ExploreNEOs I: Description and first results from the Warm Spitzer NEO Survey

We have begun the ExploreNEOs project in which we observe some 700 Near Earth Objects (NEOs) at 3.6 and 4.5 microns with the Spitzer Space Telescope in its Warm Spitzer mode. From these measurements and catalog optical photometry we derive albedos and diameters of the observed targets. The overall goal of our ExploreNEOs program is to study the history of near-Earth space by deriving the physical properties of a large number of NEOs. In this paper we describe both the scientific and technical construction of our ExploreNEOs program. We present our observational, photometric, and thermal modeling techniques. We present results from the first 101 targets observed in this program. We find that the distribution of albedos in this first sample is quite broad, probably indicating a wide range of compositions within the NEO population. Many objects smaller than one kilometer have high albedos (>0.35), but few objects larger than one kilometer have high albedos. This result is consistent with the idea that these larger objects are collisionally older, and therefore possess surfaces that are more space weathered and therefore darker, or are not subject to other surface rejuvenating events as frequently as smaller NEOs.Comment: AJ in pres

ExploreNEOs. II. The Accuracy of the Warm Spitzer Near-Earth Object Survey

We report on results of observations of near-Earth objects (NEOs) performed with the NASA Spitzer Space Telescope as part of our ongoing (2009-2011) Warm Spitzer NEO survey ("ExploreNEOs"), the primary aim of which is to provide sizes and albedos of some 700 NEOs. The emphasis of the work described here is an assessment of the overall accuracy of our survey results, which are based on a semi-empirical generalized model of asteroid thermal emission. The NASA Spitzer Space Telescope has been operated in the so-called Warm Spitzer mission phase since the cryogen was depleted in 2009 May, with the two shortest-wavelength channels, centered at 3.6 μm and 4.5 μm, of the Infrared Array Camera continuing to provide valuable data. The set of some 170 NEOs in our current Warm Spitzer results catalog contains 28 for which published taxonomic classifications are available, and 14 for which relatively reliable published diameters and albedos are available. A comparison of the Warm Spitzer results with previously published results ("ground truth"), complemented by a Monte Carlo error analysis, indicates that the rms Warm Spitzer diameter and albedo errors are ±20% and ±50%, respectively. Cases in which agreement with results from the literature is worse than expected are highlighted and discussed; these include the potential spacecraft target 138911 2001 AE_2. We confirm that 1.4 appears to be an appropriate overall default value for the relative reflectance between the V band and the Warm Spitzer wavelengths, for use in correction of the Warm Spitzer fluxes for reflected solar radiation

ExploreNEOs. III. Physical Characterization of 65 Potential Spacecraft Target Asteroids

Space missions to near-Earth objects (NEOs) are being planned at all major space agencies, and recently a manned mission to an NEO was announced as a NASA goal. Efforts to find and select suitable targets (plus backup targets) are severely hampered by our lack of knowledge of the physical properties of dynamically favorable NEOs. In particular, current mission scenarios tend to favor primitive low-albedo objects. For the vast majority of NEOs, the albedo is unknown. Here we report new constraints on the size and albedo of 65 NEOs with rendezvous Δv <7 km s^(–1). Our results are based on thermal-IR flux data obtained in the framework of our ongoing (2009-2011) ExploreNEOs survey using NASA's "Warm-Spitzer" space telescope. As of 2010 July 14, we have results for 293 objects in hand (including the 65 low-Δv NEOs presented here); before the end of 2011, we expect to have measured the size and albedo of ~700 NEOs (including probably ~160 low-Δv NEOs). While there are reasons to believe that primitive volatile-rich materials are universally low in albedo, the converse need not be true: the orbital evolution of some dark objects likely has caused them to lose their volatiles by coming too close to the Sun. For all our targets, we give the closest perihelion distance they are likely to have reached (using orbital integrations from Marchi et al. 2009) and corresponding upper limits on the past surface temperature. Low-Δv objects for which both albedo and thermal history may suggest a primitive composition include (162998) 2001 SK162, (68372) 2001 PM9, and (100085) 1992 UY4

Niche as a determinant of word fate in online groups

Patterns of word use both reflect and influence a myriad of human activities and interactions. Like other entities that are reproduced and evolve, words rise or decline depending upon a complex interplay between {their intrinsic properties and the environments in which they function}. Using Internet discussion communities as model systems, we define the concept of a word niche as the relationship between the word and the characteristic features of the environments in which it is used. We develop a method to quantify two important aspects of the size of the word niche: the range of individuals using the word and the range of topics it is used to discuss. Controlling for word frequency, we show that these aspects of the word niche are strong determinants of changes in word frequency. Previous studies have already indicated that word frequency itself is a correlate of word success at historical time scales. Our analysis of changes in word frequencies over time reveals that the relative sizes of word niches are far more important than word frequencies in the dynamics of the entire vocabulary at shorter time scales, as the language adapts to new concepts and social groupings. We also distinguish endogenous versus exogenous factors as additional contributors to the fates of words, and demonstrate the force of this distinction in the rise of novel words. Our results indicate that short-term nonstationarity in word statistics is strongly driven by individual proclivities, including inclinations to provide novel information and to project a distinctive social identity.Comment: Supporting Information is available here: http://www.plosone.org/article/fetchSingleRepresentation.action?uri=info:doi/10.1371/journal.pone.0019009.s00

The Pan-STARRS Moving Object Processing System

We describe the Pan-STARRS Moving Object Processing System (MOPS), a modern software package that produces automatic asteroid discoveries and identifications from catalogs of transient detections from next-generation astronomical survey telescopes. MOPS achieves > 99.5% efficiency in producing orbits from a synthetic but realistic population of asteroids whose measurements were simulated for a Pan-STARRS4-class telescope. Additionally, using a non-physical grid population, we demonstrate that MOPS can detect populations of currently unknown objects such as interstellar asteroids. MOPS has been adapted successfully to the prototype Pan-STARRS1 telescope despite differences in expected false detection rates, fill-factor loss and relatively sparse observing cadence compared to a hypothetical Pan-STARRS4 telescope and survey. MOPS remains >99.5% efficient at detecting objects on a single night but drops to 80% efficiency at producing orbits for objects detected on multiple nights. This loss is primarily due to configurable MOPS processing limits that are not yet tuned for the Pan-STARRS1 mission. The core MOPS software package is the product of more than 15 person-years of software development and incorporates countless additional years of effort in third-party software to perform lower-level functions such as spatial searching or orbit determination. We describe the high-level design of MOPS and essential subcomponents, the suitability of MOPS for other survey programs, and suggest a road map for future MOPS development.Comment: 57 Pages, 26 Figures, 13 Table

Detection of Semi-Major Axis Drifts in 54 Near-Earth Asteroids: New Measurements of the Yarkovsky Effect

We have identified and quantified semi-major axis drifts in Near-Earth Asteroids (NEAs) by performing orbital fits to optical and radar astrometry of all numbered NEAs. We focus on a subset of 54 NEAs that exhibit some of the most reliable and strongest drift rates. Our selection criteria include a Yarkovsky sensitivity metric that quantifies the detectability of semi-major axis drift in any given data set, a signal-to-noise metric, and orbital coverage requirements. In 42 cases, the observed drifts (~10^-3 AU/Myr) agree well with numerical estimates of Yarkovsky drifts. This agreement suggests that the Yarkovsky effect is the dominant non-gravitational process affecting these orbits, and allows us to derive constraints on asteroid physical properties. In 12 cases, the drifts exceed nominal Yarkovsky predictions, which could be due to inaccuracies in our knowledge of physical properties, faulty astrometry, or modeling errors. If these high rates cannot be ruled out by further observations or improvements in modeling, they would be indicative of the presence of an additional non-gravitational force, such as that resulting from a loss of mass of order a kilogram per second. We define the Yarkovsky efficiency f_Y as the ratio of the change in orbital energy to incident solar radiation energy, and we find that typical Yarkovsky efficiencies are ~10^-5.Comment: Accepted for publication by The Astronomical Journal. 42 pages, 8 figure

The Science Case for an Extended Spitzer Mission

Although the final observations of the Spitzer Warm Mission are currently scheduled for March 2019, it can continue operations through the end of the decade with no loss of photometric precision. As we will show, there is a strong science case for extending the current Warm Mission to December 2020. Spitzer has already made major impacts in the fields of exoplanets (including microlensing events), characterizing near Earth objects, enhancing our knowledge of nearby stars and brown dwarfs, understanding the properties and structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to study galaxy birth and evolution. By extending Spitzer through 2020, it can continue to make ground-breaking discoveries in those fields, and provide crucial support to the NASA flagship missions JWST and WFIRST, as well as the upcoming TESS mission, and it will complement ground-based observations by LSST and the new large telescopes of the next decade. This scientific program addresses NASA's Science Mission Directive's objectives in astrophysics, which include discovering how the universe works, exploring how it began and evolved, and searching for life on planets around other stars.Comment: 75 pages. See page 3 for Table of Contents and page 4 for Executive Summar

LSST Science Book, Version 2.0

A survey that can cover the sky in optical bands over wide fields to faint magnitudes with a fast cadence will enable many of the exciting science opportunities of the next decade. The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a total point-source depth of r~27.5. The LSST Science Book describes the basic parameters of the LSST hardware, software, and observing plans. The book discusses educational and outreach opportunities, then goes on to describe a broad range of science that LSST will revolutionize: mapping the inner and outer Solar System, stellar populations in the Milky Way and nearby galaxies, the structure of the Milky Way disk and halo and other objects in the Local Volume, transient and variable objects both at low and high redshift, and the properties of normal and active galaxies at low and high redshift. It then turns to far-field cosmological topics, exploring properties of supernovae to z~1, strong and weak lensing, the large-scale distribution of galaxies and baryon oscillations, and how these different probes may be combined to constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at http://www.lsst.org/lsst/sciboo