331 research outputs found
Phenomenological and statistical analysis of fracture in polycrystalline aluminum oxide
Phenomenological and statistical analysis of fracture in polycrystalline aluminum oxid
Chasing Nomadic Worlds: A New Class of Deep Space Missions
Nomadic worlds, i.e., objects not gravitationally bound to any star(s), are
of great interest to planetary science and astrobiology. They have garnered
attention recently due to constraints derived from microlensing surveys and the
recent discovery of interstellar planetesimals. In this paper, we roughly
estimate the prevalence of nomadic worlds with radii of . The cumulative number density
appears to follow a heuristic power law given by . Therefore, smaller objects are probably much more numerous
than larger rocky nomadic planets, and statistically more likely to have
members relatively close to the inner Solar system. Our results suggest that
tens to hundreds of planet-sized nomadic worlds might populate the spherical
volume centered on Earth and circumscribed by Proxima Centauri, and may thus
comprise closer interstellar targets than any planets bound to stars. For the
first time, we systematically analyze the feasibility of exploring these
unbounded objects via deep space missions. We investigate what near-future
propulsion systems could allow us to reach nomadic worlds of radius in a
-year flight timescale. Objects with km are within the purview
of multiple propulsion methods such as electric sails, laser electric
propulsion, and solar sails. In contrast, nomadic worlds with
km are accessible by laser sails (and perhaps nuclear fusion), thereby
underscoring their vast potential for deep space exploration.Comment: 22 pages including "Highlights" page; accepted by Acta Astronautic
Moduli-Space Approximation for BPS Brane-Worlds
We develop the moduli-space approximation for the low energy regime of
BPS-branes with a bulk scalar field to obtain an effective four-dimensional
action describing the system. An arbitrary BPS potential is used and account is
taken of the presence of matter in the branes and small supersymmetry breaking
terms. The resulting effective theory is a bi-scalar tensor theory of gravity.
In this theory, the scalar degrees of freedom can be stabilized naturally
without the introduction of additional mechanisms other than the appropriate
BPS potential. We place observational constraints on the shape of the potential
and the global configuration of branes.Comment: 10 pages, 1 figur
Report of the panel on earth rotation and reference frames, section 7
Objectives and requirements for Earth rotation and reference frame studies in the 1990s are discussed. The objectives are to observe and understand interactions of air and water with the rotational dynamics of the Earth, the effects of the Earth's crust and mantle on the dynamics and excitation of Earth rotation variations over time scales of hours to centuries, and the effects of the Earth's core on the rotational dynamics and the excitation of Earth rotation variations over time scales of a year or longer. Another objective is to establish, refine and maintain terrestrial and celestrial reference frames. Requirements include improvements in observations and analysis, improvements in celestial and terrestrial reference frames and reference frame connections, and improved observations of crustal motion and mass redistribution on the Earth
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