Satellite

The prime contractor for ESA is Thales Alenia Space Italy, which is also the satellite prime contractor.

The Euclid spacecraft will have a launch mass of 2,160 kg, and be 4.5 metres tall and 3.1 metres across (figure 1). It comprises two modules:

• The Payload Module (PLM) consisting of the telescope and optical systems (“cold” part) of the instruments
• The Service Module (SVM) housing the satellite’s subsystems. This consists of a large carbon fibre reinforced polymer (CFRP) sunshield coated with Kapton® to protect the PLM. Its Sun-facing side comprises three solar panels delivering between 1,780 and 2,430 watts, depending on the satellite’s orientation and solar cell ageing. The energy required to power the satellite, particularly during launch, is stored in a Li-ion battery with a capacity of 300 Wh and able to deliver 419 watts. The platform also accommodates the Attitude and Orbit Control System (AOCS), communication system, propellant tanks, thrusters, avionics (power electronics, command and control, and data handling systems) and thermal control systems (radiators, multilayer insulation, heating elements). Attitude control manoeuvres not related to science operations are performed by ten hydrazine thrusters. The SVM also houses the “warm” electronics for the instruments, i.e. the units for powering and communicating with the optical and optoelectronic systems in the PLM.

         Euclid satellite in environmental testing at Thales Alenia Space France, Cannes. ©Rémy Decourt

In addition to its own housekeeping operations, the satellite must also meet the mission’s data transmission and pointing requirements.

The unprecedented volume of science data from Euclid at the L2 Lagrange point—on the order of 800 to 850 Gbits per day, for four hours a day—and housekeeping telemetry (to keep a check on the instruments), will be communicated via a high-speed K-band link (25-27 GHz) offering a nominal data rate of 73.85 Mbps or 36.92 Mbps in degraded mode. These systems will provide cold redundancy and are coupled to a high-gain antenna (HGA) with a span of 70 centimetres and two degrees of freedom, pointed permanently at Earth. The satellite also has four Tbits of mass memory able to store three days of science data and 20 days of telemetry data. Telecommands and satellite housekeeping data will be transmitted via an X-band link (8-8.4 GHz) with a slower data rate and redundant systems coupled to three low-gain antennas (LGA).

The pointing specifications are fairly tight, requiring an absolute pointing error (APE) of 7.5 arcseconds and relative pointing error (RPE) of 75 milliarcseconds with a level of confidence of 99.7%. This calls for a very effective Attitude and Orbit Control System (AOCS), made up of three star trackers, a high-performance gyro, four inertia wheels and a cold-gas (nitrogen) microthruster system. The AOCS is complemented by two Fine Guidance Sensors (FGS) coupled to the Focal Plane Assembly (FPA) of the VISible Instrument (VIS).