Mission overview

BeppoSAX ([Boella et al. 1997a,Piro et al. 1995,Scarsi 1993]) is a space mission dedicated to X-ray astronomy of the Italian Space Agency (ASI, Agenzia Spaziale Italiana) with participation of the Netherlands Agency for Aerospace Programs (NIVR). The project started about 15 years before the launch and involves several Italian institutions (IAS-CNR and IOA-'La Sapienza' University in Rome, IFCAI-CNR and GIFCO in Palermo, IFCTR-CNR and GIFCO in Milan, ITESRE-CNR in Bologna, Department of Physics-University of Ferrara), one Dutch institution (SRON in Utrecht) and the SSD-ESA in Noordwijk (NL). ASI prime contractors are Alenia Aerospazio, Torino (Italy), for the spacecraft and Telespazio, Roma (Italy), for the ground segment. The original mission name was SAX (Satellite per l 'Astronomia X); it was renamed BeppoSAX after launch in honor of the Italian physicist Giuseppe "Beppo" Occhialini. The spacecraft carries a set of instruments allowing simultaneous high sensitivity ($\sim$10 mCrab) observations of sources in the unprecedented wide energy band 0.1- 300 keV, the monitoring (at $\sim$10 mCrab sensitivity) of large regions of the sky in 1.5-26 keV with 3 arcmin angular resolution, and the detection and spectral study in 40-700 keV of high energy transients (GRB, SGR). The satellite (sketched in Fig. 1.1) is composed of two basic structures: the service module, in the lower part, housing subsystems and electronics of the scientific instruments, and the payload module, with scientific instruments and star trackers. A thermal shade structure surrounds the payload module. The total mass is 1400 kg (480 kg the payload alone), for an height of 3.6 m and a diameter of 2.7 m (with solar panels closed).

  

Figure 1.1: Schematic representation of the BeppoSAX satellite (from the BeppoSAX Observers' Handbook)

BeppoSAX was successfully launched from Cape Canaveral on April 30, 1996 into a 600 Km nearly equatorial (3.9$^{\rm ^{\circ}}$ inclination) orbit by an Atlas G-Centaur rocket. This orbit was chosen in order to minimize background intensity (by taking advantage of the reduction of the flux of cosmic -rays contribution due to the screening effect of Earth's magnetic field) and because of the marginal passage through the South Atlantic Geomagnetic Anomaly (SAGA) and of the very low modulation of the Earth magnetic field rigidity cut-off and consequently of the particle induced background. The time needed to cover a complete orbit is 97 minutes. The Earth' s angular diameter at this distance is about 130$^{\circ}$. The pointing is three axis stabilized with an accuracy of the order of 1 arcmin, which is therefore the limiting angular resolution for BeppoSAX observations. During each orbit up to 450 Mbits of data are stored to the on-board mass memory and transmitted to ground during the radio contact with the Malindi (Kenya) ground station. The time window for data retrieval and commands transmitting is $\sim$10 minutes per orbit. The Operational Control Center (OCC), the Scientific Operation Center (SOC) and the Scientific Data Center (SDC) are all located in Telespazio, Rome, where the data are finally received and commands transmitted to Malindi through a dedicated intelsat bi-directional link ([Bruca et al. 1998]). The OCC performs orbital management and telecommands uplinking. The SOC is responsible for scientific instruments monitoring, planning of the on-board scientific activities, archiving of the orbit-by-orbit raw telemetry data for quick look analysis. The SDC manages the observation proposals and sends them to the Time Allocation Committee (TAC), collects and checks approved observations, performs long period observation planning. It is responsible for raw data final archiving and the release of the Final Observation Tapes (FOT) to the final users after quality check and standard analysis. The overall Ground Segment (GS) operations are resumed in Fig. 1.2.

  

Figure 1.2: BeppoSAX ground segment operations (from Bruca et al. 1998)

The observational strategy is to dedicate most of the time to Narrow Field Instruments (see next subsection) pointings, with the Wide Field Cameras (see next subsection) monitoring the galactic plane in secondary mode. The flexibility of the planning allows for several Target of Opportunity (TOO) observations. The observing program is divided in a Core Program (CP, assuming an optimal use of the satellite), open for proposals from Italian and Dutch institutions, ESA/SSD and MPE/Garching, and the Guest Observer Program (GOP) open to the worldwide scientific community. Data of every observation are made public after 1 year. Every year an Announcement of Opportunity is issued; at the time of writing (December 1998), BeppoSAX is successfully completing AO2 observations, and the AO3 has already been issued.

The first 2 year and a half of operation (the nominal duration of the mission was 2 year, expected 4 years) have demonstrated the high quality of BeppoSAX scientific instrumentation and servo-systems, with standard of performances above the average of other missions. The only real problems have been the loss of one of the three medium energy concentrators (see next subsection) and the shorter than expected duration of four (out of six) gyroscopes, supplied by the development of software able to manage satellite operations in 1-gyro and 'gyro-less' mode. From the scientific point of view, in addition to the well-known discoveries on GRBs, BeppoSAX is providing a large number of outstanding scientific results on the several classes of X-ray sources.