Mission to Mercury

R. Grard

Mercury orbiter homepage http://www.estec.esa.nl/spdwww/future/html/mercury.htm

A proposal for a Mercury Orbiter was submitted to ESA by a team of 19 scientists in 1993. Following the initial results of an assessment study conducted in 1994, this mission was identified as a new Cornerstone in the Horizon 2000 plan with a possible launch in 2009. A model payload tentatively defined during this study consisted of the following instruments: multispectral camera, gamma- and X-ray spectrometers, magnetometer, ion and electron analysers, electric and magnetic field and wave analysers, ion gun for spacecraft potential control, radio propagation experiment, Doppler tracking and ranging.

The scientific payload's overall requirements are presently about 50 kg and 44 W. It is hoped, however, that the technology developments anticipated for the next decade will make the inclusion of a complementary set of instruments feasible: UV spectrometer, altimeter and neutral atom imager. The possible addition of independent systems, such as a penetrator or a subsatellite, will also be investigated. The system design that emerged out of the assessment study relied on inheritance from Cluster and Giotto. The spacecraft has a dry mass of about 630 kg and a launch mass of 1620 kg; it is stabilised at 10 rpm, but the telemetry antenna is despun in order to point constantly towards Earth.

The spacecraft would be launched from Kourou on Ariane 5 and reach its destination after approximately 3.8 years. The nominal orbit is polar with periherm and apherm altitudes of 400 km and 16 800 km, respectively. The mission proper around Mercury would last for at least 9 months, i.e. 3 Hermean years. During this period, the science data would be continuously recorded in an onboard memory and telemetered to Earth daily.

The sequence of preparatory events leading to a possible launch in January 2009 will start with Phase A-level mission and system definition studies in 1997, supported by a science team. In parallel, technology studies will be conducted on items critical to the mission, such as:

• thermal subsystem components (louvre, insulation material) capable of protecting equipment under high heat flux conditions;
• lightweight, low power Stirling cooler to enable the utilisation of germanium detectors for gamma-ray spectroscopy;
• high gain mesh antenna, despin mechanism, feeds, subreflector and RF rotary joints;
• solar cells with special coating and high temperature bonding materials;
• low mass, non-magnetic, high temperature battery technology;
• low mass components and semiconductor mass memory;
• X-band for both up and down telecommunication links, optional Ka-band for dual-frequency telemetry links;
• spacecraft operating autonomy.

GAIA: Global Astrometric Interferometer for Astrophysics

M.A.C. Perryman

GAIA homepage http://astro.estec.esa.nl/SA-general/Projects/GAIA/gaia.html

GAIA is a preliminary concept for a second space astrometry mission, proposed by L. Lindegren (Lund) and M.A.C. Perryman (ESTEC), and recommended for consideration as a Cornerstone mission in the context of ESA's Horizon 2000 Plus long-term scientific programme. It is aimed at the broadest possible astrophysical exploitation of optical interferometry using a modest baseline length. The experimental concept is estimated to lead to positions, annual proper motions and parallaxes of some 50 million objects, complete to about V=15 mag, with an accuracy of better than 10 µarcsec (see Fig. 3.3.3.2/1), along with multi-colour multi-epoch photometry of each object. Many millions of fainter objects, as faint as 20 mag, would also be measurable with somewhat lower accuracy. The scientific case for such a mission is dramatic: at this accuracy level, distances and kinematical motions for tens of millions of objects throughout our Galaxy would be obtained. The expected accuracy is such that direct (trigonometric) distance estimates to the galactic centre would be accurate to 10%, with transverse motions accurate to about 1 km/s at 20 kpc. Direct distances and luminosities would be measurable throughout and beyond our Galaxy, and every constituent star would have its motion measured, along with details of its binary or multiple star nature.

Figure 3.3.3.2/1: Examples of the galactic astrometry accessible to the GAIA mission.

As by-products, the global measurements would yield unprecedented information on the space-time metric (g to a precision of about 1 part in 10⁶ or better, close to values that might distinguish between currently competing theories of gravity), angular diameters of hundreds of stars, and a vast body of information on double and multiple systems. Perhaps the most dramatic of these subsidiary goals would be the possibility of screening several hundred thousand stars within 100-200 pc for periodic photocentric motions, which would provide the most powerful and systematic method of detecting possible planetary companions proposed to date.

GAIA follows many of the principles employed by ESA's highly successful Hipparcos space astrometry mission. A continuously scanning satellite provides a global astrometric system in which positions, proper motions and absolute trigonometric parallaxes are determined for all stars, with direct connection to an extragalactic reference system being obtained by GAIA directly. The reason that GAIA can provide such a significant improvement in accuracy, limiting magnitude and numbers of stars compared with Hipparcos follows from three principal considerations:

• the larger baseline (provided by the small non- deployable interferometer),
• the more efficient detector system (a CCD rather than an image dissector tube),
• parallel rather than sequential observations of all objects within the field of view, made possible by the use of a CCD detector.

Following the recommendations of the survey committee, an international workshop devoted to GAIA and the future of space astrometry was held at the Royal Greenwich Observatory, Cambridge, UK, in June 1995, organised by F. van Leeuwen (RGO) and M.A.C. Perryman. This work-shop generated many new scientific and technical ideas that may be included in the final GAIA mission. ESA has now established a science team to pursue the scientific and technical aspects of the mission in further detail. If selected as Cornerstone 5, launch could be in 2009. If selected as Cornerstone 6, launch could be in 2014.

Darwin: a search for extra-solar terrestial planets

M. Fridlund

Darwin http://astro.estec.esa.nl/SA-general/Projects/Irsi/

Darwin is a mission that intends to provide an answer to some of mankind's great questions: are there other worlds like the Earth in our Galaxy, and can they sustain life as we know it?

Recently, the first exo-planets have been detected through ground-based radial velocity measurements, finding about one new system per month. Masses of the planets found are, however, of Jupiter-size and upwards. Although it now appears that some fraction of low mass stars are surrounded by planetary systems and/or brown dwarfs, there are fundamental reasons why radial velocity techniques will not be able to find planets with masses less than about Neptune's. Techniques for finding Earth-type planets involve detection of micro-lensing. This was suggested as part of the STARS mission and is presently being proposed to NASA for the Kepler mission. Alternatively, several ground-based projects are planned. This method could, however, detect Earth analogues only many kpcs away, and further characterisation of the objects would be impossible.

To find out if relatively nearby (tens of pc) solar analogues (late F to early M type stars) have Earth-type planets, whether these planets have atmospheres, and if these atmospheres could sustain life as we understand it, proposals for using 'nulling interferometers' for spectroscopy have recently been put forward both in the US to NASA (Exploration of Neighboring Planetary Systems, ExNPS) and in Europe to ESA.

The European proposal, Darwin, consists of 5-7 space-craft, each carrying a 1 m-class telescope operating in the near-IR (6- 17 µm) and connected into a 'nulling interferometer' with a baseline of order 50 m. Such an interferometer will achieve a suppression of about 10⁵-10⁶ in the central part of the synthesised beam and thus remove the interfering stellar light.

Analysis shows that about 100 suitable systems could be detected in 1 year, with at least three observations needed for each to map out the planetary motions. The rest of the mission would be dedicated to trying to detect, by spectroscopic methods, CO₂ (common to all atmospheres in the solar system), H₂O (indication of a planet where life as we know it is possible) and O₃ (evidence of photosynthesis) in the observed systems.

It is expected that ESA will perform a feasibility study of the mission in the 1997-98 time frame. Major topics for this study include the interferometric method (e.g. delay lines) and whether the mission should consist of free-flying telescope units or a mechanically joined system.

A major problem relates to the zodiacal light in the inner part of our own solar system, which will have a very strong emission at the wavelengths in question. The mission would have to operate 4-5 AU from the Sun (where the temperature is lower and the zodiacal emission peaks at longer wavelengths) in order to be able to use 1 m-class telescopes (8 m-class telescopes could possibly do the observations from 1 AU). The technical problems associated with such a mission (e.g. power generation, communication systems, launch methods), as well as the option of launching the mission into an orbit with a high inclination relative to the ecliptic, will have to be studied.

LISA: Laser Interferometer Space Antenna

Y. Jafry

LISA http://www.estec.esa.nl/spdwww/future/html

LISA's primary objective is to detect and observe gravitational waves from massive black holes and galactic binary stars in the frequency range 10^-4-10^-1 Hz. Useful measurements in this range cannot be made on the ground because of the unshieldable background of local gravitational noise.

Conceptual ideas for interferometers in space using separate spacecraft were suggested in the US in 1978 and 1981. The concept was further developed over the next decade, leading ultimately to the LISA proposal to ESA in 1993. A 4-spacecraft mission was studied at assessment level as an M3 mission candidate, but it turned out that the cost was clearly above the limit for a medium-size project. However, many of the details that are now established regarding the design of the gravitational wave detector in space were worked out during this assessment phase. For the Horizon 2000 Plus proposal, two spacecraft were added, essentially for redundancy.

LISA now consists of six identical spacecraft, forming a large equilateral triangle in space (Fig. 3.3.3.4). The length of each side of the triangle is 5x10⁶km, defining the interferometer arm length. For reasons of compactness, stability and reliability, LISA will use solid state diode-pumped monolithic miniature Nd:YAG ring lasers, which generate a continuous 1 W IR beam at 1.064 µm.

Figure 3.3.3.4: Schematic diagram of the LISA configuration.

h main 1 W beam is transmitted to the corresponding remote spacecraft via a 38 cm-aperture f/1 Cassegrain telescope. The same telescope on each spacecraft is used to focus the very weak return beam from the distant spacecraft, directing the light to a sensitive photodetector where it is superimposed with a fraction of the original local light. The interference signals thus obtained from each arm are combined in software by the onboard computers to perform the multiple-arm interferometry required to cancel the phase-noise common to all arms. With the triangular configuration, the three arms provide two almost independent interferometers and also provide redundancy in case of the failure of up to two spacecraft (though not at the same vertex).

the heart of each spacecraft is a vacuum enclosure containing a polished platinum-gold cube (test mass) that serves as an optical reference (mirror) for the light beams. When a gravity wave passes through the system, it causes a strain distortion of space, which, in turn, causes fluctuations in the separation of the test masses. This leads to fluctuations in the optical path between the masses, causing the phase-shifts that are detected by the interferometry. The distance fluctuations are measured to sub-Å recision, which, when combined with the large separation between the spacecraft, allows LISA to detect gravitational wave strains down to a level of order 10^-23 in 1 year of observation, with a signal-to- noise ratio of five. Capacitive sensing is used to monitor the relative motion between each spacecraft and its test mass. These position signals are used in a feedback loop to command Field Emission Electric Propulsion (FEEP) thrusters to enable the spacecraft to follow its test mass precisely and without introducing disturbances in the bandwidth of interest. The same thrusters are used for precision attitude control relative to the incoming optical wavefronts.

LISA is included as the third Cornerstone in the Horizon 2000 Plus long-term programme, with a launch in 2017. Currently, activities are underway to reduce the cost of the project by:

• reducing the size of the telescope from 38 cm to 30 cm (without significant science loss);
• making drag-free control part of the payload (which is already the case with all other fundamental physics missions);
• reducing the number of spacecraft from six to three by combining each corner pair into a single spacecraft.

A 1-year system level study is planned to begin in late 1997.