The Venus ExpressMissionThe Venus ExpressMission

esa bulletin 124 - november 2005 11

Venus Express Mission

Donald McCoy, Thorsten Siwitza & Roy GoukaVenus Express Project, ESA Directorate ofScientific Programmes, ESTEC, Noordwijk, The Netherlands

V enus evokes the ever-attractive imageof a goddess from antiquity, and yet

our sister planet, although attractive, isfar from hospitable. The reasons for such agreat difference between Earth and Venus havestill to be understood and so, considering thatthey are very close in terms of astronomicaldistances, a mystery is invoked. Whether Earthis a unique planet, for which life was destined,or whether both planets were created undersimilar circumstances and subsequently evolvedin different manners, is fundamental to theunderstanding of our place in the Solar Systemand, indeed, perhaps the Universe.

IntroductionBuilding on what we know from theRussian and American spacecraft thatvisited Venus in the seventies and eighties,Venus Express will continue the quest tounderstand the fundamental mysteries ofthe planet, but now using the latest state-of-the-art scientific instrumentation.Similar to Mars Express, the technicalprecursor of the Venus Express mission,the information produced around Venuswill allow scientists to compare our nearestneighbour with our home planet. The linkto Mars Express goes beyond the science,since the satellite is essentially derivedfrom the same design.

12 esa bulletin 124 - november 2005 www.esa.int

Venus Express is a mission that wasproposed to ESA in response to a Call forIdeas to re-use the Mars Express platform,issued in March 2001. The Venus Expressproposal competed with nine others andwas selected as one of three candidates fora preliminary three-month study byindustry to establish the feasibility of re-using the Mars Express spacecraft bus.That study, performed in the period July-October 2001 by ESA, Astrium and a teamof scientific institutes, demonstrated that anorbiter mission to Venus could indeed becarried out by adapting the Mars Expresssatellite. At the completion of the study,ESA’s Solar System Working Group andSpace Science Advisory Committeerecommended to the Science ProgrammeCommittee (SPC) in November 2001 thatthe Venus proposal be chosen for furtherinvestigation. As a result, the SPCrecommended that a Pre-Phase-B study beimplemented to prepare the Venus Expressmission for an implementation decision in2002. Finally, on 4 November 2002 theVenus Express project was fully acceptedby the SPC for implementation, with amodification to exclude the VENSIS radarexperiment due to lack of financial support.

An enabling factor for embarking on theVenus mission was the availability ofinstruments from previous missions suchas Mars Express and Rosetta, both ofwhich were projects already managed bythe Scientific Projects Department. Thespacecraft design modifications were keptto an absolute minimum to satisfy theVenus Express mission requirements. As aconsequence, the Venus Express spacecraftmaintains large similarities to MarsExpress. The system concept, such as thestructural design, propulsion subsystem,avionics units and operational concept,have been maintained while some VenusExpress mission characteristics, such asthe proximity to the Sun, the constellationof planets, and the distance to Earth, haveled to unavoidable design changes,primarily in the areas of thermal control,communications and electrical power.

The following articles in this Bulletindescribe the spacecraft, the ground system

and the payload in more detail. Thisintroductory article is intended to providethe reader with an overall impression of themission and how it was created in aresource-critical environment with a shortdevelopment time and limited budget.

ScienceVenus, Mars and Earth, which representthree out of the four inner or rocky planetsof our Solar System, have much incommon: a solid surface of comparablecomposition, an atmosphere, a weathersystem and a location in space where thesolar energy flux is moderate. However,the differences between the planetsthemselves are striking, particularly in thecase of Venus, which is very similar toEarth in terms of size and gravity, andthere are radical differences in theenvironments of the planets. Surfacepressures on Venus are 90 times greaterthan those on Earth and surfacetemperatures reach 470°C (about ten timeshigher than the hottest temperatures onEarth), although the Sun's energy fallingon Venus is only double that of the Earth's.Clearly, the ‘greenhouse effect’ is at workon Venus! The evolution and behaviour ofthe Venusian atmosphere must be of keeninterest to us, if we believe that the sameeffect is beginning on Earth. The ESAmission will therefore study Venus ingreater detail than ever before, using it as a‘living laboratory’ to gain better insightinto the life cycles of planets like our ownand perhaps help us predict the future forthe Earth's environment.

Although the distances to our planetaryneighbours, Mars and Venus, are hugerelative to those experienced in our daily

lives, they are not so great when comparedto the distances to the giant planetsorbiting our star. Why then is there such agreat difference between the environmentsof our planets? Mars is cold with a verythin atmosphere composed primarily ofcarbon dioxide, while Venus is very hotwith a massive atmosphere also consistingprimarily of carbon dioxide. Earth, on theother hand, balances the composition of itsatmosphere with a small amount of carbondioxide mixed in with a large amount ofoxygen in an atmosphere dominated bynitrogen.

Through a regular and extended periodof global observations, the Venus Expressinstruments will provide scientists with abroad range of spectral data in the infraredand ultraviolet spectral bands.Furthermore, in-situ measurements ofatomic particles at the boundary betweenthe Venusian atmosphere and space willprovide insight into the interaction with thesolar wind. Magnetometer measurementswill support the measurements of plasmaaround the planet.

The multi-disciplinary science packageonboard Venus will enable scientists tocorrelate many physical phenomenaaffecting the planet and will provide cross-correlation of these phenomena to helpthem understand why Venus is so radicallydifferent from its neighbours.

The Mission The Venus Express spacecraft waslaunched by a Soyuz-Fregat launcher fromthe Baikonur Cosmodrome in Kazakhstanon 9 November at 04:33 CET. Thelauncher put the combined spacecraft andFregat upper stage onto a trajectory that

Science

Venus Express’s journey to Venus

esa bulletin 124 - november 2005www.esa.int 13

allowed the Fregat to circularise the orbitinto a temporary parking orbit about 190 km above the Earth. After about onerevolution around the planet, the upperstage was reignited to put the composite onan interplanetary trajectory towards Venus.Separation of the spacecraft from the upperstage was commanded by the Fregat 90 minutes after launch and this actioninitiated the deployment sequence onboardthe spacecraft. Once the solar arrays hadbeen deployed and the propulsion systemprimed, the spacecraft was controlled by

the Venus Mission Operations Centre atESA’s European Space Operations Centre(ESOC) in Darmstadt, Germany.

Venus Express will spend approximately150 days on its interplanetary journey,during which time any necessary trajectorycorrections will be made using thespacecraft's thrusters. At least onecorrection, planned to take place aboutsixty days after launch, will be required.On arrival at Venus, a significantdeceleration manoeuvre will be madeusing the spacecraft’s own engine. The 53 minute burn will reduce the spacecraft'sarrival speed by about 1.3 km/sec,sufficient for it to be inserted initially intoa highly elliptical polar orbit around theplanet, with a pericentre altitude of about250 km and a period of about ten Earthdays. Smaller engine burns will then beused to lower the apocentre and reach theoperational orbit.

The nominal arrival of the spacecraft atVenus is planned for 11 April 2006. Aftersome time for trimming the orbitparameters to achieve the required 24 hourpolar orbit, the commissioning of thespacecraft and the scientific payload willstart. Full nominal operations will thencommence for a period of 2 Venusian days,corresponding to 486 Earth days. Amission extension beyond this time is apossibility since the onboard consumableshave been sized to cope with that.

The major events of the mission are

listed in the accompanying panel, togetherwith the nominal planning dates for eachevent.

The Ground SegmentThe concept for controlling Venus Expressis based on the use of a single controlcentre in conjunction with ESA’s newCebreros 35 m station near Madrid(Spain). The New Norcia 35 m station nearPerth (W. Australia) will be used to supportthe Venus Orbit Insertion phase and fordata acquisition in support of the RadioScience investigations. The baselineoperations philosophy is to acquirescientific data primarily during the 95minute pericentre planetary passes, store itonboard, and downlink it during a singlepass each day.

All phases of the mission will becontrolled from the Venus Express MissionOperations Centre (VMOC) located atESOC. The Launch and Early Orbit Phase(LEOP) will use the Main Control Room(MCR) at ESOC and will be supported fortracking, telemetry and commanding bythe ESA ground stations in Kourou(French Guiana) and New Norcia.

The VMOC is the primary interface withthe spacecraft through the groundstation(s) and will be responsible formonitoring and control of the completemission. The principal mode of operationsis that all routine payload operations mustbe pre-planned and executed according toan agreed Science Activity Plan (SAP).There are no real-time payload operationsforeseen, other than the near-real-timeinteractive operations at the time ofcommissioning (initial turn-on,calibration) and/or during contingencysituations. The respective procedures arecontained in the Flight Operations Plan(FOP). Following launch, the performanceof each spacecraft subsystem will bechecked out, followed by a sequentialswitch-on/commissioning of eachexperiment. Cruise operations will followthis checkout phase.

From launch until the end of themission, facilities and services will beprovided to the scientific community forthe planning and execution of scientificdata acquisition. This will include thegeneration and provision of complete raw-

Venus Express Mission

Main Events and Mission PhasingM a i n E v e n t s a n d M i s s i o n P h a s i n gMain Events and Mission Phasing

Event/Mission phase Nominal date(s) or typical duration

Launch window 26 October 2005 to 24 November 2005 Launch and Early-Orbit Phase (LEOP) ~3 daysNear-Earth Commissioning Phase ~21 daysInterplanetary Cruise Phase ~3.5 monthsOrbit Insertion Phase About 5 weeks starting in March 2006- Venus Capture Manoeuvre 11 April 2006- Venus Apocentre Lowering ~15 daysVenus Payload-Commissioning Phase ~45 days Routine Operations Phase 2 Venusian days (486 Earth days)- End of nominal mission September 2007Extended Operations Phase September 2007 to January 2009Total Mission Duration 1185 days

By digging into the atmosphere, Venus Express will provide cluesabout the whole planet

14 esa bulletin 124 - november 2005

data sets and the necessary auxiliary datato the Principal Investigators (PIs).

A Venus Express Science OperationsCentre (VSOC) will support scientificmission planning and experimentcommand request preparation forconsolidated onward submittal to theVMOC. The VSOC is managed by theResearch and Scientific SupportDepartment at ESTEC in Noordwijk (NL).The VSOC undertakes the short-termscience coordination and missionplanning, and the Data Handling andArchive Service (DHAS) will make pre-processed scientific data and the scientificdata archive available to the scientificcommunity.

Express-Mission ExperienceThe Venus Express mission’s feasibilitywas totally reliant on making maximumreuse of already developed spacecraftelements and subsystems. For VenusExpress, ESA had the unique opportunityto reuse a complete spacecraft bus.Contrary to previous Announcements ofOpportunity to the scientific community,this one specifically asked the question:‘What science can be done with a MarsExpress-type spacecraft and within a veryshort period of time?’ The reuse conceptwas even extended to the ESA andindustrial personnel involved, with some

teams working on both Mars Express andVenus Express, for more than a year duringan overlapping period.

While the reuse approach certainly hasmany advantages, there are also some side-effects: at the programmatic level, reducedcost and shortened development time aretraded against a repetition and consequentamplification of deviations in thegeographical-return targets. The com-mensurate reduction of team sizes andpartly simultaneous working allows amaximum of knowledge transfer, but canat times lead to an overload on the teams.Finally, the concept of reusing entirespacecraft and offering them to the sciencecommunity cannot compete with aprogramme approach of setting scientificobjectives that drive state-of-the-artpayload instruments along with mission-specific spacecraft.

The limitations of the Express approachhave also to be recognised whenconsidering a programmatic approach inwhich larger missions pave the way fortechnological development of spacecraftand payload units that cannot beaccommodated on fast, low-cost missions.For example, the very compressed VenusExpress schedule probably represents thelimit in terms of reducing developmenttime. Any attempt at further reductionrequire a much different approach to the

way of building scientific spacecraft. Thisis demonstrated by the accompanyinggraph, which shows a number of ESAscience missions and their relativedevelopment times divided into theduration of the Phase-B, which is typicallya design phase, and of the Phase-C/D,which is typically a phase of buildinghardware models and the final flightspacecraft. This plot clearly shows that theVenus Express development schedule wasthe shortest for science missionsconducted to date. More interestingly,however, it shows that the time for thePhase-C/D over the family of missionsupon which the Express approach wasapplied is not drastically different. ThusRosetta, which was the source spacecraftfor the units of Mars Express and VenusExpress, shows a Phase-C/D manufactureand verification time only a few monthslonger than Mars Express, while the VenusExpress Phase-C/D is shorter by about 7 months. More significant reductions aredemonstrated in the Phase-B designduration from Rosetta to Venus Express.

The three projects had quite differentapproaches in terms of model philosophy,with Rosetta being the most extensive, MarsExpress a modified proto-flight approach,and Venus a pure proto-flight approach.Clearly, the lower limit of time for buildinga spacecraft has been approached while stillfollowing the standard practices for goodspacecraft manufacturing and testing. Thisis indeed further reinforced whencomparing with the other science-projectdevelopment times.

The rebuilding of the Cluster-IIspacecraft shows a similar developmenttime to the Express missions, whilesingular one-off missions show a range ofdevelopment times similar to Rosetta.Particular exceptions to this are ISO andIntegral, which suffered from veryparticular technological problems thatresulted in stretched developmentschedules. In the case of ISO, it was thetechnology for handling the cryogenicfluids, while for Integral it was the

Science

ESA scientific mission development times

www.esa.int

esa bulletin 124 - november 2005 15

complex sensor technologies needed forthe instruments.

The development of the instruments forscientific satellites is a very demandingtask by virtue of the need to exploitcutting-edge technologies to meetadvancing scientific goals. The Expressmissions have had the benefit of reusingexisting instruments through controlledmodifications in most cases. This is a keypoint in the setting of programme goalswhere a series of spacecraft are planned tobe produced. For any follow-on spacecraft,a series of instruments based on thosedeveloped for a first mission is virtuallyobligatory if the development times are tobe maintained at the level of the VenusExpress mission.

In conclusion, the Express experience,which the Agency has just completed, canbe a future model for a programmeapproach with multiple spacecraft

developments, if the programme carefullyplans instrument and unit pre-developments along with multiple copiesof the flight models. It is highly unlikelythat, using the current best-practicesapproach for a scientific mission, the timeto build the satellite can be furthercompressed. Attempts to further reduce thedevelopment duration will require adifferent approach, without increasing therisk for mission success. The provenintegration and verification methodsshould therefore not be sacrificed for thesake of time savings.

Some possibilities for improving thedevelopment for a series of spacecraftcould look at having several units withrather large requirement envelopes builtconcurrently even without the missionbeing selected. This would be a true off-the-shelf approach, which would alsorequire careful review of mission

differences and of whether deltaqualifications are needed for utilisation infollow-on missions.

All in all, the Mars and Venus Expressprojects have allowed the ESA ScienceDirectorate to procure two interplanetarymissions with minimal resources while stillproviding a world-class scientific return.

AcknowledgementThe challenging development of the Venus Express satellite and its groundsegment has been successfully achievedwithin a tight schedule and budgetaryenvelope thanks to the efficient teamworkby ESA, the scientific Principal Investi-gators and Industry. It was their skill anddedication that turned this challenge into asuccess. r

Venus Express Mission

Combined spacecraft and launcher teams from Alenia, Astrium, ESA, Europe Assistance, Lavotschkin and Starsem in front of the Venus Express spacecraft mounted on the Fregat upper stage prior toencapsulation

www.esa.int