Large-scale impact cratering on the terrestrial planets

Impact cratering as a geologic process on the terrestrial planets is addressed. The crater densities on the Earth and Moon form the basis for a standard flux-time curve, which can be used to date unsampled planetary surfaces and constrain the temporal history of endogenic geologic processes. The attached uncertainties and the shape of the flux curve (a rapid exponential decay for the period 4.6 – 4.0 by, followed by the establishment of a constant fluid by 3.5 – 3.0 by which continues more or less to the present) are such that only very old (3.8 by) and very young ( 1.0 by) surfaces can be dated with some confidence. Dating of intermediate-aged surfaces is more imprecise; a problem which is most significant for the geologic history of Mars.

The cratering mechanics of simple craters are fairly well understood. A transient cavity of roughly parabolic cross-section results from the combined excavation and displacement of the target rocks by the cratering flow-field, which can be approximated by the Z-model derived from shallow-buried explosive events. The walls of the transient crater are unstable and slump inwards, resulting in a final bowl-shaped crater partially filled by breccia. The formation process of larger, shallow complex structures is less well understood. Recent models favor the complete collapse of the initial cavity, with the dynamic uplift of the excavated cavity floor. Regardless of the driving force for uplift, yield strength of the target rocks must be drastically reduced during cavity modification by an, as yet, imprecisely known process.

The formation of large impact basins had a profound effect on planetary evolution. They define the basic tectonic and stratigraphic framework of the Moon and their secondary effects lasted for 10⁸ y. The evidence is less compelling from other planets, but a general feature appears to be the concentration of later endogenic activity in and around basins. On Earth, it is possible that basin-formation contributed to the establishment of the dichotomy between proto-continental and proto-oceanic crusts. The effects of impact continue into recent geologic history and may be linked to major biological changes on Earth, such as at the Cretaceous-Tertiary boundary.     

Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments

Ongoing research on martian meteorites and a new set of observations of carbonate minerals provided by an unprecedented series of robotic missions to Mars in the past 15 years help define new constraints on the history of martian climate with important crosscutting themes including: the CO₂ budget of Mars, the role of Mg-, Fe-rich fluids on Mars, and the interplay between carbonate formation and acidity. Carbonate minerals have now been identified in a wide range of localities on Mars as well as in several martian meteorites. The martian meteorites contain carbonates in low abundances (<1 vol.%) and with a wide range of chemistries. Carbonates have also been identified by remote sensing instruments on orbiting spacecraft in several surface locations as well as in low concentrations (2–5 wt.%) in the martian dust. The Spirit rover also identified an outcrop with 16 to 34 wt.% carbonate material in the Columbia Hills of Gusev Crater that strongly resembled the composition of carbonate found in martian meteorite ALH 84001. Finally, the Phoenix lander identified concentrations of 3–6 wt.% carbonate in the soils of the northern plains. The carbonates discovered to date do not clearly indicate the past presence of a dense Noachian atmosphere, but instead suggest localized hydrothermal aqueous environments with limited water availability that existed primarily in the early to mid-Noachian followed by low levels of carbonate formation from thin films of transient water from the late Noachian to the present. The prevalence of carbonate along with evidence for active carbonate precipitation suggests that a global acidic chemistry is unlikely and a more complex relationship between acidity and carbonate formation is present.     

Towards a One Percent Measurement of Frame Dragging by Spin with Satellite Laser Ranging to LAGEOS, LAGEOS 2 and LARES and GRACE Gravity Models

During the past century Einstein’s theory of General Relativity gave rise to an experimental triumph; however, there are still aspects of this theory to be measured or more accurately tested. Today one of the main challenges in experimental gravitation, together with the direct detection of gravitational waves, is the accurate measurement of the gravitomagnetic field generated by the angular momentum of a body. Here, after a brief introduction on frame-dragging, gravitomagnetism and Lunar Laser Ranging tests, we describe the past measurements of frame-dragging by the Earth spin using the satellites LAGEOS, LAGEOS 2 and the Earth’s gravity models obtained by the GRACE project. We demonstrate that these measurements have an accuracy of approximately 10%. We then describe the LARES experiment to be launched in 2010 by the Italian Space Agency for a measurement of frame-dragging with an accuracy of a few percent. We finally demonstrate that a number of claims by a single individual, that the error budget of the frame-dragging measurements with LAGEOS-LAGEOS 2 and LARES has been underestimated, are indeed ill-founded.     

Human and robotic repair of a solar array wing during ISS assembly mission 10A

With the installation of a new module and the relocation of three other modules, including multiple hand-offs from the station arm (SSRMS) to the shuttle arm (SRMS), International Space Station (ISS) assembly mission 10A/STS-120 was anticipated to be one of the most complicated ISS assembly missions ever attempted. The assembly operations became even more complex when a solar array wing (SAW) on the relocated Port-6 (P6) truss segment ripped while being extended. Repairing the torn SAW became the single most important objective for the remainder of STS-120, with future ISS assembly missions threatened by reduced power generation capacity if the SAW could not be repaired. Precise coordination between the space shuttle and ISS robotics teams led to an operational concept that combined the capabilities of the SRMS and SSRMS robotic systems in ways far beyond their original design capacities. Benefits of consistent standards for ISS robotic interfaces have been previously identified, but the advantages of having two such versatile and compatible robotic systems have never been quite so spectacular. This paper describes the role of robotics in the emergency SAW repair and highlights how versatility within space robotics systems can allow operations far beyond the intended design scenarios.     

NASA Human Health and Performance Center: Open innovation successes and collaborative projects

In May 2007, what was then the Space Life Sciences Directorate published the 2007 Space Life Sciences Strategy for Human Space Exploration, setting the course for development and implementation of new business models and significant advances in external collaboration over the next five years. The strategy was updated on the basis of these accomplishments and reissued as the NASA Human Health and Performance Strategy in 2012, and continues to drive new approaches to innovation for the directorate. This short paper describes the successful execution of the strategy, driving organizational change through open innovation efforts and collaborative projects, including efforts of the NASA Human Health and Performance Center (NHHPC).     

Matched-Filter Responses of the Linear FM Waveform

Although the properties of the linear FM signal have been studied previously in considerable detail, such studies have involved rather narrow aspects of the theory. This paper extends the work in several respects. By presenting three-dimensional projections of the conventional ambiguity function of the linear FM signal in more detail than was available before, we can study the sidelobe behavior off as well as on the axes, without weighting, with unilateral weighting in the receiver, and with bilateral weighting. These plots reveal interesting properties related to the signal symmetry in time and frequency. The matched-filter response is then extended to include Doppler distortions of the modulation function. The results show that Woodward's ambiguity function is valid only for signals with relatively modest sophistication, even though in most practical situations one is interested only in those undistorted parts of the matched-filter response in the vicinity of the delay axis. Plots of the response are presented for various degrees of distortion, for signals with and without weighting. Lastly, we consider the effects of a mismatch in range acceleration, again for the various cases of interest. The results convey a thorough insight into the properties of chirp radar under a broad range of operational conditions.     

The extreme ultraviolet spectrum of the sun

The present state of knowledge of the spectrum of the sun from 3000 Å to 170 Å is critically reviewed. The characteristics of the different photographic and photoelectric spectroscopic instruments and the results are summarized and compared. Topics covered are line identifications, intensities, the continuum and its origin, the sun's surface in Lyman alpha, the profiles of H and K of Mg II, Lyman alpha and beta, and monitoring work conducted from rockets and satellites, including OSO-I.     

Radar Waveforms for Suppression of Extended Clutter

A common problem in waveform design is to adapt the transmitted signal to the target environment in order that the interference from extended fields of scatterers is reduced. This problem is investigated here for the special case of detection of a single target in the ``vicinity' of an extended clutter space. The paper considers the possibility of confining the matched-filter response in delay and Doppler, or ambiguity function, to a narrow strip with arbitrary orientation in the delay-Doppler plane. It is shown that strict confinement of the response is achievable only with waveforms that are unlimited in both time and frequency domain. With practical waveforms, which are necessarily of finite extent, one merely can trade close-target separability against detectability in the background clutter. Thus, one form of the resolution problem is exchanged against the other. The paper examines these effects quantitatively.     

Sulfur “concrete” for lunar applications – Sublimation concerns

Melting sulfur and mixing it with an aggregate to form “concrete” is commercially well established and constitutes a material that is particularly well-suited for use in corrosive environments. Discovery of the mineral troilite (FeS) on the moon poses the question of extracting the sulfur for use as a lunar construction material. This would be an attractive alternative to conventional concrete as it does not require water. However, the viability of sulfur concrete in a lunar environment, which is characterized by lack of an atmosphere and extreme temperatures, is not well understood. Here it is assumed that the lunar ore can be mined, refined, and the raw sulfur melded with appropriate lunar regolith to form, for example, bricks. This study evaluates pure sulfur and two sets of small sulfur concrete samples that have been prepared using JSC-1 lunar stimulant and SiO₂ powder as aggregate additions. Each set was subjected to extended periods in a vacuum environment to evaluate sublimation issues. Results from these experiments are presented and discussed within the context of the lunar environment.