Binding of precursor tRNAs (ptRNAs) by bacterial ribonuclease P (RNase P) involves an encounter complex (ES) that isomerizes to a catalytic conformation (ES*). However, the structures of intermediates and the conformational changes that occur during binding are poorly understood. Here, we show that pairing between the 5′ leader and 3′RCCA extending the acceptor stem of ptRNA inhibits ES* formation. Cryo-electron microscopy single particle analysis reveals a dynamic enzyme that becomes ordered upon formation of ES* in which extended acceptor stem pairing is unwound. Comparisons of structures with alternative ptRNAs reveals that once unwinding is completed RNase P primarily uses stacking interactions and shape complementarity to accommodate alternative sequences at its cleavage site. Our study reveals active site interactions and conformational changes that drive molecular recognition by RNase P and lays the foundation for understanding how binding interactions are linked to helix unwinding and catalysis.

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As NASA continues to make plans for future robotic precursor and eventual human missions to Mars, the need to characterize and develop designs for entry vehicles capable of delivering large masses to the surface of Mars will persist. In combination with this, NASA has recognized that the current heritage technology for Mars' Entry Decent and Landing (EDL) does not have the capability to land the required payload masses. Both the Thermal Protection System (TPS) and the Descent/Landing systems require new design approaches. Because of these needs, NASA has performed an Entry, Descent and Landing Systems Analysis (EDL-SA) study for high mass exploration and science missions to identify key enabling technology areas for further investment. One key technology area identified includes rigid aeroshell shapes for aerodynamic performance and controllability.In this investigation, a system optimization study of alternative aeroshell shapes for Mars exploration class payloads of approximately 40 metric tons has been conducted. This system optimization is accomplished using a Multi-disciplinary Design Optimization (MDO) framework which accounts for the aeroshell shape, trajectory, thermal protection system, and vehicle subsystem closure along with a Multi Objective Genetic Algorithm (MOGA) for the initial shape exploration. This is accomplished using a combination of engineering analysis and higher-fidelity physics based tools along with optimization methods and engineering judgment. The results of this process have shown that a proposed family of optimized mid lift-to-drag aeroshell shapes exhibit a significant improvement to the current reference entry rigid aeroshell configuration. Furthermore, a trade-off between the vehicle TPS and structural mass is identified for this class of aeroshell shapes and their corresponding trajectories. Balancing this trade-off can yield an overall decrease in the total vehicle mass or corresponding increase in landed payload, as compared to the current reference configuration. NomenclatureA = Effective projected vehicle area CD = Drag coefficient CDa = Drag Area, m 2 CL = Lift coefficient Cm = Pitching moment coefficient Cm α = Pitching moment coefficient curve slope variation with angle-of-attack, /deg. 0 = Earth's gravity constant (g 0 = 9.80665 m/s2) Isp = Specific impulse, sec (on earth) L B = Body Length, m L B_REF = Reference Body Length, m L/D = Lift-to-drag ratio m_Entry = Entry Mass, kg m_Entry REF = Reference Entry Mass, kg m payload = Payload mass, kg MaxQdot = Maximum Heat flux on the body, W/cm 2 qmax = Maximum dynamic pressure, Pa hload = Integrated heatload, J/cm 2 Re = Reynolds Number RCS = Reaction Control System RSS = Root Sum Square Swet = Wetted vehicle surface area, m 2 TPS = Thermal Protection System UWT_Body REF = Reference body structural areal mass M ∞ = Free-stream Mach number αtrim = Trim angle-of-attack, deg. ρ ∞ = Free-stream density, kg/m 3

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Abstract-Mars Sample Return (MSR) is the highest priority science mission for the next decade as recommended by the recent Decadal Survey of Planetary Science. This paper presents an overview of a feasibility study for a MSR mission.The objective of the study was to determine whether emerging commercial capabilities can be used to reduce the number of mission systems and launches required to return the samples, with the goal of reducing mission cost. The major element required for the MSR mission are described and include an integration of the emerging commercial capabilities with small spacecraft design techniques; new utilizations of traditional aerospace technologies; and recent technological developments.We report the feasibility of a complete and closed MSR mission design using the following scenario that can start in This work shows that emerging commercial capabilities can be effectively integrated into a mission to achieve an important planetary science objective.