Solar Anomalous and Magnetospheric Particle Explorer A NASA satellite designed to study the energy, composition, and charge states of four classes of charged particles that come from beyond the Earth: galactic cosmic rays (from supernova explosions in our galaxy), anomalous cosmic rays (from interstellar gas surrounding our solar system), solar energetic particles (from explosions in the Sun’s atmosphere), and magnetospheric electrons (particles from the solar wind trapped by Earth’s magnetic field). The four SAMPEX instruments are a complementary set of high resolution, high sensitivity, particle detectors used to conduct studies of solar, anomalous, galactic, and magnetospheric energetic particles. The instrument hardware is integrated throughout the primary structure and consists of three sensor assemblies, an 8-bit instrument interface microprocessor, and a tank of isobutane for use by one of the sensors. The instruments weight totals 40 kg and occupies most of the upper half of the spacecraft. The total observatory mass is 157 kg. The SAMPEX mission is operated in a 550 x 675 km orbit with an 82? inclination. SAMPEX is a momentum-biased, sun-pointed spacecraft that maintains the experiment-view axis in a zenith direction as much as possible, especially while traversing the polar regions of the Earth. It points its solar array at the Sun by aiming the momentum vector toward the Sun and rotating the spacecraft one revolution per orbit about the Sun/spacecraft axis. The Attitude Control System (ACS) uses a combination of three orthogonal torque rods to react against the Earth’s magnetic field and one momentum biased reaction wheel to provide the bias momentum. A two-axis digital Sun sensor, a three-axis magnetometer and a set of five course Sun sensors are used for attitude determination. During orbit eclipse the ACS estimates the location of the Sun in order to continue computing a three-axis attitude solution. Attitude determination is always better than 2? accuracy except for when the Sun and Earth magnetic vectors become co-aligned. When this occurs the spacecraft simply coasts until the co-alignment condition passes. This ACS approach provides the quasi anti-nadir pointed attitude required by the science investigation. Use of horizon sensors to directly measure nadir was rejected due to the extra power weight and volume of these sensor devices. Two deployable, fixed solar arrays containing 1.7 m2 of solar cells provide an orbit average power of 100 W to the spacecraft and the instrument. The orbit average power consumption of the spacecraft hardware is 60 W. The instruments consume 22 W. The data system for the SAMPEX mission contains 30 Mbps of memory. It utilizes the fiber optic implementation (1773) of the MIL-STD-1553 data bus to connect the subsystems. Two hemispherical coverage quadrifilar helix antennas are used for ground communication. The average science data rate for the mission is 3 Kbps. The spacecraft is configured to operate with two ground contacts per day, typically of 10 minutes duration. The stored data is transferred to the ground stations at the downlink data rate of 900 Kbps. Commands are uplinked at 2 kbps. Thanks Magik