In January 2004 the Mars Exploration Rover mission will land two rovers at two different landing sites that show possible evidence for past liquid-water activity. The spacecraft design is based on the Mars Pathfinder configuration for cruise and entry, descent, and landing. Each of the identical rovers is equipped with a science payload of two remote-sensing instruments that will view the surrounding terrain from the top of a mast, a robotic arm that can place three instruments and a rock abrasion tool on selected rock and soil samples, and several onboard magnets and calibration targets. Engineering sensors and components useful for science investigations include stereo navigation cameras, stereo hazard cameras in front and rear, wheel motors, wheel motor current and voltage, the wheels themselves for digging, gyros, accelerometers, and reference solar cell readings. Mission operations will allow commanding of the rover each Martian day, or sol, on the basis of the previous sol's data. Over a 90-sol mission lifetime, the rovers are expected to drive hundreds of meters while carrying out field geology investigations, exploration, and atmospheric characterization. The data products will be delivered to the Planetary Data System as integrated batch archives.
 Seven years after the Mars Pathfinder mission, the two Mars Exploration Rovers will provide a significant advance in our robotic and science instrument capability on Mars. The Mars Exploration Rover (MER) mission will also benefit from a much better understanding of Mars and a regional geologic context of the landing sites, resulting from the analysis of data from Mars Global Surveyor and Mars Odyssey. Each rover will be capable of driving at least 600 m during its expected 90 sol minimum lifetime and will acquire visible and infrared multispectral panoramas from several separate locations in the vicinity of each landing site. A “sol” is one Martian day, which is approximately 24 hours and 40 minutes long. These panoramas will allow the science team to characterize and map the diversity of rock and soil types at each landing site and to select representative and scientifically promising samples for close-up examination with the instruments on the rover's robotic arms. The MERs will provide more advanced “ground truth” than what was obtained at the Pathfinder landing site, for two new locations that can be tied to existing and future orbital remote-sensing data sets.
 The MER mission has a set of science and technology objectives. The science is closely aligned with the Mars Exploration Program objective of determining the degree to which Mars provided conditions necessary for formation and preservation of prebiotic compounds and whether life started and evolved. This objective can be broadly stated as defining habitability of Mars and providing an understanding of roles of tectonic and climatic processes in possibly providing the conditions that led to life. The presence of water and its interaction with crustal materials is of fundamental importance. Thus three of the MER objectives focus on exploring for evidence of water in the past: (1) to investigate landing sites which have a high probability of containing evidence of the action of liquid water, (2) to search for and characterize a diversity of rocks and soils that hold clues to past water activity, and (3) to extract clues related to the environmental conditions when liquid water was present and assess whether those environments were conducive for life. The selection of a favorable landing site is requisite to meeting these particular science objectives. Golombek et al.  describe the candidate landing sites and the selection process. The clues suggesting possible past water activity at MER candidate landing sites rely on mineralogic or geomorphologic evidence, and the rovers can be used to test the various water-process hypotheses.
 The other MER science objectives are related to the Mars Exploration Program objective of determining the nature and sequence of the various geologic processes that have created and modified the Martian crust and surface: (4) to determine the spatial distribution and composition of minerals, rocks and soils surrounding the landing sites, (5) to determine the nature of local surface geologic processes from surface morphology and chemistry, (6) to calibrate and validate orbital remote-sensing data and assess the amount and scale of heterogeneity at each landing site, (7) for iron-containing minerals, to identify and quantify relative amounts of specific mineral types that contain H20 or OH, or are indicators of formation by an aqueous process, and (8) to characterize the mineral assemblages and textures of different types of rocks and soils and put them in geologic context. These are basic field geology objectives that can be carried out at any landing site, but will provide the basis for addressing the first three objectives related to past water and thus habitability.
 Three additional objectives for MER are technology related: (9) to demonstrate long-range traverse capabilities by mobile science platforms to validate long-lived, long-distance rover technologies, (10) to demonstrate complex science operations through the simultaneous use of multiple science-focused mobile laboratories, and (11) to validate the standards, protocols and capabilities of NASA-provided and internationally-provided orbiter-based Mars communications infrastructure. These objectives will provide experience, lessons-learned, and technology feed-forward that will enable improved Mars science missions in the future. While not part of the formal mission objectives, the rovers' remote-sensing instruments can also look skyward, allowing them to make scientific observations of the Martian atmosphere [Squyres et al., 2003].
3. Mission and Spacecraft Description
 Between late May and mid-July of 2003, two Mars Exploration Rover spacecraft will be separately launched to Mars, on trajectories shown in Figure 1. The spacecraft design is based on the Mars Pathfinder configuration for cruise and entry, descent, and landing. They will arrive at Mars on 4 January and 25 January 2004, will enter the Martian atmosphere directly from their interplanetary trajectories, and will land on the surface at two distinct landing sites. Each will then shed its landing shell, and reconfigure itself into a roving science laboratory with the ability to explore the landing sites' geology using a suite of remote and in situ instrumentation and tools. The vehicles are designed to operate for more than three months each on the surface of Mars through April of 2004. Depending on the Martian environment and on the survival of the systems, their missions may extend into the summer of 2004.
 The two missions, Spirit (MER-A) and Opportunity (MER-B), were launched separately on Boeing Delta II launch vehicles, both from the Cape Canaveral Air Force Station in Florida. Spirit launched on a Delta II 7925 on 10 June 2003. Opportunity launched on a Delta II 7925H (Heavy) on 7 July 2003. At launch, the spacecraft consisted of an atmospheric entry vehicle attached to a cruise stage with a launch mass of 1077 kg, including propellant. Spirit launched to an injection specific energy, or C3 of 9.25 km2/s2. The later MER-B launch required the heavy version of the Delta II due to its more demanding C3 of 10.35 to 18.0 km2/s2. The launch vehicles utilized a spinning solid upper stage for interplanetary injection. The spacecraft were separated with a residual spin rate of 12 RPM, which was reduced to 2 RPM one to two days after launch.
3.3. Cruise and Approach
 The cruise stage (Figure 2) provides solar power, propulsion, attitude sensors, communications antennas, and a heat rejection system for the interplanetary transit. Spirit and Opportunity each take approximately seven months to get from Earth to Mars. Spacecraft attitude is adjusted through this transit to maintain adequate power from the Sun and communications with the Earth. Additional solar array segments are switched on as the spacecraft move further from the Sun in order to provide the required power. Antenna configurations are changed to provide the required data rates as the spacecraft move further from the Earth.
 Each spacecraft's trajectory is corrected with a series of five to six translational propulsive maneuvers (timing shown in Figure 1). The first maneuver corrects both for errors in the launch vehicle injection and for a deliberate initial bias away from Mars for planetary protection. The remaining maneuvers adjust for improved knowledge of the trajectory through radiometric tracking and to correct errors in previous maneuvers as well as other tiny disturbances to the trajectories. The last 45 days before entry is the Approach phase. The two to three maneuvers in this phase, the last of which may be as late as three to six hours before entry, combined with intensive radio tracking during this phase provide for accurate targeting of the vehicles to their designated landing sites.
 NASA's Deep Space Network of 70-meter and 34-meter antennas and precision radio equipment will track the MER spacecraft and communicate with them. In addition to the standard Doppler and ranging radiometric-tracking techniques, a high-precision interferometric tracking technique using extragalactic quasar radio sources and widely separated antennas on Earth will be relied on for the approach navigation [Portock et al., 2002]. This technique has been successfully demonstrated with the Voyager 1 and 2, Ulysses, Galileo, Mars Observer, Magellan, Phobos, Vega 1 and 2, Viking, Pioneer Venus Orbiter, Mars Global Surveyor, Mars Odyssey, Nozomi, and Deep Space 1 missions.
 During the Cruise phase, various checkout, characterization, and maintenance activities will be performed, including the checkout of equipment to be used in later phases of the mission. Approximately three hours before entry there is a final update of the entry, descent, and landing software parameters based on the best trajectory solution following the final maneuver. This is the last uplink to the vehicle before entry. The spacecraft turns to the entry attitude about one hour before entry, and separates the cruise stage 15 minutes before entry. Through all of this, communications to Earth is maintained for diagnostic purposes.
3.4. Entry, Descent, and Landing (EDL) Through Egress
 After separation of the cruise stage, the 840 kg entry vehicle consists of a backshell and heatshield enclosing the lander. The primary components of the spacecraft are shown in Figure 3. The 550 kg lander has a tetrahedral structure that the air bags are deployed from and surround, and that houses the gas generators for the airbags, the RADAR altimeter, motors for the unfolding of the tetrahedron's sides or “petals”, and the rover lift mechanism for standing up the rover. The rover is stowed within the lander, and contains most of the power, computing, and communication electronics for all phases of the mission. The backshell and heatshield provide thermal protection from the hyperbolic entry into the Martian atmosphere through the use of ablating materials. Mounted inside the backshell is the parachute, the deceleration and transverse impulse solid rocket motors, an inertial measurement unit, and thermal batteries for the entry and descent phase.
 The Mars Pathfinder landing system used a set of deceleration stages and a lander surrounded by large airbags to protect the enclosed payload and avionics system from the first surface impact and the subsequent bouncing until eventually rolling to a stop. The airbag system has the benefit of being tolerant to common terrain features on Mars, such as rocks up to a meter in diameter. The Mars Exploration Rover mission will use essentially the same system, with some scaled-up components for its more massive lander, and with some landing reliability enhancements described later.
 Entry begins at a defined radius from the center of Mars, which is approximately 128 km above the surface. As shown in Figure 4, a rapid series of critical events and activities unfolds over the next six minutes culminating in the first impact with the surface. Due to the round-trip light time to Mars of approximately twenty minutes, no control from Earth is possible, so all of the events are autonomously controlled within the vehicle. Diagnostic information during this period is transmitted to Earth using an X-band signal change every ten seconds, with each change conveying up to eight bits, as well as the information that can be extracted from the Doppler shifts observed in the X-band carrier. For the final portion of the descent, diagnostic information is also transmitted to the Mars Global Surveyor (MGS) orbiter using a high-rate, eight kilobit per second UHF link. MGS is maneuvered to fly over each landing site during the entry, descent, and landing (EDL) events.
 The vehicle enters the Martian atmosphere with an atmosphere relative velocity of 5400 m/s. The subsequent stages of EDL are designed to reduce that velocity to zero in a controlled manner. Four minutes before impact, the entry vehicle has gone through peak heating and is at peak deceleration. Two minutes later and two minutes before impact, the heat shield has completed its job of slowing the vehicle to about 400 m/s. Given the uncertainty in the a priori atmospheric density and in the entry flight path angle, onboard accelerometers are used to determine the vehicle's deceleration through the atmosphere and to decide when to deploy the parachute. At the appropriate derived dynamic pressure, a supersonic parachute is deployed to decelerate the vehicle further, and the bottom portion of the heat shield is separated and drops away. The lander then descends on a bridle below the backshell. The parachute slows the vehicle to approximately 75 m/s over the final two minutes of descent.
 A RADAR altimeter locks onto the surface about 35 seconds before impact and provides the measurements required to decide when to initiate the following events, which depend on the altitude above the surface and the descent rate. Eight seconds before impact, airbags are inflated that completely surround the lander in order to protect it from the first and subsequent impacts. Two seconds after airbag inflation, three solid rocket motors mounted in the backshell are fired to further slow the vehicle to close to zero velocity relative to the surface at a target altitude of 15 meters above the surface. The bridle connecting the lander to the parachute and backshell with the still-firing rockets is severed to allow those articles to fly away from the lander. The lander wrapped in its protective airbags falls the last several meters to the first impact. Due to uncertainties in the measurements and in the thrust and direction of the rocket firings, the actual altitude and velocity at bridle cut will vary, resulting in impact velocities in the range of 10 to 20 m/s.
 A system new to MER and not present on Mars Pathfinder is a set of three outward-pointing transverse-impulse solid rockets in the backshell that are employed under some environmental conditions to control the backshell attitude during the main solid rocket firing. These smaller rockets are used to avoid or cancel a horizontal velocity at the first impact that would have been induced by winds. This system makes use of both the inertial sensors to determine the backshell attitude, and a descent imager to estimate the horizontal velocity relative to the surface. Golombek et al.  describes all of the EDL system constraints on landing site selection.
 After the first impact, the lander will bounce many times and finally roll to a stop as the energy of the first impact is dissipated and the airbags begin to deflate. This may take as long as a few minutes and could cover a few hundred meters of distance before the lander comes to rest. Throughout these events, the X-band transmission continues, and may be received by Earth so long as Earth is in view of the antennas. The airbags are retracted, releasing any remaining gas in the bags. The retraction is completed about one hour after landing. The lander petals are then opened exposing the folded rover inside, which takes about half an hour. The opening of the petals provides a righting mechanism in case one of the side petals is facing down. In that instance, the down petal is opened first which then forces the base of the tetrahedron to fall onto the ground. As the lander structure opens, it deploys three egress aids between each pair of petals, which allow for a smooth egress of the rover under a wide range of landed conditions and environments (see Figure 5). The landings occur between one and two o'clock in the afternoon, local solar time. Shortly after landing, three key critical deployments must occur to permit communications and to survive the night. First, the lander petals must open, exposing the rover. Second, the three solar panels must open, similar to the three petals of the lander. Lastly, two of those solar panels have one more unfolding to expose all of the solar cells to the sky. Assuming the critical deployments go as expected, the rover will proceed to deploy the panoramic camera mast to its upright position. The last of the deployments rotates the high-gain antenna out of its launch locks to provide it the freedom of motion needed to point to and track the Earth. At this point, the rover is in a power and thermally safe state, and will be able to communicate directly with Earth the following morning.
 Over the next few sols, and under control by the flight team on Earth, the rover will complete its deployment by standing up using the lift mechanism on the lander and simultaneously locking the rocker-bogie suspension in place, rotating the front wheels out and down, and lastly driving the rear wheels back to the final mobility configuration. The 180 kg rover will cut its own umbilical to the lander, and will then be able to drive off to begin its surface mission. The rover is completely independent of the now discarded shell that delivered it to the surface of Mars.
 The MER rover (Figure 6) is a 6-wheeled drive, 4-wheel-steered vehicle 180 kg in mass, including the science package. At its wheelbase, the rover is approximately 141 cm long and 122 cm wide. At the height of the solar panel, the rover is approximately 225 cm wide by 151 cm long. In its deployed configuration with the Pancam Mast Assembly (PMA) deployed, the rover is 154 cm tall. Figure 7 points out some of the main components of the rover.
 The rocker bogie configuration gives the rover the ability to drive over obstacles approximately one wheel diameter (26 cm) in size while providing a stable platform for instrument measurements. The distribution of mass of the vehicle allows the vehicle to be stable at nearly a 45° tilt. Each wheel and steering degree of freedom is independently actuated which allows the vehicle to turn in place (turning diameter 1.9 m), to skid steer to a tighter angle (turning diameter as small as 0.9 m) and to drag wheels that effectively trench the Martian regolith. When moving on flat terrain, the vehicle can achieve a top speed of 4 cm/s. Under autonomous control using its hazard avoidance system, the rover can achieve an average speed of about 1 cm/sec.
 Our ability to carry out the science mission is critically dependent on being able to get the rover to a selected in situ target in as few sols as possible. This requires good navigational accuracy. Details of the navigation error budget, including the capabilities of the hardware, onboard software, and ground operations software, are discussed by Maki et al. . For expected rock and terrain conditions, when the rover is within 2 meters of a target, it will be able to autonomously navigate within one sol to position itself so that the target is reachable by the instruments on the robotic arm. This is accomplished through the execution of ground commands developed from measurements taken within the stereo image sets provided by rover cameras. These commands, a series of turns and movements corrected using onboard sensors and estimation techniques, are designed to place the rover in a position with respect to the target for the instrument placement.
 The rover is powered by a combination of solar arrays and rechargeable batteries. The solar panel provides 30 strings of sixteen-to-eighteen 26-cm2 triple junction cells, which produce about 800 to 900 W hours at the beginning of the MER mission. Due to the change in seasons and expected degradation in performance due to dust deposition, this array should produce about 600 W h 90 sols after landing. Energy can be stored in two 8 A h Li-ion rechargeable batteries to provide over 400 W h of energy that can be used to support rover peak power operations and provide auxiliary heating and operations overnight.
 Temperature-sensitive electronics are housed in the rover warm electronics box (WEB) which is a box built with composite materials and insulated with 2.5 cm of opacified aerogel. A combination of radioisotope heater units, waste heat from electronics, and auxiliary heating by survival heaters ensures that the internal electronics are maintained between +50°C and −40°C as the external Mars environment cycle ranges from 0°C to −97°C. Survival heating in the WEB requires not more than 100 W h of energy during the coldest environment conditions. The rechargeable batteries housed in the WEB supply this energy.
 The rover receives commands and transmits data to the earth through two distinct systems: a direct-to-Earth X-band system supported by both a low-gain antenna and a steerable high-gain antenna, and a UHF system supported by a monopole antenna which enables relay communication to orbiters at Mars. Early in the surface mission, the X-band system through the high-gain antenna will support up to 28.8 kbps to a 70 m station, although the current demonstrated spacecraft flight computer capability is 11.8 kbps. Commands can be received through the high-gain antenna at a rate of up to 2000 bps. The X-band system through the low-gain antenna provides a minimum capability of transmitting telemetry at 40 bps and receiving commands at 40 bps throughout the MER missions (evaluated within about an hour of local noon). The UHF system can support telemetry rates of up to 128 kbps during orbiter passes which may last up to 8 minutes. The UHF system also supports a command receipt capability of 8 kbps (through Odyssey only).
 The computing, command, and data handling functions of the rover are supported by a 20 MHz 32-bit RAD6000 processor housed in a Versa Module Europa (VME) card cage. This processor has access to 128 Mbytes of DRAM and 256 Mbytes of nonvolatile flash memory that supports a multiprocess C-coded software architecture. This system, supported by auxiliary processing functions housed on boards within the VME card cage, can acquire images from pairs of 10 cameras, drive up to 10 motors simultaneously from 35 motors located on the vehicle, and process data from three spectrometers. The multiprocess architecture allows communication, image acquisition, and operation of payload elements to proceed simultaneously.
 During the mission the rover will communicate at least 2 hours per sol on X-band, generally reporting once each day on its status and the results of the execution of commands transmitted that day. Data can also be relayed through the UHF communication system to the Mars Global Surveyor or Mars Odyssey orbiters. Useful over-flights by these orbiters at the landing site can occur as frequently as twice per day per orbiter.
3.6. Science Payload
 Each rover carries the Athena Science Payload consisting of two remote-sensing instruments that look out on the terrain from the top of a mast 154 cm above the ground, four devices for in situ analysis on a turret on the end of a robotic arm and several magnets and calibration targets. Squyres et al.  describe this payload, the mast, the robotic arm, and the plans for science investigation in more detail. Azimuth and elevation actuators permit the collection of data sets for specific targets, regions, or full 360-degree panoramas from the mast instruments, which are the stereo multispectral Panoramic Camera (Pancam) and Miniature Thermal Emission Spectrometer (Mini-TES). The five degree-of-freedom robotic arm can position the following devices on rocks and soils for in situ analysis or rock abrasion: Alpha Particle X-ray Spectrometer (APXS), Mössbauer Spectrometer (MB), Microscopic Imager (MI), and Rock Abrasion Tool (RAT).
3.7. Engineering Sensors of Interest to Science
3.7.1. Engineering Cameras
 All of the cameras on the rover have identical electronics and produce 1024 × 1024 full-frame images. The stereo navigation cameras (Navcams) on the top of the mast and the stereo hazard detection cameras (Hazcams) pointed toward the ground beneath the solar panels in the front and rear of the rover are required for engineering purposes, but will also be used for science analysis [Maki et al., 2003]. Compared to the Pancam cameras that have a 16.8° field of view (FOV) and angular resolution of 0.28 mrad, the Navcam offers more quickly obtained and transmitted monochrome panoramas due to its cameras' 45° FOV and consequent lower angular resolution of 0.76 mrad averaged over the entire field of view. The Hazcams will provide monochrome close-up views of rock and soil textures and wheel-surface interaction, with a 120° FOV and 2 mrad resolution. In order to plan robotic arm movements and rover traverses on Mars, engineering images will be acquired for safety evaluation purposes. The engineering cameras have undergone rigorous geometric and radiometric calibration, so the images will be useful to the science team for their studies involving examination of surface texture and morphology.
3.7.2. Gyros, Accelerometers, Wheel Torque, and Other Sensors on the Rover
Arvidson et al.  provides more detail on the plans for using rover engineering sensors for assessing terrain and soil physical properties, dust accumulation, and other related investigations. All of the rover engineering data will be archived in the Planetary Data System along with Athena Payload data sets and documentation for access by the science community.
 During traverses and other types of rover motions, engineering data associated with wheel motor operation and positions of moving components and other related performance data are collected upon selection by command from Earth. In general, during each step of a rover traverse a minimum set of data collected by the onboard computing system includes rover position and direction achieved, the position of moving parts (bogies, differential, steering actuators), tilt vectors, and average current from each motor. Upon command, additional data can be reported at a frequency of 8 Hz. These data can include steering angles, motor encoder readings, current, voltage, joint speed, and controller status.
 Due to the autonomous hazard avoidance feature of certain commanded rover drives, the data from the onboard estimator of hazards and the onboard path planner can be reported at each step of the rover motion. Again, depending on command, the data can include the images from the hazard cameras at each step. Thus an engineering data and imaging record can be provided for use in reconstructing the drive and the terrain traversed by the rover.
 The solar panels can be used to measure atmospheric dust accumulation. Each rover has two reference solar cells, one that measures short circuit current and another that measures open circuit voltage. The reference cell measurements, along with the array current, bus voltage, and other power-related measurements, will be stored every 5 to 10 minutes in non-volatile memory in the battery controller board (one board in the VME card cage), and these data will be sent back to Earth each day. An assessment of the settling rate of atmospheric dust can be made similar to what was done for the Sojourner rover [Landis and Jenkins, 2000] and the Pathfinder lander solar panels [Crisp et al., 2003].
3.7.3. IMU Measurements During EDL for Atmospheric Temperature Profile
 Each MER spacecraft has two Litton LN-200S inertial measurement units (IMUs) on board, one in the rover body and one on the backshell. These two sensors provide some redundancy (in acceleration) and provide the engineers with an onboard estimation (in rate) of the lander-to-backshell relative orientation while the landers are swinging during entry. Each IMU contains a set of 3-axis accelerometers and 3-axis gyroscopes. The measurements from the IMUs will be returned for engineering purposes and will be archived in the Planetary Data System for access by the science community. As was done for Pathfinder [Magalhães et al., 1999] and Viking [Seiff and Kirk, 1977], the measurements from the MER accelerometers can be used to reconstruct the atmosphere encountered during passage through the upper atmosphere. Using the drag properties of the entry capsule and the resulting accelerations, a vertical profile of atmospheric density can be reconstructed. Through the use of the ideal gas law and hydrostatic equation, the density profile can be used to derive the atmospheric temperature profile at high resolution. This is currently the only method for in situ measurements of the bulk of the Martian atmosphere. Pathfinder and each Viking Lander provided similar entry profiles, but all three were in the northern hemisphere midlatitude. The MER landers will enter in the equatorial regions, 2 and 15 degrees south of the equator. Furthermore, they will sample a new season and local time. Beagle 2 will provide another profile from the midlatitude northern hemisphere, at this new season [Bridges et al., 2000].
 The IMUs have a dynamical range of 80 gn and a resolution of 2.4 mgn, with a noise of 1.6 mgn, but they produce measurements at 400 Hz, much faster than can be used by the spacecraft. Therefore the IMU data will be summed and effectively sampled at 8 Hz. This also reduces the effective noise to 300 μgn, and should produce an effective resolution of 50 μgn. This performance is significantly worse than the < 2 μgn noise and resolution produced by the Pathfinder accelerometers [Magalhães et al., 1999]. The MER accelerometers should first detect the atmosphere around 120 km. Even with the poorer performance, it should still be possible to use the accelerometer results to reconstruct an atmospheric profile between about 100 km and the deployment of the parachute which is expected to occur about 8.5 km above the surface. Given the expected spacecraft velocities and entry angles, the reconstructed profile in this altitude range should have a vertical resolution better than 250 m.
 Once the lander is descending on the parachute, it is expected to start swinging, with several of the modes being quite fast. This made it impossible to reconstruct the Pathfinder profile below the point of parachute deployment [Magalhães et al., 1999]. In addition to the accelerometers, each MER IMU has a set of gyroscopes that are also read out and stored. These data can be used to determine the orientation of the IMU, and to remove the effects of the swinging on the accelerometers, thus allowing the atmospheric reconstruction for MER to proceed below the parachute deployment altitude. Each lander uses RADAR to determine its altitude during the last few kilometers of descent. This will give an independent source of position and velocity, which will reduce the accumulated uncertainty in the reconstruction. The MER rovers do not have atmospheric pressure or temperature sensors to record the near-surface conditions after landing.]
3.8. Operational Constraints
3.8.1. Telecommunication Constraints
 During the MER missions, X-band telecommunication coverage will be governed by antenna assignment from the Deep Space Network (DSN). A request has been submitted for 70 m antenna coverage for 8 hours each day for each rover. Due to the number of missions with critical activities occurring during the first few months of 2004, less coverage than that requested is expected. While daily 70 m antenna coverage is planned, scenarios for evaluating mission return estimates have assumed that 70 m antenna coverage for 8 hours per day will be available on about 90% of the days of the MER missions. Eight percent of the mission days were assumed to have 34-m antenna coverage. The remaining 2% of the mission days were assumed to have no antenna coverage.
 During the MER surface missions, the Mars Global Surveyor and Mars Odyssey orbiters will be used to relay non-critical science and engineering data to Earth. Each sol, up to two overflights of the MER landing sites per orbiter result in a pass geometry suited for communication. An overflight is defined as suitable when an orbiter receiver and a MER transmitter are in contact for at least 2 minutes above 20° elevation. These overflights occur at times centered at about 0400 and 1600 Local True Solar Time (LTST) for Odyssey, and at about 0130 and 1330 LTST for Mars Global Surveyor.
 The average data return volume is estimated to be about 56 Mbits per sol per rover for Odyssey, and about 49 Mbits per sol per rover for Mars Global Surveyor. In agreement with the Mars Odyssey Project, as much as 30 MBytes of data can be transmitted from the MER rovers and stored onboard the Odyssey spacecraft for subsequent downlink to Earth. Latency in the collection and transmission of data via Odyssey may result in data receipt at earth up to five hours after transmission from the rover. Data relayed through Mars Global Surveyor can have a latency up to 2 days depending on transmission mode, data buffering on the spacecraft (up to 80 Mbits per overflight), and antenna coverage. As a result, only data that are not required for next-day planning use by the ground team are transmitted through these relay orbiters.
3.8.2. Thermal Constraints
 The rover thermal design ensures that sensitive electronics in the Warm Electronics Box (WEB) are maintained above −40°C survival temperatures during the Martian diurnal cycle. However, this design is vulnerable to overheating during daytime operation. Although the flight operations team will model commanded operations to ensure such overheating will not take place, the rover will also monitor temperatures inside the WEB and initiate a shutdown prior to overheating above +50 °C. Planning constraints associated with preventing overheating conditions include: avoiding more than 2 hours of continuous X-band communication especially in the warmer mid-day periods, and ensuring electronics in the WEB do not exceed an expenditure of about 650 W h in a given day.
 Devices external to the WEB are designed to survive the temperatures of a Mars diurnal cycle. However, most of these devices operate within specification above −55 °C and operations during portions of the night are not possible without prior warm-up to operational temperatures. Special heaters are provided for these components and preheating times vary depending on expected operation during the night.
3.8.3. Power Constraints
 The power system is designed so that on each sol of the 90 sol surface mission, solar illumination of the rover array is sufficient to allow the rover to wake up, conduct routine data sampling establishing system status, communicate for at least one hour at X-band, communicate at least twice at UHF, shut down, and survive the nighttime portion of the diurnal cycle to wakeup on the next day. This engineering “floor” requires not more than about 430 W h of the more than 600 W h available from the solar array energy production.
 These power constraints are relevant for an expected nominal environment associated with an atmospheric opacity at visible wavelengths (τvisible) of 0.5. Higher atmospheric dust loadings corresponding to τvisible greater than 0.5 lead to a reduction in solar illumination, which can further constrain the energy balance. Models of the system performance indicate that science operations can occur through conditions of τvisible less than 1.0 (a 13% reduction in energy production from τvisible = 0.5 conditions), and the rover system can survive with a τvisible less than 2.0 (a 30% reduction in energy production from τvisible = 0.5 conditions).
 Planning for an energy balance during the mission involves planning operations that meet the thermal constraints while ensuring that the battery achieves a sufficient state of charge by the end of daytime so that the system survives through the night. Not more than 200 W h of the 400 W h of the battery is required for nighttime engineering support (survival heating, UHF communication, etc.), so that significant nighttime science operations can occur each night through the end of the mission.
4. Mission Operations
4.1. Surface Mission Operations
 Under the control of the flight team on Earth, each rover will investigate its landing site for at least 90 sols. It will drive potentially hundreds of meters from the lander during its mission. The rover will perform remote-sensing science with its panoramic imager and thermal emission spectrometer to provide information on the geological context of the landing site and information on nearby rock and soil targets deserving closer inspection. The rover will then drive close enough to the chosen target to be able to reach it with a robotic arm that holds in situ instruments. The arm will carefully place those instruments on the target to obtain microscope images and Mössbauer, alpha, and X-ray spectra. An abrasion tool on the arm will abrade up to 5 mm deep into some of the rock targets to expose their interior to the in situ and remote-sensing instruments.
 A representative sol of rover activities is shown in Figure 8. Being solar powered, the rover will wake up in the morning around 0900 Local Solar Time (LST), may shut down for energy conservation and thermal control reasons around noon, and then go to sleep for the night around 1400 to 1500 LST to conserve energy. While asleep, the rover's computer, communications, and almost all other devices and instruments are powered off. What remains powered on is electronics for battery charging, switch position maintenance, excess energy shunting, alarm clock functions for computer wakeup, and thermostatic component thermal control. Independent of the alarm clock wakeup, there is a wakeup function controlled by power developed by illumination of the solar arrays. When sufficient power becomes available from the arrays to support the operations, the computer is awakened through an automatic power-up. Two of the instruments, the Mössbauer and Alpha Particle X-Ray Spectrometers, can be left on while the rover computer is off in order to permit long integration times while conserving rover energy. Those spectrometers each have their own memory in which to accumulate data for later transfer to the computer and subsequent downlink.
 The rover will also wake up for periods of several minutes at a time to relay data through the Odyssey and MGS orbiters as they fly overhead. Section 3.8.1 describes these communication links in more detail, and Figure 8 shows when these communication links occur in a typical sol. Other short nighttime wakeups may be planned to perform specialized observations, such as night-sky thermal measurements.
 A typical rover sol begins with a communications session directly with Earth through the rover's high-gain X-band antenna. During this session, the instructions for that sol through wakeup the following sol are transmitted from Earth. Contingency instructions for the subsequent sol are also transmitted, and the rover communicates back to Earth what happened overnight. The rover is then on its own to perform the requested activities. Before going to sleep, the rover will communicate directly to Earth again for one to two hours, this time with the primary objective of conveying how the requested activities went, the state of the rover, and the images and other data required back on Earth to plan the next sol's activities. Normally there would be no commanding of the rover during this session.
 Approximately a third to a half of the total data volume from each surface mission will be sent by direct communication with Earth. The rest will be through the short orbiter passes at high data rates. The orbiters will later relay those data to Earth. Data that are critical for next-sol planning will be communicated directly to Earth during the afternoon pass before earthset. The orbiter passes will typically contain large amounts of image data for later science and engineering analyses that are not needed immediately for planning. It is possible to command the rovers through the Odyssey orbiter, though that capability will not be used in normal operations.
 The Odyssey orbiter will also provide Doppler-shift information from some of its passes over each rover, which will be used to determine the locations of the rovers on the surface to an accuracy of a few tens of meters. This will be used to locate the rovers in high-resolution orbiter images to better direct the rovers to areas of scientific interest and to provide specific regional geologic context.
 Most of the activities that the rovers will be instructed to do fall into four generic categories: panorama, drive, approach, and target science. These different types of activities are described by Squyres et al. . The timescales for assessing the results of a previous sol and generating commands for the next, the uncertainties in the outcomes of commanded activities, and the limited resources available for energy and communication drive the surface operations design of the MER mission.
4.2. Surface Mission Tactical Process
 Due to the limited time between the receipt of the data from the previous sol and the commanding of the rover for the next sol, approximately 19 hours, a series of activities are planned to facilitate high-quality decisions based on the available information, and to generate a set of commands that will complete with high reliability and provide the best science return within the available rover resources. The rover has a limited life on the surface of Mars, and the available energy and communications resources are dwindling as the weeks and months go by. Therefore it is important to make the best use of this incredibly valuable and wasting scientific resource on Mars. For this reason, an intense overnight process will react to the unpredictable events of the previous sol and create a new set of instructions to the rover for the next sol. This process timeline is shown in Figure 9.
 The overnight tactical process begins with the automatic mining of the received data to extract, convert, and place the telemetry and data products in locations and forms usable by the team. The engineering team immediately begins to assess the health and state of the rover as well as the resources available for the next sol, while the science teams determine what results were achieved and what answers are available to guide the strategy for the next sol. 3-D visualization tools developed for MER are used to assess the rover state, the immediate and distant environment, and the science data. Engineering activities are generated to provide required diagnostic information, perform maintenance if needed, and retransmit lost data as desired. This assessment phase takes approximately 8.5 hours.
 At a pivotal Science Operations Working Group (SOWG) meeting, the rover's health, state, and resource constraints for the next sol, the results, successes, and failures of the previous sol's activities, and the long-range plan and objectives are all brought together for the team to discuss and decide on what observations, traverses, arm activities, and data return priorities to direct on the next sol. After this meeting a detailed activity plan is generated to implement those decisions in a set of activities that are likely to meet the resource constraints. This planning phase takes approximately 4 hours.
 Last, a sequence of spacecraft commands to execute that plan is generated and validated. A set of instructions written in the rover's sequencing language is developed to perform the observations, traverses, arm activities, and communication sessions, often with some of those executing in parallel. These instructions are validated against the resource constraints, the position and orientation of the rover with respect to nearby obstacles and hazards, and the rules of operation of the rover. The plan is adjusted as needed to fit within and utilize the available rover resources. A set of automated tools developed for MER are used to aid the rover sequence developers, drivers, and instrument operators in completing the command development and validation in approximately 5 hours.
4.3. Science Team Operations
 During MER surface operations, the Athena science team will be divided into two separate groups, one supporting each rover. Some team members will stay with one rover all the way through the mission, while others will switch back and forth as their science interests dictate. The overall job of the science team during operations is to analyze data, generate scientific hypotheses for testing, and devise daily and longer-term rover activity plans that can be used to test these hypotheses.
 The science team for each rover is supported by a Science Operations Support Team (SOST). This group is composed of Athena science team members who have payload responsibilities, their collaborators, and other JPL payload experts. The job of the SOST during operations is to assess payload health, generate data products, and develop payload command sequences to be sent to the rover. For each rover there is also a Spacecraft/Rover Engineering Team (SRET) composed of JPL engineers that is responsible for assessing rover health and safety, and for developing rover command sequences.
 The science team for each rover is subdivided into several Science Theme Groups (STGs). At the time of the Spirit landing there will be five STGs: Geology, Mineralogy & Geochemistry, Rock & Soil Physical Properties, Atmospheric Science, and Long-Term Planning. This group structure may be adjusted in response to discoveries made during the mission. On each sol, the STGs begin their work as soon as the first data products become available. The job of the first four STGs in the list above is to analyze SOST-produced data products relevant to their area of interest, assess the implications of these data for existing hypotheses, develop new hypotheses, and generate preliminary sets of rover and payload activities for the coming sol which can be used to test hypotheses.
 The job of the Long-Term Planning STG is different. They track the key hypotheses developed by all the STGs, and consider how best to test them in the broader strategic context of the 90-sol mission. On a given sol, the Geology STG might focus on the particular scientific opportunities and tactical challenges posed for the next sol by a rock outcrop immediately in front of the rover. The Long-Term Planning STG, however, is charged with considering how to conduct the next sol's overall activities in the way that is most consistent with achieving overall mission goals.
 After the STGs have met both individually and jointly, the SOWG meeting begins. At this meeting, rover and payload state of health are first summarized by the SRET and the SOST respectively. These summaries are followed by a description of the key operational constraints for the coming sol, including any necessary engineering activities. Each STG then presents their activity requests for the coming sol and the scientific rationale for them. Most requests deal with the specific scientific opportunities presented by the rover's situation on that sol. Others are for activities that should occur at regular intervals, including monitoring atmospheric conditions, performing instrument and rover engineering calibrations, etc. After all of the activity requests have been presented, the SOWG Chair leads a discussion of priorities and guides the group to a consensus. The product of the SOWG meeting is a single merged activity plan for the coming sol. The rest of the tactical timeline for that sol is devoted to turning this merged activity plan into a validated command sequence to be transmitted to the rover.
 Several processes are used to assure that the science strategy for each rover is kept consistent with mission goals. There is a Mission Planning Team (MPT) whose responsibilities include tracking progress against quantifiable goals like number of observations made and distance traversed. In addition, after each SOWG meeting, the head of the Long-Term Planning STG leads a discussion involving all the STGs for that rover in which the working hypotheses are discussed in light of all data collected to date, including orbital data. This discussion helps assure that all STG members are kept abreast of the progress made toward achieving the mission's scientific goals. And the two SOWG Chairs and other key personnel meet regularly to assure that important scientific and operational lessons learned are shared by both rover teams.
 Some aspects of rover operations can be predicted many sols into the future. These include expected availability of electrical power, anticipated data volumes and timing of communication opportunities, and other similar items. These projections are the responsibility of the MPT. It is the responsibility of the Long-Term Planning STGs to generate and continuously update the “sol trees” [Squyres et al., 2003] that forecast the science activities that will be conducted for each rover in the coming sols. Sol trees are high-level multisol activity plans that have multiple branches to account for the uncertainty in the outcome of each sol's activities. Experience with operation of the FIDO rover [Arvidson et al., 2002] in Mars-like settings has shown that sol trees can typically be projected four or five sols into the future before they become too branched to be useful.
5. Expected Rover Capability
 Using models of the system engineering performance, a surface mission of 90 sols has been planned, demonstrating the capability of the system to achieve the MER mission objectives. The engineering and science requirements on mission return capability for the MER mission have been defined as a set of observations and mission achievements. At any given landing site, a MER rover will likely be capable of the following:
 1) Returning 2 color stereo Pancam and 2 Mini-TES panoramas.
 2) Driving to at least 4 distinct locations and performing in situ measurements at each location.
 3) Returning measurements using the full instrument suite of at least 1 soil sample, 4 rocks, and 1 abraded rock.
 4) Driving at least 600 m.
 5) Lasting for 90 sols with full use of the complete instrument suite.
 6) Performing one soil mechanics experiment and returning the associated measurements necessary for characterizing soil physical properties.
 7) Returning the supporting calibration measurements, imaging and Mini-TES measurements that enable characterization of the context and diversity of the landing sites.
 This is not the list of required achievements on Mars for mission success, which is a smaller subset. Instead, this list provides an example of the expected capability of each rover. When a representative mission of 90 sols was planned under nominal environmental conditions, the system demonstrated the capability to perform this set of activities within a combination of energy, data, and operational margins consistent with conservative mission practice. The actual capability of the rovers on the surface of Mars may turn out to be better or worse than this, and will depend on a variety of factors such as hardware and operations testing pre-landing, environmental conditions, and the terrain. The actual activities carried out by the rovers on Mars will depend on the science discoveries made, the terrain characteristics, and the operational risk assessments made after landing. Given that, the design is capable of addressing the science objectives of the mission in realistic scenarios.
6. Data Management
 As detailed in the Mars Exploration Program Data Management Plan (R. E. Arvidson et al., Mars Exploration Program Data Management Plan, Rev. 3, 20 March 2000, http://wufs.wustl.edu/missions/mep/dmp.html) and the MER Project Archive Plan (R. E. Arvidson and S. Slavney, Mars Exploration Rover Project Archive Generation, Validation, and Transfer Plan, JPL D-19658, 22 March 2002, http://wufs.wustl.edu/missions/mer/docs/mer_archive.pdf), the MER Project will generate, validate, and deliver all of the mission raw and derived data and supporting documentation to the Planetary Data System within six months of data acquisition. In keeping with this requirement, the Project will deliver to the Planetary Data System (PDS) two integrated archives for each rover mission, the first one no later than six months after Sol 30 data are received on Earth, and the second one no later than six months after Sol 90 data are received on Earth. If there is an extended mission, then a third release will occur no later than six months after the last data from the extended mission are received on Earth. Table 1 summarizes the standard products to be archived with the PDS. Standard products are products generated according to a standard “pipeline” procedure. Special products are generated outside the pipeline, perhaps as a one-time effort or in response to a special request.
Table 1. MER Standard Data Products
Instrument Data Product
Camera Experiment Data Records (EDRs)
Raw image data
Radiometrically calibrated camera images
Image data expressed as radiance on sensor
Pancam, Navcam and MI mosaics
Mosaicked image files
Spectral/Spatial data sets
Atmospheric opacity determinations
Optical depth as a function of time derived from Pancam images of the Sun
Digital elevation models
Terrain maps derived from Pancam stereo data
Mini-TES Experiment Data Records (EDRs)
Raw Mini-TES data
Mini-TES radiometrically calibrated spectra
Mini-TES data expressed as radiance on sensor
Mini-TES atmospheric sounding profiles
Microscopic Imager (MI)
MI Experiment Data Records (EDRs)
Raw microscopic imager data
MI geometrically and radiometrically calibrated images
Image data expressed as radiance on sensor with camera distortions removed
MI merged focal sections
Images produced by merging data acquired at different distances from target
APXS Experiment Data Records (EDRs)
Raw APXS data
APXS summed spectra in counts
Data binned by counts
APXS summed spectra in energy
Data binned by energy
APXS peak area and elemental concentrations
MB Experiment Data Records (EDRs)
Raw MB data
MB summed spectra in counts
Spectra binned by counts
MB summed spectra in velocity
Spectra binned by velocity
MB Fe-mineral and Fe-state components
Fe mineralogy and composition estimates
Rock Abrasion Tool (RAT)
RAT Experiment Data Records (EDRs)
Currents and motions associated with the RAT
 For planning purposes, the expected total downlinked data volume from both rovers is approximately 4 gigabytes for the primary mission, based on sample mission scenarios. Optimistic outcomes could result in twice as much downlinked data. Experiment Data Records (EDRs) are the raw, unprocessed, uncalibrated data products. The total volume of EDR products after decompression is estimated to be the downlink volume times 16. The total volume of Reduced Data Record (RDR) products, which are higher-level products derived from the EDRs, is not yet determined. However, it could be as much as 30 times the EDR volume, which would correspond to a total expected archive data volume (EDR and RDR) of 2 terabytes.
 The concept of integrated archives is the key to making the best use of the data returned by the various science instruments on MER. Unlike previous orbital and landed missions in which the instruments were operated mostly independently of one another, in a rover mission the instruments must operate in close coordination. Furthermore, a rover mission is non-deterministic; a decision to conduct a sequence of observations may be driven by recently acquired data rather than by a plan determined in advance. The Athena Team and the general science community will require access to science data archives that are integrated across instruments by time, by location, and by observation target, at a minimum. Two complementary systems, the Planetary Data System PDS-D online service and the MER Analyst's Notebook, will provide the desired accessibility. The Analyst's Notebook (example shown in Figure 10) is a Web-based tool for correlating data products from various Athena instruments on the basis of time, location, observation target, and other criteria. The Notebook will provide detailed views into operational decisions, results, and access to raw and derived data and instrument calibration information. Using the Notebook, a scientist will be able to virtually replay mission events to better select and understand data products of interest. The PDS-D online service (http://starbrite.jpl.nasa.gov/pds/), which recently made its debut with the first release of data from the 2001 Mars Odyssey mission, will be a complementary tool to the Analyst's Notebook. PDS-D will provide access to all standard products and will be used by the general science community and the public to satisfy requests for locating and downloading data products. The Analyst's Notebook will probably be used by a smaller set of scientists who need access to the most detailed information available about the data.
 MER science archives will be validated before being released to the PDS. Validation is accomplished in two parts: validation for scientific integrity and validation for compliance with PDS standards. Science team members will conduct validation for scientific integrity in the course of their analysis of EDRs and their production of RDRs. Validation for compliance with PDS standards will also be the responsibility of each Payload Element Lead on the science team, with help from the MER Data and Archive Working Group and PDS. This validation includes a pre-landing peer review of the design and labeling of data products as laid out in the Data Product Software Interface Specification (SIS) documents, and validation of the PDS labels using sample data. Reviewers will consist of a small group of scientists who represent typical users of the data. After the start of operations, when generation of standard products has begun, each individual product will be validated to see that it conforms to the design specified in the SIS.
7. Synergy With Other Mars Missions
 Building on what we have learned from the Viking and Pathfinder landers, this dual-rover mission will significantly extend our mobility “reach” on the surface while carrying a more capable set of instruments. Our landings will come soon after the British-led Beagle 2 spacecraft lands in Isidis Basin on Mars in late December 2003. The Beagle 2 mission, carried to Mars by the European Space Agency's (ESA) Mars Express spacecraft, is focused on searching for evidence of past or present life and characterizing the geology and environmental conditions. It carries a large assortment of instruments on a robotic arm, and a sampling robotic device called a “mole” that can burrow and crawl up to 3 m from the lander to retrieve a sample. Results from the Beagle mission will be highly complementary to MER's, which focuses on the nature of the past environment and habitability.
 The Viking, Mars Global Surveyor, and Mars Odyssey orbiter missions have provided crucial information needed to select safe and scientifically interesting landing sites for the Mars Exploration Rovers. These also supply a regional context for the geology of the landing sites and may influence the choice of direction for driving the rovers. In return, MER will provide “ground truth” information that will be valuable in the interpretation of the current (MGS, Mars Odyssey) and future orbiter data sets from missions such as ESA's Mars Express and NASA's Mars Reconnaissance Orbiter.
 The Mars Exploration Rover mission will represent the first extended exploration of the surface of another planet with a mobile robotic science laboratory, and it will lay the groundwork for the next generation of surface science missions. The rovers' engineering capability in power, thermal performance, and data management will support 90 sols of continuous science operation and data transmission to Earth. The flexible computing architecture will allow the rovers to conduct simultaneous payload and engineering operations, efficiently using the time each day when the rovers can be awake. Each rover is equipped with a set of tools that will enable it to carry out a field geology investigation of each landing site and to address a set of science objectives aimed at understanding the role of water in the past environment on Mars. Measurements relevant to Mars atmospheric science will be obtained from gyro and accelerometer readings taken during atmospheric entry of the spacecraft, and from remote-sensing measurements of the sky acquired after landing. Engineering instrumentation will enable the use of wheels, cameras, robotic arm, and solar array surfaces to supply auxiliary scientific investigation opportunities for the mission such as regolith excavation, inspection, rock motions, and dust accumulation. The science and engineering team will have the tools and training necessary to acquire, analyze, and plan daily operations simultaneously for two rover vehicles. The lasting legacy of the mission will be an estimated several tens of gigabytes of uncompressed raw data products and approximately 1.8 terabytes of derived products, which will be delivered to the NASA Planetary Data System as validated and integrated archives. This data set will allow the science community to continue making more in-depth analyses and to produce new results and discoveries long after the mission has ended.
 This work was carried out at and for the Jet Propulsion Laboratory, California Institute of Technology, sponsored by the National Aeronautics and Space Administration. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement by the United States Government, Cornell University, Washington University in St. Louis, or the Jet Propulsion Laboratory, California Institute of Technology.