Artist's concept of lander and microrover on Mars
The Mars Pathfinder Mission is the second launch in the Discovery Program, a NASA initiative for small planetary missions with a maximum three year development cycle and a cost cap of $150M (FY92) for development. The Mars Pathfinder Project is managed for NASA by the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California.
The mission was conceived primarily as an engineering demonstration of key technologies and concepts for future landed science missions: during its rapid development, science has been greatly enhanced within the original cost constraint. The instruments delivered to the surface of Mars investigate the structure of the Martian atmosphere and surface meteorology, surface geology, form, and structure, and the elemental composition of Martian rocks and soil. A free-ranging surface rover is deployed to conduct technology experiments and to serve as an instrument deployment mechanism.
This description was developed from the NASA "Proposal Information Package", consisting of Project and Investigator documents and links, updated during and subesquent to the 10th Project Science Group meeting, JPL, Feb 5-7, 1997, with the assistance of Project Staff and Science team members. The project home page can be found at the NASA JPL Mars Pathfinder site.
The flight system was launched on a Delta II-7925 launch vehicle which includes a Payload Assist Module (PAM)-D upper stage, from the Cape Canaveral Air Force Station, December 4, 1996.
After launch, the spacecraft requires 7 months to reach Mars. During this phase, a series of four Trajectory Correction Maneuvers (TCMs) are performed, in order fine tune the flight path. Tracking, telemetry, and command operations with the spacecraft are conducted using the giant dish antennas of the NASA/JPL Deep Space Network (DSN). Upon arrival at Mars on July 4, 1997, the cruise stage is jettisoned and the spacecraft properly orients itself for atmospheric entry (entry angle is 14.2° from horizontal) at a velocity of 7.65 km/s. The spacecraft then enters the Martian atmosphere, and deploys the parachute, rocket braking system, and air bag system for a soft, upright landing.
Entry, descent and landing
The lander enters the atmosphere behind the aeroshell which slows the vehicle (peak deceleration at about 20 g's at 30 km altitude). The aeroshell is jettisoned just prior to parachute deployment (at mach 1.8 at about 6-10 km altitude). During descent on the parachute at a terminal velocity about 65 m/s, the lander is lowered beneath the back cover on a 20 m long bridle. Signals from a radar altimeter, mounted in one of three triangular openings at the base of the lander, are used to initiate the firing of three small solid tractor rockets at about 50 m above the surface, reducing the vertical component of velocity to near zero. Large six-lobed airbags inflate around each face of the lander and the bridle is cut prior to final rocket burn, insuring that the parachute and back cover are carried away. The airbags, which provide about 0.5 meters of energy absorbing stroke, are not vented during landing. Peak decelerations of 40 g's will be experienced as the lander makes several bounces before coming to rest.
Accelerometers signal that the lander has come to rest, at which time interior, filtered, peal-away patches vent the airbags, with minimal surface contamination. The lander accelerometers determine which petal is down and automatically initiate the lander deployment sequence in which the upper airbags retract (internal tendons pull the airbags close to the petals) and the petals open in the proper sequence to assure an upright configuration. In addition, all engineering and science data obtained during the entry, descent, and landing phase are recorded for playback at the initiation of lander surface operations. The duration of the entry phase is approximately 5 minutes. Landing occurs in darkness, very early in the morning at about 3 am local Mars time, just after Earth rise. Opening of the lander takes a few hours before the spacecraft can be commanded from the earth (about 7 am local Mars time).
The events to be accomplished on the landing sol are to achieve an upright landed configuration, to return the recorded entry science and engineering data, to establish a high rate telecommunications link, to acquire and return a panoramic image of part of the surrounding terrain, to deploy the meteorology mast, and to drive the rover off its petal. Constraints on the operation of the lander during this period include limited power (the batteries have been partially depleted by the entry descent and landing operations), limited telemetry capability, and the need for flight controllers on Earth to verify key events before proceeding with subsequent operations. Surface operations for the remainder of the mission are focused on extensive use of the rover and science instruments. The rover will explore a region within ten or a few tens of meters from the lander during the first week of the rover mission After that time the rover may be commanded to acquire APXS spectra from magnetic targets on the rover ramps and to explore regions of the surface up to 500 meters from the lander (the approximate limit of its communication range).
Mars Pathfinder will land in Ares Vallis, Chryse Planitia (19.5°N, 32.8°W), within a 100 km by 200 km error ellipse determined by navigational uncertainties during cruise and variable atmospheric entry conditions. This site lies near the mouth of the catastrophic outflow channels Ares Vallis (and Tiu Vallis), which drained from the highlands to the south. Selection of this site was made after an exhaustive search and selection process involving engineering analyses of site safety and an assessment of the science potential of the several candidate sites. Landing at this site may permit sampling a diversity of rock types that make up the ancient cratered terrain on Mars (some of which may reflect primary differentiation and early evolution of the crust), the ridged plains and a variety of reworked channel materials (reflecting later crustal evolution and the development of weathering products). Even though the exact provenance of the samples would not be known, data from subsequent orbital remote sensing missions could then be used to infer the provenance for the "ground truth samples studied by Pathfinder.
The lander (250 kg mass) is a tetrahedron shaped structure containing the science instruments, rover, and all electronic and mechanical devices required to operate on the surface of Mars (Figure 4). The tetrahedron consists of three, similarly-shaped, 1 m triangular panels attached to a central base panel. The meteorology mast is the very thin, white line slightly to the left of the right panel where the technician is working. All lander equipment except the solar arrays, rover and meteorology mast are attached to a single center panel. The other three panels are attached to the edges of the center panel by actuators that are used to right the lander after touchdown. This active self-righting mechanism is needed because of the passive nature of the Pathfinder deceleration subsystems. All thermally sensitive electronics are contained in an insulated enclosure on the center panel. Specific hardware components inside this enclosure include a high performance central computer that controls the spacecraft during cruise, entry, descent, landing and surface operations (a 32 bit, radiation-hardened work station-RAD 6000 with roughly a gigabit of memory, programmable in C with a VxWorks operating system), a Cassini heritage transponder, a solid state power amplifier for telecommunications, and a high capacity rechargeable battery (40 amp-hr). Hardware outside the thermal enclosure includes a steerable high gain X-band antenna capable of approximately 5.5 kilobits per second into a 70 m Deep Space Network antenna and solar arrays (3.3m2 gallium arsenide array generating 1100 W-hr/day) capable of providing enough power to transmit for 2-4 hours per sol and maintain 128 megabytes of dynamic memory through the night. The lander is designed to survive for a minimum of 30 sols, with an extended mission lifetime of up to a year.
A comprehensive description of the Rover program is provided by the above link.
The Microrover "Sojourner"
The Mars Pathfinder rover (named Sojourner) is a six wheel drive rocker bogie design vehicle, which is 65 cm long by 48 cm wide by 30 cm high (Figure 5). Total mass of the rover is about 10.5 kg (including payload), lander-mounted support equipment is an additional 5.5 kg. The rocker bogie chassis has demonstrated remarkable mobility, including the ability to climb obstacles that are a full wheel diameter in height and the capability of turning in place. The vehicle communicates through the lander via a UHF antenna link and is intended to operate entirely within view of the lander cameras, or within a few tens of meters of the lander (extended mission traverses up to 500 m from the lander are possible, limited by the UHF link). It is a solar powered vehicle, generating 16 W peak power, with a (non-rechargeable) primary battery back-up (~300 W-hr). The rover moves at 0.4 m/min, and carries 1.5 kg of payload. The payload consists of the APXS, its deployment mechanism and the material adherence and wheel abrasion experiments. In addition, the rover has monochrome stereo forward cameras for hazard detection and terrain imaging, and a single rear color camera.
The alpha proton x-ray spectrometer (APXS) is mounted on a
deployment device on the rear of the vehicle (Figure 5) that enables placing the
APXS sensor head up against both rocks and soil in a continuum of
orientations (from horizontal on the ground to vertical rock faces at
rover height). The rear facing camera will image 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 for safing the vehicle in potentially hazardous situations
(forward laser light stripers for detecting obstacles or crevasses,
potentiometers for detecting bogie tilts).
The rover will also perform a number of technology experiments designed to provide information that will improve future planetary rovers. These experiments include: terrain geometry reconstruction from lander/rover imagery; basic soil mechanics by imaging wheel tracks and wheel sinkage; dead reckoning, path reconstruction and vision sensor performance; vehicle performance; rover thermal characterization; UHF link effectiveness; material abrasion by sensing abrasion of different thicknesses of paint on a rover wheel; and material adherence by measuring dust accumulation on a reference solar cell with a removable cover and by directly measuring the mass of the accumulated dust on a quartz crystal microbalance.
This Mars Pathfinder Instrument description serves as the primary project document. The scientific goals of the Imager for Mars Pathfinder were developed as an integral part of the instrument and those of the Alpha-Proton-X-Ray Spectrometer were developed in conjunction with the Rover program. As there was no Atmospheric Structures/Meteorology proposal, its goals were developed by the ASI/MET Science Advisory Team and the project. Information from this and associated documents are provided below.
IMAGER FOR MARS PATHFINDER
IMP Camera System.
Goals of the Investigation
These scientific goals and planned investigations have been abstracted from other documents as provided below. The major science objectives of the IMP experiment include. (1) mapping the morphology and terrain of the landing site. (2) Determination of the mineralogy of the exposed crustal rocks and soil as well as the mineralogy of the weathering products in the soil, dust, and on the surface of the rocks. (3) Observation of time-variable phenomena at the landing site including cloud formation, wind velocity and direction, frost, and the formation/evolution of eolian features. (4) Observation of the properties of the martian atmosphere including measurement of the quantity of atmospheric water vapor and the quantity and size distribution of atmospheric dust. (5) Study the magnetic properties of the martian dust by multispectral imaging of dust accumulations on the magnetic arrays. (6) Multispectral studies of the nighttime martian atmosphere by imaging bright stars and the multispectral study of the mineralogy of the martian moons Phobos and Deimos.
Investigations Planned by the IMP Team
Geomorphology, Photoclinometry, and Topography:: Panoramas of the landing site will be taken both before and after the camera mast deployment. These images will map the landing site for rover operations, as well as study the large-and small-scale structure of the landing site, rock and dune features, and any erosional features. Stereo ranging will determine the topography of the landing site and support rover operations. Images of the same areas taken at different solar elevation angels will permit topographic analysis by shadow length and photoclinometry. Additional images will be taken to study the nature of the martian soil. This will include imaging the rover wheel tracks to determine soil strength and compaction properties. Observations of the calibration targets and the lander surfaces will measure the rate of dust outfall.
Geology and Mineralogy: Filter-wheel spectral mapping using 12 filter wavelengths spanning 0.45 to 1.0 mm will determine the compositional variation of the landing site and identify mineralogical units as targets for further investigation using the rover-based Alpha/Proton/X-ray Spectrometer. Spectral mapping will also study weathering processes and products in the dust, soil, and rock. Of particular interest is the possibility of a "grab-bag" of mineralogies at the Ares Vallis landing site. This site was chosen in a catastrophic outwash area to explore the possibility that a range of mineralogies, including ancient martian highlands materials, were deposited in this area by the flood event(s). Nighttime multispectral observations of Phobos and Deimos can enhance the limited spectral data available on these small moons. The IMP should be able to detect bright (0 visual magnitude) stars and use these objects as standards for removal of the atmospheric signature from the Phobos and Deimos observations. These observation can also be used for the nighttime study of the martian atmosphere.
Magnetic Properties of Martin Soil and Dust: The scientific goal of the IMP Magnetic Properties Experiment, comprehensively described here, is to identify the magnetic minerals in the martian soil and airborne dust. Imaging the distribution of magnetic material on the different strength magnets will be diagnostic of material's mineralogy (Figure 14). Spectral images of the accumulated dust on these magnets should provide diagnostic mineralogical identification of these magnetic species. The magnets built into the rover deployment ramps will put magnets in close proximity to the soil and allow the rover to measure the accumulated magnetic dust with the Alpha Proton X-ray Spectrometer. The magnet mounted on the camera tip plate is less than 10 cm from the camera windows and the IMP will use a diopter lens to allow close-up viewing of this magnet. This magnet will collect magnetic dust settling from the atmosphere and its proximity to the camera will provide 200 µm per pixel resolution of the dust grains. The dust grains are probably much smaller than 200 µm, but the camera will be able to resolve chains that grains may form in the magnetic force lines of the tip plate magnet. The morphology of these chains can be diagnostic of the mineralogy of the magnetic species.
Atmospheric Water Vapor and Dust: Atmospheric water vapor will be measured by ratioing solar images through two narrow band filters, one at the 0.935 µm water band and one on the continuum at 0.925 µm. Additional solar images using ratios of narrow band filters at 0.45 and 0.925 µm will provide data on the size distribution and quantity of atmospheric dust. These measurements will be made at a number of airmasses from zenith to the horizon to provide an hourly history of water vapor, dust loading, and particle size variability for each sol of the Pathfinder mission. The 12 different geology filter wavelengths can be used to take multispectral sky images for several applications. Sky brightness measured at various angles from the Sun at multiple wavelengths can determine atmospheric particle size and shape. As the sun sets, illumination is restricted to progressively higher zones in the atmosphere and a series of multispectral sky brightness measurements can give us a picture of the vertical structure of the aerosols in the martian atmosphere. Finally multispectral measurements of Phobos and its aureole can be used to detect nighttime condensation and early morning fogs.
Wind Speed, Direction, and Gradient: The IMP Wind Sock Experiment will measure the direction, speed, and the boundary-layer velocity profile of the local martian wind by imaging the deflections and azimuth of the socks. These measurements will allow us to characterize the eolian processes at the Pathfinder landing site including particle threshold and the aerodynamic surface roughness. The socks will be imaged by the IMP every daylight hour to provide a continuous record of wind parameters. The windsock images will be sub-framed, processed on-board, and compressed to minimize the downlink data requirements. The deflections of the socks under wind velocities have been measured to provide quantitative data about the wind speed ( Figure 16). The azimuth that the sock "points" indicates the direction of the wind.
The objective of the Alpha-Proton-X-ray Spectrometer (APXS) for the
Mars Pathfinder mission is to provide a complete and detailed chemical
elemental analysis of martian soil and rocks near the landing
site. The APXS technique is well established, and can measure all
major and minor elements except hydrogen. Because all other major
elements are determined to high accuracy, even H can often be
estimated from stoichiometry. We presently have only coarse soil
chemistry of Mars at the two Viking landing sites. Viking, however,
had no means of determining C, N, O, and Na which are vital to
understanding the history and the evolution of the planet, so the APXS
experiment will fill important gaps in our knowledge of Mars.
The primary focus of the APXS investigation is new measurements of the chemistry of martian rocks. The APXS instrument is carried aboard the Pathfinder microrover, which will provide transportation to places of interest on the surface. The possibility to transport the APXS to an arbitrary location, pre-selected on Earth, and to perform in-situ analysis at it, constitutes one of the most exciting scientific aspects of the Pathfinder mission. Chemical analyses of several rocks at the landing site will therefore shed light on a variety of important processes that have operated on Mars.
The principle of the APXS technique is based on three interaction of alpha particles from a radioisotope source with matter: (a) simple Rutherford backscattering, (b) production of protons from reactions with the nucleus of light elements, and (c) generation of characteristic X-rays upon recombination of atomic shell vacancies created by alpha bombardment.
Measurement of the intensities and energy distributions of these three components yields information on the elemental chemical composition of the sample. In terms of sensitivity and selectivity, data are partly redundant and partly complementary: Alpha backscattering is superior for light elements (C, O), while proton emission is mainly sensitive to Na, Mg, Al, Si, S, and X-ray emission is more sensitive to heavier elements (Na to Fe and beyond). A combination of all three measurements enables determination of all elements (with the exception of H) present at concentration levels above typically a fraction of one percent.
APXS Data and Analyses
Martian Surface Soils and Rock Measurements: The APXS will analyze at multiple rock and soil samples in the vicinity of the landing site. The strategy is to obtain the first soil analysis right after the landing on the first night on the surface. On the second day, after analyzing the images of the landing site, an appropriate rock sample will be chosen and the rover will be commanded to place the APXS to analyze it. Most of the time the APXS will be operating during the night time, when there is no other activity going on. Later in the mission, daylight measurements will be also performed to see if there are any condensation processes happening during the night time. Under nominal rover conditions, it may be possible to obtain a new APXS spectrum every night during the first week of operation.
Magnetic Target Measurements: During operations on the surface of Mars, the APXS will analyze many samples of martian soil and rocks. It is, however, also anticipated that the APXS will analyze one of the magnetic targets mounted on the rover ramps to provide information about the magnetic properties of the martian surface material. By analyzing the collected material with the APXS it will be possible to determine if there is any preferential separation of the collected material according to the magnetic properties of the material. The spectra of the magnetic targets obtained will be compared with the spectra of the soil as a whole ( and with rocks). From these measurements, it is expected to determine some of the iron mineralogy of the martian surface material.
Atmospheric Structure Instrument/Meterology Package
Science and Measurement Objectives.
The Atmospheric Structure Investigation/Meteorology (ASI/MET) experiment is designed to obtain measurements of the martian atmosphere during the entry, descent, and landed phases of the Mars Pathfinder mission. It has been developed at JPL under the supervision of a Science Advisory Team (SAT) with considerable experience in in-situ atmospheric measurements and martian meteorology.
The overall scientific goals and measurement objectives of the investigation during the Pathfinder spacecraft Entry, Descent, and Landing (EDL) are to derive a vertical density, pressure and temperature structure profile for the upper atmosphere from measurements of entry vehicle acceleration in three orthogonal axis during the entry phase of the mission; derive a vertical density, pressure and temperature structure profile for the lower atmosphere from measurements of atmospheric pressure and temperature during the descent phase of the mission; and to characterize boundary layer meteorology and accumulate a climatological atmospheric data set at the Pathfinder landing site using measurements of pressure, temperature and wind, during the landed phase of the mission.
The ASI/MET experiment is performed by accelerometer and meteorology (MET) instruments using hardware distributed throughout the Pathfinder lander (Figure 6). The accelerometer instrument consists of three science and three engineering accelerometers, with supporting electronics, mounted on two boards in the Pathfinder Integrated Electronics Module (IEM). The MET instrument consists of pressure, temperature and wind sensors and a single IEM electronics board. The pressure sensor is mounted outside the IEM, but within the lander thermal enclosure and is connected to an aperture in the lander vehicle via a pitot tube. All the temperature sensors and the wind sensor are mounted on a meteorological mast 1 cm in diameter and 1.1 m high, which is deployed after landing. Temperature is measured by thin wire thermocouples, (Figure 24), mounted on a meteorological mast that is deployed after landing. (Figure 25) shows the top of the mast which includes a thermocouple is positioned to measure atmospheric temperature during descent, the wind sensor, and the top of three more thermocouples to monitor atmospheric temperatures 25, 50, and 100 cm above the surface during the landed mission. The mast base is located on the end of a lander petal to minimize the thermal contamination of temperature and wind measurements by the spacecraft. (In addition to pressure, temperature, and wind sensor electronics, the MET board also carries the signal processing electronics for the temperature sensors of the Aeroshell Instrumentation Package (AIP).)
The Deep Space Network (DSN), by using two-way X-Band and doppler tracking of the Mars Pathfinder lander will be able to address a variety of orbital and rotational dynamics questions. Ranging involves sending a ranging code to the lander on mars and measuring the time required for the lander to echo the code back to the Earth-based station. dividing this time by the speed of light results in an accurate measurement (within 1-5 meters) of the distance from the station to the spacecraft. As the lander moves relative to the tracking station, the velocity between the spacecraft and Earth causes a shift in frequency (doppler shift). Measuring this frequency shift provides an accurate measurement of the distance from the station to the lander. Within a few months of observing these features, the Mars Pathfinder lander location can be determined within a few meters. Once the exact location of Pathfinder has been identified, the orientation and precession rate (regular motion of the pole with respect to the ecliptic) of the pole can be calculated and compared to measurements made with the Viking landers 20 years ago. Measurement of the precession rate allows direct calculation for the moment of inertia, which is in turn controlled by the density of the martian rock with depth. Measurements similar to these are used on earth to determine the makeup of the earth's interior.
NASA Lewis Research Center has been involved in the Mars Pathfinder Mission nearly from the beginning. The Mars Solar Energy Model (G. Landis and J. Applebaum) provided an early demonstration that sufficient solar energy is available at Mars to provide operating power for a spacecraft and lander. This solar energy model was incorporated into the computer model used by JPL to design solar arrays for the Pathfinder lander, and the Sojourner Rover. Since then, Lewis scientists have designed, built, and delivered hardware for three sensors incorporated onto the rover. Also delivered were several small tungsten points for removing electrostatic charge accumulated during rover surface operations.
The Material Adherence Experiment (MAE), will quantify how much dust settles out of the martian atmosphere. The Viking landers showed that the atmosphere of Mars contains a large amount of suspended dust. The MAE consists of two sensors. The first sensor is a solar cell (G. Landis and P. Jenkins) which will measure how much light is obscured from the cell by dust that settles on it. Results will show how opaque Mars dust is, and how rapidly it settles out of the martian atmosphere. The second sensor is a quartz crystal monitor (G. Hunter and L. Oberle) which will use a vibrating quartz crystal. An adhesive surface on the crystal accumulates the dust, and resultant changes in crystal frequency indicate its mass. Together, the two sets of measurements will provide excellent information on dust properties and deposition rates.
The Wheel Abrasion Experiment (WAE) will assess wheel wear (D. Ferguson and J. Kolecki). WAE uses atomically thin metal films deposited on black anodized aluminum strips attached to a rover wheel. A photocell monitors changes in film reflectivity as the rover moves and the surfaces wear. Twice each martian day, all the other rover wheels will be locked stationary while the test wheel alone is spun and allowed to dig into the martian surface. Marked abrasion will indicate a surface composed of hard, possibly sharply edged grains. Lack of abrasion would suggest a somewhat softer surface. WAE results will be correlated with ground simulations to determine which terrestrial materials behave most like Mars dust. This knowledge will enable a deeper understanding of erosion processes on Mars and the role they play in martian surface evolution. All of the above results will be significant to future Mars designs.
Rover electrostatic charging will be controlled by fine, tungsten points mounted on the rover antenna base (J. Kolecki and M. Siebert). Tests and calculations have confirmed the possiblity that the rover will accumulate a large static charge during its surface operations. (the charge is though to occur when the dry martian dust is compacted by the rover wheels.) Once this charge is accumulated, disruptive electrical discharges on our around the rover become possible. Since actual martian conditions are unknown, discharge points have been added to the rover as a precaution. If the rover accumulates electric charge, some, or all of it will be removed to the atmosphere through the discharge points. Atmospheric dust, blowing by the rover, will collect this charge, and eventually return it to the martian surface. Discharge currents through the points are predicted to be non-disruptive.
NASA Lewis Research Center overview of their contributions to the Pathfinder mission.
J E Tillman
University of Washington