Traditionally interplanetary spacecraft are expensive, and have development schedules lasting up to 10 years. Being so high cost few concepts reach the manufacture phase. As a result of the infrequency of these missions, more experiments and supporting systems become incorporated, further driving up the cost, and delaying the schedule. Programmes rapidly become unaffordable. The Venus Entry Probe is one of ESA's technology reference studies. It aims to identify the technologies required to develop a low cost science-driven mission for in-situ exploration of the atmosphere of Venus, and the philosophy which can be adopted. The mission includes a science gathering spacecraft in circular polar Venus orbit, a relay satellite in elliptical Venus orbit, and an atmospheric entry probe delivering a long duration aerobot, which will drop several microprobes during its operational phase. This paper will focus on the design of the mission, spacecraft and aerobot, with particular emphasis on system level trade-offs. Recognising that resource reduction (mass, power, volume etc.) will be a key requirement to achieving low recurring cost spacecraft, the project has identified a variety of innovative mission enabling and mission enhancing technologies.

The Venus Entry Probe study is one of ESA's technology reference studies. It aims to identify; the technologies required to develop a low-cost, science-driven mission for in-situ exploration of the atmosphere of Venus, and the philosophy that can be adopted. The mission includes a science gathering spacecraft in an elliptical polar Venus orbit, a relay satellite in highly elliptical Venus orbit, and an atmospheric entry probe delivering a long duration aerobot which will drop several microprobes during its operational phase. The atmospheric entry sequence is initiated at 120 km altitude and an entry velocity of 9.8 kms. Once the velocity has reduced to 15 ms the aerobot is deployed. This consists of a gondola and balloon and has a floating mass of 32 kg (which includes 8 kg of science instruments and microprobes). To avoid Venus’ crushing surface pressure and high temperature an equilibrium float altitude of around 55 km has been baselined. The aerobot will circumnavigate Venus several times over a 22-day period analysing the Venusian middle cloud layer. Science data will be returned at 2.5 kbps over the mission duration. At scientifically interesting locations 15 drop-sondes will be released. This paper focuses on the final mission design with particular emphasis on system level trade-offs including the balloon and pressurisation system, communications architecture, power system, design for mission lifetime in a hostile and acidic environment. It discusses the system design, design drivers and presents an overview of the innovative missionenabling and mission-enhancing technologies. Introduction The Venus Entry Probe is one of ESA’s Technology Reference Studies (TRS). These are model sciencedriven missions that although not part of the ESA science programme are able to provide focus to future technology requirements. This is accomplished through the study of several technologically demanding and scientifically meaningful mission concepts, which are strategically chosen to address diverse technological issues. The TRSs complement ESA’s current mission specific development programme and allow the ESA Science Directorate to strategically plan the development of technologies that will enable potential future scientific missions. Key technological objectives for future planetary exploration include the use of small orbiters and in-situ probes with highly miniaturized and highly integrated payload suites. These low resource, and therefore potentially low cost, spacecraft allow for a phased strategic approach to planetary exploration, thus reducing mission risks compared to a single heavy resource mission. The aim of the Venus Entry Probe (VEP) TRS is to study approaches for low cost in-situ exploration of the Venusian atmosphere. The mission profile consists of two minisatellites, one dedicated to atmospheric remote sensing and the other specialised for entry probe deployment as well as data relay . This two-satellite configuration is required in order to commence the remote sensing atmospheric investigations prior to the aerobot deployment. The additional advantage is that through the use of a data relay satellite, the other minisatellite can practically continuously perform remote sensing investigations of the atmosphere. The Low Venus Orbiter (LVO) enters low Venus orbit (6000km x 2000km) and contains a highly integrated remote sensing payload suite primarily dedicated to support the in-situ atmospheric measurements of the aerobot and to address the global atmospheric science objectives. The Venus Relay Satellite (VRS) enters a highly elliptical orbit (215,000 x 250 km), deploys the Venus Entry Vehicle (VEV) and subsequently operates as a data relay satellite (and may provide navigational support). The aerobot consists of a long-duration balloon and gondola (depicted below) that will analyse the Venusian middle Venusian middle cloud layer at an altitude of ~55 km, where the environment is relatively benign. The balloon will deploy a swarm of active ‘ballast’ micro-sondes, which, once deployed, will determine vertical profiles of the lower atmosphere . Figure 1 Venus aerobot mission (gondola and balloon) Mission Objectives The objective of the Venus Entry Probe TRS is to establish a feasible mission profile for a low-cost insitu exploration of Venus. The primary scientific objectives of the mission are to study: 1. Origin and evolution of the atmosphere 2. Composition and chemistry of the lower atmosphere 3. Atmospheric dynamics 4. Aerosols in the cloud layers A more detailed description of the scientific rationale is detailed by . The strategy for this mission development is to meet the science requirements at lowest overall mission cost. The study will determine the mission cost, the system drivers and determine if the instrument duty cycle is viable. It will also identify technologies required to develop such a mission. Mission Design MISSION REQUIREMENTS In order to address the science objectives, the following mission requirements have been imposed on the Venus Entry Vehicle: Mission launch in 2014 onwards Planetary protection requirements: None Support ~4 kg payload suite as well as ~4 kg microprobes, including a ranging and navigation system (DALOMIS-C). This generates science data at a rate of 2.5kbs for the duration of the mission Deploy swarm of fifteen 115 g drop sondes or microprobes. These will either be deployed individually or in groups of 3 in a drop campaign Nominal mission duration: 15 days Extended mission duration: to 30 days Ballistic or orbital entry is permitted, entry must be 20±5 latitude either north or south Maximum entry deceleration is ~200 G Entry sequencing must be dual redundant Aerobot float altitude extremes are 53-62 km. For the first 8 days the balloon must be at an equilibrium float altitude of 55 km