JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, E00B33, doi:10.1029/2008JE003290, 2009
Venus Express mission
H. Svedhem,1 D. Titov,2 F. Taylor,3 and O. Witasse1
Received 30 October 2008; revised 18 December 2008; accepted 5 January 2009; published 19 March 2009.
[1] Venus Express is well and healthy and has now been providing exciting new data from
Venus, our nearby twin planet, for over 2 years. Many of the new results are presented and
discussed in the subsequent papers in this special section. The overall scientific objective
of Venus Express is to carry out a detailed study of the atmosphere of Venus, including the
interaction of the upper atmosphere with the solar wind and the interaction of the lowest
part of the atmosphere with the surface of the planet. In addition, the plasma environment
and magnetic fields as well as some aspects of the surface of the planet are addressed.
For the first time, investigations make systematic use of the transparent infrared spectral
windows in order to probe the atmosphere in four dimensions: three spatial dimensions plus
time. The spacecraft design is taken from Mars Express with some modifications necessary
owing to the specific environment around Venus. The payload is composed of three
spectrometers, a camera, a magnetometer, an instrument for detecting energetic particles,
and a radio science package. The orbit is polar and highly elliptic, with a pericenter altitude
of about 200 km over the northern polar region and an apocenter altitude of 66,000 km.
Presently, the coverage of the southern hemisphere is very good, but important gaps still do
exist. The coverage of the northern hemisphere is much less dense. Venus Express is a part
of the European Space Agency’s program for the exploration of the inner solar system,
which includes missions to study the Sun, Mercury, Venus, the Moon, Mars, and comets
and asteroids.
Citation: Svedhem, H., D. Titov, F. Taylor, and O. Witasse (2009), Venus Express mission, J. Geophys. Res., 114, E00B33,
doi:10.1029/2008JE003290.
1. Introduction
[2] Venus was a forgotten planet for more than a decade
after the emphasis for investigations of the terrestrial planets
shifted from Venus toward Mars during the late 1980s. There
were, however, still a large number of fundamental questions
to be answered about the past, present and future of Earth’s
sister planet, and for an improved understanding of the
general evolution of the terrestrial planets better knowledge
of Venus is essential [Taylor, 2006; Titov et al., 2006a; Moroz,
2002]. The European Space Agency (ESA) launched Venus
Express to open up opportunities for new investigations with
a combination of instruments employing completely new
techniques and improved versions of conventional instruments. The response to the initial results and findings has
been very enthusiastic, generating renewed interest both in
the scientific community and in the major space agencies,
with new missions being developed or planned in Japan,
the United States, and Russia. Venus has come back into the
forefront in planetary science.
1
ESA, ESTEC, Noordwijk, Netherlands.
Max-Planck Institute for Solar System Research, Katlenburg-Lindau,
Germany.
3
Department of Atmospheric, Oceanic, and Planetary Physics, Oxford
University, Oxford, UK.
2
Copyright 2009 by the American Geophysical Union.
0148-0227/09/2008JE003290
[3] Venus Express is an independent follow-up to the
successful Mars Express mission, launched a few years earlier, and is reusing much of the basic design for spacecraft,
launcher and ground system in order to keep costs down
[Titov et al., 2001; Svedhem et al., 2007a, 2009]. It is the third
spacecraft in the family that started with the (much larger)
Rosetta spacecraft. Both Venus Express and Mars Express
benefit greatly from the generic developments carried out for
that mission. With only 3 years between mission approval
and the launch date, plus about a year of preparatory work
preapproval, Venus Express is by far the most rapidly
developed scientific project of ESA. The short developments
time and the significant design heritage have enabled a
powerful mission to materialize at a substantially lower cost
compared to a single mission developed in isolation.
[4] With the new data from Venus Express, there is a
picture emerging of Venus where the existing conditions and
the ongoing processes are becoming clearer. Comparisons
with the Earth will continue and intensify. Venus and Earth
are very different twin planets, and while they will remain
different, we are moving closer toward understanding how
and why the two planets have their distinct characteristics,
particularly with regard to climate [Svedhem et al., 2007b].
[5] This paper describes the mission objectives of the
Venus Express mission and gives an overview of the main
features of the spacecraft, with emphasis on the differences
compared to Mars Express, and the scientific instruments on
board. It discusses the mission scenario and the operational
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orbit and the specific questions related to the choice of orbit
and it gives an introduction to the various activities of the
scientific operations. It discusses the coverage achieved to
date by the different instruments with respect to longitude,
latitude and local solar time, for the atmospheric and surface
observations as well as the coverage of the different regions
in and around the induced magnetosphere. Temporal coverage is discussed for a few special cases but the principles
are applicable to most of the measured parameters. It outlines
the place of Venus Express in the scientific program of the
European Space Agency and finally it discusses topics to be
studied in an extended mission, beyond May 2009, that was
requested during the latter part of 2008.
2. Mission Objectives and Investigations
[6] The overall scientific objective of Venus Express is to
carry out a detailed and systematic study of a large number of
aspects of the atmosphere of Venus. This includes the
interaction of the upper atmosphere with the solar wind and
the interaction of the lowest part of the atmosphere with the
surface of the planet. In addition, dedicated instruments are
focusing on the study of the plasma environment and magnetic fields. Some aspects of the surface are addressed. The
objectives have been structured and organized under the following seven themes: (1) atmospheric structure, (2) atmospheric dynamics, (3) atmospheric composition and chemistry,
(4) cloud layer and hazes, (5) energy balance and greenhouse
effect, (6) plasma environment and escape processes, and
(7) surface properties and geology.
[7] The evolution of the planet and in particular the
atmosphere and the climate are topics of great interest that
span over all the themes. Most themes include aspects of
comparative planetology and comparison with the other
terrestrial planets, the Earth in particular, is an important
objective. Direct comparison of data from Mars Express and
Venus Express are enabled by the fact that several instruments are identical or similar on the two spacecraft. Also,
several of the Venus Express scientists are involved in both
missions. This opens up possibilities not found before on
interplanetary spacecraft.
2.1. Atmospheric Structure
[8] Knowledge about the atmospheric structure is of great
importance for the understanding of the state of the atmosphere and the processes active therein. Previous missions
provided a basic understanding of the thermal structure, but
many questions remained to be answered [Taylor et al., 1980;
Seiff et al., 1985]. The atmosphere can be roughly divided
into three main layers based on the temperature distribution
and the various processes that govern the state of the layers:
the troposphere (0 –60 km), the mesosphere (60 – 100 km),
and the thermosphere (above 100 km). The isothermal region
between the troposphere (where temperature falls with
height) and the mesosphere (where temperature increases
with height) that would correspond to the stratosphere on the
Earth is of very limited vertical extent on Venus and is not
usually described as a separate layer.
[9] Below 30 km the temperature is believed to be fairly
constant all over the planet [Seiff et al., 1985] but the
latitudinal coverage of the data is fairly poor. The mesosphere
shows a much more variable temperature, especially in
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latitude, but what the dominating processes are is not clear
[Lellouch et al., 1997].
[10] The thermosphere turned out to be surprisingly cool,
especially at night (leading to the coining of the term ‘‘cryosphere’’ for the nighttime thermosphere). While it is clear that
radiative processes play a major role, additional, and more
densely sampled data covering all latitudes and local times
are needed to achieve a good understanding [Fox and Bougher,
1991].
[11] Venus Express studies the thermal structure of the
atmosphere by means of global, simultaneous, and spatially
resolved spectroscopic observations in the wavelength range
from UV to thermal IR. Specifically, it (1) investigates the
upper atmosphere (140– 90 km) at high vertical resolution
through solar and stellar occultation measurements [Bertaux
et al., 2007b], (2) sounds the temperature of the middle
atmosphere (100– 60 km) using spectroscopic observations
of the 4.3 mm CO2 bands, with a coverage in latitude and local
solar time corresponding to a spatial resolution of a few tens
of kilometers or better [Formisano et al., 2006; Drossart
et al., 2007a], (3) sounds the temperature in the altitude range
80– 40 km by radio occultation, providing vertical resolution
of a few hundred meters [Häusler et al., 2006; Pätzold et al.,
2007], and (4) maps the surface temperature on the nightside
as a function of surface elevation [Drossart et al., 2007a].
[12] Together, these different techniques cover the range
from 140 km down to 35 km. The optical measurements can
be made only during nighttime since it would be very difficult
to discriminate between thermal radiation and scattered
sunlight. Unfortunately the PFS instrument, which would
have used the 15 mm CO2 band for daytime profiling, is not
operating. The use of other instruments has been rescheduled
in order to as much as possible minimize the impact of this
loss.
2.2. Atmospheric Dynamics
[13] Data from previous missions and ground-based observations have shown that the general circulation of the
atmosphere can be divided into two regimes: A retrograde
zonal superrotation in the troposphere and mesosphere
[Gierasch et al., 1997] and a solar to antisolar component
across the terminator in the thermosphere [Bougher et al.,
1997]. The zonal superrotation has a maximum wind velocity
of about 100 m/s at the cloud top level (70 km), decreasing
to almost zero at the surface. At the same time, there is a
‘‘Hadley-like’’ slower (about 10 m/s) overturning of the
atmosphere from the equator to the high latitudes, with giant
vortices at each pole recycling the air downward. No attempt
to model the superrotation has been completely successful so
far, indicating that the basic mechanisms of the phenomenon
are unclear. More and better data, with improved spatial and
temporal coverage, are needed in order to understand the
detailed physical condition in this region and to modify the
models to better reflect the reality.
[14] Venus Express investigates the atmospheric dynamics
by observing clouds at different levels [Drossart et al.,
2007a; Markiewicz et al., 2007a], by deriving thermal winds
from thermal profiles, and by monitoring airglow of different
species. Specifically, it (1) measures the global wind fields in
three dimensions and investigates whether the meridional
circulation is one large basic ‘‘Hadley’’ cell extending from
the surface to the upper atmosphere, or a variation of such a
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Figure 1. The abundance of the most important minor gases in the atmosphere of Venus as determined by
missions before Venus Express. The individual gases are color coded as shown in the bottom left corner. The
sizes of the blocks indicate the spread in several measurements rather than measurement errors. The bars
with arrows indicate the limits of detection for the Venus Express instruments.
cell, or something completely different; (2) studies the
temporal behavior of the polar vortices: how they evolve
and how they couple to the two main components of the
global circulation [Piccioni et al., 2007]; and (3) studies how
the transition between the retrograde superrotation and the
solar to antisolar circulation takes place in the interface
between the mesosphere and the thermosphere. [Drossart
et al., 2007b].
2.3. Atmospheric Composition and Chemistry
[15] The main components of the Venus atmosphere are
CO2 (96.5%) and N2 (3.5%). Sulfur-bearing trace gases,
carbon and chlorine compounds, and water vapor are present
in the atmosphere in amounts from few to few hundred parts
per million (ppm) [de Bergh et al., 2006]. Figure 1 shows
the mean abundance of the main trace gases and their vertical profiles measured in the Venus atmosphere by earlier
missions.
[16] Although present in small amounts, the trace gases are
involved in important complex chemical cycles. In the region
around the cloud tops photochemical reactions between CO2,
SO2, H2O, and chlorine compounds lead to the formation of
sulfuric acid, which is the main component of the cloud particles. The chemistry of the lower atmosphere is dominated
by thermal decomposition of sulfuric acid, and thermochemical cycles that include sulfur and carbon species and water
vapor. Surface minerals can also play a significant role in
buffering the abundance of certain gases in the lower atmosphere [Fegley et al., 1997]. The main sulfur-bearing gas,
SO2, is present in the Venus atmosphere in amounts of a
few hundred ppm, which is much more than expected from
thermal equilibrium with the surface minerals. Pioneer Venus
measured a strong continuous decline of the SO2 abundance
at the cloud tops during its 14 years of operation, indicating
possible recent volcanic activity.
[17] Venus Express measurements of the chemical composition of the atmosphere address the following: (1) The
abundance and spatial and temporal variation of SO2, SO,
H2O, HCl, and CO at the cloud tops, to improve the understanding of the physical and chemical processes in this region,
including the production of sulfuric acid aerosols [Bertaux
et al., 2007a], (2) vertical profiles of SO, SO2, H2O and HDO,
HCL, and HF between 80 km and the cloud tops, and vertical
profiles of CO from the cloud tops up to about 120 km,
by stellar and solar occultation [Bertaux et al., 2007b], and
(3) the abundance and spatial variation of H2O, SO2, COS,
CO, H2O, HCl, and HF in the lower atmosphere, to improve
the understanding of chemistry, dynamics, and radiative
balance of the lower atmosphere, and to search for local volcanic activity [Drossart et al., 2007a; Svedhem et al., 2007b].
2.4. Cloud Layer and Hazes
[18] Venus appears completely featureless in visible light
owing to a thick cloud layer located between 50 km and
70 km altitude. However, images made in the UV blue
spectral range show much structure, both at large scale and
at small scale (Figure 2). It has important implications for the
energy balance since about half of the solar energy entering
into the atmosphere is absorbed in this region. The nature
of the UV absorbing matter remains one of the mysteries of
Venus. Earlier observations have shown that the upper cloud
layer consists of micron-sized droplets of 75% sulfuric acid.
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radiation penetrates through the atmosphere and heats the
surface [Taylor et al., 2007]. The radiation balance, however,
is not well constrained and in particular the spectral characteristics and the spatial and temporal variations of the outgoing
radiation are not well known.
[21] Venus Express measures the outgoing radiative flux
(reflected solar light and thermal radiation from the atmosphere and the surface) over the near-IR spectral range and
maps the temperature field and the cloud layer in three dimensions. This will constrain the optical models of the atmosphere and give an insight into the radiative and dynamical
heat transport, and the role of the different species in the
greenhouse mechanism and the planetary heat balance.
Figure 2. An image of Venus made by the Venus
Monitoring Camera (VMC) instrument in ultraviolet light
at 365 nm from a distance of about 35,000 km. The south pole
is at the bottom right in the image and the equator at the top
left. The cloud structure is shown to be very different in
the polar region compared to the equatorial region. A bright
elongated cloud is shown in the middle southern latitudes.
These droplets make up a major portion of the cloud mass but
the processes and conditions for their formation are poorly
understood, while the physical and chemical processes forming the rest of the cloud population are virtually unknown.
[19] Venus Express sounds the structure, composition,
dynamics, and variability of the cloud layer. Specifically, it
(1) images the spatial distribution and temporal variability of
the unknown UVabsorber [Markiewicz et al., 2007b; Drossart
et al., 2007a]; (2) measures the vertical structure and microphysical properties of the upper cloud layer and the hazes
above it, by stellar and solar occultation, limb observations,
and nadir sounding [Bertaux et al., 2007a]; (3) measures
the spatial variations of the opacity and particle sizes on
the nightside (through the IR ‘‘spectral windows’’); this data
together with information on the composition will help to
constrain models of the cloud formation and evolution [Titov
et al., 2006b]; and (4) searches for correlation in the spatial
distribution of the unknown UV absorber with the abundance
of SO2 and other trace gases.
2.5. Energy Balance and Greenhouse Effect
[20] With a surface temperature of more than 730 K Venus
has the strongest greenhouse effect in the solar system. The
dense CO2 atmosphere is responsible for this extreme behavior, but water vapor and sulfur dioxide and the cloud layer
also play a role [Crisp and Titov, 1997; Titov et al., 2007]. It is
even more impressive when considering that Venus, in fact,
absorbs less energy from the Sun than the Earth does, owing
to the high albedo (76%), which is caused by the thick cloud
cover of the planet. Less than 10% of the incoming solar
2.6. Plasma Environment and Escape Processes
[22] Today Venus has only very little water, mostly in the
form of water vapor. If it all were condensed it would form a
global layer of about 3 cm, compared with 3 km on the Earth.
There is no good reason why initially the two planets should
have been significantly different and it is expected that Venus
indeed has had large quantities of water, and possibly other
volatile species, perhaps as much as the Earth. If so, how,
when and why did this disappear? Pioneer Venus found that
deuterium is enhanced relative to hydrogen about a factor 150
compared to the Earth’s value [Donahue et al., 1997]. This is
an indication that hydrogen has been lost, while deuterium,
having twice the mass, does not escape as easily as hydrogen.
Therefore information on the present abundance of deuterium, hydrogen and oxygen and their escape rates are essential to the understanding of the history of water on Venus.
[23] The lack of an internal magnetic field causes the solar
wind to act on the upper atmosphere in a much more violent
way than it does on the Earth. The solar wind interacts with
the top of the ionosphere to form a complex system of plasma
clouds, tail rays, filaments, and ionospheric holes on the
nightside through which a substantial amount of material can
leave the planet [Brace and Kliore, 1991]. The escape mechanisms induced by the solar wind are the dominant ones
for the loss of heavy atmospheric gases such as oxygen
because the gravitational force inhibits both Jeans escape
and nonthermal escape.
[24] To address the problems of atmospheric escape and
investigate the plasma environment, Venus Express (1) determines the positions of the plasma boundaries for the different
domains in the planetary environment, and their dependence
on the solar activity [Zhang et al., 2008a, 2008b; Martinecz
et al., 2008], (2) measures in situ, for the different domains,
directional flux and energy of energetic neutral atoms, ions
and electrons; of particular importance are the measurements
of the escape rate of key species like hydrogen, deuterium and
oxygen [Barabash et al., 2007a, 2007b], (3) measures the
magnetic field in the different domains in the planetary
environment [Zhang et al., 2006, 2007], (4) determines the
vertical structure of the ionosphere, by radio occultation
measurements [Häusler et al., 2006; Pätzold et al., 2007],
and (5) measures the composition and energies of the undisturbed solar wind, as a reference for the Venus measurements and for comparison with similar measurements around
the other planets of the solar system. The Venus Express
measurements are taken at solar minimum activity, thus
complementing the Pioneer Venus plasma studies that were
acquired during solar maximum conditions.
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2.7. Surface Properties and Geology
[25] Venus has a geologically young surface at an age of
700 Ma ± 200 Ma, as estimated on the basis of impact crater
statistics. In addition there are indications that the age of the
different regions do not differ much from each other. Volcanic
and tectonic activities have strongly affected the surface
[Solomon et al., 1992] forming highly deformed plateaus,
tesserae, and extensive lowlands, planitiae. Many scientists
favor a catastrophic global resurfacing of the crust, a mechanism unique among the terrestrial planets [Strom et al.,
1994]. Important open questions are related to the surface
mineralogy, the existence of present volcanic activity and the
role of chemical interaction at the surface-atmosphere interface. Despite the fact that Venus Express is not carrying
any instruments specifically devoted to surface science, it
contributes to these studies in several ways [Titov et al., 2006b],
by
[26] 1. Investigating regions of high radar reflectivity, by
bistatic measurements, focusing on highland regions like
Aphrodite Terra, Beta and Atla Regio, and Maxwell Montes
[Häusler et al., 2006]. These areas have shown anomalously
high reflectivity in the Magellan radar images. Unfortunately,
these measurements have had to be discontinued since mid2007 owing to a still unexplained loss of power in the S band
transmitter chain.
[27] 2. Investigating the mass distribution in and around
Atalanta Planitia by radio science gravity studies, through
orbital trajectory analysis.
[28] 3. Mapping the surface temperature and estimate
the surface emissivity by observations in the 1 mm spectral
window.
[29] 4. Searching for surface hot spots and local deviations
in chemical abundance in the lower atmosphere, in particular
SO2, indicating possible volcanic activity.
[30] 5. Searching for atmospheric waves generated by
seismic activity and coupled to the atmosphere owing to
the high atmospheric density at the surface [Drossart et al.,
2007a].
3. Spacecraft
[31] Venus Express uses the basic design of the Mars
Express spacecraft, which was launched in 2003, adapted
for the specific conditions at Venus [Sivac and Schirmann,
2009]. The main modifications to the spacecraft are related to
the thermal control system and the power system. The most
important characteristics are summarized in the following
paragraphs.
3.1. Structure and Propulsion
[32] The Venus Express spacecraft is based on a box-like
structure with the dimensions 1.7 m 1.7 m 1.4 m. The
distance from tip to tip of the deployed solar panels is about
8 m. The principal mechanical structures inside the spacecraft
are the two fuel tanks, with MonoMethyl Hydrazine (MMH)
as the fuel and Nitrogen TetrOxide (NTO) as the oxidizer, and
a smaller helium tank for main tank pressurization. On both
sides of these tanks two internal shear walls are built. The
spacecraft, without solar panels, can be seen during the solar
illumination testing in Figure 3 and the fully integrated
spacecraft during its mating to the Fregat upper stage can
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be seen in Figure 4. The mass of the dry spacecraft is 633 kg,
including 94 kg for the payload. The tanks were filled at
launch with 570 kg of propellant, most of which was
consumed during the orbit insertion maneuvers. Together
with the launch vehicle adapter this made up the total launch
mass of 1241 kg, leaving a comfortable margin to the maximum allowed mass of 1270 kg for the Soyuz Fregat launcher
combination. Venus Express as well as Mars Express has
been designed to sustain soft aerobraking, at a maximum
dynamic pressure of 0.3 N/m2.
3.2. Thermal Control
[33] The closer distance to the Sun, and the higher albedo
of Venus compared to Mars, made it necessary to completely
redesign the thermal control system. A new thermal blanket is
used, giving the spacecraft a golden finish, in contrast to the
black appearance of Mars Express. Several instruments and
subsystems are connected to radiators mounted on different faces on the outside of the spacecraft. One side of the
spacecraft, the -X side, carries the largest radiators which act
as heat sinks for the coolers for the instruments requiring
cryogenic temperatures. This side must never be exposed to
the sun at any angle. The -Z side, where the main engine
nozzle and the launch vehicle adapter interface reside, also
must avoid illumination by the sun since these will absorb the
solar heat very quickly. This design, together with specific
operational constraints, ensures a low maximum temperature
even in Venus orbit. Under some conditions the temperature
would be too low, and therefore a set of electrical heaters
are fitted to the most critical units. These heaters are either
switched by mechanical thermostats or by the on board
computer.
3.3. Power System
[34] As the solar input in Venus orbit is about 2.6 kW/m2,
which is about four times that of Mars or twice that of the
Earth, the size of the solar panels could be significantly
reduced. The original silicon-based solar cells have been
replaced by gallium arsenide, optimized for operation at
higher temperature. To keep the temperature in the proper
range these cells are mounted in rows interleaved with optical
solar reflectors, giving the solar panels a striped appearance.
The electrical output in Venus orbit is approximately 1400 W.
The remaining part of the power system, including the
batteries, required only minor modification from the Mars
Express version.
3.4. Telecommunication System
[35] The spacecraft uses X band communications for both
the telecommand uplink and the telemetry downlink. Fully
redundant cross-strapped communication chains, including
two 65 W traveling wave tube power amplifiers, feed the
signal to the two high-gain antennas of 1.3 m and 0.3 m
diameter. S band communications, with dual 5 W solid state
power amplifiers, are included as a backup system and for use
near the Earth during the first weeks following the launch.
The data rate varies between 15 kbps to 228 kbps, corresponding to a downlink capability of between 400 Mbit to
6.5 Gbit per day, depending on the actual distance between
Venus and the Earth. The antennas are body fixed and thus the
spacecraft will turn toward the Earth for dumping the data
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Figure 3. The compete spacecraft excluding solar panels during the solar illumination test at Intespace,
Toulouse, France. The facility was upgraded for this test in order to be able to produce the required beam
with an intensity 2.6 kW/m2 over the full surface of the spacecraft. The front side shows the 1.3 m diameter
high-gain antenna covered with its solar shield, a thin foil with germanium deposit to keep the antenna
within its operational temperature during all spacecraft attitudes. The back side of the smaller high-gain
antenna is the white dish seen on the top platform where all remote sensing instruments are also located. The
top and front sides are allowed to be continuously illuminated by the Sun at Venus, while all other sides have
strict limitations on the duration of solar illumination. The height of the structure is about 1.4 m.
each telemetry session, usually 8 – 10 h per day. The data are
stored in a 12 Gbit solid state data recorder during the
observation passes.
[36] The radio science investigation makes full use of
the telecommunication system and an ultrastable oscillator
has been added to one chain of the system to enhance the
performance for this purpose. The S band system experienced
a drop in signal level during the solar conjunction in July
2006 and has therefore only been used occasionally since
then. The reason for this anomaly is still not known.
[37] The main ground station is the Cebreros station west
of Madrid, Spain, which is used for all uplink and most data
downlink activity. The New Norcia station in Australia is
used for radio science activities. Occasionally the NASA
DSN assists by giving extra coverage, for radio science
activities in particular, but also in connection with periods
of high data rate requirements, for example, when measurements at high temporal resolution are made, such as highresolution movies of the cloud motions.
3.5. Operational Constraints
[38] While pointing to the Earth from Mars during communications the Sun is always within a cone of 40° from the
Earth, making it easy to avoid illumination of protected
surfaces. In addition the high-gain antenna acts a thermal
shield of the spacecraft. From Venus, however, the Sun can
appear anywhere in the sky, making it impossible to avoid
illumination of specific surfaces. To deal with this a second,
smaller, high-gain antenna has been mounted on top of the
+Z face of the spacecraft, pointing in the opposite direction
from the main antenna. Then it is possible always to find an
attitude where illumination of the forbidden faces can be
avoided. Figure 5 shows that inside the quadrature, where the
Venus-Sun-Earth angle is less than 45°, the small antenna
is used; that is, the spacecraft needs to rotate 180° about the
z axis at entry and exit of the quadrature. In addition the
spacecraft will need to rotate 180° about the x axis at inferior
and superior solar conjunction.
4. Payload
[39] The payload is composed of seven instruments in three
different categories; spectrometers and (spectral) imagers
for remote sensing, plasma and magnetic field instruments
for in situ measurements, and the ultrastable oscillator used for
radio science.
[40] Great care was taken in selecting the instrument complement in order to make optimum use of the limited resources
on board. A complication was the short time available for
development since the project schedule was very compressed.
As it turned out, a very well balanced set of instruments could
be assembled from existing designs and even from spare parts
and units from other recent missions, mainly Mars Express
and Rosetta. In addition, two completely new instruments
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Figure 4. The fully integrated spacecraft is being mounted
on top of the Fregat upper stage. The black conical structure is
the launch adapter, which remains on the Fregat stage after
separation. Protective covers are still mounted on the folded
solar panels and on the instrument apertures and the two
cylindrical baffles of the star trackers. The white recessed
surface on the front is the VIRTIS thermal radiator.
were designed, developed and built in this short time, namely
the VMC and the SOIR.
[41] Figures 6 and 7 show how the different instruments
complement each other with respect to their fields of view
and their spectral range and resolution. Venus Express is
the first dedicated atmospheric mission to Venus since the
discovery of the near infrared spectral windows [Allen and
Crawford, 1984; Baines et al., 2006]. The payload was composed in order to maximize the benefit from this new opportunity to study the atmosphere in three dimensions. Table 1
lists the individual instruments and their main functions
and Figure 8 shows the locations of the instruments on the
spacecraft.
4.1. Remote Sensing Instruments
[42] Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus/Solar Occultation in the
Infrared (SPICAV/SOIR) [Bertaux et al., 2007a, 2009] is a
set of three spectrometers optimized for providing thermal
profiles and composition in the upper atmosphere by observations in stellar and solar occultation mode. It is also used
in nadir mode, together with other instruments. The solar
and stellar occultation technique provides very good vertical
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resolution and high sensitivity for species of low abundance
(like SO2, COS, CO, HCL, HF, and others). Particularly
important molecules to measure are H2O and HDO, since the
ratio between the two carries information on the history of the
total amount of water on the planet. Temperature and density
profiles are retrieved by analyzing the absorption in various
CO2 bands. The instrument covers three wavelength bands:
110 –310 nm, 0.7 –1.7 mm and 2.2 – 4.4 mm (SOIR). The
SOIR unit utilizes a miniaturized Stirling-cycle engine to
cool the detector to 90 K, and an acoustooptic tunable filter,
together with a grating used at a high order of diffraction, to
achieve a spectral resolution (l/dl) of more than 20,000.
[43] The basic SPICAV unit has an important heritage from
the SPICAM instrument on Mars Express, while the SOIR
channel is a completely new development. Important recent
findings include the determination of the D/H ratio as a
function of altitude, the discovery of a warm layer on the
nightside at the base of the thermosphere (90 km) and the
discovery of a set of previously unobserved absorption lines
of the CO2 isotope 16O12C18O [Bertaux et al., 2008; Wilquet
et al., 2008].
[44] Visible and Infrared Thermal Imaging Spectrometer
(VIRTIS) is a combination of a visible (0.27– 1.1 mm) and an
infrared (1.05– 5.2 mm) medium resolution imaging spectrometer (VIRTIS-M) with a high-resolution infrared (1.8 –
5.0 mm) spectrometer (VIRTIS-H) [Piccioni et al., 2009;
Drossart et al., 2007a]. The infrared detectors are actively
cooled to 80 K, while the visible sensor is passively cooled
to below 200 K. The instrument is almost identical to the
VIRTIS instrument presently flying on the Rosetta spacecraft. The imaging spectrometer has a field of view of
64 mrad 64 mrad, with a pixel size of 0.25 mrad 0.25 mrad. The images have a spatial dimension of 256 256 pixels and a spectral dimension of 432 lines. From near
apocenter, the field of view covers about 1/3 of the diameter of Venus. Mosaics of 3 3 frames are constructed by
repointing of the spacecraft in order to generate global images
of the southern hemisphere. VIRTIS is addressing a large
number of scientific questions and has generated dramatic
images and video sequences of the south polar vortex, nonLTE emission patterns, profiles of abundance of several
atmospheric gases, wind field maps and temperature profiles
over the southern hemisphere, images of cloud structure
at several altitudes and surface temperature and emissivity
maps, to mention just a few.
[45] Venus Monitoring Camera (VMC) is a small but efficient camera operating simultaneously in four narrow spectral bands at 365, 513, 965, and 1000 nm [Markiewicz et al.,
2007a, 2009]. The UV band is mapping the cloud tops in
reflected sunlight during daytime, where structure is visible
owing to the still unknown UV absorber. These images are
used for deriving global wind fields at this altitude and for
studying the cloud morphology. The near-IR 1mm band is
used for mapping the surface brightness during nighttime.
The remaining two bands include O2 airglow emission and
possible water vapor absorption. VMC has an unusual design
with four optical chains, each one with its own permanent
filter, sharing one CCD, with each optical system having its
own dedicated quadrant on the CCD. When the CCD is read
out in normal mode each image frame actually contains four
images. The field of view is 17.5° (0.3 rad), resulting in a
pixel footprint size on the surface ranging from 200 m at
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Figure 5. The orbit of Venus in a Sun-Earth fixed frame, with the Sun at the center and the Earth at the left.
The attitude of the spacecraft while Earth pointing will change depending on the relative position in this
frame in a way such that the Sun is illuminating only two of the sides of the spacecraft (+X and +Z). To
achieve this, the spacecraft is rotated 180° around the z axis at the points marked quadrature and 180°
around the x axis at inferior and superior conjunction.
pericenter to 45 km at apocenter. VMC has sampled a large
number of images, and important findings include the determination of global wind fields at different times, the characterization of the cloud morphology in different regions and
synthesis of surface maps.
[46] Planetary Fourier Spectrometer (PFS) is a high spectral resolution double pendulum spectrometer with two
channels, a short wave channel (0.9 – 5.5 mm) and a long
wave channel (5.5 – 45 mm) [Formisano et al., 2006, 2009].
The history of this instrument goes back to the Russian
Mars-96 mission that unfortunately was lost in a launch
failure in 1996. It flew again on Mars Express where it
performs very well and has made important discoveries.
Some of the main objectives for Venus were determination
of the temperature field in the altitude range 55– 100 km, both
on the nightside and on the dayside, mapping of the surface
temperature, and making profiles of the abundance of a large
number of gases in the middle and lower atmosphere. It was
also to measure the outgoing thermal flux to determine the
global radiation budget. Unfortunately, a mechanism that
controls the mirror that switches the beam between the
different calibration targets and the view toward the planet
is stuck in its launch position, pointing toward a blackbody
target. In spite of many attempts it has not been possible to
move the scanner since the launch.
4.2. In Situ Instruments
[47] Analyzer of Space Plasma and Energetic Atoms
(ASPERA-4) is the fourth of this series of instruments to
fly in space, but the first of its kind around Venus [Barabash
et al., 2007a, 2009]. ASPERA-4 is identical to ASPERA-3
presently in orbit around Mars on Mars Express. The instrument has four different sensors, housed in two separate units.
The ion mass analyzer, IMA, measures ions separated by
mass up to m/q = 40 for energies from 10 eV/q to 36 keV/q.
The ions are mapped at 22.5° resolution over 360° in azimuth
and at 4.5° resolution over ±45° in elevation with respect to
the instrument orientation. The Neutral Particle Imager (NPI)
measures the integral flux of neutral particles between 100 eV
and 60 keV with an instantaneous field of view of 9° by 344°
at an angular resolution of 4.6° by 11.5°. The Neutral Particle
Detector (NPD) consists of two identical units that are
basically pinhole cameras for the energy range 0.1 – 10 keV,
each with a field of view of 9° by 90° and a resolution of 5° by
30° with separation of hydrogen and oxygen. The Electron
Spectrometer (ELS) is a compact electrostatic analyzer for
the energy range 1 eV to 15 keV with an energy resolution of
7%. The field of view is 10° in elevation and 360° in azimuth
with a resolution of 22.5° in azimuth. ASPERA has made
a large number of measurements in all accessible domains
around Venus and in the solar wind and has characterized the
different regions and their boundaries with respect to flux,
energy and composition of the detected particles. A major
result is the absolute determination of the escape rates of H+,
O+ and He+ which is important for estimating the historic
water content on the planet.
[48] Magnetometer (MAG) is a dual sensor fluxgate instrument with the main sensor mounted on the tip of a 1 m
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Figure 6. A comparison of the fields of view (FOV) of the remote sensing instruments onboard and the
size of Venus corresponding to a distance of 20,000 km. The background image was taken by VMC at
ultraviolet wavelengths from about 35,000 km. The south pole is at the bottom of the image. At apocenter
the planetary disk is smaller than the VMC FOV and about three times the VIRTIS-M FOV.
long boom and an auxiliary sensor mounted at the foot of the
boom on the outer surface of the spacecraft [Zhang et al.,
2006, 2009]. This dual sensor configuration has proven very
useful since the spacecraft was developed without a specific
magnetic cleanliness program and therefore disturbances
from the spacecraft itself are unavoidable. The data from
the auxiliary sensor are used to clean the data of the main
sensor, and a performance similar to that of a magnetically
clean spacecraft is achieved. Venus Express is the first spacecraft where this concept has been used. MAG is the only
instrument on board that is making continuous measurements.
Samples are taken at 1 Hz frequency when the spacecraft is in
the solar wind and at up to 128 Hz around the pericenter. The
results include detailed models of the bow shock and the
induced magnetopause, and detection of foreshock activities
and upstream waves. Particularly interesting is the detection
of whistler waves that are interpreted as evidence of lightning
in the atmosphere.
4.3. Radio Science
[49] Venus Radio science (VeRa) is the radio science investigation [Häusler et al., 2006, 2009]. The main objective is
to determine the temperature and the density of the lower
atmosphere in the altitude range 40– 90 km and to determine
the electron density of the ionosphere up to the ionopause by
means of radio occultation of the telemetry signal. Additional
objectives are to carry out bistatic radar measurements over
selected areas of the surface, including those highly reflective
(at radar wavelength) areas discovered by the Magellan radar,
in particular in areas of high elevation, like Maxwell Montes
and Thetis Regio. Measurements are made by directing the
main S/C antenna toward the area of interest on the surface,
and receiving the reflected signal on a large antenna back on
Earth. This was attempted successfully several times until the
power in the S band channel was lost, as described above.
The absorption in the atmosphere at S band is manageable but
at X band it is too high for reliable operation.
[50] VeRa occultation profiles have been used to map the
thermal structure in the lower atmosphere at a high vertical resolution and medium horizontal resolution, separating
the daytime and the nighttime regions. The lower parts show
little difference between day and night but in the higher
regions variations are clearly visible.
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Figure 7. A comparison between the wavelength ranges and spectral resolutions of the Venus Express
remote sensing instruments.
Figure 8. The Venus Express spacecraft in a semitransparent view, showing the positions of the seven
scientific instruments. The top face, the +Z platform, accommodates all the apertures of the remote sensing
instruments. This side is turned toward Venus during observations. The two ASPERA units are mounted on
the bottom side ( Z platform).
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Table 1. The Seven Instruments of the Venus Express Payloada
Name
Function
SPICAV/SOIR
UV-IR spectrometer
VIRTIS
VMC
PFS
ASPERA-4
MAG
VeRa
Measured Parameters
Major Topics of Interest
Wavelength 110 – 310 nm,
Solar and stellar occultation. Upper atmosphere:
0.7 – 1.65 mm, 2.2 – 4.4 mm
Composition, D/H ratio, thermal profiles.
Resolving power up to 20,000
UV-Vis-IR
Virtis-M, 0.27 – 5.2 mm imaging
Atmospheric dynamics, structure and composition.
Imaging spectrometer
Virtis-H 1.8 –5.0 mm single point at resolving power 2000
Surface temperature, emissivity.
4 band camera
Wavelength 365 nm, 513 nm, 965 nm, 1000 nm
Wind speed, clouds, surface, global context
Fourier spectrometer
Wavelength 0.9 – 45mm, resolving power 1200
Thermal profiles, composition.
Presently non operational
Energetic particles
Electrons 1eV – 15 keV, ions 0.01 – 36 keV/q,
Atmosphere solar wind interaction,
neutral particles 0.1 – 60 keV
atmospheric escape.
Magnetic fields
B field, 8 pT – 262 nT, at 128 Hz
Induced magnetosphere.
Plasma boundaries. Lightning
Occultation of telecom link
X- and S-band, Doppler shift,
Lower atmosphere density and thermal structure.
polarization, amplitude
Ionosphere. Surface radar reflectivity.
a
The total mass is 92 kg. The instruments together produce between 1 Gbit and 6 Gbit data per day, adapted to the telemetry transfer rate which depends on
the actual Venus-Earth distance.
[51] The radio measurements also contribute to surface
science by precise tracking of the spacecraft trajectory, from
which data the gravity field is determined and gravity anomalies are derived. The gravity field of Venus is in general
well known from previous missions, but a few anomalies
need to be further investigated, for example in the region
of Atalanta Planitia. A secondary objective is to study the
solar corona at times near Venus superior conjunction, when
the signal from Venus passes close to the disk of the Sun,
revealing the structure, density and dynamics of the solar
corona.
[54] Many scientific and technical constraints were considered when the operational orbit was selected. To achieve
good global coverage, a polar obit was deemed to be essential. The need for a global view and long duration measure-
5. Mission Scenario and Operational Orbit
[52] Venus Express was launched by a Soyuz-Fregat combination from Baikonur, Kazakhstan, on 9 November 2005
and arrived at Venus on 11 April 2006 (Figure 9). The injection into the heliocentric transfer orbit by the launcher was
very precise and very little fuel had to be spent for trajectory
corrections during the cruise to Venus. During the first part of
the 153 days cruise all spacecraft and payload elements were
checked out for proper operation.
[53] The Venus orbit insertion was divided into several
steps, starting with a capture maneuver which included a
52 min long burn by the 400 N main engine, to achieve the
required delta-v of 1251 m/s [Warhaut and Accomazzo,
2009]. This delta-v is significantly higher than that required
for the capture of Mars Express into Mars orbit. The reason is
twofold; the differential velocity between the spacecraft and
the planet is larger at Venus, and the larger mass of Venus
requires more energy to reduce the apocenter height once
captured. The insertion burn placed the spacecraft in a 9-day
orbit with an apocenter height of 330,000 km. During this
initial ‘‘capture’’ orbit, six blocks of scientific observations
were included to benefit from the unique opportunity of
having a very high apocenter distance that allowed global
observations and dynamic studies on a large scale. After
finishing the capture orbit, a sequence of smaller engine
burns took place to reduce the apocenter height to the
operational value. During the capture maneuver and the
subsequent orbit reduction burns a total of 482 kg of fuel
was consumed. The final operational orbit was reached on
6 May 2006.
Figure 9. Venus Express liftoff on a Soyuz-Fregat launcher
from Baikonur Cosmodrome, Kazakhstan, 0333 UT,
9 November 2005. The spacecraft will subsequently be
inserted into a perfect heliocentric trajectory toward Venus.
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ments for dynamic studies, together with close-up studies and
near-planet in situ measurements, drove the requirement
toward a highly elliptical orbit. A low circular orbit was not
desired but would also not be feasible owing to the high fuel
demands. An orbit with a pericenter height of 250 km and an
apocenter height of 66,000 km, resulting in a period of 24 h,
was selected. Such an orbit also had the advantage of allowing operators work at the same time each day and, since the
main ground station is located in Spain, this turns out to be
normal daytime working hours, saving a significant amount
in the operational costs.
[55] The remaining parameter to decide was the latitude
of the pericenter. Since Venus rotates very slowly, only one
revolution in 243 Earth days, there is very little polar flattening and consequently very little drift in the pericenter latitude, only about 3° per year. This makes the decision more
critical for Venus than for Mars or the Earth. The orbital
trajectory at arrival left basically only two options free for the
choice of pericenter latitude, either 15° or +78°. In order to
study one hemisphere at large distance and the other at short
distance +78° was selected. This choice also nicely complements Pioneer Venus, which had its pericenter around the
equator. The final orbit is shown in Figure 10.
[56] A specific feature with a highly elliptical, polar orbit
around Venus is the drift in pericenter altitude, due to perturbations by the solar gravity, which causes the pericenter
altitude to drift upward or downward at a rate of 1 – 3 km per
day. For the case of Venus Express the drift is upward until
May 2009, then downward. This drift is compensated for
by regular thrusting by four 10 N thrusters, to maintain the
pericenter height between 250 km and 400 km and, after
August 2008, between 175 km and 275 km. This will ultimately lead to the depletion of the fuel and an end of the
mission in late 2013 if no countermeasures are taken. A
possible alternative is to use aerobraking in the upper atmosphere of Venus to reduce the apocenter height (and the
orbital period), as well as the rate of the drift of the pericenter height. A 12 h (37,000 km apocenter height) or 8 h
(27,000 km apocenter height) orbit would dramatically
reduce the drift rate and allow operation of the mission for
another decade if required.
6. Science Planning and Operations
[57] The planning of the scientific operations is a complex
process that is carried out at different levels in a sequential manner. At the highest level, governed by the Science
Requirements Documents, the Venus Express Science Working Team develops a long-term Science Activity Plan (SAP)
typically covering a year or more of observations [Titov et al.,
2006b]. This plan identifies specific orbital, and other, characteristics like solar illumination, accessible surface targets,
solar eclipse periods, and Earth occultation periods. A scheme
for the operations is then designed, on the basis of 10 different building blocks, called Science Cases, which can be put
together in different combinations.
[58] A medium-term plan (MTP) covering a period of
4 weeks, is agreed between representatives of the instrument
teams on the basis of the guidelines in the SAP, approximately 3 months before its execution.
[59] The lowest level in the planning cycle is the short-term
plan (STP) which is prepared on a weekly basis. Here the
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Figure 10. The operational 24 h orbit around Venus. There
are four major operational phases: (1) Around apocenter, at
10 h to 14 h, full disk imaging is made by VIRTIS and VMC,
(2) along the ascending branch, between 14 h and 22 h, longduration observations are made for studies of the dynamics
and cloud evolution, and (3) around pericenter, 23 h to 1 h,
high spatial resolution observations and occultation measurements are performed. (4) Data transmission to the Earth takes
place with the spacecraft Earth pointing from 2 h to 10 h
orbital time.
detailed command files with all settings for the instruments
are included. The whole process is based on an exchange of a
large number of files of different complexity between the PI
teams, the science operations center (VSOC) and the mission
operations center (VMOC), following a well-defined scheme
[Koschny et al., 2009].
[60] The orbit is normally split into four parts where
different activities take place (Figure 10). The region around
apocenter is used for global mapping by VMC and synthesis
of 3 3 frame mosaics for full disk coverage by VIRTIS. The
ascending branch, from 10 h before pericenter to about 2 h
before pericenter, has a long duration view of the same area
of the planet and is used for studies of atmospheric dynamics.
At times of high data rate, time lapse movies are composed
here. The pericenter region, between about 1 h before and
1 h after pericenter, is shared between high spatial resolution
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Figure 11. The present coverage of the VIRTIS-M images of the southern hemisphere as a function of
latitude and local solar time is shown as a color-coded image. The coverage of the latitudes between 50°S
and the south pole is obviously very good, while the coverage at the equator and northward of the equator is
very limited.
nadir measurements and stellar occultation measurements.
If in a solar eclipse period or an Earth radio occultation
period, solar and radio occultation measurements also take
place here. The descending branch of the orbit, from 2 h after
pericenter to apocenter, is normally reserved for telecommunication and data downlink.
7. Coverage of Observations
[61] A global and systematic survey of the atmospheric and
plasma phenomena is a major goal of the Venus Express
mission. To separate spatial variations from temporal variations, good coverage in a multiparameter space is essential
for most measurements. Therefore priority is given to areas
with poor coverage as far as possible. In general the coverage
of the southern hemisphere is good and the coverage of the
northern hemisphere is limited. This is inherent in the present
orbit that has a larger distance to the planet, allowing more
time for measurements over the southern hemisphere.
[62] The coverage of VIRTIS spectral images over the
southern hemisphere is shown in Figure 11. It can be seen that
the polar region is well covered and the nightside of the
southern hemisphere is reasonable well covered south of 50°
southern latitude. On the day side, the equatorial region and
the midlatitudes have very poor coverage. In addition the
coverage of the northern hemisphere is very poor. Figure 12
shows the VIRTIS surface coverage from measurements dur-
ing nighttime. These data are essential for making surface
maps. It can be seen that the coverage at midlatitudes between
longitudes 150° and 30° is fairly good. The rest of the
southern hemisphere is less well covered, in particular the
longitudes between +45° to +150°. Also the surface coverage
in the northern hemisphere is very poor.
[63] The VMC dayside images provide uniform coverage
of the southern hemisphere. These observations are used for
cloud morphology and wind tracking studies. In the north
only small-scale imaging is possible owing to the close proximity of the planet and the fast motion of the spacecraft. The
VMC surface observations are limited to low latitudes (±40°).
These images are made during nighttime, but owing to stray
light only images taken during eclipse are useful. The observations complement well VIRTIS thermal mapping of the
southern hemisphere. However, full coverage of all longitudes at low latitudes would require about 7 years since the
part of the surface seen in eclipse drifts slowly because the
rotation period of the planet is close to a Venus year.
[64] The coverage of the SPICAV stellar occultation profiles depends on the availability of sufficiently bright UV
stars. Presently, about 30 stars are used by SPICAV UV for
occultation studies of the upper atmosphere, covering latitude
ranging from 50°S to 40°N. A given star will always occult
Venus at the same latitude but at varying local time, resulting
in a coverage appearing as horizontal lines (Figure 13). The
SPICAV and SOIR solar occultations by definition always
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Figure 12. The present coverage of the VIRTIS-M nighttime images of the southern hemisphere as a
function of latitude and longitude is shown as a color-coded image. The coverage of the latitudes between
40°S and 80°S for longitudes between 150° and 30° is fairly good, while the coverage of other locations is
limited. These data are required for seeing the lower atmosphere and the surface.
occur at the terminator, i.e., 0600 or 1800 local solar time
(Figure 14b), except very close to the poles owing to the
slight tilt of the axis of rotation of the planet. Full coverage in
local solar time is thus not possible, although improved
temporal coverage, to study both short-term and long-term
variations, can be achieved by more measurements.
[65] Figure 14a shows the coverage of the VeRa Earth
radio occultations. It can be seen that even after three full
occultation seasons the coverage is not very dense, in particular in the northern hemisphere, from the equator up to
about 60° northern latitude.
[66] ASPERA has a very good coverage inside the bow
shock, except for in a small corridor straight south of the
planet. It also has a good set of reference data taken in the
undisturbed solar wind, well outside the bow shock (Figure 15).
[67] MAG is the only instrument that is almost constantly
on. In the solar wind it samples at a fixed frequency of 1Hz
and close to pericenter it samples at 128Hz for a few minutes.
This thorough coverage provides data for a multitude of processes within and outside the induced magnetosphere.
8. Mission Extension Topics
[68] Presently, the Venus Express mission is funded for
operations until May 2009. Most of the original primary
objectives will have been met by then, but a number of old
and new questions that have arisen would benefit from more
data and longer total mission duration. As described above,
the coverage for many parameters both in local solar time and
in latitude and longitude is not yet sufficient for detailed
analysis. The present solar minimum has surprised us by
being longer than normal, and the solar activity is expected to
increase soon. It is very valuable to monitor the response
by the atmosphere to this increase and to compare it to the
situation with the low solar activity we have had over the past
2 years, and to the Pioneer Venus measurements at high solar
activity in the years around 1980. An improved knowledge on the influence of the solar activity on the escape rate
of various species may have important implications for the
understanding of the evolution of the atmosphere.
[69] Long-duration monitoring of key species, for example
SO2, and of cloud properties and wind fields is essential to
make conclusions on secular variation of composition and
dynamics. Likewise, long-duration observations of the surface increase the likelihood of finding local hot spots and/or
volcanic activity. An extended operational life will also allow
time for a further reduction of the pericenter altitude, perhaps
to 170 km in order to improve the basis for the in situ measurements of energetic particles and magnetic fields. The
thermal structure and density in the otherwise inaccessible
region between 170 and 190 km can be studied by atmospheric drag measurements.
[70] An extension beyond 2010 would open up the possibility for joint observations with the Japanese Planet-C
spacecraft. Planet-C is focusing on small-scale atmospheric
dynamics and weather phenomena [Nakamura et al., 2007],
while Venus Express aims at a more general study of the
atmosphere. Planet-C will be placed in a highly elliptical
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Figure 13. Coverage of the SPICAV stellar occultations in the ultraviolet channel; latitude versus local
time. From these measurements, thermal profiles in the altitude range 90 km to 140 km are derived together
with and abundances of several atmospheric gases. The coverage is poor at high latitudes, and more data
will be needed for a full coverage. These measurements are only effective over the dark hemisphere.
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equatorial orbit, complementing well the polar orbit of Venus
Express. Joint observations would allow simultaneous highresolution imaging and global context imaging, and detailed
3-D imaging of the morphology of the upper clouds. This
would be of particular interest for the studies of cloud
dynamics and cloud evolution.
[71] As discussed above an attractive way to extend the
lifetime of the mission is to significantly lower the apocenter
altitude by aerobraking. This would allow both an extended
duration of the present observations and allow new investigations from the vantage of a different orbit. In addition
density and temperature of the upper atmosphere can be
retrieved from the drag exerted on the spacecraft by the
atmosphere, parameters that in this altitude (140– 170 km)
range cannot easily be measured remotely. Aerobraking
in 2012 is under consideration and will be studied in detail
in the near future.
9. Venus Express and the ESA Solar System
Science Program
Figure 14. (a) Coverage of VeRa radio occultation data;
latitude versus local time. From these measurements, profiles
of atmospheric density and temperature are derived in the
altitude range 35 km to 90 km. Additional data will be needed
for a full coverage. (b) Coverage of the SOIR solar occultation measurements. By principle, only data along the terminator are available. More data will be needed for coverage
at high southern latitudes and between 40°N and 80°N.
[72] Since Venus Express is very similar to Mars Express,
with several instruments in common and simultaneous operations, interesting comparisons can be made. Together with
data from Earth orbiting science satellites like Envisat and the
Cluster spacecraft, an important basis for the comparative
planetology of the terrestrial planets with atmospheres is laid.
Possibilities also open up for simultaneous investigations
of the solar wind and atmosphere interactions, and ‘‘space
weather’’ studies on the three planets. The Solar System
Science program of ESA is addressing all major bodies of
the inner solar system from the Sun to Mars, and asteroids
and comets. The three planets mentioned above all have
ESA spacecraft presently orbiting them and the Sun is being
observed, jointly with NASA, by the SOHO spacecraft
from L1.
[73] The last of the terrestrial planets, Mercury, will be
studied in depth, jointly with Japan, by the dual spacecraft
mission BepiColombo, which will be launched in 2014. The
Rosetta spacecraft is already on its way to a comet and is due
to arrive at its target, comet 67P/Churyumov-Gerasimenko,
in 2015. During the cruise two asteroid flybys are scheduled.
Thus, a fairly complete portion of the inner solar system is
being researched, and synergistic effects can also be expected
even if not predictable in advance. The information gathered
will form a basis for the definition of other missions still
to come.
[74] ESA’s science program for the future, ‘‘Cosmic Vision,’’
with launches foreseen in the timeframe 2015 – 2025, is
addressing a number of basic scientific themes with questions
of a fundamental nature, like ‘What are the conditions for
life and planetary formation?’ and ‘How does the Solar System work?’. A large number of missions were proposed for
Cosmic Vision and several of them are now being studied in a
competitive phase for a final selection in 2009. As discussed
above, Venus Express already contributes to parts of these
objectives, in particular in the field of comparative planetology and planetary evolution.
[75] The ESA Planetary Science Archive, PSA, stores the
data from all missions and makes them available to the world
wide community. The first data from Venus Express, as well
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Figure 15. The ASPERA coverage shown in Venus solar cylindrical coordinates (positive x axis pointing
toward the Sun and the y axis showing the distance toward the Venus-Sun line). The coverage is excellent in
most of the volume inside the bow shock, with a slightly weak coverage only straight south of the southern
hemisphere. A very good set of solar wind reference data are available from measurements around the
apocenter.
as the other missions, are available for immediate electronic
download at www.rssd.esa.int/psa.
10. Conclusions
[76] Originally born as a ‘‘mission of opportunity’’ reusing
the Mars Express design and available instruments, Venus
Express has proven to be the most powerful mission ever in
Venus orbit. A fortunate combination of a versatile spacecraft, a state of the art payload, and an efficient ground segment allows scientists to carry out a complex and systematic
survey of the planet from the surface to the thermosphere and
above. The observations include nadir and limb geometry,
stellar, solar and Earth occultation measurements, and in situ
plasma investigations. These studies have unveiled details of
the atmospheric structure, composition, cloud morphology,
dynamics and escape processes never observed before.
[77] The papers following this one in this special section of
JGR report on the most important findings to date. Many of
the original objectives have been addressed to a significant
depth, while additional objectives are being formulated for
the extended mission presently being planned. The analysis
of the large amounts of data collected is a formidable task,
and the science teams are engaged with both operations and
processing the data. There is much more to be done with the
data sets already acquired and new types of observations still
to be carried out, such as atmospheric drag measurements and
coordinated measurements with Planet-C, are promising new
exciting results.
[78] Everyone involved in the mission is appreciating the
worldwide revival of the interest in Venus science that Venus
Express appears to have triggered. The spacecraft and the
instruments are in a good condition, and should continue to
provide new data for scientists worldwide for several years
to come.
[79] Acknowledgments. The authors are grateful for the professionalism and enthusiastic commitment shown by all colleagues in the experimental teams, at EADS-Astrium, ESTEC, ESOC, and ESAC, which
contributed greatly to the positive spirit and the success of the Venus Express
mission.
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