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Home , Cluster (spacecraft), Cluster II (spacecraft), Venus Express

The Express Mission Mission

Donald McCoy, Thorsten Siwitza & Roy Gouka Venus Express Project, ESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands

enus evokes the ever-attractive of a goddess from antiquity, and yet Vour sister planet, although attractive, is far from hospitable. The reasons for such a great difference between and Venus have still to be understood and so, considering that they are very close in terms of astronomical distances, a mystery is invoked. Whether Earth is a unique planet, for which life was destined, or whether both planets were created under similar circumstances and subsequently evolved in different manners, is fundamental to the understanding of our place in the Solar System and, indeed, perhaps the Universe.

Introduction Building on what we know from the Russian and American that visited Venus in the seventies and eighties, Venus Express will continue the quest to understand the fundamental mysteries of the planet, but now using the latest state- of-the-art scientific instrumentation. Similar to Express, the technical precursor of the Venus Express mission, the information produced around Venus will allow scientists to compare our nearest neighbour with our home planet. The link to goes beyond the science, since the satellite is essentially derived from the same design.

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Venus Express is a mission that was and the payload in more detail. This lives, they are not so great when compared proposed to ESA in response to a Call for introductory article is intended to provide to the distances to the giant planets Ideas to re-use the Mars Express platform, the reader with an overall impression of the orbiting our star. Why then is there such a issued in March 2001. The Venus Express mission and how it was created in a great difference between the environments proposal competed with nine others and resource-critical environment with a short of our planets? Mars is cold with a very was selected as one of three candidates for development time and limited budget. thin composed primarily of a preliminary three-month study by carbon dioxide, while Venus is very hot industry to establish the feasibility of re- Science with a massive atmosphere also consisting using the Mars Express spacecraft bus. Venus, Mars and Earth, which represent primarily of carbon dioxide. Earth, on the That study, performed in the period July- three out of the four inner or rocky planets other hand, balances the composition of its October 2001 by ESA, and a team of our Solar System, have much in atmosphere with a small amount of carbon of scientific institutes, demonstrated that an common: a solid surface of comparable dioxide mixed in with a large amount of orbiter mission to Venus could indeed be composition, an atmosphere, a weather oxygen in an atmosphere dominated by carried out by adapting the Mars Express system and a location in space where the nitrogen. satellite. At the completion of the study, energy flux is moderate. However, Through a regular and extended period ESA’s Solar System Working Group and the differences between the planets of global observations, the Venus Express Space Science Advisory Committee themselves are striking, particularly in the instruments will provide scientists with a recommended to the Science Programme case of Venus, which is very similar to broad range of spectral data in the Committee (SPC) in November 2001 that Earth in terms of size and gravity, and and ultraviolet spectral bands. the Venus proposal be chosen for further there are radical differences in the Furthermore, in-situ measurements of investigation. As a result, the SPC environments of the planets. Surface atomic particles at the boundary between recommended that a Pre-Phase-B study be pressures on Venus are 90 times greater the Venusian atmosphere and space will implemented to prepare the Venus Express than those on Earth and surface provide into the interaction with the mission for an implementation decision in temperatures reach 470°C (about ten times solar . measurements 2002. Finally, on 4 November 2002 the higher than the hottest temperatures on will support the measurements of Venus Express project was fully accepted Earth), although the 's energy falling around the planet. by the SPC for implementation, with a on Venus is only double that of the Earth's. The multi-disciplinary science package modification to exclude the VENSIS radar Clearly, the ‘greenhouse effect’ is at work onboard Venus will enable scientists to experiment due to lack of financial support. on Venus! The evolution and behaviour of correlate many physical phenomena An enabling factor for embarking on the the Venusian atmosphere must be of keen affecting the planet and will provide cross- Venus mission was the availability of interest to us, if we believe that the same correlation of these phenomena to help instruments from previous missions such effect is beginning on Earth. The ESA them understand why Venus is so radically as Mars Express and , both of mission will therefore study Venus in different from its neighbours. which were projects already managed by greater detail than ever before, using it as a the Scientific Projects Department. The ‘living laboratory’ to gain better insight The Mission spacecraft design modifications were kept into the life cycles of planets like our own The Venus Express spacecraft was to an absolute minimum to satisfy the and perhaps help us predict the future for launched by a - launcher from Venus Express mission requirements. As a the Earth's environment. the in consequence, the Venus Express spacecraft Although the distances to our planetary on 9 November at 04:33 CET. The maintains large similarities to Mars neighbours, Mars and Venus, are huge launcher put the combined spacecraft and Express. The system concept, such as the relative to those experienced in our daily Fregat upper stage onto a trajectory that structural design, propulsion subsystem, avionics units and operational concept, have been maintained while some Venus Express mission characteristics, such as the proximity to the Sun, the constellation of planets, and the distance to Earth, have led to unavoidable design changes, primarily in the areas of thermal control, communications and electrical power. The following articles in this Bulletin describe the spacecraft, the ground system

Venus Express’s journey to Venus

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the Venus Mission Operations Centre at listed in the accompanying panel, together ESA’s European Space Operations Centre with the nominal planning dates for each (ESOC) in Darmstadt, Germany. event. Venus Express will spend approximately 150 days on its interplanetary journey, The Ground Segment during which time any necessary trajectory The concept for controlling Venus Express corrections will be made using the is based on the use of a single control spacecraft's thrusters. At least one centre in conjunction with ESA’s new correction, planned to take place about Cebreros 35 m station near Madrid sixty days after launch, will be required. (Spain). The New Norcia 35 m station near On arrival at Venus, a significant Perth (W. Australia) will be used to support deceleration manoeuvre will be made the Venus Insertion phase and for using the spacecraft’s own engine. The data acquisition in support of the Radio 53 minute burn will reduce the spacecraft's Science investigations. The baseline arrival speed by about 1.3 km/sec, operations philosophy is to acquire sufficient for it to be inserted initially into scientific data primarily during the 95 a highly elliptical orbit around the minute pericentre planetary passes, store it planet, with a pericentre altitude of about onboard, and downlink it during a single 250 km and a period of about ten Earth pass each day. By digging into the atmosphere, Venus Express will provide clues days. Smaller engine burns will then be All phases of the mission will be about the whole planet used to lower the apocentre and reach the controlled from the Venus Express Mission operational orbit. Operations Centre (VMOC) located at allowed the Fregat to circularise the orbit The nominal arrival of the spacecraft at ESOC. The Launch and Early Orbit Phase into a temporary parking orbit about Venus is planned for 11 April 2006. After (LEOP) will use the Main Control Room 190 km above the Earth. After about one some time for trimming the orbit (MCR) at ESOC and will be supported for revolution around the planet, the upper parameters to achieve the required 24 hour tracking, telemetry and commanding by stage was reignited to put the composite on polar orbit, the commissioning of the the ESA ground stations in an interplanetary trajectory towards Venus. spacecraft and the scientific payload will (French Guiana) and New Norcia. Separation of the spacecraft from the upper start. Full nominal operations will then The VMOC is the primary interface with stage was commanded by the Fregat commence for a period of 2 Venusian days, the spacecraft through the ground 90 minutes after launch and this action corresponding to 486 Earth days. A station(s) and will be responsible for initiated the deployment sequence onboard mission extension beyond this time is a monitoring and control of the complete the spacecraft. Once the solar arrays had possibility since the onboard consumables mission. The principal mode of operations been deployed and the propulsion system have been sized to cope with that. is that all routine payload operations must primed, the spacecraft was controlled by The major events of the mission are be pre-planned and executed according to an agreed Science Activity Plan (SAP). There are no real-time payload operations MainM Events and Missiona Phasing i n E v foreseen,e othern than the neart -real-times a n d M i s s i o n P h a s i n g interactive operations at the time of Event/Mission phase Nominal date(s) or typical duration commissioning (initial turn-on, calibration) and/or during contingency 26 October 2005 to 24 November 2005 situations. The respective procedures are Launch and Early-Orbit Phase (LEOP) ~3 days contained in the Flight Operations Plan Near-Earth Commissioning Phase ~21 days (FOP). Following launch, the performance Interplanetary Cruise Phase ~3.5 months of each spacecraft subsystem will be Phase About 5 weeks starting in March 2006 checked out, followed by a sequential - Venus Capture Manoeuvre 11 April 2006 switch-on/commissioning of each - Venus Apocentre Lowering ~15 days experiment. Cruise operations will follow Venus Payload-Commissioning Phase ~45 days this checkout phase. Routine Operations Phase 2 Venusian days (486 Earth days) From launch until the end of the - End of nominal mission September 2007 mission, facilities and services will be Extended Operations Phase September 2007 to January 2009 provided to the scientific community for Total Mission Duration 1185 days the planning and execution of scientific data acquisition. This will include the generation and provision of complete raw- www.esa.int esa bulletin 124 - november 2005 13 Science

ESA scientific mission development times

way of building scientific spacecraft. This is demonstrated by the accompanying graph, which shows a number of ESA science missions and their relative development times divided into the duration of the Phase-B, which is typically a design phase, and of the Phase-C/D, which is typically a phase of building hardware models and the final flight spacecraft. This plot clearly shows that the Venus Express development schedule was the shortest for science missions conducted to date. More interestingly, however, it shows that the time for the Phase-C/D over the family of missions upon which the Express approach was applied is not drastically different. Thus data sets and the necessary auxiliary data teams working on both Mars Express and Rosetta, which was the source spacecraft to the Principal Investigators (PIs). Venus Express, for more than a year during for the units of Mars Express and Venus A Venus Express Science Operations an overlapping period. Express, shows a Phase-C/D manufacture Centre (VSOC) will support scientific While the reuse approach certainly has and verification time only a few months mission planning and experiment many advantages, there are also some side- longer than Mars Express, while the Venus command request preparation for effects: at the programmatic level, reduced Express Phase-C/D is shorter by about consolidated onward submittal to the cost and shortened development time are 7 months. More significant reductions are VMOC. The VSOC is managed by the traded against a repetition and consequent demonstrated in the Phase-B design Research and Scientific Support amplification of deviations in the duration from Rosetta to Venus Express. Department at ESTEC in Noordwijk (NL). geographical-return targets. The com- The three projects had quite different The VSOC undertakes the short-term mensurate reduction of team sizes and approaches in terms of model philosophy, science coordination and mission partly simultaneous working allows a with Rosetta being the most extensive, Mars planning, and the Data Handling and maximum of knowledge transfer, but can Express a modified proto-flight approach, Archive Service (DHAS) will make pre- at times lead to an overload on the teams. and Venus a pure proto-flight approach. processed scientific data and the scientific Finally, the concept of reusing entire Clearly, the lower limit of time for building data archive available to the scientific spacecraft and offering them to the science a spacecraft has been approached while still community. community cannot compete with a following the standard practices for good programme approach of setting scientific spacecraft manufacturing and testing. This Express-Mission Experience objectives that drive state-of-the-art is indeed further reinforced when The Venus Express mission’s feasibility payload instruments along with mission- comparing with the other science-project was totally reliant on making maximum specific spacecraft. development times. reuse of already developed spacecraft The limitations of the Express approach The rebuilding of the -II elements and subsystems. For Venus have also to be recognised when spacecraft shows a similar development Express, ESA had the unique considering a programmatic approach in time to the Express missions, while to reuse a complete spacecraft bus. which larger missions pave the way for singular one-off missions show a range of Contrary to previous Announcements of technological development of spacecraft development times similar to Rosetta. Opportunity to the scientific community, and payload units that cannot be Particular exceptions to this are ISO and this one specifically asked the question: accommodated on fast, low-cost missions. Integral, which suffered from very ‘What science can be done with a Mars For example, the very compressed Venus particular technological problems that Express-type spacecraft and within a very Express schedule probably represents the resulted in stretched development short period of time?’ The reuse concept limit in terms of reducing development schedules. In the case of ISO, it was the was even extended to the ESA and time. Any attempt at further reduction technology for handling the cryogenic industrial personnel involved, with some require a much different approach to the fluids, while for Integral it was the

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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 to encapsulation complex sensor technologies needed for developments, if the programme carefully differences and of whether delta the instruments. plans instrument and unit pre- qualifications are needed for utilisation in The development of the instruments for developments along with multiple copies follow-on missions. scientific satellites is a very demanding of the flight models. It is highly unlikely All in all, the Mars and Venus Express task by virtue of the need to exploit that, using the current best-practices projects have allowed the ESA Science cutting-edge technologies to meet approach for a scientific mission, the time Directorate to procure two interplanetary advancing scientific goals. The Express to build the satellite can be further missions with minimal resources while still missions have had the benefit of reusing compressed. Attempts to further reduce the providing a world-class scientific return. existing instruments through controlled development duration will require a modifications in most cases. This is a key different approach, without increasing the Acknowledgement point in the setting of programme goals risk for mission success. The proven The challenging development of the where a series of spacecraft are planned to integration and verification methods Venus Express satellite and its ground be produced. For any follow-on spacecraft, should therefore not be sacrificed for the segment has been successfully achieved a series of instruments based on those sake of time savings. within a tight schedule and budgetary developed for a first mission is virtually Some possibilities for improving the envelope thanks to the efficient teamwork obligatory if the development times are to development for a series of spacecraft by ESA, the scientific Principal Investi- be maintained at the level of the Venus could look at having several units with gators and Industry. It was their skill and Express mission. rather large requirement envelopes built dedication that turned this challenge into a In conclusion, the Express experience, concurrently even without the mission success. r which the Agency has just completed, can being selected. This would be a true off- be a future model for a programme the-shelf approach, which would also approach with multiple spacecraft require careful review of mission

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