Comet C/2013 A1 (Siding Spring) will have a close encounter with Mars on October 19, 2014. We model the dynamical evolution of dust grains from the time of their ejection from the comet nucleus to the Mars close encounter, and determine the flux at Mars. Constraints on the ejection velocity from Hubble Space Telescope observations indicate that the bulk of the grains will likely miss Mars, although it is possible that a few-percent of grains with higher velocities will reach Mars, peaking approximately 90-100 minutes after the close approach of the nucleus, and consisting mostly of millimeter-radius grains ejected from the comet nucleus at a heliocentric distance of approximately 9 AU or larger. At higher velocities, younger grains from sub-millimeter to several millimeter can reach Mars too, although an even smaller fraction of grains is expected have these velocities, with negligible effect on the peak timing. Using NEOWISE observations of the comet, we can estimate that the maximum fluence will be of the order of 10 −7 grains/m 2 . We include a detailed analysis of how the expected fluence depends on the grain density, ejection velocity, and sizefrequency distribution, to account for current model uncertainties and in preparation of possible refined model values in the near future.

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In 2007 a JPL Rapid Mission Architecture (RMA) analysis team identified and evaluated a broad set of mission architecture options for a suite of scientific exploration objectives targeting the Saturnian moon Enceladus. Primary science objectives were largely focused on examination of the driving mechanisms and extent of interactions by the plumes of Enceladus recently discovered by Cassini mission science teams. Investigation of the architectural trade space spanned a wide range of options, from high-energy flybys of Enceladus as a re-instrumented expansion on the Cassini mission, to more complex, multielement combinations of Enceladus orbiters carrying multiple variants of in-situ deployable systems. Trajectory design emerged as a critical element of the mission concepts, enabling challenging missions on Atlas V and Delta IV-Heavy class launch vehicles. Various Enceladus Flagship-class mission concepts identified were analyzed and compared against several first-order figures of merit, including mass, cost, risk, mission timeline, and associated science value with respect to accomplishment of the full set of science objectives. Results are presented for these comparative analyses and the characterization of the explored trade space.

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Since the selection of the proposed Mars 2020 mission as a Rover with the capability of sample collection and caching, there has been renewed interest in subsequent mission concepts to return Mars samples to Earth. The general architecture for this series of missions is outlined in the Planetary Science Decadal Survey of 2011. The role of the Sample Return Orbiter (SRO) in The 2011 Decadal Survey MSR architecture was to collect an orbiting sample (OS) from low Mars orbit and deliver it to Earth's surface. The architecture focused on chemical propulsion orbiters with ballistic and aerobraking trajectories that were dedicated entirely to the capture of orbiting samples and returning them to the surface of the Earth. Recent concepts have explored the use of Solar Electric Propulsion (SEP) to Mars and for the return to Earth. SEP could enable significant mission flexibility which includes: lower launch mass or increased mass delivery capability to Mars orbit and return to Earth; longer launch periods for both launch and Earth return; consistency of design across launch opportunities; access to both high and low Mars orbitaltitudes; increased on-orbit ∆V budgets for orbit changes and sample rendezvous; and greater control over Earth arrival speed and geometry. With this flexibility come opportunities to: save launch cost; add functions such as remote sensing observations, secondary payload deployment, and relay telecommunications; and choose between direct return of Mars samples to the Earth's biosphere or capturing them to a stable long-term orbit around the Earth. This paper compares the previous SRO chemical-ballistic concepts with the recent SEP orbiter concepts. We will show the potential benefits gained by the inherent flexibility of SEP as applied to launch mass, launch periods, Earth return opportunities, on-orbit ∆V and other architectural drivers.