Download Mysterious Mercury

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
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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).