SPACE-BASED IMAGING: Clementine II targets asteroids

The proposed Clementine II deep-space mission may furnish data on the composition and structure of Earth-orbit-crossing asteroids while demonstrating technologies for future small satellites. The project is spearheaded by the US Air Force Phillips Laboratory (Kirtland AFB, Albuquerque, NM) under the mantra of "better, faster, cheaper" space-technology development; the spacecraft payload is being formulated by Lawrence Livermore National Laboratory (LLNL, Livermore, CA). The original Clementine I lunar probe mission demonstrated miniaturized digital sensors for multispectral remote sensing (see Laser Focus World, May 1994, p. 38). The second spacecraft, for which funds are still pending, will further integrate such technology into an entire spacecraft system with high-power computers and compact communication components that can perform complex operations autonomously.

Imaging and probing

Not only will Clementine II furnish remote images, but it will release probes several hours before the high-speed flybys of three asteroids; the probes will impact each body. Images and data will be relayed from the probes to the main spacecraft, or "mothership," for later transmission to Earth. The mothership will also image the impact events to determine the structure and composition of the planetoids and the impact dynamics. The three encounters will take place over the span of approximately one year from launch in April 1998, although the target asteroids have yet to be selected.

Sensors to be provided for the mission by LLNL will include a Clementine I star tracker and a high-resolution 0.3- to 1.0-µm CCD camera for each probe. The mothership will be outfitted with two star trackers, a wide-field ultraviolet/visible (UV/Vis) camera, a laser rangefinder, and a high-resolution detector suite using a common 30-cm aperture. The latter includes a beamsplitter to allow imaging by three separate cameras—another 0.3- to 1.0-µm CCD camera and two indium antimonide (InSb) cameras, one for near-infrared (1.0 to 2.5 µm) and one for mid-infrared (2.5 to 5.0 µm) imaging. Additional instruments might be added including a UV/Vis flash spectrometer and a wide-field short-wavelength UV camera.

Imaging resolution is predicted to be 5 m from the mothership and 0.5 m from the probes. The impact of the probes is expected to create a high-temperature plasma of the surface and underlying material whose composition will be determined by mothership sensors. Images of the resulting crater will reveal the asteroid structure.

Mission cost is estimated at less than $120 million, including launch and spacecraft operations. The Clementine II effort is seen by its proponents as complementing the NASA NEAR (Near Earth Asteroid Rendezvous) mission to fly by one asteroid and then fly a long-term formation with another such body, the latter requiring greater spacecraft-propulsion capability.