MISSION

The European Spaceport

The Guiana Space Centre (CSG) is located in French Guiana, a French overseas department. It lies on the Atlantic coast of the Northern part of South America, close to the equator, between the latitudes of 2° and of 6° North at the longitude of 50° West. The European spaceport is located between the two towns of Kourou and Sinnamary and is operational since 1968. The CSG is governed under an agreement between France and the European Space Agency and is managed by the French National Space Agency (Centre National d’Etudes Spatiales – CNES) on behalf of the European Space Agency.

EnVision launch and transfer options

The proposed mission will launch in late November 2032 on Ariane 62 for delivery to Venus. Following orbit insertion and periapsis walk-down in 2034, orbit circularisation will be achieved by aerobraking over a period of several months, with the main mission phase starting around end 2035. Various trade-offs on the transfer options were performed for the ESTEC Phase 0 / CDF study ending in fall 2018, to allow more margin on the spacecraft mass, which was particularly constraining following a first assessment of the M5 proposal.

Our baseline launch in November 2032 encompasses a comparably short transfer duration. A launch in HEO (High Earth Orbit) followed by two escape manoeuvres gives more flexibility with available launch mass of 2870kg. In order to have a feasible mission, this HEO launch strategy – about 100,000 km apogee, nearly a third of the Earth-Moon distance – is considered as baseline, with the drawback of having a time-critical escape manoeuvre that adds a risk to be further managed at study and project level.

Launch in High-Earth Orbit (HEO) in November 2032, transfer and orbital insertion manoeuvers, after final ESA Concurrent Design Facility (CDF) mission study in Dec. 2018. A backup launch is foreseen in May 2033 and is significantly longer due to an Earth swing-by. Both launch options are compatible with the programmatic mission aspects and are both feasible in terms of mass budget. Acronyms : B/L = baseline ; B/U = backup ; VOI = Venus Orbit Insertion ; HEO = High Earth Orbit.
EnVision deployment video – M5 EnVision design based on ESA-ESTEC Concurrent Design Facility Nov. 2018. Credit : © VR2Planets – François Civet

Operational orbit for Venus science

One of the main trade-offs involved revisiting the operational orbit options between quasi-circular, periodic eccentricity, highly elliptical.

The selected science orbit is a quasi-polar slightly elliptical low Venus orbit. Its Apocytherion and Pericytherion are evolving in the [220, 470] km altitude range. Its specificity is that it has a periodic eccentricity. This means that the eccentricity vector, describing the eccentricity and argument of pericentre, evolves in time, but the initial value is such that at the end of the cycle it is back to its initial point. This characteristic enables cycle-to-cycle InSAR measurements, which require that the repeat and the reference orbits are such that the angular separation between the spacecraft positions at the acquisitions of two SAR images of a given InSAR pair, as seen by any point imaged on the surface of Venus within the targeted Regions of Interest, remains smaller than the critical angular baseline or 1.4 deg (this includes the maximum frequency shift capability of the SAR to compensate for spectral baseline shift), provided this viewing angle is known better than 0.5 mrad.

From a nominal frozen eccentricity ~250 km altitude elliptical orbit, the imaging radar will operate in three different modes across four cycles. Each four-orbit sequence will consist of one stereo pair stripmap; two interferometric pass-to-pass stripmaps; and a targeted acquisition of high resolution multipolar images. 

The imaging strategy permits collection of nested image data products: SAR imagery at 30 m resolution of the 20% of the planet, repeated interferometric pairs and multi-polarimetric imagery, for change detection and surface material mapping, over this same 20%, and high resolution (<5 m) imaging of approximately 2% of the planet’s surface. Core mission instruments will also provide subsurface sounder profiles, and global infrared emissivity mapping from nightside observations and spectroscopic data to identify key volcanic gases. 

EnVision mission propulsion

Chemical, electric and hybrid propulsion options were assessed, even if the hybrid ones were discarded early on in the study, due to added complexity and mass. In order to compare the two options (chemical and electric), they were selected to be assessed as system options. The electric propulsion option would see a transfer, Venus orbit insertion and science orbit acquisition with the electrical thrusters, while the station keeping and AOCS would be executed with monopropellant. The chemical propulsion option has every manoeuvre done through a bi-propellant system and uses aerobraking for the science orbit acquisition.

The EnVision mission description in the chemical propulsion option (CP). The chemical propulsion option has every manoeuvre done through a bi-propellant system and uses aerobraking for the science orbit acquisition. Launch, lifetime, orbit, ground stations, mass (with margins) were assessed during the Concurrent Design Facility (CDF) study ending in Dec. 2018.

Ariane 62 fairing

The Ariane 62 usable volume beneath payload fairing in single launch structure. This volume constitutes the limits that the static dimensions of the spacecraft, including manufacturing tolerance, thermal protection installation, etc shall not exceed.

Considering the volume available on the launcher fairing, different configuration options were considered during the CDF study, taking into account especially the solar array and SAR antenna configuration and accommodation (stored and deployed). Other factors to take into consideration were the mission profile, potential interferences, antennas position (VenSAR and SRS antennas wrt. HGA), and the radiators configuration in order to sustain the thermal requirements of the spacecraft.

Spacecraft stowed configuration

EnVision stowed configuration is shown without the launcher fairing. Approximate size is 2.0 x 2.0 x 2.0 m. The satellite has a compact body and the Solar Arrays, the SAR antenna and the SRS antenna are the elements stowed during launch and deployed in orbit. At the bottom right is the 420 N Main Engine. Also visible are 8 + 8 10 N thrusters for AOCS.

Spacecraft deployed configuration

EnVision deployed configuration and the position of the main deployable elements (i.e. SAR antenna, SRS antenna and Solar Arrays) in their selected positions as consolidated during the discussions in the CDF study sessions. Solar arrays are 97 kg with a 15.7 m2 area (57% solar cells, 40% optical reflectors). A fixed, 3-m HGA requires dedicated slews. Ka-Band downlink is 4.2 Mbps @ 1.7 AU [Venus superior conjunction], 75 Mbps @ 0.3 AU [Venus inferior conjunction] at transponder saturation limit.

Instrument locations

The location of the instruments are on the top panel of the spacecraft. This panel faces the launcher flight direction of the satellite in launch configuration. The top panel of the s/c will be nadir looking during observations.

Spacecraft top panel with mission instruments.
A detail of the EnVision Mission Instrument panel shows the distribution of the units.The top panel of the s/c will be nadir looking during observations.