Welcometo the Cryptic Planet

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Welcometo the Cryptic Planet

AA bb out ESAout ESAThe European Space Agency (ESA) was formed on 31 May 1975. It currently has 15 Member States: Austria, Belgium, Denmark, Finland,France, Germany, Ireland, Italy, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom.

The ESA Science Programme has launched a series of innovative and successful missions. Highlights of the programme include:

Cluster, which is a four-spacecraft missionto investigate in unprecedented detail theinteraction between the Sun and the Earth’smagnetosphere.

Cassini-Huygens is one of the mostambitious planetary exploration efforts everattempted. After a seven-year journey, theCassini orbiter began studying the Saturniansystem in great detail, and the Huygens probedescended onto Saturn’s giant moon Titan,unveiling an amazing cold but Earth-like world.

Giotto, which took the first close-uppictures of a comet nucleus (Halley) andcompleted flybys of Comets Halley and Grigg-Skjellerup.

Double Star, following in the footsteps ofthe Cluster mission, with its two spacecraft itstudies the effects of the Sun on the Earth’senvironment.

Rosetta, Europe’s comet chaser, will be thefirst mission to fly alongside and land on acomet, probing the building blocks of the SolarSystem in unprecedented detail.

Hipparcos, which fixed the positions ofthe stars far more accurately than ever beforeand changed astronomers' ideas about thescale of the Universe.

Hubble Space Telescope,a collaboration with NASA on the world's mostimportant and successful orbital observatory.

ISO, which studied cool gas clouds andplanetary atmospheres. Everywhere it looked,it found water in surprising abundance.

IUE, the first space observatory everlaunched, marked the real beginning ofultraviolet astronomy.

SOHO, which is providing new views of theSun's atmosphere and interior, revealing solartornadoes and the probable cause of thesupersonic solar wind.

Ulysses, the first spacecraft to fly over theSun’s poles.

XMM-Newton, with its powerful mirrors,is helping to solve many cosmic mysteries ofthe violent X-ray Universe, from enigmaticblack holes to the formation of galaxies.

For further information on the ESA Science Programme visit the ESA Science WebSite at: http://www.esa.int/science

Integral, which is the first spaceobservatory that can simultaneously observecelestial objects in gamma rays, X-rays andvisible light.

Mars Express, Europe’s first mission toMars, which consists of an orbiter and a landerlooking for signs of water and life on the RedPlanet.

SMART-1, Europe’s first mission to theMoon, which will test solar-electric propulsionin flight, a key technology for future deep-space missions.

Prepared by: ESA Science Programme Communication Service

Written by: Monica Talevi

Published by: ESA Publications Division

ESTEC, PO Box 299

2200 AG Noordwijk, The Netherlands

Editors: Bruce Battrick & Carl Walker

Design and layout: Jules Perel

Cover image: Christophe Carreau

Copyright: © 2005 European Space Agency

ISSN No: 0250-1589

ISBN No: 92-9092-486-1

Price: 7 Euros

Contents

Tribute to Venus, the cryptic planet 4

To Venus in record time 7

Rendezvous with Earth’s mysterious sister 10

Penetrating an impenetrable world 14

Planetary answers from atmospheric clues 17

Venus Express: a technological jewel 24

The launcher 26

Operating Venus Express 27

Venus ExpressWelcome to the Cryptic Planet

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Venus ExpressWelcome to the Cryptic Planet

Contents

Tribute to Venus, the cryptic planet 4

To Venus in record time 7

Rendezvous with Earth’s mysterious sister 10

Penetrating an impenetrable world 14

Planetary answers from atmospheric clues 17

Venus Express: a technological jewel 24

The launcher 26

Operating Venus Express 27 2

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Venus, the ‘Morning Star’, the second closestplanet to the Sun after Mercury and ourclosest planetary neighbour, has fascinatedmankind from the earliest times. Namedafter the Roman goddess of love andbeauty, it is the third brightest object in thesky after the Sun and the Moon.

Since the beginning of the space era, Venushas been an attractive subject for planetaryscience and it was one of the first naturaltargets to explore, due to its proximity toEarth – half as far as Mars at closestapproach. Very similar to Earth in size andmass, Venus was expected to be very like toour own planet when the first Russian andAmerican space probes approached it inthe early 1960s and started returning thefirst data about its atmosphere.

Observers soon realised that Venus is anentirely different, exotic and inhospitableworld, hidden behind a curtain of denseclouds of noxious gases. It has anatmosphere composed mainly of carbondioxide, with a crushing surface pressureand burning-hot temperatures. Thequestion then arose: Why did a planetapparently so similar to Earth evolve in such

a radically different way over the last fourthousand million years?

By the mid-1990s, many groundobservatories and more than twentyspacecraft had attempted to explore Venus.A set of orbiters and descent probes - theSoviet Venera series and the Vega balloonsand landers, the US Mariner, Pioneer Venusand Magellan missions - tried to penetratethis hostile world to add pieces to the bigjigsaw that Venus represented for scientists.

Many missions were lost, many landers weredestroyed by the high pressure andtemperature of the Venusian atmospherewhile trying to gather the first informationabout the planet. For years, the denseatmosphere with its thick cloud coverprevented scientists from seeing what laybelow and from understanding the natureof the Venusian surface. Only when modernradar imaging systems became operationalon space probes and at groundobservatories did the first glimpses of thereal face of Venus start to emerge.

NASA’s Pioneer Venus mission (1978), theSoviet Union’s Venera 15 and 16 missions

Tribute to Venus,the cryptic planet

Thick cloud coverage as shown by thisMariner 10 mosaic image, taken in February 1974.

Venera 14 lander images of the surface of Venus taken in March 1982. The lander survived for 60 minutes.

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Tribute to Venus,the cryptic planet

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Composite radar image obtained by the Magellan mission, displaying the whole Venusian surface. At the top, left of centre,the bright region is Maxwell Montes, the highest mountain range on Venus. Extending along the equator to the right ofcentre is Aphrodite Terra, a large highland region on Venus.

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Mariner 2 (USA, 1962) Fly-by First spacecraft at Venus. Closest distance 35 000 km. No magnetic field detected.

Venera 4 Atmospheric probe First probe to return data about atmospheric composition (90-95% carbon dioxide), surface temperature (500°C) (USSR, 1967) and pressure (75 bar). It was crushed by the pressure before it reached the surface.

Venera 5 & 6 Atmospheric probes It detected the presence of atmospheric nitrogen (2.5%) and oxygen (~4 %).(USSR, 1969) It was crushed by atmospheric pressure a few kilometres before reaching the surface.

Venera 7 Lander The first successful soft landing of a spacecraft on another planet. It measured a surface temperature of 475°C(USSR, 1970) and a surface pressure of 90 bar.

Venera 8 Lander The first to measure wind speeds as it descended through the atmosphere (from 100 m/s above 48 kilometres(USSR, 1972) to 1 m/s below 10 kilometres). Surface composition measured by gamma-ray spectrometer.

Mariner 10 Fly-by spacecraft Flew by Venus in 1974 on its way to Mercury. It was the first spacecraft to have an imaging system,(USA, 1974) recording circulation in the Venusian atmosphere and the temperature of the cloud tops to be -23°C.

Venera 9 & 10 2 orbiters with First spacecraft in orbit around Venus. They photographed the clouds and looked at the upper atmosphere,(USSR, 1975) 1 lander each while the landers returned the first black and white panoramic images of the surface.

Pioneer Venus 1 & 2 1 orbiter and Longest mission in orbit around Venus (14 years). First orbiter to conduct radar mapping of the surface.(USA, 1978-1992) 4 atmospheric probes Radio bursts possibly caused by lightning were detected. No magnetic field was detected. Over a decade, it

recorded a 90% decrease in sulphur dioxide, possibly indicating a large volcanic eruption just before the orbiter’s arrival. The probes measured structure, composition and cloud properties down to 12 km altitude.

Venera 11 & 12 Fly-by, 2 landers During the descent, the landers investigated the structure and composition of the atmosphere and clouds,(USSR, 1978) measured scattered solar radiation. Atmospheric dynamics were studied by Doppler tracking.

Venera 13 & 14 Fly-by, 2 landers The landers returned the first colour panoramic views of the surface. They conducted soil analysis and found(USSR, 1982) leucite basalt, a rock type rare on Earth, and tholeiitic basalt similar to that found at mid-ocean ridges on Earth.

Venera 15 & 16 2 orbiters Radar mapping of the surface. Detailed study of the mesosphere and cloud tops in the morning and evening(USSR, 1983) sectors by high-spectral-resolution thermal emission spectroscopy.

Vega 1 & 2 Fly-by spacecraft en route First balloons in the atmosphere of another planet recorded winds running at 240 km/h. Landers provided (USSR, 1985) to Comet Halley, carrying precise temperature profiles down to the surface and in situ measurements of cloud composition.

1 lander and 1 balloon each

Magellan Orbiter First almost-global radar mapping of the surface (300 m/pixel resolution).(USA, 1990-1994)

Galileo Fly-by en route Spectral imaging of the night-side near-infrared emissions. Detection of radio waves possibly emitted by (USA, 1990) to Jupiter lightning.

Cassini-Huygens Fly-by en route Spectral imaging of the night-side near-infrared emissions.(USA, ESA, I, 1998, 1999) to Saturn

Major Historical Missions to Venus

(1983-1984), and NASA’s Magellan radar-mapping mission (1990-1994) togetherprovided a comprehensive picture of anarid world, with landscapes shaped byintense volcanic and geological activity.There were vast plains marked by lavaflows, bordered by highlands andmountains. The later discovery of the night-side thermal emissions from deep insidethe atmosphere of Venus provided aneffective tool with which to peek throughthe dense clouds and study the loweratmospheric layers. The Galileo and Cassinispacecraft were the first to use thisapproach during their short fly-bys en routeto Jupiter and Saturn.

Despite this past intensive exploration ofthe planet, our knowledge of Venus is stillvery limited. Almost all of the planet’speculiar features remain unexplained, andso a new phase of exploration is longoverdue. This important scientific call hasbeen picked up by Europe and, after beingforsaken for a decade, the ‘Morning Star’ willreceive a new visitor in the form of VenusExpress, an orbiter mission developed bythe ESA and due for launch in October2005.

Thanks to a set of state-of-the-artinstruments for planetary investigations,Venus Express will delve into the secrets ofthe Venusian atmosphere. It will study itscomplex dynamics and chemistry, and theinteractions between the atmosphere andthe surface, which will provide clues aboutthe surface’s characteristics. It will also studythe interactions between the atmosphereand the interplanetary environment (solarwind) to improve our understanding of theplanet’s evolution.In the most comprehensive study ever ofthe Venusian atmosphere, Venus Expresswill address many unanswered questions:

• How do the complex global dynamicsof the planet work?

What causes the super-fast atmospheric rotation and the hurricane-force winds?What maintains the double atmospheric vortex at the poles?

• How does the cloud system work?How do clouds and haze form and evolve at different altitudes?What is at the origin of mysterious ultraviolet markers at the cloud tops?

• What processes govern the chemicalstate of the atmosphere?

• What role does the ‘greenhouse effect’play in the global evolution of theVenusian climate?

• What governs the escape processes ofthe atmosphere?

• Are there water, carbon-dioxide orsulphuric-acid cycles on Venus?

• What caused the global volcanicresurfacing of Venus 500 thousandmillion years ago?

• Why are some areas on the surface soreflective to radar?

• Is there currently volcanic or seismicactivity on the planet?

The Magellan radar mapping mission.

The Pioneer Venus mission.

The Venera 15 spacecraft.

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In the sidereal dance of the planets, Venusreaches its closest point to Earth everynineteen months. At the next approach, ESAwill be ready to launch Venus Express to thestar-bright planet. The launch window willopen on 26 October 2005, and close onemonth later on 24 November. Everythinghas been planned to exploit the planet’sproximity to make the journey as short aspossible.

A Russian Soyuz-Fregat launcher will boostVenus Express into space from the BaikonurCosmodrome in Kazakhstan. The Soyuzrocket will lift the autonomous upper stage‘Fregat’, with Venus Express mounted ontop, into a sub-orbital trajectory. Then, withtwo consecutive burns, Fregat will inject thespacecraft onto an interplanetary or ‘escape’trajectory that, with few corrections, willtake it directly to Venus.

For about five months (162 days), VenusExpress will cruise in the peace of space.Once at Venus, in April 2006, it will fire itsmain engine to slow down and counteractthe predominant pull of the Sun and ofVenus, to be captured into orbit around theplanet. A large velocity change is requiredfor the initial capture manoeuvre, which willrequire the engine to burn for 53 minutes.Starting about five days later, a two-week-long series of manoeuvres will put thespacecraft into its final operating orbit,circling the poles of the planet once every24 hours. At its closest, the spacecraft’s orbitwill drop to 250 kilometres altitude overnorthern latitudes, and at its furthest it willbe 66 000 kilometres from the planet’ssurface.

To Venus in record timeTo Venus in record time

Venus Express leaves Earth

The orbit of Venus Express hasbeen specifically chosen to learn asmuch as possible about the planetand to maximise the scientific returnfrom the mission. It prescribes anellipse which is ‘locked’ in space withrespect to fixed stars, while the planetslowly rotates below it. During thecourse of the first full Venusian day(243 Earth days), the spacecraft willmake a global observation from orbit ofthe Venusian atmosphere, of the surfacecharacteristics and of the interaction withthe solar wind. On the second Venusian day,Venus Express will have the opportunity tofill possible observation gaps experiencedin the course of the first day and to study inmore detail items of particular scientificinterest selected during the first day. Thesecond day also allows observation of howthe phenomena seen on the first day varywith time.

The spacecraft’s very short distance fromVenus at pericentre (250 kilometres) allowsin particular study of the northern

hemisphere atvery highresolution. Onespecially interestingfeature is ‘Ishtar Terra’,a continent-sized highlandthat may provide clues as to whether thesurface of Venus is actually ‘moving’ ongeological time scales. The greater distanceof Venus Express from the planet aroundapocentre (66 000 kilometres) will allowgood global coverage of the atmosphericconditions in the southern hemisphere,including the tracking of cloud features.

Venus Express’s journey to Venus.

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Venus Express re-uses some of Mars Express’s and Rosetta’s instrumentation.

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‘Fast’ is Venus Express’s watchword

Not only will Venus Express reach the planet in a very short time, but the mission has also been developed in an incrediblyshort time. Actually, Venus Express is the fastest ESA mission ever developed, taking less than four years from concept tolaunch.

In 2001, in consultation with the European scientific community, ESA saw a very favourable and creative opportunity forplanetary research. By reusing the Mars Express spacecraft design and the same industrial team, as well as some of thescientific instruments developed for Rosetta (mission to study a comet) and Mars Express (the ESA mission to Mars), Europewould be provided with an excellent chance to have an almost fully equipped mission to explore Venus.

Venus Express, along with the ESA missions to Mars and Mercury, will complete a comprehensive European study of theterrestrial planets, resulting in a global and coherent programme of planetary research and providing Europe with a leadingrole in this field.

The development of Venus Express was started in 2002, with a global investment by ESA of 220 million Euro, coveringdevelopment of the spacecraft, launch and operations. This amount also includes about 15 million Euro allocated to researchinstitutes for building the instruments. Together with Rosetta and Mars Express, Venus Express makes up a family of missionsin which costs are shared.

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With Venus Express, Mars Express orbiting Mars andBepiColombo to reach Mercury, ESA will complete

the study of terrestrial planets.

Orbit Highly elliptical

Orbit inclination 90˚ over Venusian equator (polar orbit)

Pericentre (min. distance from the planet’s surface) 250 km

Apocentre (max. distance from the planet’s surface) 66 000 km

Pericentre latitude ~80 to 90˚N

Period 24 hours

Planned mission lifetime 2 Venusian sidereal days (486 Earth days).It is possible to extend the mission by another two Venusian days.

Mariner 10, Apollo 17 andViking images showingthe relative dimensions ofVenus, Earth and theterrestrial planets.

Venus, Mars and Earth, which constitutethree of the four inner or ‘rocky’ planets ofthe Solar System, have a lot in common – a solid surface you could walk on, acomparable surface composition, anatmosphere, and a weather system. If youare looking for a twin sister to Earth, thatwould be Venus. Or so it seems!

It is amazing to compare some of theplanetary features of Venus with those ofEarth. Their masses are basically the same,and their densities also. Their radii seem justto be copied from one planet to the other.Their distances from the Sun are not sodifferent – Venus is about 108 millionkilometres away and Earth 150 millionkilometres. Their rocks are both largely

basaltic, the result of intense volcanism andof a similar solidification process, initiated atabout the same time, four and a halfthousand million years ago, when theplanets of the Solar System first started toform from the solar ‘proto-planetary nebula’.

However, looking more closely, Venus andEarth are truly worlds apart in many major

respects. Walking on Venus’ssurface would be difficult, likewalking under water at adepth of 900 metres - theatmospheric pressure onVenus is 90 times greater thanon Earth at sea level - and asuncomfortable as sitting in anoven at 465ºC ! Venus’s surfaceis the hottest in the SolarSystem. Despite only 10% of

the solar flux reaching the surface, enoughenergy is trapped by gases and particlespresent in the lower atmosphere to raisethe surface temperature dramatically. Acatastrophic ‘greenhouse effect’ is takingplace.

Venus’s atmosphere is composed almostentirely of carbon dioxide, with very little

Rendezvous withEarth’s mysterious sisterRendezvous withEarth’s mysterious sister

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Only thick cloud cover could be seenfrom this image taken in February 1990by NASA’s Galileo spacecraft.

Venus Express will orbit over Venus’s poles. It will map the North Pole in highresolution.

water vapour. It contains significantamounts of corrosive sulphur-bearing gasesand rapidly moving clouds of sulphuric-aciddroplets. The dense clouds scatter back tospace about 80% of the radiation receivedfrom the Sun. Scientists translate thisconcept by saying that Venus has a veryhigh albedo. This is the reason why it is soeasy to see with the naked eye and waslong considered to be a star rather than aplanet.

Venus is almost 70% covered by gentlyrolling uplands, about 20% by very flatlowlands called ‘planitia’, and 10% bycontinent-size highlands, rising above thelowland regions, called ‘tesserae’. On Earth,continents are associated with the bordersof tectonic plates, which drift and collideover time, giving birth to mountains andcausing earthquakes. On Venus there doesnot seem to be any tectonic-plate activityto move and shape the surface. On the

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Venus EarthMass 4.87x1024 kg 5.98 x 1024 kg

Equatorial radius 6052 kilometres 6378 kilometres

Density (mean) 5250 kilograms per cubic metre 5520 kilograms per cubic metre

Average distance from the Sun 108 million km (0.7 Astronomical Unit) 150 million km (1 Astronomical Unit)

Rotation period 243 Earth days (retrograde) 23 hours 56 minutes

Year length (orbital period) 224.7 (Earth) days 365.2 days

Surface temperature (mean) 465 ºC 15 ºC

Atmospheric pressure on the surface 90 bars 1 bar (sea level)

Visual albedo (reflectivity) 0.76 0.37

Highest point on the surface Maxwell Montes (17 kilometres) Mount Everest (8.8 kilometres)

Major atmospheric components 96% carbon dioxide, 3% nitrogen 78% nitrogen, 21% oxygen, 1% argon

Surface materials Basaltic rock and altered materials Basaltic and granitic rock and altered materials

Orbit inclination 3.4 º 0 º (per definition)

Obliquity of rotation axis 178 º 23.5 º

Surface gravity (at equator) 8.9 m/s2 9.8 m/s2

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Three-dimensional view of Alpha Regio, one of Venus’shighlands. It is 1300 kilometres across and it was the firstfeature on Venus to be identified from Earth-based radar.

Rolling uplands, showing signs of lava flows.

contrary, it seems that there is a single platecovering the whole planet. Whether thiscrust is thick or thin, and whether it stillmoves, no one yet knows.

It is sure that the surface of Venus is shapedby deformation of the crust and byvolcanism. Lava flows extend hundredsmiles across the surface. Volcanismindicates that, at least in the past, the crusthas been lying on a liquid mantle, like onEarth. However, on Venus there are signs ofmany different kinds of volcanism, such asvolcanic plains and volcanic rises, besidesthe classic cones we find on Earth. So arevolcanoes still active on Venus?

High mountains and many impact craterspepper the surface. Unlike other worlds,however, it is almost impossible to findcraters less than two kilometres across. Thisis possibly due to the high density of theplanet’s atmosphere, which pulverizessmaller meteorites before they hit theground.

The oldest craters seem not to be olderthan 500 million years, which may indicatethat the planet behaves like a volcanic‘pressure cooker’. On Earth, the constant,steady eruption of volcanoes and theshifting of the surface ensures that theenergy released is dissipated gradually.Instead, on Venus pressure builds up insidethe planet until the whole world is engulfedin a global eruption, resurfacing the planetand destroying many craters that haveformed.

Magellan images showing different kinds of volcanism atVenus: group of four overlapping domes at the edge ofAlpha Regio, showing possible viscous eruptions (top); Sif Mons volcano (centre); cluster of small volcanic conesoverlying a fracture network in Niobe Planitia (bottom).

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Venus rotates in the opposite direction toEarth, so the Sun appears to rise in the west.It also rotates very slowly, with one rotation,or sidereal day, taking 243 Earth days. This islonger than the Venusian year (orbitalperiod), which is 225 Earth days long. Itswinds, however, are so fast that they cancircumnavigate the planet in only four Earthdays: it is still not known why this happens.

Venus does not have its own planetarymagnetic field to protect it from the solarwind. On Earth, the magnetic field isgenerated by movements in the metalliccore coupled with the planet’s relatively fastrotation. On Venus, the core may be solidand the rotation of the planet is very slow,which may explain the lack of a magneticfield, but scientists are still debating thereasons.

In studying Venus, we are trying tounderstand not only how the Solar Systemformed and evolved, but also the complexset of forces and mechanisms that havecaused such dramatic differences betweenthe internal structures, surfaces andatmospheres of Venus and Earth. Anunderstanding of Venus can therefore leadeventually to a better understanding of ourown planet, and especially the long-termevolution of Earth’s climate.

The Venus Express spacecraft.

Venus rotates in the opposite direction to Earth.

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Probing the mysteries of Venus with aprecision never before achieved is the goalof Venus Express.

Compared with previous missions, thescientific instrumentation onboard VenusExpress is enormously improved. This willallow us to study Venus’s atmosphere ingreat detail, to understand its complexdynamics and its close relationship with the

surface and with the planetary spaceenvironment around it. Venus Express datawill answer many of the unresolvedquestions about Venus and will allow thedefinition of new objectives for futuremissions to the planet. This will be achievedusing mainly instruments inherited fromthe scientific packages already developedfor the Mars Express and Rosetta missions.The ‘eyes’ of Venus Express are extremely

Penetrating animpenetrable worldPenetrating animpenetrable world

Instrument Purpose Heritage

ASPERA The Analyser of Space Plasma and Energetic Atoms will investigate the interaction between the solar wind Mars Express(led by Institute of Space Physics, and the atmosphere of Venus by measuring outflowing particles from the planet’s atmosphere and theKiruna, Sweden) particles making up the solar wind. It will study how the molecules and ions escape from the planet.

MAG Venus has no detectable internal magnetic field, and the field that exists around the planet is entirely due Newly developed for(led by IWF, Graz, Austria) to the interaction between the solar wind and the atmosphere. The MAG magnetometer will study this Venus Express (but

process and will help in understanding the effect this has on Venus’s atmosphere, for instance the re-using sensor designatmospheric escape process. from the Rosetta lander)

PFS The Planetary Fourier Spectrometer will be able to measure the temperature of the atmosphere at Mars Express(led by IFSI-INAF, Rome, Italy) altitudes of 55–100 kilometres and 0–10 kilometres at very high resolutions. It will also be able to

measure the surface temperature and therefore be able to search for volcanic activity. In addition to itstemperature measurements, PFS will be able to make composition measurements of the atmosphere.

SpicaV/SOIR The Ultraviolet and Infrared atmospheric spectrometer for stellar and solar occultation assists in the analysis SpicaV inherited from(led by Service d’Aeronomie du CNRS, of Venus’s atmosphere. In particular, it will search for the small quantities of water expected to exist in Mars Express, newVerrieres, France; Institute for Space the Venusian atmosphere. It will also look for sulphur compounds and molecular oxygen in the atmosphere. development for SOIRAeronomy, Belgium; IKI, Russia) It will determine the density and temperature of the atmosphere at 80–180 kilometres altitude.

VeRa The Venus Radio Science experiment uses the powerful radio link between the spacecraft and Earth to Rosetta(led by Univ. der Bunderswehr, investigate the conditions prevalent in the ionosphere of Venus. Scientists will also use it to study theMunich, Germany) density, temperature, and pressure of the atmosphere from 35–40 km up to 100 km from the

surface, and to determine roughness and electrical properties of the surface. It will also allow investigationsof the conditions of the solar wind in the inner part of the Solar System.

VIRTIS The Ultraviolet/Visible/Near-Infrared mapping spectrometer will be able to study the composition of the lower Rosetta(led by CNR-IASF, Rome, Italy, atmosphere between 40 kilometres altitude and the surface. It will track the clouds at both ultraviolet andObservatoire de Paris, France;) infrared wavelengths and allow scientists to study atmospheric dynamics at different altitudes.

VMC The Venus Monitoring Camera is a wide-angle multi-channel camera that will be able to take images of the Newly developed for(led by MPS, Katlenburg-Lindau, planet at near-infrared, ultraviolet and visible wavelengths. VMC will be able to make global images and will Venus Express (reusingGermany) study the cloud dynamics and image the surface. In addition, it will assist in the identification of phenomena parts of Mars Express’s

seen by other instruments. High Resolution StereoCamera)

sensitive to a wide range of electro-magnetic wavelengths, from the ultravioletto the infrared. They will be able to peerdown through the atmosphere byexploiting for the first time the so-called‘spectroscopic windows’, discovered in thelate 1980s thanks to ground observations.The radiation leaking from the deepatmosphere through these ‘windows’ can

be observed on Venus’s night side.Surprisingly, infrared radiation from thelower atmosphere and even the surface canescape to space here, carrying preciousinformation that Venus Express will collectfrom orbit. In this way, many phenomenapreviously hidden below the thick cloudswill be seen by Venus Express.

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Venus Express ‘firsts’

• First global monitoring of the composition of the lower atmosphere via near-infrared transparency ‘windows’.• First coherent study of atmospheric temperature and dynamics at different levels of the atmosphere, from the surface up to ~200 km.• First measurements from orbit of global surface-temperature distribution.• First study of middle- and upper-atmosphere dynamics from oxygen (atomic and molecular) and nitrogen-oxide emissions.• First measurements of non-thermal atmospheric escape.• First coherent observations of Venus in the ultraviolet to thermal-infrared spectral range.• First application of the solar/stellar occultation* technique at Venus.• First use of a 3D ion mass analyser, high-energy resolution electron spectrometer and energetic neutral atom imager.• First sounding of Venusian top-side ionospheric structure.

* Occultation: looking at an object like the Sun, Earth or a star through the atmosphere from a limb perspective allows us to analyse how the light emitted by thisobject is absorbed by the atmosphere, and this tells us about the characteristics of the atmosphere itself.

• Atmospheric escape into space:ASPERA, in combination with SPICAV toassess how this process evolves overtime

• Interaction of solar wind withatmosphere: ASPERA, MAG and VeRa.

• Surface roughness, temperature andother properties: VeRa, together withVIRTIS, PFS and SPICAV

• Volcanic activity: VIRTIS, PFS and VMC.

• Signs of seismic activity: VIRTIS andPFS.

Synergy is VenusExpress’s strength

The underlying philosophy of theVenus Express investigations is to

observe the same target with severaldifferent instruments at the same time.

This provides a comprehensive, versatileand complete view of the different

phenomena taking place on Venus:

• Temperatures: PFS, VIRTIS, VeRa andSPICAV

• Chemical composition: VIRTIS, SPICAVand PFS

• Climate, weather and globalatmospheric dynamics: VIRTIS and VMC,complemented by temperaturesretrieved by PFS, VeRa and VIRTIS

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SAInfrared view of Venus, as imaged by the NIMSexperiment on board NASA’s Galileo mission.

Ultraviolet image of Venus from the NASA/ESA Hubble SpaceTelescope, taken in January 1995, showing mysterious dark

atmospheric features.

Venus’s atmosphere is so dense and sostrictly ‘coupled’ to the planet’s surface thatstudying it will provide information aboutthe nature of the whole planet. Not only willthe study of the atmosphere allow us tounderstand the complex Venusianenvironment and climate system, and theway it evolved, but it can also provide anenormous amount of information about thesurface, its geology and, indirectly, aboutthe interior of the planet. Are there gaseousemissions from the surface that could tell uswhether there are active volcanoes? Is therea carbon dioxide (or even water) cyclesimilar to the water cycle on Earth? Or it iscompletely replaced by the cycle ofsulphuric acid and other sulphurcompounds? Is there any similarity betweenthe ocean-atmosphere relationship onEarth and the surface-atmosphererelationship on Venus? Can mechanicalwaves propagating through the denseatmosphere reveal the presence of seismicactivity? Deeper than ever before

Looking through the infrared ‘windows’ inthe atmosphere will allow Venus

Express to study its lower layers –between the surface and 60

kilometres altitude – whosephysics is virtually unknown.Venus Express will be able todetermine the surfacetemperature as a function ofaltitude at all planetarylatitudes and longitudes.Measuring a local increase in

surface temperature mayindicate the presence of

volcanic activity. These

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Planetary answers fromatmospheric cluesPlanetary answers fromatmospheric clues

Cutaway view of the possible internal structure of Venus. Previousspacecraft data suggest that Venus is differentiated with a basaltic

crust extracted from the mantle, and that it has a single tectonic plate,unlike the Earth.

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ESA By digging into the

atmosphere, VenusExpress will provide cluesabout the whole planet.

measurements will also allow us to betterunderstand the local weather, including forinstance the mechanisms that causeturbulences close to the surface.Furthermore, Venus Express will be able togather information about the surfacetopography, allowing us to understand whysome regions are particularly reflective toradio waves: is it due to the presence ofparticular materials in a specific state, likesnow or ice on Earth?

We do not know anything about the natureof the chemistry in the lower atmospheric

layers. Indeed, from the little data availablefrom previous missions, we know only thatthe thermal decomposition of sulphuricacid, caused by the high surfacetemperature (465ºC), is the dominantprocess in these layers. Chemical cyclesinvolving sulphur, carbon monoxide andwater vapour are also at work. These aregases that are especially important forunderstanding the greenhouse effect onVenus. In addition, they can also be markersfor volcanic activity and global atmosphericdynamics. Venus Express will make anunprecedented global map of thecomposition of the planet’s loweratmosphere, telling us exactly what gasesexist, in what quantity, and how theirconcentrations may vary in space and time.

Acid clouds and lightning

At around 60 kilometres altitude is a verythick cloud layer – a 20 kilometre-deep

Clouds moving to the west (left) at about 100 m/s at the equatorare visible in this Galileo image of Venus. The bright spot on theright shows the local noon. The evening terminator is on the left.

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Are the radio flashes recorded at Venus due to lightning ?

blanket surroundingthe planet. It marks the limitbetween Venus’s lower andmiddle atmospheric layers. It isthis yellowish layer that for along time prevented Earth-basedobservatories and previous orbiter missionsfrom seeing the surface. Today it is knownthat the upper part of this layer is mostlycomposed of tiny droplets of sulphuric acid,but what is happening chemically in thelower clouds is still unknown. For instance,what is the origin of the large solid particlesfloating in the lower clouds observed byPioneer-Venus? The Venus Express remote-sensing instruments will be able to see howthe clouds are shaped and structured, howthey form and evolve with time, how theiropacity varies and what molecules they aremade of. Local and global weather patternswill keep no secrets from Venus Express.

During previous ground and satelliteobservations, visible flashes in theatmosphere have been observed, andlocalised emissions of radio waves have alsobeen reported. Are they due to lightning?The sulphuric-acid droplets can beelectrically highly charged, and so they offerthe potential for lightning. It isvery important not only to learnwhether it is possible thatlightning has an influence onVenus’s atmospheric chemistry,but also to understand how theatmospheres of all terrestrialplanets become electrified andthen discharge. Venus Expresshas the ability to address thesestill unanswered questions.

Greenhouse effect

On the global scale, Venus’s climate isstrongly driven by the most powerfulgreenhouse effect found in the SolarSystem. The agents sustaining it are watervapour, carbon dioxide and sulphuric-acidaerosols. About 80% of the incoming solarradiation is reflected back to space by thecloud layer, and only 10% manages to getthrough it and heat the surface. However,the thermal radiation emitted by thesurface gets trapped by the sameatmosphere. The result is an amazing 500ºCdifference between the surface and cloud-top temperatures. Are there other gasessustaining such an efficient planetarygreenhouse? How is the heat transportedfrom the surface to the cloud layer: throughradiation or dynamic mechanisms such asturbulence? Answering these questions isone of the primary goals of Venus Express.

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A dramatic greenhouse effect is in place at Venus.

Mysterious ultraviolet markingson the clouds

Looking at Venus’s thick cloud layer byoptical means would not reveal much.However, by looking at the clouds in otherwavelengths, such as the infrared andultraviolet, a very different world appears.The clouds on Venus are veryinhomogeneous in all directions, duepossibly to the formation of enormouscumulus-type clouds. Furthermore, theupper clouds are marked by areas visible inthe ultraviolet that mysteriously absorb halfof all the solar energy received by theplanet. What is the origin of these ultravioletmarkings? What makes their absorbingpower so high? By revealing thetemperature and the chemical compositionof these regions, Venus Express will help toprovide the answers.

Hurricane-force winds and hugeatmospheric vortexes

The lower atmosphere of Venus has adramatic and peculiar behaviour pattern. Atthe level of the cloud tops, the atmosphererotates at a formidable velocity, with windspeeds of up to 360 kilometres per hour.The speed of the winds then progressivelydecreases to almost zero at the planet’ssurface, where it becomes a gentle breeze,barely able to kick up dust. Whatmechanisms cause this ‘zonal super-rotation’? Furthermore, two enormous

vortices, with very complex shapes andbehaviours, rotate vertically over the poles,recycling the atmosphere downwards. Thevortex at the north pole, the only onepreviously observed in some detail, has apeculiar ‘double-eye’ shape, surrounded bya collar of cool air. It completes a fullrotation in only three Earth days. How arethe super-rotation and the polar vorticeslinked? How does the global atmosphericcirculation on Venus work? No model is sofar able to simulate the dynamics of Venus’satmosphere as too few data are available.Venus Express observations will finally fillthe gaps.

Where the sulphuric acid forms

The higher layers of the atmosphere,between 60 and 200 kilometres altitude, arealso still relatively unknown. We know thatin the middle atmosphere (60 to 110kilometres), above the cloud tops, carbonmonoxide is in great abundance among theminor species. It is formed by dissociation ofcarbon dioxide by ultraviolet radiationcoming from the Sun. Photo-dissociation ofsulphur dioxide leads to the formation ofsulphuric-acid molecules that eventuallyform the cloud aerosols. Limited amounts ofoxygen and water vapour are also presentin the middle atmosphere. Study of thechemical cycles of these elements isimportant in order to understand theformation of the clouds below.

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Looking at Vultraviolet m

Higher in the atmosphere, above 110kilometres, the mysteries continue. Why, inthe higher atmosphere of a planet as closeto the Sun as Venus, do we measuretemperatures as low as 30ºC on the dayside, and even –160ºC on the night side?And why do such differences between theday and night sides not cause the strongwinds we would expect? This behaviour isunique in the Solar System.

Venus Express will study the globalcirculation, chemistry, temperature anddynamics of the upper atmospheric layersin great detail.

A planet with no magnetic shelter

It is very likely that Venus and Earth, whichare similar in mass and density, initiallycontained the same amount of ‘volatile’substances. However, they have becometwo worlds apart. The secret must lie in theway their atmospheres evolved. How muchof the atmosphere of Venus escaped underthe bombardment of the solar wind andhow much did it combine with the surfacematerial?

Venus is a planet with no intrinsic magneticfield, and so it has no shield to protect itfrom the continuous attack of thecapricious and violent solar wind. Theplanet is embedded only in a localmagnetic field induced by the solar wind

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nus’s atmosphere in different wavelengths shows very inhomogeneous clouds, and mysteriousrks at the cloud tops.

By observing the ionosphere, Venus Express will make the first global study of the atmospheric escape processes at Venus.

itselfandperhaps bylocal magnetismderived from the surface.This peculiar situation gives birth toa complex set of mechanisms throughwhich atmospheric gases are lost to space,including water vapour. This may well havehad effects on Venus’s climate on geologicaltime scales and driven the evolution of thetwo planets’ atmospheres in differentdirections.

Very little water vapour can presently befound in the Venusian atmosphere. It ispossible that hydrogen escaped from it inlarge quantities during early epochs.Equally, the same may have happened withthe oxygen, or it may have been involved inhuge oxidation processes with surfacematerials. Maybe a primordial ocean wasvapourised at a certain point in Venus’shistory. Otherwise, we could conclude thatthe little water vapour present was justsupplied by comets or volcanism. VenusExpress will make the first global study ofhow the atmosphere’s escape processeswork, which is fundamental to under-standing its evolution and, indirectly, that ofEarth’s atmosphere.

Alien volcanoes and ‘Venus-quakes’

Venus, as seen by previous space missions,appears to be among the most geologicallyactive planets in the Solar System. Itssurface doesn’t record signs of its pasthistory earlier than 500 million years ago - itis as if the planet was completely resurfacednot so long ago by some massive planetary-crust phenomenon. Moreover, it also seemsthat some of its surface features have onlyrecently been formed, which suggests thatthe planet may still be active internally.

Venus Express will address this questionfrom several perspectives. First of all, it willanalyse the gases in the lower atmosphere,where a certain abundance of sulphuric

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Artist’s impression of the Venusian surface,the hottest in the Solar System.

acid and sulphur dioxide may, for instance,indicate the presence of volcanic activity.This can also be revealed by possibletemperature variations at correspondinglatitudes. Venus Express is also able toobserve local variations in the temperatureand pressure that may indicate thepropagation of seismic waves through theatmosphere. Signatures of ‘Venus-quakes’may be observed at the top of the planet’satmosphere, which is so dense that itsability to transmit waves is thirty timesgreater than on Earth. Venus Express couldidentify the most active parts of the planet,determine the rate of this activity, and eventhe positions and magnitudes of thestrongest quakes.

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Danilova crater is an exampleof Venusian impact crater, asseen by the Magellanspacecraft.

Smooth features may show that the planet could have beenresurfaced by intense geological activity not long ago.

A volcano on the plains between Artemis Chasma andImdr Regio displays a sheet of broad flow lobes. Thistype of flow is called a ‘festoon’ and only three havebeen found on Venus.

Venus Express:a technological jewelVenus Express:a technological jewel

The VenusExpress spacecraft

is a virtual twin of MarsExpress. The spacecraft

body itself, termed a ‘bus’, is ahoneycomb aluminium box about

1.5 metres wide. With its solar arraysextended, it measures about 8 metres

across. Only minor changes from MarsExpress were required to accommodate therevised instrument payload, which isconcentrated on three sides of thespacecraft bus.

However, the fact that the environment ofVenus is very different from that of Marsrequired some design changes to makeVenus Express more suited to operatingaround this particular planet.

Venus: hot and harsh

Flying to an inner Solar System planet likeVenus, at half the distance to the Sun ascompared to Mars, means that the effects ofsolar illumination and ionising radiation onthe spacecraft are much higher. In fact, theheating of the spacecraft is four timesgreater at Venus than at Mars. To keepVenus Express at safe temperatures, theradiators on its surfaces have thereforebeen increased in both area and efficiency.The spacecraft’s protective jacket, known as‘multi-layer insulation’ or MLI, is composedof 23 layers, packaged differently fromthose on Mars Express. Moreover, the MLI onVenus Express is gold instead of black incolour, which allows it to reflect moreradiation. In general, Mars Express wasdesigned to keep warm, while VenusExpress needs to stay cool.

Plenty of electrical power

The Sun is twice as strong at Venus as onEarth, so there is plentiful solar radiation topower the spacecraft. Venus Express’s solararrays could therefore be made smaller(almost half the size) than those on MarsExpress. The two symmetric solar arrays,with a total area of about six square metres,are based on a ‘triple-junction’ galliumarsenide (GaAs) technology, rather than thesilicon-based technology used for MarsExpress. The solar cells, each composed offour layers of gallium arsenide, are moretolerant to high temperatures (up to around120ºC), as well as being able to exploit awider spectrum of solar radiation. The solarcells are separated by aluminium strips thathelp to reject heat. Altogether, thesealuminium strips account for half of thesolar array’s total surface area.

When the spacecraft is in shadow (eclipse)or when its power demand exceeds thecapacity of the solar arrays, electrical poweris supplied by three lithium-ion batteries,which are charged by the solar generatedpower.

More propellant to catch Venus

The gravity of Venus, which is almost thesame as Earth’s, is about eight timesstronger than that of Mars. This, plus thefact that the gravitational pull of the Sun isstronger at Venus, means that VenusExpress needs more energy to brake and becaptured into orbit around Venus. Thisenergy is provided by part of the 570kilograms of propellant onboard (about20% more than for Mars Express), whichconstitutes almost half of the spacecraft’slaunch weight!

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Major spacecraft manoeuvres, such as itsinjection into orbit around Venus, areperformed by firing the main enginelocated at the base of the spacecraft, whileminor manoeuvres are effected using fourpairs of thrusters located at its four bottomcorners. The thrusters are used for smalltrajectory corrections, for spacecraftattitude changes, and to correct the altitudeof the orbit’s pericentre approximately onceevery 50 days. In fact, due to thegravitational pull of the Sun while thespacecraft is further away from the planet,the pericentre naturally drifts upwards at arate of about 1.5 kilometres per day.

Two antenna dishes

For a spacecraft in orbit around Venus, it is not always possible to point a singleantenna dish at Earth while always keepingthe cold face of the spacecraft, hostingdelicate instruments, away from the Sun. Toovercome this pointing constraint, VenusExpress has two high-gain antennasmounted on different spacecraft faces. Themain high-gain antenna, used for most ofthe communications with Earth, is a 1.3metre-diameter dish. The second, smallerhigh-gain antenna (30 centimetresdiameter) is used when the spacecraft is inthe part of its orbit closest to Earth (lessthan 0.78 AU* away). There are also twolow-gain antennas onboard, which are usedto communicate with Earth during launch,during the first days of the cruise phaseand, should the need occur, during severe‘safe modes’.

Navigation and attitude control

Venus Express is a three-axis-stabilisedspacecraft. Three different onboard systemscan acquire data about its position and

attitude (orientation) in space, and anychange in its velocity. They include two startrackers, two Sun sensors, and a set of threelaser gyroscopes and three accelerometers,which provide the data for re-orienting thespacecraft and the solar arrays whennecessary. The actual spacecraft re-orientation manoeuvre or trajectorycorrection is performed by means of so-called ‘reaction wheels’ or by firing thethrusters.

Storing data on board

Venus Express’s onboard computer isresponsible for supervising and managingthe overall functioning of the spacecraft, forhandling all data acquired by itsinstruments and sensors, and fordistributing commands throughout thewhole spacecraft. The computer’s SolidState Mass Memory has a capacity of 12 Gigabits and it stores all scientific datacollected by the instruments until they canbe transmitted to Earth during theappropriate orbital phase. -2524

Profile of Venus Express

Spacecraft dimensions 1.5 x 1.8 x 1.4 m (excluding solar panels)

Spacecraft mass 1270 kg (including 93 kg of payload and 570 kg of fuel)

Main engine 400 Newton (N) thrust

Thrusters 10 Newton (N) thrust

Solar arrays Two triple-junction gallium arsenide; 5.7 square metres in total; generating 800 Watts in the vicinity of Earthand 1100 Watts at Venus

Power storage 3 lithium-ion batteries

Antennas 2 high-gain antenna dishes (HGA1 = 1.3 m diameter; HGA2 = 0.3 m diameter); 2 low-gain antennas

Prime Contractor Astrium, Toulouse (France), leading an industrial consortium of several European companies

*One Astronomical Unit (AU) is defined as the mean distance between the Sun and Earth, which is about 150 million kilometres

The launcherThe launcher

A Soyuz-Fregat launcher will set VenusExpress on its course to Venus from theRussian Baikonur Cosmodrome inKazakhstan. Soyuz, first launched in 1963,has made more than 1600 flights, and its98% success rate makes it one of the mostreliable launchers in the World. The Soyuz-Fregat is being procured through theEuropean-Russian company Starsem.

The Soyuz vehicle itself has three lowerstages, to which the Fregat upper stage andits fairing which houses Venus Express areattached via a conically shaped adapter.Venus Express will use an upgraded versionof Fregat, which is both lighter and morepowerful than the previous one.

Fregat will be ignited twice. The first firingwill move the Fregat - Venus Expresscomposite into an almost circular ‘parkingorbit’ from the sub-orbital trajectory intowhich it is to be delivered by the Soyuzthird stage. The second ignition will putVenus Express onto its interplanetary flighttrajectory.

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Souyz-Fregat on the launch pad.

All communications to and from VenusExpress will be controlled from a singlecentre, namely the Venus Express MissionControl Centre (VMOC), located at ESA’sEuropean Space Operations Centre (ESOC)in Darmstadt, Germany. Immediately afterlaunch, the antennas at ESA’s deep-spaceground stations at Villafranca, Spain (15 m),New Norcia, Australia (35 m) and Kourou,French Guiana (15 m) will be used forcommunication and orbit determination.Once the spacecraft is in orbit aroundVenus, all communications will be routedvia the 35 m antenna at the new ESAground station at Cebreros near Madrid,Spain, and the New Norcia antenna will beused to support the Venus Radio Scienceexperiment (VeRa).

Once in orbit around the planet, VenusExpress essentially follows the ‘look-store-downlink’ mission scenario alreadyimplemented forthe Mars Expressand Rosettamissions. Thespacecraft willcollect most of itsscientific dataduring a one and ahalf hour passagethrough the orbit’spericentre, when it is closestto the surface of the planet.The part of the orbit where thespacecraft is further from theplanet will be shared betweenremote-sensing observations,in-situ observations and periodsof data transmission. All datacollected are to be transmitted toEarth for about eight hours a day (oneorbit around Venus lasts 24 hours). Eighthours of transmission correspond to thedownlinking of between 100 and 800

Megabytes of data, depending on theactual distance between Earth and Venus.Throughout the mission, ESOC will providethe Venus Express Principal Investigatorswith the complete sets of raw data acquiredfrom their instruments, together with anyother spacecraft data needed, for furtherprocessing and analysis.

The ESA Venus Express Science OperationsCentre (VSOC), which is located at ESTEC,ESA’s Space Research and TechnologyCentre in Noordwijk, The Netherlands, iscollecting and co-ordinating all observationrequests from the Principal Investigators.It also generates and cross-checks theinstrument command files that are thenpassed to ESOC for uploading to thespacecraft. The VSOC is also managing thePlanetary Science Archive, where all datawill be stored and made available to thewide scientific community once the six-month period during which the PrincipalInvestigators have proprietary rights haselapsed. The data will be archived in thesame standardised format that is nowapplied for all new ESA planetary missions,

including Mars Express andRosetta.

ESA’s New Norcia antennain Australia

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OperatingVenus ExpressOperatingVenus Express

From 23:00 hours to 01:00 hours High (spatial) resolution observations of atmosphere and surface in the Northern Hemisphere (near pericentre)

From 01:00 hours to 09:00 hours Communication with Earth and transmission of the scientific data

From 11:00 hours to 13:00 hours Global mapping and study of large-scale phenomena in the Southern Hemisphere (around apocentre)

From 15:00 hours to 23:00 hours Study of the dynamics of the atmosphere and the cloud systems

Various orbital phases High (vertical) resolution studies of the atmosphere, through solar, stellar and Earth occultation

N.B. 00:00 hours corresponds to the passage over the pericentre

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Contact: ESA Publications Divisionc/o ESTEC, PO Box 299, 2200 AG Noordwijk, The NetherlandsTel. (31) 71 565 3400 - Fax (31) 71 565 5433

■ ESA Headquarters8-10 rue Mario-Nikis75738 Paris Cedex 15Tel. (33) 1.53.69.71.55Fax (33) 1.53.69.76.90

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■ ESOC DarmstadtGermanyTel. (49) 6151.90.2696Fax (49) 6151.90.2961

■ EAC CologneGermanyTel. (49) 2203.60.010Fax (49) 2203.60.0166

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