1. Orbit and Launcher

The best orbit for e-ASTROGAM is an equatorial LEO of altitude 550 - 600 km. Particle background properties are optimum for this orbit, as already determined by the AGILE mission (which has an orbit of altitude 520 - 550 km and 2.5deg; inclination with respect to the equator). An equatorial orbit(required to have an inclination i < 2:5°, and eccentricity e < 0:01) will make use of the ESA groundstation at Kourou as well as the possible use of the ASI Malindi station in Kenya. The foreseen launcher for e-ASTROGAM is Ariane 6.2.

2. Spacecraft and system requirements

The e-ASTROGAM system is composed of a satellite in an equatorial Low Earth Orbit and a ground segment that includes the ESA ground station at Kourou and possibly the ASI Malindi station in Kenya. These stations are in charge of performing the spacecraft control, monitoring, and the acquisition of scientific data. The e-ASTROGAM spacecraft is observing the sky according to a predefined pointing plan uploaded from ground. Different pointing profiles can be selected in order to observe selected sky regions or to perform a scanning that can cover a large fraction of the sky at each orbit.
The spacecraft platform is made of a structure that mechanically supports the e-ASTROGAM instrument and hosts internally the payload electronic units and all the platform subsystems. The payload is attached to a mechanical structure at a distance of about 90 cm from the top of the platform. The space between the payload and the platform is used to (i) host the time-of- ight (ToF) unit of the payload (P/L) Anticoincidence (AC) system, (ii) host the P/L PSU, PDHU and BEE modules (Tracker, Calorimeter and AC BEEs), and (iii) accommodate the two fixed radiators of the thermal control system. In addition, this mechanical design has the advantage of signi cantly reducing the instrument background due to prompt and delayed gamma-ray emissions from fast particle reactions with the platform materials.

Figure 1

Figure 1 e-ASTROGAM spacecraft in deployed configuration.

Deployable and steerable solar panels are required to support the payload operating profile and the platform pointing and communication requirements. Figure 1 shows the spacecraft configuration in flight with deployed solar arrays. Figure 2 shows e-ASTROGAM under Ariane 6.2 fairing in upper position of a dual launch configuration.
A precise timing of the payload data (1 μs at 3σ) is required to perform a proper on ground data processing able to guarantee the scientific performances of the mission. The required timing performance is obtained by a GPS delivering a pulse-per-second (PPS) signal both to the P/L PDHU and the Spacecraft Management Unit time management in order to allow a fine synchronization with the time reference.
Communication with the ground is ensured by an X-band TM/TC subsystem. The average orbital contact time with the ground stations of Kourou and Malindi is about 10 min for each of them (for an orbit altitude of 550 km and 2.5° inclination).

2.1 Attitude and Orbital Control Systems (AOCS)

The AOCS performance requirements for the e-ASTROGAM mission are not critical, with an absolute Pointing Error of less than 1 degree, a relative Pointing Error of less than 0.01 deg/s and an Absolute knowledge Error of less than 30 arcseconds. In addition, to fulfill the scientific requirements, the spacecraft shall be placed in a nearly equatorial low Earth orbit (550 - 600 km) without the need to perform orbit maintenance during nominal lifetime. The propulsion subsystem is therefore provided with the only purpose to have the possibility to correct for launcher dispersion, perform debris avoidance and, as required by the international regulations and because of the construction of the instrument, to execute a direct controlled re-entry at the end of the mission. The proposed subsystem design mis a monopropellant hydrazine, blow-down mode propulsion system. The hydrazine is contained in a diaphragm tank, together with the pressurant. From a preliminary estimation of the propellant budget the amount of hydrazine required to fulfill the mission needs are about 266 kg, among which more than 190 kg are allocated to the end of mission disposal.
The spacecraft is able to provide the following attitude pointings to support the payload observation requirements:

  • zenith pointing to perform at each orbit a scan of the sky;
  • nearly inertial pointing (with the possibility to slowly rotate around the payload boresight) to observe continuously a selected area of the sky;
  • fast payload repointing during eclipse periods to avoid the presence of the Earth in the payload FoV (allowing 2 pointings per orbit).
  • The required pointing accuracy (± 1 deg), stability (0.01°/s), and attitude knowledge of 30 arcsec (to be reached after ground processing) can be obtained using standard class sensors and actuators. The 3-axis stabilized attitude control is achieved mainly using a set of four reaction wheels used in zero momentum mode ensuring the possibility to perform fast repointing manoeuvres. Magnetic torquers are provided to perform wheels desaturation and to support a safe attitude pointing based on a basic subset of AOCS items.

    Figure 2

    Figure 2 e-ASTROGAM under Ariane 6.2 fairing in upper position.

    2.2 Attitude and Orbital Control Systems (AOCS)

    The e-ASTROGAM P/L detector has an optimal performance in the temperature range -10° - 0°. In order to guarantee the required environment, the P/L power dissipation is evacuated towards external space by two fixed large radiators located on the two solar array panels, below the instrument. The total radiative area is 11.6 m2. The radiator is a heat transfer device based on a Loop Heat Pipe (LHP) with a condenser as a part of a radiation heat exchanger. Large radiators are necessary because of the large payload thermal dissipation, variable external environmental conditions, and limited heat transport capability within the payload. However, thanks to the large Ariane 6.2 fairing (4.5 m diameter), a deployable radiator can be avoided, as shown in Figure 1.

    3. Ground segment
    3.1 Mission Control Center and ground stations

    The e-ASTROGAM satellite will be managed by an ESA Mission Control Center (MOC). Standard operations and activities will be performed by the MOC (satellite control, flight dynamics, mission planning). The ESA ground station in Kourou will be the standard communication base. No specific requirements are envisioned regarding satellite telemetry, tracking and command. However, considering the relatively high telemetry throughput of the mission, for the scientific data download we propose to use also the ASI Malindi ground station in Kenya. The MOC will be in charge of (i) commanding the instrument and spacecraft; (ii) mission planning; (iii) flight dynamics; (iv) instrument and satellite health monitoring; (v) telecommand and telemetry (TM) history; (vi) spacecraft TM management; (vii)auxiliary data production (attitude reconstruction, time calibration, orbit reconstruction) and transmission to the SOC; (viii) scientific TM acquisition and transmission to the SOC.

    3.2 Scientific ground segment

    The e-ASTROGAM scienti c ground segment consists of a Science Operations Center (SOC) and a Science Data Center (SDC).
    Science Operations Center (SOC) | The SOC is to be located in an ESA facility under ESA supervision. It is the official interface among the MOC, the SDC, and the scientific community(users and guest observers). The SOC is in charge of: (i) TM acquisition from the MOC; (ii) scientific mission planning, including management of Target of Opportunities; (iii) running data preprocessing (from level L0 to L1); (iv) running quick-look analysis; (v) issuing scientific alerts (GRBs, transient sources, TGFs);(vi) managing the Guest Observer programme (including scientific tool delivery); (vii) managing the science data archive. The SOC is coordinated by ESA, which enacts the planning/operational decisions regarding the mission.
    Science Data Center (SDC) | The SDC will be based on both ESA and Science Team contributions. The SDC is in charge of (i) L1 data acquisition from the SOC; (ii) running data pipeline and data reduction (from L1 to L2); (iii) instrument monitoring and support to payload diagnostic; (iv) instrument calibration and calibration archive; (v) final stage of scientific software development and testing; (vi) development of scientific products and data analysis (imaging, lightcurves, spectral and temporal analysis, etc.); (vii) production of tools for quick-look analysis and alert generation; (viii) production and validation of Guest Observer analysis tools. The variability aspect of the gammaray sources above 300 keV is a key factor for e-ASTROGAM and special focusing and resources will be dedicated to it. An automated Science Alert System will be implemented. Target of Opportunity observations (ToOs) are required to follow particularly important transient events that need a satellite repointing. The e-ASTROGAM mission requirement for ToO execution is within 6-12 hours, with the goal of reaching 3{6 hours. The e-ASTROGAM scienti c ground segment will use a sophisticated data reduction and analysis system that merges the Compton and the pair production regimes. It will be based on advanced techniques of pattern recognition for the classi cation of di erent event topologies originated by two interaction processes. The e-ASTROGAM Consortium team will be in charge of developing, testing and SDC implementation of the main SW tools and data analysis algorithms. Based on our experience from ongoing gamma-ray/particle missions (AGILE, Fermi, PAMELA, AMS) we envision for event classi cation and background rejection an implementation based on neural networks,multivariate analysis techniques like Boosted Decision Tree, and Bayesian techniques.

    3.3 Data policy, Guest Observer programme, scienti c mission planning

    The e-ASTROGAM scientific program will be open to the international community through a Guest Observer (GO) programme. A science management plan will regulate the programmatic activities between ESA, the e-ASTROGAM team and the scientific community. e-ASTROGAM will operate as an observatory, and standard guidelines will be applicable. The mission will be operated by ESA following its standard rules in term of Announcement of Opportunities, data right, data distribution, etc. Hundreds of sources will be made available for GO investigations, following the operational examples of INTEGRAL, AGILE and Fermi.
    We envision the implementation of a core science programme that should guarantee that the mission key objectives are met. In particular: (i) a performance verification phase is foreseen at the beginning of the operations, then a fraction of the observing time will be routinely reserved for in-flight calibrations. All these data will be available to the community after they have been validated by the Collaboration and the SOC. (ii) Mission planning (regular pointings and ToOs) will be regulated by a Mission Planning Committee following the indications of a Users' Committee. (iii) All data will become public after 1 year of proprietary right. (iv) Guest Observers will be supported by the e-ASTROGAM Science Data Center with data and analysis software.