European Space Agency

Phase A studies

  • Intermarsnet
  • MORO: a moon-orbiting observatory
  • STARS: the asteroseismology mission


    A. Chicarro

    Intermarsnet homepage

    The Intermarsnet study represents a joint ESA/NASA network and orbital scientific mission to investigate the interior, surface and atmosphere of Mars (SCI(96)2). It was designed to allow Europe to join the growing international movement to explore Mars, which has recently been accelerated after the announcement of potential ancient lifeforms on the planet. The ESA contribution would have been the M3 medium-size mission in the Horizon 2000 plan, and the NASA contribution would have been part of the Mars Surveyor programme. ESA would have provided a full Ariane 5 launcher, a Mars Orbiter and a launch support structure, while NASA would have provided three Landers based on the Mars Surveyor-98 lander design.

    As the natural next phase of Mars exploration in 2003 and in accordance with the recommendations of the International Mars Exploration Working Group (IMEWG), the Intermarsnet mission was to establish the first network of scientific stations on the surface to study the interior, surface and atmosphere of the planet, and to be a precursor to more detailed surface exploration (e.g. sample return). With multiple stations, it is possible to perform seismological and meteorological measurements in order to infer the internal structure of the planet, the atmospheric circulation and the weather patterns. These long-term investigations require an operational lifetime of at least one Martian year (687 days). Other Intermarsnet scientific goals were to study the morphology and geology of the landing sites, the geochemical and mineralogical composition of Martian rocks, soils and volatiles, and other physical properties of the surface materials, the atmospheric circulation and structure, the magnetic field, geodesy and exobiology. These scientific objectives of the surface network were to be complemented by orbital data on the atmosphere, surface roughness and plasma environment.

    Each Lander carried a scientific model payload of about 15 kg, consisting of a core payload and site-specific instruments. The core payload on each Lander contained instruments dedicated to seismology, meteorology, geology and geochemistry, including a 3-axis ultra-broadband long/short period seismometer coupled to a magnetometer, panoramic and descent cameras, a-proton-X-ray spectrometer, an atmospheric descent experiment, a comprehensive meteorological instrument package and a permanent magnet array.

    The site-specific payload varied depending on the expected geological environment of each landing site (e.g. sedimentary rocks and volatiles or igneous rocks and soils). It included an appropriate suite of instruments such as a thermal analysis/evolved gas analyser, a Mössbauer spectrometer, a thermal emission spectrometer and a close-up imager. Sampling, drilling and deployment mechanisms were included, in particular a mobile instrument deployment device capable of reaching and analysing rocks several metres from the Lander.

    A scientific payload of about 30 kg was accommodated on the Orbiter, including an atmospheric sounder, a synoptic atmospheric imager, a reflectivity and roughness radar and a plasma physics package to study certain aspects of the Martian environment (see Table

    Intermarsnet model payload
    Table Intermarsnet model payload

    A set of three landing sites was selected to satisfy the scientific and technical requirements of a Lander network, and to begin to provide a global perspective of Mars from the surface.

    The baseline mission design included three Free Flyer Landers and a Mars Orbiter, launched together on a dedicated Ariane 5 in June 2003. After insertion of the common launch support structure into an interplanetary hyperbolic trajectory to Mars, the Orbiter and the Landers were to separate and travel independently to Mars. The cruise time was approximately 7 months. The Free Flyers' atmospheric entry altitude was 125 km, while the descent time was about 12-15 min. The Orbiter was to be placed in a circular Sun-synchronous polar orbit of 582 644 km altitude, 93.6° inclination and 128 min period. Each Free Flyer mass was up to 415 kg, including propellant, cruise hardware and separation systems. It was 2.4 m wide and 1.4 m high. The Orbiter was spin-stabilised at 5 rpm; its mass was 1212 kg, including 657 kg of propellant. For the mission operations, each agency was responsible for its own hardware. The data volume was to be up to 10 Mbit/Lander/sol and up to 158 Mbit/day for orbital scientific data.

    MORO: a moon-orbiting observatory

    B.H. Foing

    MORO homepage address

    A European Moon Orbiting Observatory (MORO) was studied in Phase A for the global mapping of lunar topography, mineralogy, geochemistry and gravity (SCI(96)1). The main scientific goals of MORO are: to constrain theories describing the formation of the Earth-Moon system, to study the Moon's origin, its thermal evolution and geological history, to measure quantitatively on the lunar surface processes relevant for solar system studies (impact cratering, volcanic activity, tectonics, erosion and volatiles), and to survey resources for further lunar exploration.

    These goals are addressed by the MORO payload with a global geophysical and geochemical multispectral lunar mapping instrument package of unprecedented resolution. The growing need to reduce the cost of space missions led the study team to look for several approaches to limit the costs. It therefore decided to study a 'small-satellite' version of MORO which would address only the most important scientific issues, complementing Clementine, Lunar Prospector and Lunar-A. These are gravimetry and high accuracy stereo imaging, topography, mineralogy and some elemental composition capability. This approach resulted in halving the payload mass (742 kg) and substantially reducing the spacecraft dry mass (5300 kg). Such a small satellite could be placed directly into a lunar transfer orbit by a number of emerging new small/medium-class launchers. The new approach allowed large cost reductions for the mission and spacecraft design. The Solar System Working Group and the SSAC assessed the MORO mission in April 1996 and urged ESA 'to explore all ways to expeditiously implement a mission like MORO in the framework of the ESA Lunar initiative, recommended at the Toulouse Ministerial Conference'.

    After the Earth, the Moon is the most important planetary body for mankind. It can be considered as an outer continent of Earth containing mantle material from the primitive Earth. Its origin is related to Earth's earliest environment, and its old crust allows studies of the very early history of a planet. The scientific study of the Moon is needed to improve our understanding of the evolution of the solar system, terrestrial planets, the Earth-Moon system and the Moon itself. It provides excellent opportunities for geophysics, geochemistry, a unique astronomical window on the Universe, a field base for new sciences on another planet and for space technology and exploitation (see Mission to the Moon, ESA SP-1150, and the Proceedings of the International Lunar Workshop in Beatenberg, ESA SP-1170).

    Why the Moon and why a new lunar orbiter? There are strong reasons for further studies of the Moon with a new lunar orbiter:

    These open problems and limitations from previous lunar exploration require a lunar polar orbiter with novel instruments, covering geochemical, geophysical and geodetic investigations of the whole Moon at unprecedented resolution. The prime science objectives for the MORO orbiter are to:

    Four core instruments were considered in the Phase A model payload:

    MORO's novel instruments, with no equivalent ever flown around the Moon, would deliver:

    MORO was conceived as a lunar-orbiting observatory to fill the many fundamental gaps of lunar science left open by Apollo and the recent technology-oriented Clementine. It would undertake remote sensing observations of the lunar surface and measurements of the Moon's global topography and gravimetry, with several instruments working in synergy.

    STARS: the asteroseismology mission

    F. Favata & M. Fridlund

    STARS homepage

    Stellar structure is one of the classical branches of astro- physics, yet some fundamental issues in stellar structure and evolution, which are of great relevance to astrophysics at large, are still unresolved. Theory has not made any dramatic advances in the last few years because of the lack of new observational data. Stars are optically thick, point sources when observed from Earth, so the study of their interiors is based on their integrated, external properties. As it has long been shown that very similar external properties can be predicted by very different interior models, the fundamental unresolved issues of stellar structure and evolution thus cannot be addressed by traditional techniques.

    While stars are opaque to light, they are essentially transparent to sound waves. The study of acoustical oscillations of stars therefore offers the unique possibility of directly probing their interiors. Stellar oscillations, through their slight deformation of a star's photosphere, will produce tiny, periodic variations in the integrated light output. Very accurate photometry and subsequent Fourier-like analysis can recover the frequencies and intensity of the fundamental oscillation modes and thus shed light on the structure.

    The STARS mission is designed to perform very accurate, month- long stellar photometric observations, producing a database of accurate light curves for tens of thousands of stars. This will provide the basic observational material used to calibrate, through asteroseismological techniques, stellar structure and evolution theory, finally providing it with firm observational foundations. Such photometric observations are perturbed at a fundamental level, from the ground, by atmospheric scintillation. In fact, notwith-standing the large efforts of several teams, and the increasing tantalising evidence, no confirmed detection of asteroseismological signatures by photometric or spectroscopic techniques have yet been reported.

    The primary element of the STARS payload is a 1 m-diameter, 3 3° field of view, triply-reflecting telescope. At its primary focus is a mosaic CCD camera designed to perform precise photometry at 3500-7500 Å with a large dynamic range. A far- UV instrument at the secondary focus will monitor a 50 Å band centred on the C II line at 1335 Å, with an effective area of ~~100 cm², providing information on surface rotation and activity. The telescope will point at each target field for at least 1 month, yielding frequency measurements to a precision of at least 0.3 µHz.

    To gain an understanding of stellar physics, it is necessary to observe a wide range of stars of different masses and in different stages of evolution. The chemical and luminosity evolution of our Galaxy, and others, is primarily determined by the more massive stars, but the record of that chemical evolution is in the lower mass stars. They provide the dating of globular clusters and of the other old populations in the Galaxy. Stars of different masses and in different stages of evolution are used for distance calibration.

    STARS will thus observe a wide variety of fields to cover targets spanning a wide range in stellar masses and ages in most of the Hertzsprung-Russell diagram. Cool stars of solar and sub- solar mass, hot stars of intermediate and large mass, stars with high and low metallicities, will all be observed by STARS and become accessible to the refined techniques of asteroseismology. The very high photometric sensitivity coupled with STARS' wide field of view will be used in parallel for a number of other observing programmes with no impact on the asteroseismological science goals. Foremost among these latter are the search for and study of Earth-like planets around other stars and the study of low surface brightness structures in galaxies.

    The STARS spacecraft is based on the XMM bus operating in a highly eccentric 24 h orbit, reached with an Ariane 5 in dual configuration. The spacecraft, telescope and launch will be provided by ESA, while the two focal plane instruments will be provided by nationally-funded PI consortia. Spacecraft operations will be the responsibility of ESA, while scientific operations, as well as the data reduction, will be carried out by the nationally-funded STARS Science Consortium. The Consortium will comprise an Input Catalogue Collaboration, responsible for the optimisation of the final observing programme, two parallel Data Reduction Collaborations, responsible for the processing of the data to the best possible precision, the Telescope Scientist and the two Instrument PI Consortia.


    R. Reinhard

    Gravitation, electromagnetism and the weak and strong interactions are the four known fundamental forces of Nature. Einstein's theory of gravity, General Relativity, provides the basis for our description of the Big Bang, the cosmological expansion, gravitational collapse, neutron stars, black holes and gravitational waves. It is a 'classical' non-quantum field theory of curved spacetime, constituting an as-yet unchallenged description of gravitational interactions at macroscopic scales. The other three interactions are dealt with by a quantum field theory called the 'Standard Model' of particle physics, which accurately describes physics at short distances where quantum effects play a crucial role. But, at present, no realistic theory of quantum gravity exists. This fact is the most fundamental motivation for pursuing our quest into the nature of gravity. The Equivalence Principle, the contention that different bodies fall with the same acceleration in a gravitational field, is both the historical foundation stone of General Relativity and its most precisely testable element. The central aim of STEP is to test the Equivalence Principle to 1 part in 1017, an unprecedented improvement of five orders of magnitude over the most precise experiments performed to date.

    two concentric cylindrical test masses
    Figure Relative motion of two concentric cylindrical test masses in low Earth orbit in the case of an Equivalence Principle violation. The signal (relative displacement) is periodic at orbital frequency.

    The Standard Model successfully accounts for all existing non- gravitational particle data. However, just as in the case of General Relativity, it is not a fully satisfactory theory. Its complicated structure lacks an underlying rationale. Even worse, it suffers from unresolved problems concerning the violation of the charge conjugation parity symmetry between matter and anti- matter and the various unexplained mass scales. Purported solutions of these shortcomings typically involve new interactions that could manifest themselves as apparent violations of the Equivalence Principle and/or novel 'mass-spin' couplings between two bodies, one of which is spin-polarised.

    The STEP model payload consists of five differential accelerometers and two single accelerometers (SCI(96)5). The payload is accommodated in a quartz block, which itself is accommodated in a cryogenic superfluid helium dewar which keeps the payload at 2 K. Four differential accelerometers, using different pairs of test mass materials, are devoted to a test of the Equivalence Principle (EP experiment). One differential accelerometer is used to search for any forces between quantum- mechanical spin and ordinary unpolarised matter (MSC experiment). Two single accelerometers mounted at either end of the quartz block form a gradiometer for geodesy (geodesy experiment).

    All three experiments employ similar highly sensitive measurement techniques (superconducting accelerometers), but the driving force is different in each case. For the EP experiment, any differential motion between two test masses would be due to different materials being accelerated differently in the Earth's gravity field. For the MSC experiment, any motion of the test masses would result from the hypothetical short-range force between spin-polarised and ordinary matter within the experiment.

    For the geodesy experiment, the driving force is the local gradient of the Earth's gravitational field. In the EP and MSC experiments, the signal is periodic, at a low frequency (~10-3-10-4Hz) that is specific for each experiment. This enables spectral separation from the noise sources. Differential sensing is employed to attenuate residual satellite motion by a factor of 10-4.

    The cryogenic dewar is accommodated in a 3-axis stabilised drag-free spacecraft which uses the helium boil-off from the dewar to feed a number of proportional thrusters to compensate for the residual air drag at orbital altitude. STEP's orbit is circular and at low altitude (~400 km). A Sun-synchronous (i.e. almost polar) orbit was chosen to avoid eclipses, thus providing a highly stable thermal environment. During flight, the spacecraft rotates about its long axis at a small multiple of the orbital frequency in order to spectrally shift the science signal from orbit-fixed systematic error sources.

    STEP was not selected as ESA's third medium-size project (M3) as at about the same time there were three other mission concepts under study of a test of the Equivalence Principle in space, all at much lower cost, and ESA was invited to participate in any one of them: the NASA-led MiniSTEP and the CNES-led GEOSTEP, both quite similar to STEP, and the Galileo Galilei (GG) concept pursued by ASI, which uses orbiting test masses at room temperature in high speed co-rotation. After two very thorough rounds of evaluation, the Fundamental Physics Advisory Group (FPAG) expressed a clear preference for ESA participation in MiniSTEP. Essentially, ESA (ESOC) would be responsible for the mission operations, ESA's SSD is invited to provide the drag-free control subsystem (see Section Drag-free control), and many other European groups are invited to provide parts of the payload.

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    Right Left Up Home SP1211
    Published August 1997.