Roving for Rocks on the Red Planet: The Mars Pathfinder Mission

M. Golombek

Mars is the most Earth-like planet in our solar system. After Earth, it is the only other planet that is capable of supporting life. Recent scientific evidence suggests that life did once exist on Mars and makes the planet a doubly exciting target for exploration. Mars is a unique terrestrial planet. Evidence suggests it underwent major climatic changes and has a geologic record of surface rocks that spans the entire history of the solar system. The geologic record suggests that early climate on Mars was warmer and wetter, and that liquid water a requirement for life may have been present. Studying the geological, climatological, and exobiological conditions of Mars may provide the data science needs to address the almost theological question of: "Are we alone in the universe?"

Artist's rendition of the Mars Pathfinder lander and rover exploring the surface of Mars (courtesy of Mike Carroll). The lander base is attached to triangular panels (solar cells are shown in blue). Actuators allow the tetrahedral-shaped lander to open and right itself. Airbags that cushion the landing are beneath the panels and are shown in yellow. The gold-covered enclosure on the base panel houses the central computer and batteries. Above it are the high-gain antenna, the imager, and three vertical rods, which are, from largest to smallest, the low gain antenna, the UHF antenna for rover communications, and a radiometric calibration target for the imager. The other mast is used to study meteorology; it has temperature sensors, wind socks, and an electronic wind sensor. The rover uses ramps that unfurl to drive off the panel. The small square blue magnetic targets at the center of the end of the ramps allow the APXS to measure the elements that make up the magnetic dust on Mars. The rover is shown in the foreground; its top is a solar panel and the crooked rod is the UHF antenna.

Despite the great interest in Mars, Pathfinder will be the first spacecraft to land on the Red Planet since the Viking landers of more than 20 years ago. When Mars Pathfinder lands on July 4, 1997, an exciting era of exploration will begin. The Pathfinder will be followed closely by the Mars Global Surveyor orbiter, which will systematically study the atmosphere, surface, and interior and use remote sensing to relay data about Mars' surface back to Earth.

Mars Pathfinder, however, will be the first mission to explore a landing area on Mars with a mobile platform that can measure the chemical composition of surface materials. Its mobility will allow Pathfinder to explore a landing area that is hundreds of square meters, much more than the few square meters accessible by lander-mounted mechanical arms. Pathfinder's instruments and rover will characterize martian rocks and surface materials over a substantial area, providing "ground truth" for our current view of Mars, which is based largely on global remote sensing data.

The Mars Pathfinder Project began in October 1993 as one of the first Discovery-class missions, which are low cost ($171 million) and short development time (about 3 years) missions that focus on engineering, science, and technology objectives. In addition to the development cost, the launch vehicle costs about $55 million, the rover development and operations cost $25 million, and the mission operations and data analysis cost $14 million. The spacecraft has a novel design that combines the cruise, entry, descent and landing functions into a single "free-flyer." It was designed as a low-cost system that could safely place a payload on the martian surface, which will deploy and operate a microrover and science instruments.

The Mars Surveyor '98 lander and orbiter currently in development extensively use Pathfinder components and systems, including the aeroshell, backshell, parachute, flight computer and software, aspects of the command and data handling systems, and the solid-state power amplifier. The Jet Propulsion Laboratory of the California Institute of Technology, which is NASA's lead center for the robotic exploration of the solar system, built and manages the spacecraft and rover, with most subsystem components contracted out to industry. Existing flight-qualified hardware is used wherever possible to reduce cost, but at the expense of increasing the mass of the spacecraft, which is 890 kg.

After Pathfinder is launched on an expendable Delta rocket in December 1996, the spacecraft will embark on a 7-month cruise to Mars. Surface operations are planned for up to one Earth year. The spacecraft enters the martian atmosphere directly from approach behind its aeroshell. A parachute unfurls, slowing the vehicle, and the aeroshell is jettisoned. During descent, the lander is lowered on a bridle from the backshell; an altimeter near the surface triggers the firing of three small, solid tractor rockets to further slow the lander. Giant airbags inflate around each face of the four-sided lander, the bridle is cut, and the airbag-enshrouded lander bounces onto the Martian surface. Entry, descent and landing take about 5 minutes.

Once the Pathfinder has landed, the airbags retract and the triangular panels open to right the lander. The lander is solar powered, with rechargeable batteries for nighttime operations. It is operated by a 32-bit, high-performance central computer with about a gigabit of memory; the computer communicates with the Earth via a steerable, high-gain antenna that is capable of transmitting many kilobits of data per second. The lander carries a multispectral stereoscopic imager (Imager for Mars Pathfinder, IMP) on an extendable mast and an atmospheric structure instrument/meteorology package (ASI/MET), as well as a free-ranging rover, with forward stereo and rear color cameras, an alpha proton X ray spectrometer (APXS), and sensors for a number of technology experiments.

The rover on Mars Pathfinder is a small, six-wheel drive "rocker bogie" design vehicle that is 65 cm long, 48 cm wide, and 30 cm high. The rocker bogie chassis provides a stable platform for mounting instruments and is remarkably mobile; it can climb high obstacles and turn in place. The vehicle is solar-powered, but there is also a battery back-up that allows APXS measurements at night. The vehicle communicates with the lander via a UHF antenna link and will operate almost entirely within view of the lander cameras, or within a few tens of meters of the lander. Extended mission traverses up to hundreds of meters from the lander (limited by the UHF link) are possible.

The APXS is mounted on a deployment device at the rear of the vehicle that will allow the sensor to be placed against both rocks and soil at a wide range of orientations from horizontal on the ground to vertical rock faces at rover height. The forward stereo and rear-facing color cameras will image the surroundings and the APXS measurement sites at slightly better than 1 mm per pixel resolution. The rover control system includes a variety of autonomous hazard detection systems (such as forward laser light stripers for detecting obstacles or crevasses) that protect the vehicle from potentially hazardous situations. The rover will also perform a number of technology experiments for example, rover and sensor navigation and performance, soil mechanics, and material adherence and abrasion to provide information that will improve the design and operation of future planetary rovers.

The surface imaging system will study the geologic processes and surface-atmosphere interactions at a scale currently known only at the two Viking landers sites. It will observe the general physiography, surface slopes, and rock distribution to impart a better understanding of the geological processes that created and modified the surface. The alpha proton X ray spectrometer will measure the elemental composition of surface materials, and together with the spectral filters on the imaging system and rover close up imaging will be used to infer the petrology and the mineralogy of rocks and surface materials, which can be used to address questions about the origin of crustal materials and the weathering process on Mars.

By imaging rover and lander wheel tracks, holes dug by rover wheels, and any disruptions caused by airbag retraction, researchers will learn about the near-surface stratigraphy as well as Martian soil mechanics and properties. To distinguish the magnetic component of the martian dust, researchers will image, over time, the dust deposited on a series of small magnets on the lander.

The atmospheric structure instrument will determine the pressure, temperature, and density of the atmosphere during entry and descent. Regular pressure and temperature measurements will reveal diurnal variations of the atmospheric boundary layer near the surface. Wind speed and direction versus height in the boundary layer will be determined by a wind sensor on top of a meter-high mast and three wind socks; these data will also allow calculation of aerodynamic roughness of the surface, which is important for understanding the forces acting on small particles and their entrainment in the wind. In addition, the imager will determine the characteristics and distribution of aerosols and atmospheric water vapor abundance from sky and solar spectral observations. Tracking of the lander over time will determine the location of the lander, the orientation of the pole of rotation, the precession rate, and the moment of inertia of Mars, which will help constrain the size and density of any central metallic core.

Mars Pathfinder will land within a 70 km x 200 km ellipse in Ares Vallis, Chryse Planitia (19.5 N, 32.8 W). This site lies just downstream from the mouth of the Ares and Tiu Valles catastrophic outflow channels, which drained from the highlands to the south. This site was selected after considering engineering constraints, site safety, science potential, and similarities to the Ephrata Fan and Channeled Scabland in eastern Washington, which formed from the catastrophic release of glacial Lake Missoula (see Eos, Jan. 9, 1996). This "grab bag" site offers the prospect of sampling a diversity of rock types that make up Mars' ancient, heavily cratered terrain, the ridged plains, and reworked channel materials. This will allow us to address first-order scientific questions about the primary differentiation and early evolution of the crust, the development of weathering products, and the early environments and conditions of Mars. Even though the exact provenance of the samples will not be known, data from subsequent orbital remote sensing missions will then be used to infer the provenance for the samples studied by Pathfinder.

The landing site was selected over a 3-year period, before the evidence for life in the meteorite Alan Hills 84001 was announced. In retrospect, however, the landing site may well provide new information relevant to this topic. The outflow channels drain a large area dominated by ancient, heavily cratered terrain that is likely older than 3.5 Ga; this is the same age as Alan Hills 84001 and the putative evidence for life. Examining a variety of ancient rocks at Ares Valles could yield important information about the nature of the ancient environment on Mars and whether liquid water was present at that time. By roving for these rocks, Pathfinder could radically change our view of Mars and its evolution. More information is available about the Pathfinder mission on the World Wide Web (http://mpfwww.jpl.nasa.gov/).

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