2.3    Summary of Mission Operations

      The HESSI Mission uses a single instrument consisting of an Imaging System, Spectrometer and Instrument Electronics, all mounted in a Sun-pointed, spin-stabilized spacecraft. The HESSI mission summary is shown Table 1.

      HESSI will be launched in mid-2000 on a small-fairing Pegasus XL (SELVS II) vehicle into a 600-km circular, 38 ° inclination orbit. The launch configuration will have the spacecraft ON and the instrument OFF (detectors warm). Following injection into orbit, the spacecraft will reorient towards the Sun, deploy its solar arrays, and spin up to 15 RPM.

      Once the operational attitude, orientation and spin rates have been achieved, the instrument electronics and cryocooler will be powered. Using a preprogrammed thermal profile, the cryocooler will cool the spectrometer to operational temperatures within a few days. This will be followed by a brief detector checkout period in which high voltages are turned on before normal operations begin.

      Meanwhile the spacecraft will transition from coarse to Fine Sun Sensor (FSS) pointing. The Imager’s Solar Aspect System (SAS) and Roll Aspect Sensor (RAS) will be activated and the analysis of initial data will result in commands to electronically align the FSS with the optical axis of the imager. Fine mechanical adjustment of the solar panel orientation will be used to coalign the principle moment with the optical axis. Normal operations for the remainder of the mission will consist of the spacecraft using its FSS aspect in a fully-automated closed-loop magnetic-torquing control system to keep the axis of rotation within 0.2 degrees of Sun center.

      In normal operation the germanium detectors (GeDs) are cooled to £ 75 K; GeD high voltage is on; and observations are taken continuously. Because of the large thermal mass of the GeDs the cryocooler can be cycled over time scales of hours if needed.