The design of the ATHENA observatory is driven by the need to maximise the science performance-to-cost ratio while minimising development risk. The main design drivers derive from the telescope performance requirements and, in particular, the need to accommodate the longest possible focal length without the complication of deployable structures, coupled with the need to illuminate two focal plane instruments via separate telescope optics.
The spacecraft configuration, which is similar to that of XMM-Newton, allows the volume of the Ariane 5 fairing to be fully utilised by reducing the size of the focal plane assembly and by accommodating the sensitive optics in the inner cylinder of the service module.
The spacecraft is divided into three main elements:
The total wet mass of the spacecraft will be about 4000 kg. The peak electrical power requirement will be around 5 kW.
Service module and main subsystems
The ATHENA SVM will include the fixed telescope metering structure and the actual observatory platform subsystems. The inner cylinder of the SVM will also host the mirror assembly. The fixed metering structure (FMS) will be rigidly connected to the focal plane assembly, which will host the two instruments and a radiation environment monitor. The spacecraft structure will be built around a standard Ariane 5 payload adaptor with a diameter of 2.6 metres; the inner cylinder of the SVM will have the same diameter and serve both as the supporting element for the equipment platform and as a mounting base for the conical metering structure. The MA, with the two X-ray telescopes, will be housed in the inner cylinder and protected by an external anti-contamination cover.
The FMS will be approximately 11 metres long and fabricated in two separate sections, using an aluminium honeycomb core and carbon-fibre reinforced composite skins. A dedicated venting door on the FMS, close to the FPA, will maximise removal of out-gassing products without introducing straylight.
The platform subsystems are accommodated on the lateral and bottom panels of the SVM. Thrusters and solar panels are also connected to the service module.
Focal plane assembly
The focal plane assembly (FPA) will accommodate the two focal plane instruments (Wide Field Imager, WFI, and X-ray Microcalorimeter Spectrometer, XMS), a radiation environment monitor and their ancillary equipment.
The structure is based on a horizontal platform, connected to the FMS and supporting the instruments. Three vertical shear walls will provide adequate stiffness and the surface area required to install all the electronics. The lateral panels are mostly canted to reduce the envelope size of the assembly and to maximise the focal length. Canting of the panels favours their utilisation as radiators.
The mirror assembly (MA) preliminary design comprises two telescopes based on Silicon Pore Optics (SPO). Each telescope consists of about 250 mirror modules, accommodated on a single petal. The approximate size of the petal is 1.8 × 1.0 metres, with an ellipsoidal shape. The telescopes are mounted onto a dedicated MA structure, which is then interfaced to the central cylinder of the SVM. It is currently proposed that both the telescope petals and main structure will be fabricated from ceramic material such as silicon carbide (SiC).
In the preliminary ESA design, the petals have a C-shaped beam as the petal frame, without any webs or stiffening spars in the interior, in order to preserve the optical layout.
The main MA structure is based on a circular composite sandwich, with double 'lambda' spokes to support the petals. The MA has a hyper-static support structure with eight central cylinder interfaces. This design ensures a direct and stiff attachment to the central cylinder via the lambda beam. Star trackers could be accommodated in the unused space between the lambda legs.
(For more detailed information about the ATHENA spacecraft design, please consult the ATHENA assessment study report (Yellow Book) - see link in right-hand menu.)