On February 19, 1994, a small United States spacecraft entered lunar orbit for the first time in 21 years to take scientific data on the Moon. Was this the mission for which lunar scientists had lobbied during two decades to "finish the highly successful Apollo orbital geochemistry data set? Not exactly. In fact, the implementing agency was the Department of Defense and the Clementine spacecraft was officially designated as a flight qualification of "advanced lightweight technologies developed by the Ballistic Missile Defense Organization (BMDO)."

Fortunately for planetary science, the Antiballistic Missile (ABM) Treaty forbids testing these technologies in Earth orbit. The BMDO decided to head for the Moon and recruited a NASA-funded science team for advice on how to get the best science within the technical, operational, and fiscal constraints of the mission. The end result is two months of mapping from lunar polar orbit and a stunning remote sensing characterization of an entire planet.

Within a spacecraft dry mass of only 235 kg, Clementine's instruments included a UV/Visible Camera, Short- and Long-Wavelength Infrared Cameras, a Lidar Laser Transmitter coupled with a high-resolution camera, and a Star Tracker Camera. The science team chose a combination of 11 spectral filters for the UV/Visible Camera and the Near-Infrared Camera to maximize the ability to discriminate among lunar minerals. Careful choreography of the spacecraft's elliptical orbit has provided coverage for the entire Moon at a spatial resolution of 200-300 meters. Photographs of various locations by the lidar camera provide resolutions as high as 30 meters per pixel in five spectral bands.

The lidar instrument was modified to act as a laser ranging altimeter for the lunar mission. The instrument's capability to make the measurements had been questioned, but its performance exceeded all expectations. It returned a data set of lunar topography between latitudes 70N and 70S. The altimeter was turned off over the polar regions on the expectation that the altitude of the spacecraft there was too high. Nevertheless, since Clementine passed repeatedly over the poles, sufficient imaging exists to measure topography through the use of convergent stereo photography.

NASA plans to issue an announcement of opportunity to submit research proposals for data analysis later this year. Meanwhile, the science team is working to put the data in accessible form while assessing its content and quality. The altimetry data alone is giving startling information, proving the existence of several farside basins whose existence had only been proposed. In one case, a newly confirmed basin exhibits relief of 12 km from rim to floor! The South Pole region shows a permanently dark depression that may be as much as 150 km across. A difficult and complex measurement of the dielectric constant of this polar region using bistatic radar techniques is currently being analyzed to determine whether evidence for ice can be found.

Data from Clementine is already causing us to rethink our picture of lunar evolution. Yet, as exciting and voluminous as this new information is, we are still missing major data sets for planetary study such as global geochemistry, geophysical characterization of the internal structure, absolute ages of stratigraphic provinces, and even the most elementary information on the origin of lunar regional magnetism. As the Clementine data stream is processed into information, transformed into knowledge, and incorporated as understanding over time, it will raise new debates about the origin and evolution of the Earth-Moon system and spawn new concepts for further exploration of our sister planet.