interaction of solar EUV photons with the neutral gas and the spacecraft's surface.
LAP provides information about the plasma density and temperature from the current-voltage characteristics of two electrodes. MIP is a quadrupolar probe that measures the frequency response of the coupling impedance between two dipoles; the plasma parameters are derived from the resonance peak and the interference pattern of thermal waves.
Fig. 4.2.1.8 shows the modulus and phase of the normalised mutual impedance as functions of the normalised frequency for a square array with a side 10 times larger than the Debye length of the cold electrons. The normalising factors are the values of the mutual impedance in free space and the plasma frequency of the cold electrons. The theoretical results corresponding to a mixture of hot and cold electrons with equal densities are represented by the dotted and dashed lines for temperature ratios of 100 and 50, respectively; the response of the cold plasma component alone is shown with a full line, for reference. It is concluded that, even under these extreme conditions, MIP will measure the plasma frequency and temperature of the cold population with accuracies of the order of 10%.
Figure 4.2.1.8: Rosetta/RPC-MIP: normalised modulus and
argument, in radians, of the mutual impedance of a square array
as functions of the normalised frequency. See text for detail.
Reference
Grard, R. (1997). Influence of suprathermal electrons upon
the transfer impedance of a quadrupolar probe in a plasma.
Radioscience, 32, 1091.
Figure 4.2.1.9: The CONSERT 'model comet', with the paths for
the optical ray approximation of the wave propagation through the
comet nucleus. For clarity, only a few of the measurement points
along the orbit are shown.
The Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) was selected as a payload instrument on both Rosetta's Orbiter and the Roland and Champollion landers (following the cancellation of Champollion, it will be on the Orbiter and the European Lander, see 3.1.5). This experiment will probe the interior of Comet Wirtanen with electromagnetic waves that will penetrate the nucleus because of the expected low absorption at the selected wavelength. The measurements will enable extraction of the nucleus mean permittivity, mean absorption, size distribution of internal inhomogeneities, volume scattering, and surface and internal boundaries reflection coefficients. A full inversion, comparable to that done in a tomograph to get the 3-D distribution of the complex permittivity, will not be possible with only the one Lander now available. In theory, two landers would allow an inversion but in practice many landers or a long-lived movable lander would be required to ensure that each resolution cell in the nucleus is probed from at least two different positions. However, even in the case of one lander, it will be possible to generate a low resolution image of the interior of the nucleus if sufficient data can be acquired.
The instrument's operation is controlled from the Orbiter. The Orbiter module transmits coded pulses at about 100 MHz that are received and integrated on the Lander, which then returns them. Due to reciprocity, the propagation path back and forth will be identical and thus the effect on the pulse will be twice that of a single path. Measurements of the group delay, amplitude and pulse shape will be taken at about 6000 points along each orbit. Both processed and raw data will be transmitted to Earth for analysis.
The Division's contribution includes the design and testing of a high stability frequency generator and long-term tests of temperature-controlled low power oscillators. The experiment team is led by CEPHAG, Observatoire de Grenoble (PI W. Kofman); SSD's involvement is handled by H. Svedhem, supported by A. Butler.
The SESAME instrument combines several sensors to study the electric, acoustic and seismic properties of the cometary nucleus in situ. It has been tentatively selected for the scientific payload of the Lander (originally Roland, see 3.1.5) which will be released from the Rosetta Orbiter.
The Division is specifically involved in the design and fabrication of the Acoustic Sensor (AC) and the Permittivity Probe (PP), together with their associated electronic front ends, in close cooperation with the DLR Institut für Raumsimulation (D), the Fraunhofer Institut in Saarbrücken (D), the Technical University of Braunschweig (D), the Finnish Meteorological Institute and the Laboratoire de Physique et Chimie de l'Environnement/ CNRS (F).
The mechanical and structural properties of the comet will be studied by acoustic sounding in the range 10 Hz to 10 kHz. The elastic property and density of the nucleus will be derived from the wave velocities measured with the AC. The subsurface structure below the Lander will be investigated by acoustic echo sounding down to a depth of about 10 m. The electrical conductivity and dielectric polarisability of the surface material will be evaluated with the PP by measuring the electrical coupling of two dipoles at frequencies below 10 kHz. This will yield information about water content, mantle evolution and outgassing activity. The PP can also be used as a radio receiver to measure the electromagnetic environment of the cometary nucleus. The SSD activities are carried out by R. Grard (Co-I), B. Johlander, J.-P. Lebreton (Co-I), A. Smit and H. Svedhem (Co-I).
The SOLCON/Hitchhiker experiment was prepared under the leadership of D. Crommelynck (PI) of the Institut Royal Meteorologique, Bruxelles (IRMB) for a first flight on STS-85 in July 1997. The experiment aims to build a series of absolute measurements of the solar constant at regular intervals over the 11-year solar cycle, overlapping with measurements continuously obtained by radiometers aboard SOHO and other free-flyer space missions. SOLCON uses a Differential Absolute Radiometer.
Previous flights took place on Spacelab 1 (1983) and the first three STS/ATLAS missions (1992, 1993, 1994).
The Division contributed the Digital Processing/Converter Unit (DPU), which was adapted with IRMB to provide the correct interface to the Hitchhiker Avionics (Serial Asynchronous RS422 communication for the data/commands). A data storage capacity of 1536 kbytes will allow 42.6 h of continuous SOLCON data.
An IRMB engineer (D. Loomans) was hosted in SSD for a few months in 1996 for transferring to IRMB the SSD expertise on the SOLCON/DPU, acquired on the previous ATLAS programme. The SSD support was given by U. Telljohann, T. Beaufort, B. Johlander and B.H. Foing (Co-I). The Division terminated its technical involvement in solar constant measurements, supported since Spacelab 1, in early 1997.
MUSICOS homepage http://ing.iac.es/~jht/musicos/musicos.html
The goal of the international MUSICOS project is to develop a multi-site network that permits continuous spectroscopy of variable objects during large intercontinental campaigns: support to space observations, solar-like variability, astroseismology, circumstellar environment, and solar system object variability (see 4.2.2.8).
Led by B.H. Foing, the fibre-fed cross-dispersed echelle spectrograph developed by laboratories in France was duplicated and tested in the laboratory at ESTEC, including measurements of solar spectra through the fibre. The spectrograph can give a full coverage in two exposures in the blue and red range at 380-900 nm. We also developed an interface for the 2.5 m Isaac Newton Telescope (INT) at the La Palma Observatory, using a 50 m fibre with a focal reducer adapted to the telescope and a mechanical mount for the INT CCD dewar (Fig. 4.2.1.12./1). The commissioning run in early May 1996 was successful, with first light obtained on the first night. The throughput and performances of the ESA- MUSICOS spectrometer were measured (Stempels, 1996). Specific data reduction MUSICOS software was developed and tested on the data from the commissioning run (Fig. 4.2.1.12/2) (Oliveira, 1996).
Figure 4.2.1.12/1: Functional diagram of the installation and
operation of the ESA-MUSICOS spectrograph commissioned at the 2.5
m INT Isaac Newton Telescope at La Palma Observatory.
Figure 4.2.1.12/2: Extracted spectrum of the fast rotating
star FK Comae around the H alfa line obtained with the ESA-
MUSICOS spectrograph commissioned at INT/La Palma in May 1996.
A reference star spectrum of the same type has been artificially
broadened to adjust the photospheric spectrum, leading to the
discovery of extended Balmer emission and absorption,
corresponding to giant circumstellar prominences extending up to
4 radii above the stellar surface.
Subsequently, we improved the response with a red-transmitting optical fibre and validated a mode of operation covering wavelengths up to the CCD cutoff at 1.1 µm. The ESA-MUSICOS was then used during the large multi-site campaign in November 1996 (see also 4.2.2.8). It has been offered for use by the science community at the INT for collaborative programmes in 1997.
The MUSICOS project in SSD is conducted by B.H. Foing with technical support by T. Beaufort. Several students and stagiaires assisted at various phases of the programme: J. Bohin, L. Duvet, M. Gray, J. Oliveira and H. Stempels.
References
Oliveira, J. (1996). Master's Thesis, Porto, Portugal.
Stempels, H. (1996). Master's Thesis, Leiden, The Netherlands.
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