Mars Pathfinder

The Mars Pathfinder Mission will demonstrate a cruise, entry, descent, and upright landing system for use in future missions. The landing vehicle will carry a micro-rover to the surface of Mars. Imaging, atmospheric structure and meteorological experiments will be carried out using instruments aboard the lander. An elemental analysis experiment using an alpha, proton, x-ray spectrometer will be carried on the micro-rover. The mission will characterize surface morphology and geology, acquire elemental compositions of rocks and surface materials, obtain atmospheric measurements, such as pressure, temperature and accelerations, demonstrate utility and mobility of a small rover on the martian surface and return engineering data on the condition and configuration of the lander after landing. It is currently planned for a Delta II launch in late 1996 for direct entry and descent to the surface in July, 1997.

The surface imaging system will reveal the geologic processes and surface-atmosphere interactions at a scale currently known only at the two Viking landing sites. The alpha proton x-ray spectrometer and the spectral filters on the imaging system will determine the elemental composition and mineralogy of surface materials, which can be used to address questions concerning the composition of the crust, its differentiation and development of weathering products. These investigations will represent a calibration point ("ground truth") for orbital remote sensing observations. In addition, a series of small magnets and a reference test chart will determine the magnetic component of the martian dust and any deposition of airborne dust over time. The atmospheric structure instrument will determine a pressure, temperature, and density profile of the atmosphere (with respect to altitude) during entry and descent at a new location, time, and season. Diurnal variations of the atmospheric boundary layer will be characterized by regular surface meteorology measurements (pressure, temperature, atmospheric opacity, and wind). In addition, the imager will determine dust particle size and shape and water vapor abundance from sky and solar spectral observations.

The Imager for Mars Pathfinder is a stereo imaging system with color capability provided by a set of selectable filters for each of the two camera channels. It is being developed by a team led by the University of Arizona with contributions from Martin Marietta Astronautics Group, the Max Planck Institute for Aeronomy in Germany, the University of Braunschweig in Germany, and the Neils Bohr Institute in Denmark. It consists of three physical subassemblies: (1) camera head (with stereo optics, filter wheel, CCD and pre-amp, mechanisms and stepper motors), (2) extendible mast with electronic cabling, and (3) electronics cards (CCD data card, power supply/motor drive card and interface card) which plug into slots in the integrated electronics module within the lander.

Azimuth and elevation drives for the camera head are provided by stepper motors with gear heads, providing a field of regard of +/- 180 degrees in azimuth and + 83 degrees to -72 degrees in elevation, relative to lander coordinates. The camera system is mounted at the top of a deployable mast, a continuous longeron, open-lattice type provided by Able Manufacturing, Inc. When deployed, the mast provides and elevation of 0.86 m above the lander mounting surface (1.5 m above the planetary surface).

The focal plane consists of a CCD mounted at the foci of two optical paths where it is bonded to a small printed wiring board, which in turn is attached by a short flex cable to the preamplifier board. The CCD is a front-illuminated frame transfer array with 23 micrometer square pixels. Its image section is divided into two square frames, one for each half of the stereo FOV's. Each has 256x256 active elements. A 256x512 storage section (identical to the imaging section) is located under a metal mask. The IMP focal plane and electronics are nearly identical copies of the comparable subsystem employed in the Huygens Probe Descent Imaging Spectroradiometer (DISR) using a Loral 512x512 CCD.

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 images side-by-side on the CCD focal plane. Fused silica windows at each path entrance prevent dust intrusion. The optical triplets are an f/10 design, stopped down to f/18 with 23-mm effective focal lengths and a 14.4 degree field of view. The pixel instantaneous field of view is one milliradian. The filter wheel will provide eight channels for geologic studies, two for water vapor, a blue filter for atmospheric dust and a broadband filter for stereo viewing.

Full panoramas of the landing site are acquired during the mission using the stereo baseline provided by the camera optics. Additionally, monoscopic panoramas are acquired both prior and subsequent to the mast deployment, yielding vertically displaced stereo pairs with approximately 80 cm baseline. Images of a substantial portion of the visible surface are acquired in multispectral images with as many as eight spectral bands.

A number of atmospheric investigations are carried out using IMP images. Aerosol opacity is measured periodically by imaging the sun through two narrow-band filters. Dust particles in the atmosphere are characterized by observing the sky at sunrise and sunset using multiple filters and by observing Phobos at night. Water vapor abundance is measured by imaging the sun through filters in the water vapor absorption band in the spectrally adjacent continuum. Images of wind socks located at several heights above the surrounding terrain are used to assess wind speed and direction.

A magnetic properties investigation is included as a part of the IMP investigation. A set of magnets of differing field strengths are mounted to a plate and attached to the lander. Images taken over the duration of the landed mission are used to determine the accumulation of magnetic species in the wind-blown dust. Multispectral images of these accumulations may be used to differentiate among likely magnetic mineral compositions.

This instrument is a foreign-provided copy of instruments flown on the Russian Vega and Phobos missions and planned for flight on the Russian Mars '94 and Mars '96 missions. Accordingly, the instrument has extensive, applicable flight heritage. With the mobility provided by the micro-rover and a deployment mechanism, the APX Spectrometer not only acquires spectra from the ubiquitous martian dust, but more importantly, is deployed to distinct rock outcroppings, thereby analyzing the native rock composition for the first time. The Alpha and Proton Spectrometer portions are provided by the Max Planck Institute, Mainz, Germany. The X-Ray spectrometer portion is provided by the University of Chicago.

This elemental composition instrument consists of alpha particle sources and detectors for back scattered alpha particles, protons, and x-rays. The APX Spectrometer determines elemental chemistry of surface materials for most major elements except hydrogen. The analytical process is based on three interactions of alpha particles with matter: elastic scattering of alpha particles by nuclei, alpha-proton nuclear reactions with certain light elements, and excitation of the atomic structure of atoms by alpha particles leading to the emission of characteristic x-rays. The approach used is to expose material to a radioactive source that produces alpha particles, with a known energy, and to acquire energy spectra of the alpha particles, protons, and x-rays returned from the sample. Accordingly, the instrument can identify and determine the amounts of most chemical elements.

<--- APXS Diagram (78777 bytes)

The proton spectra for alpha particles interacting with elements with atomic numbers nine to 14 are very characteristic of the individual elements, reflecting the resonance nature of the nuclear interactions involved. The proton mode allows their detection and measurement.

The addition of a third detector for x-rays results in a significant extension of the accuracy and sensitivity of the instrument, particularly for the heavier, less abundant elements. Alpha sources produce characteristic x-rays for a range of elements, giving an instrument sensitivity that can approach the ppm level.

The APXS sensor head is mounted external to the Rover chassis on a deployment mechanism. This mechanism places the APXS in contact with rock and soil surfaces. The APXS electronics are mounted within the Rover, in a temperature-controlled environment.

A deployment mechanism supports the APXS under launch and landing loads and provides a means for positioning the APXS with a single actuator. The linkage allows the APXS to be placed at a variety of elevations above normal ground level and at a variety of rotational orientations. The mounting of the APXS to the deployment mechanism permits about 20 degrees of compliance motion as the APXS is placed in contact with the sample. A set of contact closures on the APXS front aperture ring indicates to the Rover that the positioning is complete, thereby terminating the positioning motions.

The ASI/MET is an engineering subsystem which acquires atmospheric information during the descent of the lander through the atmosphere and during the entire landed mission. It is implemented by JPL as a facility experiment, taking advantage of the heritage provided by the Viking Mission experiments.

Data acquired during the entry and descent of the lander permits the reconstruction of profiles of atmospheric density, temperature and pressure from altitudes in excess of 100 km to the surface.

The accelerometer portion of the ASI is provided by the Attitude and Information Management Subsystem. It consists of x-, y-, and z-axis sensors. Several gain states are provided to cover the wide dynamic range from the micro-g accelerations experienced upon entering the atmosphere to the peak deceleration and landing events.

<--- ASI/MET Thermocouple Diagram (33338 bytes)

Sampling frequency of the sensors during the entry and descent portion of the mission is governed by the vertical rate of descent through the atmosphere. After landing, pressure and temperature measurements are made to establish the diurnal variations and day-to day variations over the operating life of the mission.

"Mars Pathfinder Mission: Science & Instruments," 4-page brochure from NASA available from the Jet Propulsion Laboratory, Pasadena, CA, 1994.