Clementine (spacecraft)

Spacecraft design

The Clementine spacecraft was built as an octagonal prism, measuring 1.88 meters tall and 1.14 meters wide. Two solar panels extended from opposite sides, aligned parallel to the prism's axis. A fixed 42-inch high-gain dish antenna sat at one end, while a 489-newton thruster was at the other. All sensor openings were grouped on a single panel, positioned 90 degrees from the solar panels, and protected by one cover.

Its propulsion system used monopropellant hydrazine for attitude control and a bipropellant mix of dinitrogen tetroxide and monomethylhydrazine for major maneuvers, offering a total delta-v of about 1,900 meters per second. Attitude control relied on twelve small jets, two star trackers, and two inertial measurement units, achieving three-axis stabilization in lunar orbit via reaction wheels with a control precision of 0.05 degrees and knowledge precision of 0.03 degrees.

Power came from gimbaled, single-axis GaAs/Ge solar panels that charged a 15 ampere-hour nickel-hydrogen common-pressure-vessel battery. Data processing used a MIL-STD-1750A computer for safe-mode, attitude-control, and housekeeping tasks, plus a 32-bit RISC processor for image processing and autonomous operations. The French space agency CNES provided an image compression system, and a data handling unit managed cameras, compression, and data flow, storing information in a 2-gigabit dynamic solid-state recorder.

Mission

Clementine launched on January 25, 1994, from Vandenberg Air Force Base using a Titan II launch vehicle. After two Earth flybys, lunar insertion occurred about one month later. Lunar mapping lasted approximately two months in two parts: first a five-hour elliptical polar orbit with periapsis near 400 km at 13 degrees south latitude and apoapsis of 8,300 km, then after one month the orbit rotated to periapsis at 13 degrees north latitude for another month.

This provided global imaging and altimetry from 60° south to 60° north over 300 orbits. After a Moon-to-Earth transfer and two more Earth flybys, the spacecraft was to head for 1620 Geographos for a flyby closer than 100 km. On May 7, 1994, a malfunction caused an attitude control thruster to fire for 11 minutes, depleting its fuel and spinning the craft to about 80 rpm, making the asteroid flyby impossible. The spacecraft was placed in a geocentric orbit through the Van Allen belts for component testing.

The mission ended in June 1994 when power dropped too low for intelligible telemetry, though ground controllers briefly regained contact between February 20 and May 10, 1995.

Science instruments

The Charged Particle Telescope (CPT) measured energetic protons (3–80 MeV) and electrons (25–500 keV) to study the Moon's interaction with Earth's magnetotail and interplanetary shocks, monitor the solar wind, and assess particle effects on spacecraft components. It was a single-element telescope under 1 kg with a 10 degree half-angle field of view, using a silicon surface-barrier detector of 100 mm² area and 3 mm thickness. The signal was divided into nine channels: six for electrons and three for protons and heavier ions. The Ultraviolet/Visible camera (UV/Vis) studied the Moon and Geographos at five wavelengths in the ultraviolet and visible spectrum, providing data on surface petrology, morphology, and cratering statistics. Most images were taken at low Sun angles, useful for petrology but not morphology. The Geographos rendezvous was canceled due to equipment malfunction.

Bistatic Radar Experiment

Radio signals from Clementine's transmitter were directed at the north and south polar regions, and their reflections were detected by Deep Space Network receivers on Earth. Analysis of the reflected signals' magnitude and polarization suggested the presence of volatile ices, including water ice, in the lunar soil. However, later Arecibo radio telescope studies found similar reflection patterns from areas not in permanent shadow, indicating the results may have been misinterpreted and could be due to surface roughness.