The Clementine Spacecraft

The Clementine spacecraft was built at the US Naval Research Laboratory in Washington, DC and carried sensors, attitude control systems and software designed and built by the Lawrence Livermore National Laboratory (LLNL). The USAF supplied advanced lightweight composite structures and the launch vehicle, a Titan II B refurbished ICBM. Several other organizations were involved, especially NASA with communications support, through the Jet Propulsion Laboratory's (JPL) Deep Space network, and orbit determination and operations support from both the Goddard Space Flight Center and JPL. Supporting these laboratories were scores of industrial contractors, both large and small.

The spacecraft consists of an octagonal prism about 2 meters high. A 110-pound thruster for delta-V maneuvers is on one end of the prism and a high-gain fixed dish antenna is on the other end. The spacecraft propulsion system consists of a nonpropellant hydrazine system for attitude control and a bipropellant nitrogen tetraoxide and monomenthyl hydrazine system for the maneuvers in space. The bipropellant system had a total capability of about 1.9 km/s with about 550 m/s required for lunar insertion and 540 m/s for lunar departure. The power system consists of a gimbaled, single axis, GaAs/Ge solar array providing a total spacecraft power of 360 watts at 30 Vdc, with a specific power of 240 w/kg, based on lightweight construction. Two arrays of the rotating solar panels protrude from opposite sides; by rolling the spacecraft and rotating the panels, full solar illumination of the panels would be achieved. The solar array was used to charge a 15 A-h, 47-w hr/kg, Nihau common pressure vessel battery. The total mass of the spacecraft in the launch configuration was 1690 kg, with most of the weight in the solid rocket motor required for translunar insertion. The spacecraft dry mass is about 227 kg, with a roughly equal mass for liquid fuel. This weight was achieved by incorporating many of the lightweight technologies developed through the research and development activities of the Strategic Defense Initiative (SDI).

The spacecraft data processing was performed by 3 computing systems. A MIL-STD-1750A computer with a capacity of 1.7 million instructions per second was used for save mode, attitude control system, and housekeeping operations. A reduced instruction set computer (RISC) 32-bit processor with 18 million instructions per second was used for image processing and autonomous operations. The Clementine mission represents the first long duration flight of a 32 bit RISC processor. Also incorporated is a state-of-the-art image compression system provided by the French Space Agency CNES. A data handling unit with its own microcontroller sequenced the cameras, operated the image compression system, and directed the data flow. During imaging operations, the data were stored in a 3 kg, 2 Gbit dynamic solid state data recorder and later transferred to the ground stations using a 128 kb/s downlink. The spacecraft was commanded from the ground using a 1 kb/s uplink from the NASA Deep Space Network and DOD stations. Demonstration of autonomous navigation including autonomous orbit determination was a major goal of the Clementine mission. Autonomous operations were conducted in lunar orbit.

Clementine was launched on January 25, 1994 from Vandenburg Air Force Base aboard a Titan IIG rocket. After two Earth flybys, lunar insertion was achieved on February 19-th. Lunar mapping took place over approximately 2 months in two systematic mapping passes over the Moon.

After successfully completing the Lunar mapping phase of the mission, Clementine suffered an on-board malfunction at 9:39 AM EST, Saturday, May 7, 1994. The result of the malfunction prevented Clementine from performing the planned close flyby of the near-Earth asteroid Geographos scheduled for August 1994. Preliminary analysis traced the cause of the malfunction to the on-board computer which controls most of the satellite's systems including the attitude control thrusters. The computer activated several thrusters during a 20 minute telemetry interrupt with the ground station, thus depleting all the fuel in the Attitude Control System (ACS) tanks. With the depletion of the ACS tanks, the spacecraft was left spinning at 80 revolutions per minute with no remaining fuel left to despin the spacecraft.

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