Laipert F.E.,AIAA - American Institute of Aeronautics and Astronautics |
Longuski J.M.,AIAA - American Institute of Aeronautics and Astronautics |
Advances in the Astronautical Sciences | Year: 2014
A study is performed to design a robotic sample return mission to the asteroid (216) Kleopatra. Kleopatra is located in the asteroid belt, is an M-type asteroid, and has two moons in orbit around it. Questions about its composition, and about M-type asteroids in general, make Kleopatra a desirable target for a sample return mission. A search is conducted for low-thrust trajectories between 2020 and 2040 with a 1 km/s departure V∞ and 20 kW solar electric propulsion system. The search covers trajectories with gravity assists from one or two bodies selected from Venus, Earth, and Mars. Regular launch opportunities are found delivering net masses of at least 1000 kg using a single gravity assist from Earth or Mars and returning to Earth in less than 10 years, with maay missions returning in 6.5 to 8 years.
Melosh H.J.,Earth |
Melosh H.J.,Purdue University
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2014
Recent comparisons of the isotopic compositions of the Earth and the Moon show that, unlike nearly every other body known in the Solar System, our satellite's isotopic ratios are nearly identical to the Earth's for nearly every isotopic system. The Moon's chemical make-up, however, differs from the Earth's in its low volatile content and perhaps in the elevated abundance of oxidized iron. This surprising situation is not readily explained by current impact models of the Moon's origin and offers a major clue to the Moon's formation, if we only could understand it properly. Current ideas to explain this similarity range from assuming an impactor with the same isotopic composition as the Earth to postulating a pure ice impactor that completely vaporized upon impact. Several recent proposals follow from the suggestion that the Earth-Moon system may have lost a great deal of angular momentum during early resonant interactions. The isotopic constraint may be the most stringent test yet for theories of the Moon's origin. © 2014 The Author(s) Published by the Royal Society.
Goldberg D.N.,Earth |
Cryosphere | Year: 2013
To date, assimilation of observations into large-scale ice models has consisted predominantly of time-independent inversions of surface velocities for basal traction, bed elevation, or ice stiffness, and has relied primarily on analytically derived adjoints of glaciological stress balance models. To overcome limitations of such "snapshot" inversions-i.e., their inability to assimilate time-dependent data for the purpose of constraining transient flow states, or to produce initial states with minimum artificial drift and suitable for time-dependent simulations-we have developed an adjoint of a time-dependent parallel glaciological flow model. The model implements a hybrid shallow shelf-shallow ice stress balance, solves the continuity equation for ice thickness evolution, and can represent the floating, fast-sliding, and frozen bed regimes of a marine ice sheet. The adjoint is generated by a combination of analytic methods and the use of algorithmic differentiation (AD) software. Several experiments are carried out with idealized geometries and synthetic observations, including inversion of time-dependent surface elevations for past thicknesses, and simultaneous retrieval of basal traction and topography from surface data. Flexible generation of the adjoint for a range of independent uncertain variables is exemplified through sensitivity calculations of grounded ice volume to changes in basal melting of floating and basal sliding of grounded ice. The results are encouraging and suggest the feasibility, using real observations, of improved ice sheet state estimation and comprehensive transient sensitivity assessments. © 2013 Author(s).
Earth | Date: 1990-12-10
sportswear, namely, T-shirts, pants, shorts, tops and shoesincluding t-shirts, pants, shorts, shirts, tops, shoes.
Bandfield J.L.,Space Science Institute |
Hayne P.O.,Jet Propulsion Laboratory |
Williams J.-P.,Earth |
Greenhagen B.T.,Jet Propulsion Laboratory |
Icarus | Year: 2015
Sunlit and shaded slopes have a variety of temperatures based on their orientation with respect to the Sun. Generally, greater slope angles lead to higher anisothermality within the field of view. This anisothermality is detected by measuring changing emitted radiance as a function of viewing angle or by measuring the difference in brightness temperatures with respect to observation wavelength. Thermal infrared measurements from the Lunar Reconnaissance Orbiter Diviner Radiometer were used to derive lunar surface roughness via two observation types: (1) nadir multispectral observations with full diurnal coverage and (2) multiple emission angle targeted observations. Measurements were compared to simulated radiance from a radiative equilibrium thermal model and Gaussian slope distribution model. Nadir observations most closely match a 20° RMS slope distribution, and multiple emission angle observations can be modeled using 20-35° RMS slope distributions. Limited sampling of the lunar surface did not show any clear variation in roughness among surface units. Two-dimensional modeling shows that surfaces separated by distances greater than 0.5-5. mm can remain thermally isolated in the lunar environment, indicating the length scale of the roughness features. Non-equilibrium conditions are prevalent at night and near sunrise and sunset, preventing the use of the equilibrium thermal model for roughness derivations using data acquired at these local times. Multiple emission angle observations also show a significant decrease in radiance at high emission angles in both daytime and nighttime observations, and hemispherical emissivity is lower than is apparent from nadir observations. These observations and models serve as a basis for comparison with similar measurements of other airless bodies and as an initial template for the interpretation of TIR measurements acquired under a variety of geometric conditions. © 2014 Elsevier Inc.