to elucidate the planet's past climate, water activity, and habitability.Science is MER's primary driver, so making best use of the scientific instruments, within the available resources, is a crucial aspect of the mission. To address this criticality, the MER project team selected MAPGEN (Mixed Initiative Activity Plan Generator) as an activity-planning tool.MAPGEN combines two existing systems, each with a strong heritage: the APGEN activity-planning tool 1 from the Jet Propulsion Laboratory and the Europa planning and scheduling system 2 from NASA Ames Research Center. This article discusses the issues arising from combining these tools in this mission's context. Combining systemsIn a most exciting development, two NASA roversnamed Spirit and Opportunity-were slated to arrive at the Red Planet in January, at two scientifically distinct sites. (Spirit arrived successfully on 3 January, with Opportunity scheduled to arrive 24 January-see Figures 1 and 2.) Each rover will have an operational lifetime of 90 sols (Martian days) or more and can traverse an integrated distance of one kilometer or more, although the maximum range from the landing site might be less. Scientifically, MER seeks to • Determine the aqueous, climatic, and geologic history of a site where on Mars conditions might have been favorable to the preservation of evidence of prebiotic or biotic processes • Identify hydrologic, hydrothermal, and other processes that have operated at the landing site • Identify and investigate Martian rocks and soils that have the highest-possible chance of preserving evidence of ancient environmental conditions and possible prebiotic or biotic activity • Respond to other discoveries revealed by rover-based exploration Each sol, operations personnel on Earth receive telemetry from the rovers. On the basis of the downloaded data, they must construct, verify, and uplink a detailed sequence of commands for the next sol to the rovers. Thus, operations personnel must formulate a viable sequence that satisfies the mission goals within tight deadlines. To help address this critical need, MAPGEN can automatically generate plans and schedules for science and associated engineering activities; assist in hypothesis testing, such as what-if analysis on various scenarios; support plan editing; analyze resource usage; and perform constraint enforcement and maintenance.APGEN has served as a multimission tool for several flight projects (including Cassini and Deep Impact), while Europa flew onboard NASA's Deep Space 1 as part of a technology experiment to demonstrate the first onboard T he Mars Exploration Rover mission is one of NASA's most ambitious science missions to date.Launched in the summer of 2003, each rover carries instruments for conducting remote and in situ observations

Objective To compare the costs of two spinal implantshook and hybrid constructs and pedicle screw constructsin posterior spinal fusion for adolescent idiopathic scoliosis (AIS) as they relate to intraoperative deformity correction. Study design and method This retrospective study examined 50 patients with AIS who were treated with posterior spinal fusion using segmental hook-hybrid constructs (23) or pedicle screws (27). Radiographic parameters measured on immediate preoperative and initial standing postoperative scoliosis films were the coronal Cobb angles of the upper thoracic, middle thoracic, lumbar, and instrumented curves; global coronal and sagittal balance; thoracic kyphosis; lumbar lordosis; and type and number of implants used. Current implant cost data were obtained from three major spinal implant manufacturers to determine the total cost of the constructs, cost per degree of correction, cost per level fused, and cost per degree of correction of the major curve. Results After surgery, the average percentage of correction for the middle thoracic curve or major curve was 57 % in the hook-hybrid group compared to 73 % in the pedicle screw group (P \ 0.001). The average amount of correction of the major curve was 31.1°in the hook-hybrid group compared to 42.7°in the pedicle screw group (P \ 0.001). The average number of fused levels was 10.7 in the hookhybrid group compared to 12.2 in the pedicle screw group (P \ 0.001). The average number of implants was 14.8 in the hook-hybrid group compared to 23.3 in the pedicle screw group (P \ 0.001). The average total cost of implants was $11,248 in the hook-hybrid group compared to $22,826 in the pedicle screw group (P \ 0.001), and the average cost per fused level was $1,058 in the hook-hybrid group compared to $1,878 in the pedicle screw group (P \ 0.001). The average cost per degree of correction of the major curve was $415 in the hook-hybrid group compared to $559 in the pedicle screw group (P = 0.0014). The global coronal balance, global sagittal balance, thoracic kyphosis, and lumbar lordosis did not differ significantly between the two groups. Conclusion Pedicle screw instrumentation was shown to be more expensive overall, per fused level, and per degree of correction. Also, more implants were used and more levels were fused in the pedicle screw group than in the hook-hybrid group. Pedicle screws showed a statistically significantly greater percentage of correction of the major curve. Physicians must evaluate each patient individually and determine if the increased percentage of correction warrants the increased cost for pedicle screw constructs.