October 9, 2014

Satellite

Spica is part of the JAXA (Japanese space agency) future science programme and is planned for launch at the end of 2018. The mission follows on both scientifically and technically from the highly successful AKARI mission. High photometric sensitivity observations in the MIR/FIR are made possible thanks to the large 3 m telescope which is actively cooled to below 5 K to effectively eliminate the non-astronomical photon noise. The thermal environment required by the telescope and the instruments will be maintained by a combination of passive cooling (via dedicated solar and thermal shields combined with radiators) and active cooling, using a number of mechanical coolers to provide base temperatures of 4.5 K and 1.7 K. High spatial resolution is achieved thanks to the large aperture, monolithic primary mirror and the appropriate tolerances on the telescope and mirror surfaces to achieve diffraction limited performance at 5 µm.

Spica satellite - © ESA - Jaxa

The platform characteristics are:

An octagonal service module at room temperature.
Dimensions: ~ 4.5 m width, 7.5 m high. Total wet mass ~ 2300 kg
Total maximum power of 2.8 kW (EOL)
Total delta-V budget: 110 m/s
Pointing requirements: Standard mode: APE < 0.135 arcsec, RPE < 0.075 arcsec/200 s; Coronagraph mode: Absolute Pointing Error < 0.135 arcsec, Pointing stability < 0.075 arcsec/10 min.
Attitude control system: 3-axis stabilised spacecraft, Reaction Wheels. AOCS sensors complemented by dedicated focal plane cameras for fine guidance.
Thermal Control System: Passive cooling via Sun-shield, 3 thermal shields, telescope shell and external telescope baffle. Active cooling via a redundant set of mechanical coolers based on 2-stage Stirling units and JT expansion units (4.5 and 1.7 K stage).
Propulsion system: Monopropellant (blow down), 4x 23N and 8x 3N thrusters. Propellant: 300 kg.
On Board Data Handling: Central DPU (common to the payload module and the service module), SpaceWire, on board mass memory > 48 Gbyte.
Telecommand, Telemetry & Communication: X-Band (HGA, MGA, and 3x LGA for downlink), S-band (2 LGA for up and down-link).

The payload characteristics are:

A cryogenic payload thermally discoupled from the service module via a dedicated, low thermal conductivity, truss structure. The cryogenic temperatures are achieved by passive cooling (Sun-shield, 3 thermal shields, telescope shell, external telescope baffle) and by active cooling via a redundant set of mechanical coolers based on 2-stage Stirling units and JT expansion units (4.5 and 1.7 K stage).

³He JT cooler with 2-stage Stirling developed for Astro-H and SMILES

a Ritchey-Chrétien, 3 m diameter cryogenic telescope (5 K), with diffraction limited performance at 5 µm (5 arcmin radius field-of-view (FOV)). The telescope will be equipped with the following instruments:

MIRACLE (5 – 38 µm, 6' x 6' FOV), a mid infrared camera and spectrophotometer,
SAFARI (34 to 210 µm, 2' x 2' FOV), a far infrared imaging Fourier Transform spectrometer
MIRMES (10.32 – 36.04 µm, R ~ 900 - 1500), a mid infrared high resolution spectrometer
MIRHES (4 – 18 µm, R ~ 20 000 - 30 000), a mid infrared high resolution spectrometer
SCI (5 (3.5) – 27 µm, R ~ 20 - 200, contrast 10^-6, inner working angle 3.3/t_el), a mid infrared coronograph
BLISS (38 – 430 µm, R ~ 700), a far infrared and sub-mm spectrometer; optional
FPC a dedicated Focal Plane Cameras for guidance