THESEUS Mission Configuration

Theseus satellite will operate in a low equatorial orbit (altitude < 600 km, inclination < 5°). This orbital configuration will guarantee a low and stable background level in the high-energy instruments. The mission has been evaluated assuming as baseline a launch with Vega-C.

 

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Figure 18. Theseus Orbit Configuration (Left) and Theseus Orbit Ground Track (Right).

 

The Theseus satellite will be equipped with a suite of three instruments: SXI, XGIS and IRT. In summary the Theseus satellite will be capable to:

  • to monitor a large sky sector for detecting, identifying and localizing likely transients/burst in the SXI and XGIS FOV;

  • of promptly (within a few tens of seconds at most) transmitting to ground the trigger time and position of GRBs and other transients of interest;

  • of autonomous (via SXI, XGIS or IRT trigger) orientation in the sky direction of interest;

  • to perform long observation of the sky direction of interest;

Theseus requires a 3-axis stabilized attitude. During its orbital period Theseus will have distinct operational modes:

Survey (burst hunting) mode: relevant to normal operation when SXI and XGIS are searching for transients. The accessible sky for this kind of operation will be determined by the requirement that:

  • Theseus will have a Field of Regards (FoR) defining the fraction of sky which can be monitored of 64°.

  • When monitoring the sky in normal operations, the number of re-pointings per orbit will be of the order of 3, resulting in observations with the SXI and XGIS of about the whole FoR every 3 orbits.

Burst mode: after detecting a GRB or other transient of interest, the satellite is triggered to this mode by SXI and/or XGIS which transmit to satellite computer the quaternion of the area of interest. The satellite will autonomously fast repoint to place the transient within the field of view of the IRT according to the following steps:

  • fast slewing (< ~60°/10min) for IRT LoS pointing to the direction of GRBs and other transient of interest for low resolution spectra,

  • satellite stabilization (RPE) within less than 0.5 arcsec

  • fast data link for GRB coordinate communication to ground within a few min

Follow-up mode: the IRT shall observes inside the full FoV of 10’x10’ the target with the pre-scripted imaging – spectroscopy sequence. In case of IRT high resolution spectra acquisition a further satellite fine slewing (based on IRT source localization) shall be activated to place the IRT LoS inside a reduced FoV of 5’x5’. XGIS and SXI are in specific follow-up data acquisition mode. In this mode the GRB observation shall be performed in a time of 30 minutes. After completion of the transient observation, Theseus will return to Survey Mode to monitor the sky.

IRT observatory mode: the IRT may be used as an observatory for pre-selected targets through a GO programme, driving the pointing of the satellite. XGIS and SXI are observing as in survey mode, with the possibility of triggering the burst mode

No specific orbit parameter change is required during the mission lifetime. Theseus mission can be supported as baseline by a dedicated ground station located in Malindi (3° S, 40° E). Another ground station, located in Alcantara (2°S, 4° W), is supposed as possible back-up in case of Brasilian participation. In conformity with the selected equatorial orbit, both stations will allow highly frequent accesses to the satellite by a contact per orbit.

Contact analysis between the satellite and the ground stations has been performed considering a minimum station elevation angle over local horizon of 10°.

Within the following tables, the average (and the maximum) single contact duration, the number of daily contacts and the resulting average daily contact duration are summarized for every ground station.

 

Table 9. Accesses to Ground Stations

Ground station

Average contact duration (sec)

Maximum contact duration (sec)

N°contacts /day

Average daily contact duration (sec)

Malindi

521

545

13 to14

7263

Alcantara

526

545

13 to 14

7322

 

Satellite Configuration

The satellite configuration and design take into account a modular approach. The spacecraft platform is divided in two modules, the Payload Module (PLM) and Service Module (SVM). The Payload Module will mechanically support the Instruments DUs (SXI, XGIS and IRT) and will host internally the Instruments ICUs. The instruments SXI and XGIS DUs are accommodated externally on the Payload Module to which they are connected by means of a structural pedestals. The Instrument DUs mechanical fixing to this structure will be designed in order to guarantee thermo-structural decoupling from the rest of satellite.

The Service Module contains all the platform subsystems and provides the mechanical interface with the Launcher. Figure 19 shows the spacecraft baseline configuration.

 

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Figure 19. THESEUS Satellite Baseline Configuration and Instrument suite accommodation

 

The Payload Module is constructed around the IRT instrument, which is partially embedded inside the PLM structure and aligned with respect the S/C symmetry axis. The module top plane is the mounting base of the other instruments DUs (SXI and XGIS) which are distributed around the IRT axis in order to minimize the satellite Moment of Inertia (MoI) but also to support efficient load transfer from the spacecraft to the launch vehicle, respecting their accommodation constraints and thermal requirements.

The IRT LoS is the reference of the overall payload. The telescope is protected from straylight by a baffle; the profile of the baffle defines the position of the entrance window plane of the other instruments.

The 4 SXI DUs are nominally mounted on the opposite side of the solar panels in order to keep them in the coldest side of the satellite and to have the largest area of the observable sky when Theseus lies between Sun and Earth. The SXI DUs positions and orientations are determined in such a way that no X-rays reflected by IRT tube or any other satellite structure can enter into SXI FOV. The 3 XGIS units are tilted in such a way that the FOV of the units partially overlap. The resulting overall FOV (i.e. that of the combination of the 3 units) covers and center the FOV of the 4 SXI modules.

 

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Figure 20. Structure of THESEUS Payload Module

 

The proposed accommodation is shown in Figure 22. This configuration guarantees the required nominal SXI, XGIS and IRT Field of view combination (REQ-MIS-050 ). Solar array proposed baseline consists of modular panels in fixed configuration, the panels are composed of a photovoltaic module and a mechanical on which photovoltaic module is mounted. At the level of the payload module top plane a Sun shield is mounted, in order to protect the instruments from solar radiation. The structure of the Payload module is provided of reinforcing shear panels and of an internal cylinder for the IRT telescope and detector accommodation.

The internal cylinder has the function of structural support and it provides also a thermal separation for IRT instrument from the rest of the module. In order to assure radiative thermal decoupling, the telescope is covered by multi-layer insulation.

At the level of focal plane of IRT, where the Detector is placed, a Miniature Pulse Tube Cooler (MPTC) system is provided, acting with a heat-pipe system and a dedicated radiator. Dedicated radiator is provided also for each of the other instruments (SXI and XIGS) on the pedestals. Every radiator is positioned on the own support structure of the single instrument. The Service module is currently conceived as composed by a single module accommodating all the platform units (with the exception of one boresight star tracker which is integrated on the IRT telescope tube to minimize relative misalignment improving pointing knowledge performances. In addition to the platform star tracker, two star trackers hare foreseen in support to IRT instrument, to allow astrometric measurements independent from the system. Figure 21 shows the configuration of Service module while Figure 22 shows Satellite accommodation within the VEGA C fairing.

 

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Figure 21. THESEUS Thermal concept (Left) and Service Module (Right)

 

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Figure 22. THESEUS accommodation within VEGA C fairing