SPACECRAFT INSTRUMENTATION: Sojourner senses surface of Mars

The Pathfinder mission that landed on the planet Mars on July 4th carried several scientific instruments, chief among them advanced imaging and sensing equipment that quickly expanded the boundaries of knowledge about Earth`s fabled neighbor.

Sojourner, the 11.5-kg self-propelled rover carried aboard Pathfinder and deployed after its landing, is about the size of a milk crate (see photo) and has an alpha proton x-ray spectrometer (APXS) as its principal scientific instrument. The device comprises, as the name implies, three different spectrometer portions for analyzing the back-scattering of alpha particles, protons, and x-rays from projected alpha particles. To contain costs, a foreign-provided derivative of instruments flown on the Russian Vega and Phobos missions was used. (Sojourner`s total cost was only $25 million.) The APXS sensor head is mounted on a deployment mechanism that places it in contact with rock and soil surfaces, permitting about 20° of compliance motion. The APXS is used to acquire dust spectra and, more important, to analyze the composition of individual Martian rocks for the first time.

Two Kodak KAI-0371 monochrome area interline sensors act as Sojourner`s stereoscopic eyes, measuring the topography and recording visual maps of the surface via a progressive-scan, interline transfer image process that, together with data from a scanning laser, allows the rover to navigate to the "seek" coordinates selected by JPL scientists. A single KAI-037M sensor, located in the rear of the rover, provides full-color images of the rocks and soil samples collected. Using a progressive-scan color charge-coupled device (CCD) with square pixels, this sensor will send uncompressed 640 ¥ 480-pixel still images to the Pathfinder lander (through a 9600-baud radio modem) for transmission to the Jet Propulsion Laboratory. The sensors are commercially available off-the-shelf.

The Imager for Mars Pathfinder (IMP), attached to the landing craft itself, is a stereo imaging system with color capability provided by a set of selectable filters for each of the two camera channels. The camera system is mounted at the top of a deployable mast of continuous longeron, open-lattice type, with a stepper motor with gear heads providing +83° to -72° in elevation (relative to lander coordinates). The focal plane of the IMP consists of a CCD mounted at the foci of two optical paths; it is a front-illuminated frame-transfer array with 23-µm pixels. Its image section is divided into two square frames, one for each half of the stereo field of view; each has 256 × 256 active elements. The stereoscopic imager includes two imaging triplets, two fold mirrors separated by 150 mm for stereo viewing, a 12-space filter wheel in each path, and a fold prism to place the images side by side on the CCD focal plane. The IMP focal plane and electronics are nearly identical copies of the subsystem employed in the Huygens Probe Descent Imaging Spectroradiometer, part of the Cassini mission that will lift-off this fall to explore Saturn`s moon Titan in 2004.