689 research outputs found
Systematic ranging and late warning asteroid impacts
We describe systematic ranging, an orbit determination technique especially
suitable to assess the near-term Earth impact hazard posed by newly discovered
asteroids. For these late warning cases, the time interval covered by the
observations is generally short, perhaps a few hours or even less, which leads
to severe degeneracies in the orbit estimation process. The systematic ranging
approach gets around these degeneracies by performing a raster scan in the
poorly-constrained space of topocentric range and range rate, while the plane
of sky position and motion are directly tied to the recorded observations. This
scan allows us to identify regions corresponding to collision solutions, as
well as potential impact times and locations. From the probability distribution
of the observation errors, we obtain a probability distribution in the orbital
space and then estimate the probability of an Earth impact. We show how this
technique is effective for a number of examples, including 2008 TC3 and 2014
AA, the only two asteroids to date discovered prior to impact
Constraints on the near-Earth asteroid obliquity distribution from the Yarkovsky effect
Aims. From lightcurve and radar data we know the spin axis of only 43 near-Earth asteroids. In this paper we attempt to constrain the spin axis obliquity distribution of near-Earth asteroids by leveraging the Yarkovsky effect and its dependence on an asteroid’s obliquity.
Methods. By modeling the physical parameters driving the Yarkovsky effect, we solve an inverse problem where we test different simple parametric obliquity distributions. Each distribution results in a predicted Yarkovsky effect distribution that we compare with a X2 test to a dataset of 125 Yarkovsky estimates.
Results. We find different obliquity distributions that are statistically satisfactory. In particular, among the considered models, the best-fit solution is a quadratic function, which only depends on two parameters, favors extreme obliquities, consistent with the expected outcomes from the YORP effect, has a 2:1 ratio between retrograde and direct rotators, which is in agreement with theoretical predictions, and is statistically consistent with the distribution of known spin axes of near-Earth asteroids
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
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The Insider\u27s Advantage: CEO Experience and The Performance of Large Diversified Firms
Much upper echelons research focuses on the effect of CEO experience on firm performance outcomes. This paper extends this research stream using human and social capital theories as a framework to examine the effect of CEO experience on the performance of large diversified companies. Our analysis of 239 Fortune “500” companies finds that larger companies are more likely to select insiders and individuals who have more firm-specific experience to be their CEO. We also find that the selection of insiders and CEOs with more firm-specific experience is associated with significantly higher firm performance. These findings highlight the importance of the human and social capital possessed by company insiders, and shed additional light on the strategic leadership of large diversified companies and other complex organizations
Multiple solutions for asteroid orbits: Computational procedure and applications
We describe the Multiple Solutions Method, a one-dimensional sampling of the six-dimensional orbital confidence region that is widely applicable in the field of asteroid orbit determination. In many situations there is one predominant direction of uncertainty in an orbit determination or orbital prediction, i.e., a ``weak'' direction. The idea is to record Multiple Solutions by following this, typically curved, weak direction, or Line Of Variations (LOV). In this paper we describe the method and give new insights into the mathematics behind this tool. We pay particular attention to the problem of how to ensure that the coordinate systems are properly scaled so that the weak direction really reflects the intrinsic direction of greatest uncertainty. We also describe how the multiple solutions can be used even in the absence of a nominal orbit solution, which substantially broadens the realm of applications. There are numerous applications for multiple solutions; we discuss a few problems in asteroid orbit determination and prediction where we have had good success with the method. In particular, we show that multiple solutions can be used effectively for potential impact monitoring, preliminary orbit determination, asteroid identification, and for the recovery of lost asteroids
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