Asteroid Diameters and Albedos from NEOWISE Reactivation Mission Years 4 and 5

The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) spacecraft has been conducting a two-band thermal infrared survey to detect and characterize asteroids and comets since its reactivation in 2013 December. Using the observations collected during the fourth and fifth years of the survey, our automated pipeline detected candidate moving objects that were verified and reported to the Minor Planet Center. Using these detections, we perform thermal modeling of each object from the near-Earth object (NEO) and Main Belt asteroid (MBA) populations to constrain their sizes. We present thermal model fits of asteroid diameters for 189 NEOs and 5831 MBAs detected during the fourth year of the survey, and 185 NEOs and 5776 MBAs from the fifth year. To date, the NEOWISE Reactivation survey has provided thermal model characterization for 957 unique NEOs. Including all phases of the original Wide-field Infrared Survey Explorer survey brings the total to 1473 unique NEOs that have been characterized between 2010 and the present

NEOWISE Reactivation Mission Year Three: Asteroid Diameters and Albedos

The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) reactivation mission has completed its third year of surveying the sky in the thermal infrared for near-Earth asteroids and comets. NEOWISE collects simultaneous observations at 3.4 um and 4.6 um of solar system objects passing through its field of regard. These data allow for the determination of total thermal emission from bodies in the inner solar system, and thus the sizes of these objects. In this paper we present thermal model fits of asteroid diameters for 170 NEOs and 6110 MBAs detected during the third year of the survey, as well as the associated optical geometric albedos. We compare our results with previous thermal model results from NEOWISE for overlapping sample sets, as well as diameters determined through other independent methods, and find that our diameter measurements for NEOs agree to within 26% (1-sigma) of previously measured values. Diameters for the MBAs are within 17% (1-sigma). This brings the total number of unique near-Earth objects characterized by the NEOWISE survey to 541, surpassing the number observed during the fully cryogenic mission in 2010.Comment: Accepted for publication in A

Uncertainties on Asteroid Albedos Determined by Thermal Modeling

We present an analysis of the accuracy of geometric albedos determined for asteroids through the modeling of observed thermal infrared radiation. We show that albedo uncertainty is dominated by the uncertainty on the measured $H_V$ absolute magnitude, and that any analysis using albedos in a statistical application will also be dominated by this source of uncertainty. For all but the small fraction of asteroids with a large amount of characterization data, improved knowledge of the $H_V$ magnitude will be fundamentally limited by incomplete phase curve coverage, incomplete light curve knowledge, and the necessary conversion from the observed band to the $V$ band. Switching the absolute magnitude standard to a different band such a $r'$ would mitigate the uncertainty due to band conversion for many surveys, but this only represents a small component of the total uncertainty. Therefore, techniques making use of these albedos must ensure that their uncertainties are being properly accounted for.Comment: 10 pages, 1 figure. Accepted to the Planetary Science Journa

Evidence for a Molecular Cloud Origin for Gamma-Ray Bursts: Implications for the Nature of Star Formation in the Universe

It appears that the majority of rapidly-, well-localized gamma-ray bursts with undetected, or dark, optical afterglows, or `dark bursts' for short, occur in clouds of size R > 10L_{49}^{1/2} pc and mass M > 3x10^5L_{49} M_{sun}, where L is the isotropic-equivalent peak luminosity of the optical flash. We show that clouds of this size and mass cannot be modeled as a gas that is bound by pressure equilibrium with a warm or hot phase of the interstellar medium (i.e., a diffuse cloud): Such a cloud would be unstable to gravitational collapse, resulting in the collapse and fragmentation of the cloud until a burst of star formation re-establishes pressure equilibrium within the fragments, and the fragments are bound by self-gravity (i.e., a molecular cloud). Consequently, dark bursts probably occur in molecular clouds, in which case dark bursts are probably a byproduct of this burst of star formation if the molecular cloud formed recently, and/or the result of lingering or latter generation star formation if the molecular cloud formed some time ago. We then show that if bursts occur in Galactic-like molecular clouds, the column densities of which might be universal, the number of dark bursts can be comparable to the number of bursts with detected optical afterglows: This is what is observed, which suggests that the bursts with detected optical afterglows might also occur in molecular clouds. We confirm this by modeling and constraining the distribution of column densities, measured from absorption of the X-ray afterglow, of the bursts with detected optical afterglows: We find that this distribution is consistent with the expectation for bursts that occur in molecular clouds, and is not consistent with the expectation for bursts that occur in diffuse clouds. More...Comment: Accepted to The Astrophysical Journal, 22 pages, 6 figures, LaTe

Simulating Ice Accretion Effects on Engine Performance

Develop a modeling tool that can be used to predict the onset of engine icing due to ice crystal ingestion. The tool will be capable of modeling the effects of ice build up as well as its effect on engine performance. Perform a parametric study of an engine with simulated ice blockage effects at altitude conditions. Using the tool, estimate the effect of blockage in the low pressure compressor due to ice buildup (accretion), and its effects on engine performance

Seasonal changes in soil organic matter after a decade of nutrient addition in a lowland tropical forest

© 2015, US Government. Soil organic matter is an important pool of carbon and nutrients in tropical forests. The majority of this pool is assumed to be relatively stable and to turn over slowly over decades to centuries, although changes in nutrient status can influence soil organic matter on shorter timescales. We measured carbon, nitrogen, and phosphorus concentrations in soil organic matter and leaf litter over an annual cycle in a long-term nutrient addition experiment in lowland tropical rain forest in the Republic of Panama. Total soil carbon was not affected by a decade of factorial combinations of nitrogen, phosphorus, or potassium. Nitrogen addition increased leaf litter nitrogen concentration by 7 % but did not affect total soil nitrogen. Phosphorus addition doubled the leaf litter phosphorus concentration and increased soil organic phosphorus by 50 %. Surprisingly, concentrations of carbon, nitrogen, and phosphorus in soil organic matter declined markedly during the four-month dry season, and then recovered rapidly during the following wet season. Between the end of the wet season and the late dry season, total soil carbon declined by 16 %, total nitrogen by 9 %, and organic phosphorus by between 19 % in control plots and 25 % in phosphorus addition plots. The decline in carbon and nitrogen was too great to be explained by changes in litter fall, bulk density, or the soil microbial biomass. However, a major proportion of the dry-season decline in soil organic phosphorus was explained by a corresponding decline in the soil microbial biomass. These results have important implications for our understanding of the stability and turnover of organic matter in tropical forest soils, because they demonstrate that a considerable fraction of the soil organic matter is seasonally transient, despite the overall pool being relatively insensitive to long-term changes in nutrient status

Mixed Phase Modeling in GlennICE with Application to Engine Icing

A capability for modeling ice crystals and mixed phase icing has been added to GlennICE. Modifications have been made to the particle trajectory algorithm and energy balance to model this behavior. This capability has been added as part of a larger effort to model ice crystal ingestion in aircraft engines. Comparisons have been made to four mixed phase ice accretions performed in the Cox icing tunnel in order to calibrate an ice erosion model. A sample ice ingestion case was performed using the Energy Efficient Engine (E3) model in order to illustrate current capabilities. Engine performance characteristics were supplied using the Numerical Propulsion System Simulation (NPSS) model for this test case