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INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 29, May 2012 Editorial Uranus was the first planet to be discovered since ancient times. Such was the luck of the young musician William Herschel in 1781. It was not just a coincidence, however, because Herschel was not only a gifted composer and organist, but also a skilled builder of telescopes which he used to observe Britain’s night sky. Later observations of Uranus revealed strange orbital irregularities that could not be explained by Kepler’s laws.This is why the planet continued to stir up interest in the astronomical community and later the French astronomer Urbain Le Verrier undertook to unveil its secret. Months of complex calculations - at that time the matter of a clear mind, much paper and a sharp pencil - brought him to the conclusion that the planet’s wobbling orbit might be caused by an additional neighbouring yet unknown planet. He quickly announced his findings to the French Academy on 31 August 1846; and he also sent them to Johann Galle of the Berlin Observatory who found the wanted planet precisely on the indicated spot.That marvellous success earned Le Verrier the nickname of the man who discovered a planet with the point of his pen. Unknown to LeVerrier, similar calculations were made at virtually the same time by a student in England; yet, they got lost and re-found again only much later, prompting lengthy disputes about the true finder of Neptune, as the newcomer was named soon after. Anyway, spurred by his success, Le Verrier set out to address a further astronomical mystery: he began observing Mercury, which also exhibits some orbital ir- SPATIUM 29 2 regularities. It stood to reason that Le Verrier should again come up with the idea that an unknown planet, which he swiftly called Vulcain, was shaking Mercury. This is now the stuff that galvanizes astronomers, and it did not last long until hasty observers claimed to have seen the hypothetical planet. Still, further analyses showed in all cases that the putative Vulcains were mere products of imagination: planet Vulcain could not be found. The mystery was only solved in 1915, when Albert Einstein explained Mercury’s anomalous orbit with his general theory of relativity. That may have been the first, yet undoubtedly not the last of Mercury’s many secrets to be unveiled. However, the more scientists look at this planet the more they get fascinated by that small, hot sphere. Peter Wurz of the Physics Prof. Institute of the University of Bern, the author of the present issue of Spatium, is no exception. On 30 March 2011, he presented the current status of Mercury research to our PRO ISSI association. We thank Prof.Wurz for his consent and support in publishing his enlightening talk, and wish our readers a hearty portion of that great fascination scientists get when studying mysterious Mercury. Impressum SPATIUM Published by the Association Pro ISSI Association Pro ISSI Hallerstrasse 6, CH-3012 Bern Phone +41 (0)31 631 48 96 see www.issibern.ch/pro-issi.html for the whole Spatium series President Prof. Nicolas Thomas, University of Bern Layout and Publisher Dr. Hansjörg Schlaepfer CH-6614 Brissago Printing Stämpfli Publikationen AG CH-3001 Bern Hansjörg Schlaepfer Brissago, May 2012 Front Cover Craters are Mercury’s trademark so to speak. The front page shows the crater Kuiper seen by NASA’s Messenger camera, interpreted by an artist. Mysterious Mercury1 by Prof. Peter Wurz, Physics Institute, University of Bern Introduction The solar system’s history begins with a huge interstellar cloud of gas and dust far out in one of the Milky Way’s several arms. Gravitational instabilities cause it to collapse into a large flat disk. Most of the matter concentrates in its centre where the later Sun will emerge. The remaining material forms the outer parts of the disk, where over a period of some 50 million years planets form together with all the other bodies populating the solar system2. Next to the Sun is Mercury, the smallest of all planets. It is known from times immemorial as it can be seen under good conditions by the naked eye notwithstanding its small size. The Babylonians call it Babo, the messenger of the Gods, an attribute reflecting its fast orbit around the Sun, faster than any other planet. Civilizations change, Mercury keeps its attribute: the Greeks call it Hermes and Apollo, which is reduced to Hermes when it becomes known that the planet in the morning sky next to the Sun is the same as that in the evening sky. Roman astronomers call it Mercury, the God of dealers and thieves, a name the planet bears down to our day. When the first astronomical telescopes appear in the 17th century, Mercury is an attractive object in the sky. Even though observation turns out to be difficult due to the glare caused by the nearby Sun, Urbain Le Verrier3 notes that its orbit slightly deviates from what is to be expected from classical New tonian mechanics.Trying to explain the planet’s mysterious behaviour, he stipulates a further hypothetical planet which by its gravitation causes the excessive shift of Mercury’s perihelion4, Fig. 1. The unknown planet even gets a name: Vulcain; yet all attempts to get hold of it are bound to fail: the suspected planet cannot be found, and Mer- Fig. 1: While Newtonian mechanics and Kepler’s laws stipulate an ellipti- cal orbit, Mercury orbits on a rosette-like pattern around the Sun as a consequence of – amongst other things – the near Sun’s powerful gravity field. The anomalous perihelion shift amounts to 43 per century, which is 120 km/year. This compares to Earth’s anomalous perihelion shift of only 3.8 per century. cury’s orbit remains a puzzle until 1915, when Albert Einstein solves the problem conclusively by means of his general theory of relativity. Einstein’s point takes into account the effect of relativity as a result of the tremendous forces of gravity reigning in Mercury’s quarters close to the Sun. A further of Mercury’s many peculiarities is found by Gordon Pettengill5 with the help of the giant 304.8 m diameter Arecibo radar telescope in Puerto Rico in 1965. He recognizes Mercury’s 3 : 2 spin/orbit resonance, meaning that whilst Mercury orbits the Sun twice, it rotates around its axis exactly three times. Such resonances occur in celestial mechanics as a result of tidal forces caused by a large body (here the Sun) acting on a smaller companion (here Mercury). The latter is slightly deformed by the larger body’s gravity field which in turn tends to synchronize it into a resonant revolution mode. That is about all that is known before the first space probes visit Mercury at close quarters. In 1974 NASA’s Mariner 10 spacecraft reaches the planet after a half year journey, followed 30 years later by NASA’s Messenger mission. Both programmes provide fascinating new insights into the secrets of Mercury, at the same time raising The present issue of Spatium reports on a lecture by Prof. Wurz for the PRO ISSI audience on 30 March 2011. See Spatium no. 6: From Dust to Planets by Willy Benz, October 2000. 3 Urbain Jean Joseph Le Verrier, 1811, Saint-Lô, Manche, France – 1877 Paris, French mathematician and astronomer. 4 The term perihelion designates the point on a planetary orbit which is closest to the Sun. Correspondingly, the aphelion is the farthest point from the Sun. If the reference is not the Sun, then the terms periapsis and apoapsis are used. 5 Gordon Pettengill, 1926, Providence, Rhode Island, USA, US-American radio astronomer and planetary physicist. 1 2 SPATIUM 29 3 many new questions. This is why the European Space Agency ESA, in collaboration with the Japanese Space Agency JAXA, is currently implementing a mission called BepiColombo in honour of the Italian space scientist Giuseppe Colombo, see text box. This mission is scheduled for launch in August 2015. As a space mission to Mercury poses uncommon, yet specific fascinating challenges to engineers and scientists, we will treat these programmes somewhat more in depth below. Mysterious Mercury inner solar system billions of years ago. So, to understand the Earth’s history, one also has to understand Mercury’s evolution. The deeper scientists look at Mercury the more they get fascinated by the small, yet unusual planet as it turns out to be an outright storybook telling the early history of the solar system, including that of the Earth. This property is owed to the fact that the planet has neither an atmosphere nor tectonic a ctivities that would tend to erode any traces of the distant past, as is the case for instance on Earth. Rather, Mercury preserves faithfully the marks left by violent events and processes in the Orbital Characteristics In addition, he participated in research at the Harvard Smithsonian Center for Astrophysics, then at Caltech and Jet Propulsion Laboratory. Giuseppe “Bepi” Colombo was born in Padua in 1920 where he attended primary and secondary school. After graduating from the University of Pisa in mathematics in 1944, he returned to Padua where he worked as an assistant and then associate professor of theoretical mechanics at the University. In 1955 he became full professor of applied mechanics at the faculty of engineering at the University of Padua. In his career, he lectured on mechanical vibrations and celestial mechanics, as well as space vehicles and rockets. Professor Colombo was a member of various advisory committees and national and international academies. He was awarded NASA’s Gold Medal for outstanding scientific achievement as well as several other prizes. He also studied new concepts concerning space transportation, large space structures and evolution of space technology for space sciences and applications. He played an important role in promoting space research at the Italian Space Agency. The Space Geodesy Centre in Matera, Italy bears his name. ESA’s mission to Mercury has been named BepiColombo in honour of this space pioneer. Giuseppe Colombo died in 1984. We have already addressed some peculiarities of the planet’s orbit. Yet, there are many more: As the innermost planet in the solar system, Mercury’s mean distance to the Sun is a mere 0.39 AU6. This leads to a very short year on the planet: Mercury completes one orbit around Sun in only 87.97 Earth days, faster than any other planet in the solar system. Furthermore, Mercury’s orbit has the largest eccentricity7 of all planets, see Fig. 2. The perihelion, the point on the orbit closest to the Sun, is 0.31 AU, while the aphelion is 0.47 AU. This feature, together with the lack of moons, and also its small size, fuels the speculation that Mercury might not have come into being as a planet proper, but as the moon of Venus, which today no longer has a moon. Fig. 2: Orbital eccentricities of the planets in the solar system. Mercury’s eccentricity is by far the largest suggesting that it might have a different origin to the other planets. AU: Astronomical Unit, equalling 149,597,870.7 kilometres, the mean distance between Earth and the Sun. In astronomy, eccentricity is the measure of deviation of an orbit from the ideal circle with = 0. 6 7 SPATIUM 29 4 Many of Mercury’s peculiarities have to do with the Sun’s vicinity and its overwhelming strength of radiation. At perihelion, the surface experiences some 14,000 W/m2 of energy flux which is ten times the value on the outer layers of the Earth’s atmosphere. Consequently, surface temperatures are extreme: at noon, they may reach as much as 450 °C on the equator, while during the night they fall down to –180 °C. This qualifies Mercury as the body with the highest day/night temperature differences in the entire solar system. In such an extreme environment no one would expect water.Yet, radar observations of the floors of deep craters near the poles in the shade of sunlight suggest the local occurrence of water ice.Where it comes from is just another of Mercury’s many secrets. Planetary Composition While Mercury is fast in orbiting the Sun, it is very slow in rotating around its own axis: it needs 58.65 Earth days to complete one revolution, exactly 2⁄3 of a Mercurian year. This is the 2: 3 spin/orbit resonance found by Gordon Pettengill. As a consequence of the spin/orbit coupling, the planet has surface points which are always far from the Sun on the aphelion while on the peri helion the opposite point on the planet’s surface always shows right to the Sun. This results in locally fixed climate zones on the surface with very hot regions alternating with less hot regions. In any case, however, less hot still means very hot as compared to Earth’s temperature regime … Fig. 3: Comparing the densities of terrestrial planets and the Moon with their radii: Mercury has an exception- Mercury presumably evolved from the same primordial disk of matter as the other terrestrial planetsVenus, Earth and Mars. One would, therefore, expect their composition to be roughly the same.Yet, this is not the case. On a plot of density versus size (Fig. 3), the terrestrial planets and the Moon stay roughly on a straight line, while Mercury has about the density of the Earth, but the size of the Moon. was then blown away by the solar wind. A third theory, promoted by Prof. Willy Benz of the Physics Institute, stipulates a catastrophic impact by a large body in the planet’s early history that jettisoned much of the planet’s former mantle containing the silicates out to space while the remaining material rebuilt the planet later.The different models lead to different chemical compositions of the planetary mantle and crust, and as soon as reliable data become available, the most prom ising theory may become apparent, or it might even turn out that a new model is needed … Surface Geology ally high density value for its size. Mercury is hence a heavy planet for its size. As the core of the terrestrial planets is made of iron, this high density implies a large iron core at the expense of a smaller mantle formed by the lighter silicon. Yet, even though Mercury possesses much iron in its core, the crust hardly contains any iron.This comes as a surprise for which the reasons are still unknown. A variety of theories have been invoked to explain this fact. First, it is possible, that the chemical composition of the primordial disk was not so homogeneous as expected. A second possibility might be that intense radiation from the young Sun evaporated much of the mantle’s silicates which Mercury’s most eye-catching feature is its uncountable craters of all sizes, Fig. 4 to 14. In the solar system’s early phase, more precisely between 4.1 and 3.8 billion years ago, the terrestrial planets experienced what is called the Late Heavy Bombardment (LHB). At that time, intensity and frequency of collisions with either asteroidal or cometary materials reached their last, high values. The impacts created craters on the planetary surfaces, and, if the collisions were strong enough, prompted volcanoes to erupt. In this case, emanating lava filled the craters again creating large plains with smooth surfaces. At the end of the LHB, the rate of impacts dropped by a factor of 1,000 to the low values we experience at present. Thus, a heavily cratered surface tends to be an old area that suffered many impacts during the LHB, while a region exhibiting only few craters tends to be younger. SPATIUM 29 5 Craters Craters on Mercury range in diameter from small cavities to multiringed impact basins hundreds of kilometres across.They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury’s stronger gravity. Fault Lines Just like all proto-planets, Mercury was very hot in the beginning after its formation. As time passed, the core cooled down resulting in a shrinking core volume. The solid crust began warping, and generating large faults, Fig. 4. Where the forces involved were strong enough, Fig. 4: When the planet’s liquid core cooled, the solid crust warped up creat- ing long, irregular fault lines that may measure as high as 3,000 m. SPATIUM 29 6 the crust broke apart. At the faults, the resulting scarps may reach heights of up to 3,000 m and lengths of 500 km. Impact Basins The largest feature on Mercury is the Caloris basin (Fig. 12), a crater excavated by the impact of a huge meteorite. The Caloris basin, featuring a diameter of some 1,550 km, has similar dimensions to the planet itself; it is even one of the largest impact basins in the entire solar system. The violent impact prompted volcanoes to emit magma that filled the Caloris basin again leading to the large smooth plains we see today. The impact also left a concentric ring system over 2,000 m high surrounding the impact crater. Yet, the most intriguing feature is found at exactly the geographical antipode of the Caloris basin, namely a large region of unusual, hilly terrain known as Weird Terrain, (Fig. 11). One hypothesis for its origin is that the shock waves generated during the Caloris impact travelled around the planet, and converged at the basin’s antipode.The resulting stresses fractured the crust, and created the weird surface. Fig. 5: Mercury’s scarred surface. The image on the opposite top left side shows a high resolution map of Mercury under uniform lighting conditions based on thousands of individual images collected by NASA’s Messenger spacecraft. (Credit: NASA) Fig. 6: Mercury forms a beautiful crescent shape in the image on top right, acquired by the Messenger spacecraft. (Credit: NASA) Fig. 7, bottom left: A double ring crater is an indication of a high-force impact, usually a very massive meteorite, causing a ripple effect in the rock, like dropping a pebble in a pond. (Credit: NASA) Fig. 8, bottom right: a colourful view of Brontë, the large crater in the top right corner, and Degas, the blue-hued crater atop Brontë. These craters are located in Sobkou Planitia, a plains region formed through past volcanic activity. The colours are artificial; they reflect the results of spectroscopic analyses helping scientists to differentiate various materials. (Credit: NASA) SPATIUM 29 7 Fig. 9 (top left): The ejecta of the crater in the centre have been deposited on older craters. Straight lines are often caused by a splash kicked up when a meteorite forms a primary crater, and some of the material tossed up is big enough to make a secondary crater when it comes back down. Instead of just one rock being kicked up and away from the primary impact, several are. They all fly off in exactly the same direction and when they come down each one makes a secondary crater – and they all occur in a line. (Credit: NASA) Fig. 10 (centre left): An unnamed old crater basin has been largely filled by magma from the planet’s interior. Later impacts created a variety of smaller craters in the smooth plain. (Credit: NASA) Fig. 11 (bottom left): The antipode of the Caloris basin: the Weird Terrain. The giant impact that formed Caloris may have had global consequences for the planet. At the exact antipode is a large area of hilly, grooved terrain, with few small impact craters that are known as the Weird Terrain. It is thought to have been created as seismic waves from the impact converged on the opposite side of the planet. (Credit: NASA) Fig.12 (right): The Caloris basin in a false colour representation.The large yellow area is the Caloris basin, featuring a diameter of some 1,550 km, among the largest impact basins in the solar system. The impact which created the Caloris basin must have occurred towards the end of the Last Heavy Bombardment, because fewer impact craters are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon, such as the Mare Imbrium and Mare Orientale, are believed to have formed at about the same time between 3.8 and 3.9 billion years ago. This image was coloured based on spectroscopic information to identify different rock types. Impact craters serve as probes into a planet’s subsurface, excavating and exposing material from depth that would be other wise unobservable. Thus the study of impact crater deposits can help to elucidate the geological history of the target region. (Credit: Science/AAAS using data from Messenger’s Dual Imaging System) Exosphere Fig. 13: The Kuiper crater: This enhanced colour view of Kuiper crater shows not just the bright rays that extend out from this relatively young crater but also the redder colour of Kuiper’s ejecta blanket. The redder colour may be due to a compositionally distinct material excavated from depth by the impact that formed Kuiper. (Credit: NASA) Fig. 14: Scarps in the Rembrandt impact basin. The long scarp trending verti- cally on the left-side of this image is located in the interior of the large 715-kilometer diameter basin Rembrandt. (Credit: NASA) The sodium-D doublet at wavelengths of 589.592 nm and 588.995 nm is the dominant spectral feature of sodium. 8 SPATIUM 29 10 Mercury is too small and too hot to retain a significant atmosphere over long periods of time.Yet, it possesses an exosphere. The term exosphere relates to a thin gaseous medium where the mean distance before an atmospheric particle hits another one is comparable to or larger than the atmospheric thickness. As a consequence of the exosphere’s low density, a molecule travelling upward fast enough can escape to space before colliding with another particle. Hence the exosphere continuously dissipates material out to space. In a steady state, it is replenished by released material from the planet’s surface. As Earth formerly had a CO2 rich atmosphere, andVenus still possesses such an atmosphere, Mercury was expected to have a CO2-dominated exosphere as well. Yet, this was not the case: no CO2 was found, but rather hydrogen, oxygen and helium from out-gassing surface material. Upon a certain filtering function, the exosphere reflects therefore the chemical composition of the surface. The solar wind continuously blows a fraction of the dissipated material from the exosphere deep out to space. This can be seen in the Na-D lines8 in Fig. 15. An enormous tail passes away from the planet, up to some 2,000 planetary radii into interplanetary space. Mercury, therefore, is continuously losing material, the surface gets depleted of certain components with time, and it may well be that in the beginning this process had been much stronger which could account for the lack of a thick mantle as we know it from the other terrestrial planets. Magnetic Field and Magnetosphere A small planet like Mercury is considered geologically dead, i.e. without any geological activity. Specifically, this planet was not expected to possess a global magnetic field, as such a field requires a dynamo to generate it, which in turn needs a liquid iron core to rotate at a different speed to the crust. Preparing for the unpredictable, NASA decided to install a magnetometer on its Mariner 10 spacecraft. The decision was rewarded: against all expectations, the instrument detected an outright magnetosphere, which is, however, small as compared to Earth’s. The magnetosphere results from the interaction of the stream of ionized particles in the solar wind with the planetary magnetic field that results in a cavity void of solar wind plasma around the planet, which extend as a long tail away from the Sun. As Mercury has no atmosphere, and therefore no ionosphere, this interaction is very direct, and undisturbed, thereby offering an ideal laboratory for investigations in space plasma physics. Fig. 15: Mercury’s tail of sodium gas in the Na-D lines in front of the Sun measures some 2.5 million km. The inserts show where on the planet the tail gases come from. (Credit: Center for Space Physics, Boston University) SPATIUM 29 11 Exploring Mercury in the Space Age Mercury is difficult to observe from Earth due to its vicinity to the Sun, which tends to outshine the small planet. On the other hand, inspecting Mercury at close range with space probes is also not an easy task, as the immense heat from the near Sun poses a critical challenge to the spacecraft’s thermal control systems. Even worse: the Sun’s gravity field accelerates the probes during their long journey leading to velocities far above the orbital speeds around Mercury. So, mission engineers have to implement appropriate braking actions en route, which may last as long as seven years. Fig. 16: The Mariner 10 spacecraft, the first space probe ever to visit Mercury in 1974/1975. (Credit: NASA) resonance with Mercury. This o rbit brings the spacecraft once around the Sun while Mercury executes two orbits. The innovative trajectory was inspired by orbital mechanics calculations by the Italian Mariner 10 NASA’s Mariner 10 (Fig. 16) was the first mission to Mercury. It was also the first spacecraft to use the gravitational pull of a planet to appropriately change its trajectory to reach Mercury, and the first to use the solar radiation pressure on its solar panels and its high-gain antenna as a means of attitude control during flight. This works just like the sails of a boat, which, when properly aligned to the wind, provide the ship with thrust in the desired direction. On 3 November 1973 Mariner 10 started to fly-by Venus to bring its perihelion down to the level of Mercury’s orbit in a 1: 2 orbit/orbit- SPATIUM 29 12 scientist Giuseppe Colombo. Mariner 10 implemented three successive fly-bys to Mercury at different distances. As a result of the resonant orbit, however, the planet’s lighted surface was almost the same on the three fly-bys, restricting the imaged area to somewhat less than half of the planet’s surface.Therefore, large portions of the planet remained unobserved. Notwithstanding Mariner 10’s inherent limitations, the mission delivered impressive amounts of scientific data from which researchers had to live for the subsequent 30 years until the second space probe, Messenger, came along. Messenger Fig. 17: The Messenger space probe during testing at Johns Hopkins University. Clearly visible is the large ceramicfabric sun shield that protects the spacecraft against solar radiation. (Credit: NASA) Messenger, an acronym for Mercury Surface, Space Environment, Geochemistry and Ranging (Fig. 17), was launched on 3 August 2004. Based on the experience with planetary fly-bys gained in the meantime, a much more sophisticated trajectory was chosen including a gravity assist with Earth, two gravity assists with Venus, three times with Mercury, to appropriately lower its speed before insertion into a polar orbit around Mercury on 17 March 2011. That was after a voyage of nearly seven years over some 8 billion km with 15 round trips around the Sun. In contrast to Mariner 10, Messenger was scheduled to enter a close orbit around Mercury with the corresponding challenges for its temperature control system. To this end, Messenger is equipped with a heat shield that is constantly aligned toward the Sun providing shade to the spacecraft proper. The orbit is highly elliptical bringing the probe as close as 200 km to the planet’s surface, and as far out as 15,193 km, where excessive heat can be dissipated. At the time of writing, Messenger continues to be operational, and is delivering fascinating science data. BepiColombo Even though Messenger is a great leap forward regarding our knowledge of Mercury, it will leave open questions, and certainly raise new ones as well. Together with the Japanese Space Agency JAXA , the European Space Agency ESA is currently preparing a mission to Mercury. Honouring the Italian space pioneer it bears the name BepiColombo, Fig. 18. The mission is scheduled for start in August 2015, and to arrive at destination about six years later. will be fixed together as part of the Mercury Composite Spacecraft (MCS). In addition to the two probes, the composite comprises the Mercury Transfer Module (MTM), providing solar-electric propulsion to secure the required braking action on the trajectory. Shortly before Mercury orbit insertion, the Transfer Module is jettisoned from the spacecraft stack.The MPO provides the MMO with the necessary resources and services until it is delivered into its own mission orbit, when control will be taken over by JAXA. BepiColombo features a new mission concept including two separate spacecraft: the Mercury Planetary Orbiter (MPO) will circle the planet on a low orbit to allow for high resolution imaging of the entire surface, while the Mercury Magnetospheric Orbiter (MMO) is intended to enter a wide elliptical orbit. During launch and the journey to Mercury, the two spacecraft BepiColombo is a high risk mission: the Planetary Orbiter will visit Mercury over its hottest zones, where thermal radiation from the hot surface adds to solar radiation to sum up to some 20 kW/m2. In contrast, the MMO will be better off: it will reach the planet’s hot inner zone only on its periapsis, while toward its apoapsis it can radiate off some of the heat received close to the planet. Mission Objectives Fig. 18: The joint ESA/JAXA BepiColombo mission in cruise configuration. It consists of a stack of four units that travel together to Mercury. The Mercury Planetary Orbiter is an ESA contribution intended for exploring Mercury on a low orbit. The Mercury Magnetospheric Orbiter is a JAXA contribution aiming at probing Mercury’s magnetosphere. The Mercury Transfer Module will provide the appropriate braking during the entire flight by means of solar electric propulsion, a technology demonstrated earlier on ESA’s SMART-1 mission. (Credit: ESA) The dual BepiColombo spacecraft is intended to further our insight in a range of directions: It is to investigate the origin and evolution of a planet close to the Sun. Other research directions call for the study of Mercury as a planet, its form, interior structure, geology, composition and craters, and the composition and origins of its polar deposits. The Magnetospheric Orbiter will probe the planet’s magnetic field and its origins, as well as the structure and dynamics of the magnetosphere. Mercury’s vestigial atmos- SPATIUM 29 13 phere will be an objective for both parts of the tandem spacecraft. Beyond planetary sciences, the mission also offers opportunities for fundamental physics experiments: as Mercury is far away from disturbing bodies, such as the asteroid belt and the giant planets Jupiter and Saturn, it is also intended to perform tests of Einstein’s general theory of relativity. Scientists do not exclude the possibility that BepiColombo could yield results that might require amendments to Einstein’s relativity theory which for science would be nothing less than a full-blown revolution. Another fundamental physics issue pertains to the old question of whether gravitational mass is really equivalent to inertial mass, which is still assumed to be the case without, however, knowing exactly why. A third topic amongst the mission’s many objectives is testing the validity of the superposition principle: Fig. 19: The BepiColombo Laser Altimeter (BELA): On the left an e ngineering drawing shows the entire system.The laser is mounted in the foreground.The laser pulse leaves the system through the small aperture. Upon reflec- SPATIUM 29 14 do two identical masses really have double the effect as compared to one single mass? Last, but not least, an orbit around Mercury also provides a vantage position from which to observe the solar system from a different and most attractive perspective, as it allows to have the Sun in the back and the objects illuminated frontally. Of special interest in this respect are small bodies with a size in the order of 100 m, which are difficult to observe from Earth in the inner solar system. Such bodies are called volcanoids. They are made of pristine material, which may hold further clues to the evolution of the solar system. In addition, BepiColombo will also provide a unique opportunity to search for Near Earth Objects (NEO’s) that eventually may cross the Earth’s orbit posing a serious danger of a catastrophic collision with our home planet. tion on the planetary surface it reaches the large receiver entrance on the left. The time lag between the laser pulse and the received signal provides precise in formation on the distance to the surface from which the elevation of the illumi- Swiss Contributions BepiColombo is a cornerstone mission in the Agency’s Horizon 2000 programme, and as such also an important endeavour for the Swiss space community. The Planetary Orbiter spacecraft features an extremely lightweight composite structure based on an aluminium honeycomb core and plastic face sheets designed and manufactured in Switzerland. The structure contains heat pipes to dissipate excessive heat from the spacecraft out to space. A further critical Swiss made subsystem is the Solar Array Drive Mechanism, controlling the appropriate position of the solar arrays to generate sufficient electrical energy while protecting them from overheating. Switzerland is also playing a prominent role on the level of scientific instruments for the Planetary Orbiter: nated point on the surface can be calculated.The electronics are contained in the boxes behind the laser system. The right image depicts the optical elements of the engineering model. (Credit: University of Bern) – The BepiColombo Laser Alti meter (BELA), (Fig. 19). For this instrument PRO ISSI’s current President, Prof. Nick Thomas, acts as Principal Investigator together with Prof. Tilman Spohn of the Deutsche Luft- und Raumfahrt, Berlin. BELA is a joint effort between Switzerland and Germany intended to produce a detailed, highly accurate elevation map of the entire surface of Mercury.Various partners of the Swiss space community are currently involved in designing and building subsystems for the BELA instrument. – The Search for Exosphere Refilling and Emitted Neutral Abundances (SERENA), Fig. 20, is a mass spectrometer, for which the University of Bern contributes to the Start from a Rotating Field mass spectrometer (STROFIO) instrument under the responsibility of Prof. Peter Wurz. STROFIO has been designed to determine the chemical composition of Mercury’s exosphere (to derive the surface composition), providing a unique tool to study the planet’s geological history. Outlook Fig. 20: The STROFIO instrument is part of the SERENA package. It is a mass spectrograph that determines the particle mass-per-charge ratio by the time-of-flight technique. (Credit: University of Bern) Missions of the size and complexity of BepiColombo are always joint endeavours involving a large number of s cientific, industrial and managerial partners. Beyond the fascination space research has per se, international and interdisciplinary co-operation provides an extremely rewarding field for space scientists and engineers.The only drop of bitterness is the long wait required until the spacecraft reaches its distant destination. SPATIUM 29 15 SPATIUM The Author Peter Wurz was born in Vienna, Austria, in 1961. Early on he showed a high interest in mathematics and technical subjects and so it was natural that he obtained an engineering degree in electronics. However, working as an engineer he soon discovered that there had to be more so he started to study physics at the Technical University of Vienna. He did his diploma in the field of solid-state physics investigating lattice defects in alkali halides and alkali-earth halide crystals. He continued his stay at the Technical University ofVienna with a doctoral thesis in the field of surface physics; he was asked to build a highly sensitive mass spectrometer for the analysis of trace elements on surfaces, which he then used to study metal alloys. During this work he had the opportunity to spend time for research at the University of Tennessee, Nashville, USA, and at Stanford, California, USA. After his doctorate he went to Argonne National Laboratory, Chicago, USA, to hold a post-doctoral position in the Chemistry division. To move from surface physics to chemistry might seem to be a large step, but the common theme was mass spectrometry at a high level. The research topic in the group at Argonne was fullerene molecules (for example the famous C60), which were discovered just at that time. Since the topic was new lots of information needed to be researched: the efficient production of these molecules, the separation of individual fullerene species, the spectrometric characterisation, and synthesis of new compounds, for example superconducting rubidium and potassium fullerenes, and crystals, for example placing an alkalineearth atom inside a fullerene cage. In fall 1992 he became research associate at the Physics Institute of the University of Bern in the solar physics group.The field of space science was new to him, but the common theme was again mass spectrometry. Solar research was at a high at that time in the Bernese group because of the SOHO mission of ESA with the CELIAS instrument to measure the chemical composition of the solar wind (principal investigator D. Hovestadt, Max-Planck-Institut, Garching, Germany,and later P. Bochsler, University of Bern, Switzerland). He studied the abundance of several minor ions in the solar wind, and in transient events, the so-called coronal mass ejections. This work allowed him to obtain the habilitation at the University of Bern in 1999. In parallel to the work in solar physics, he developed a new technology for the detection of neutral energetic atoms. The first implementation of this technology was IMAGE, a NASA mission to investigate the magnetosphere of the Earth. ESA missions to Mars andVenus were the next where this technology was used. All these missions returned exciting data on the interaction of the solar wind with planetary atmospheres.The next to come is the BepiColombo mission of ESA for which the instrumentation is currently being developed in his laboratory. Also, in preparation for future investigations, he has done theoretical studies on Mercury’s exosphere and surface, on the lunar exosphere, on Mars and Titan’s atmospheres, and a few more. In many areas of space research Swiss scientists are at the forefront, for instance in the development of highly sophisticated instrumentation in Swiss research institutions. From a small country, and a small institute, it is difficult to become an appreciated partner by big players like NASA. Given the good support in Switzerland collaborations with many space agencies have been made possible to him: with the American and European space agencies (NASA and ESA), the Russian space agency (ROSKOMOS), the Indian (ISRO) and the Japanese (JAXA).