Mission Outline

SAX ([Boella et al., 1997a,Piro, 1995]), Satellite per Astronomia X, i.e. X-ray astronomy satellite, renamed BeppoSAX after launch in honour of Giuseppe (Beppo) Occhialini, is a major program of the Italian Space Agency (ASI) with participation of the Netherlands Agency for Aerospace Programs (NIVR). The BeppoSAX consortium, supporting the development of the mission, is composed by the following institutes: Istituto di Astrofisica Spaziale (ASI, C.N.R., Rome), Istituto di Fisica Cosmica ed Applicazioni Informatiche (IFCAI, C.N.R., Palermo), Istituto Tecnologie e Studio Radiazioni Extraterrestri (ITeSRE, C.N.R., Bologna), Physics Department of the University of Ferrara, Space Research Organization of the Netherlands (SRON, Utrecht) and Space Science Department (SSD, ESA, Noordwijk, The Netherlands).

The scientific payload covers a very wide energy range ($0.1$-300 keV). This capability, in combination with a relatively large area, a medium energy resolution and an imaging resolution of $\sim 1.5$$^\prime$ (in $0.1$-10 keV), allows the simultaneous observation of the same source at different wavelengths, which is of key importance for understanding many astrophysical processes. Moreover, the presence of wide field instruments in the 2-28 keV energy band makes it possible to monitor all the X-ray transients sources: this capability has been fundamental in localizing the first prompt X-ray counterparts to GRBs.

Figure: Sketch of the BeppoSAX satellite structure (From Piro 1995).

BeppoSAX was launched on April 30, 1996, by an Atlas-Centaur into a 600 Km orbit, with a period of $\sim$ 96 min (the time between two passages over the ground station is $\sim$ 103 min) and with an inclination of $3.9$$^\circ$. The choice of this orbit presents many advantages: the shielding of the terrestrial magnetic field reducing the cosmic ray induced background (especially for high energy instruments), a slow-varying particle induced background level due to the small modulation of the Earth magnetic field rigidity cut-off, and the marginal passage above the South Atlantic Geomagnetic Anomaly (SAGA).

The spacecraft structure (fig. ) consists of three basic modules: the Service Module, in the lower part, that houses all the subsystems and the electronics of the scientific instruments; the Payload Module with scientific instruments and star trackers; the Thermal Shade Structure enclosing the Payload. The total mass is $\sim$ 1400 Kg (480 of the only payload) and the linear dimensions are $3.6$ m (height) and $2.7$ m (diameter), when the solar panels are closed. The total power is 800 W, with 260 W only for the payload.

The spacecraft is three axis stabilized and has a pointing accuracy of 1$^\prime$; when not pointing, the maximum slew rate is $\sim 10$$^\prime$/min. The need to mantain the normal direction to the solar panels within 30$^\circ$(sometimes 45$^\circ$) from the Sun is the main attitude constraint.

When, 11 min per orbit, BeppoSAX passes over the ground Telemetry and Telecommand (TT&C) station at the Italian base near Malindi (Kenya), the Operation Control Center (OCC) in Rome communicates with it through a bidirectional Intelsat link; during this time window, many tasks are performed: telecommand up-linking, data downloading, spacecraft doppler and ranging measurements and on-board time (OBT) - Universal Time (UT) synchronization.

The possibility, offered by these frequent communications, of a fast reaction and rescheduling within a few hours in favour of target of opportunity objects has turned out to be very useful in the case of GRBs.