32,496 research outputs found
The astronaut and the banana peel: An EVA retriever scenario
To prepare for the problem of accidents in Space Station activities, the Extravehicular Activity Retriever (EVAR) robot is being constructed, whose purpose is to retrieve astronauts and tools that float free of the Space Station. Advanced Decision Systems is at the beginning of a project to develop research software capable of guiding EVAR through the retrieval process. This involves addressing problems in machine vision, dexterous manipulation, real time construction of programs via speech input, and reactive execution of plans despite the mishaps and unexpected conditions that arise in uncontrolled domains. The problem analysis phase of this work is presented. An EVAR scenario is used to elucidate major domain and technical problems. An overview of the technical approach to prototyping an EVAR system is also presented
CAD-model-based vision for space applications
A pose acquisition system operating in space must be able to perform well in a variety of different applications including automated guidance and inspections tasks with many different, but known objects. Since the space station is being designed with automation in mind, there will be CAD models of all the objects, including the station itself. The construction of vision models and procedures directly from the CAD models is the goal of this project. The system that is being designed and implementing must convert CAD models to vision models, predict visible features from a given view point from the vision models, construct view classes representing views of the objects, and use the view class model thus derived to rapidly determine the pose of the object from single images and/or stereo pairs
The rotation of the planet Mercury
Rotation of planet Mercury from radar observation explained by solar gravitational torque on tidal deformation and equatorial plane asymmetr
Nonlinear r-modes in a spherical shell: issues of principle
We use a simple physical model to study the nonlinear behaviour of the r-mode
instability. We assume that r-modes (Rossby waves) are excited in a thin
spherical shell of rotating incompressible fluid. For this case, exact Rossby
wave solutions of arbitrary amplitude are known. We find that:
(a) These nonlinear Rossby waves carry ZERO physical angular momentum and
positive physical energy, which is contrary to the folklore belief that the
r-mode angular momentum and energy are negative.
(b) Within our model, we confirm the differential drift reported by Rezzolla,
Lamb and Shapiro (1999).
Radiation reaction is introduced into the model by assuming that the fluid is
electrically charged; r-modes are coupled to electromagnetic radiation through
current (magnetic) multipole moments. We find that:
(c) To linear order in the mode amplitude, r-modes are subject to the CFS
instability, as expected.
(d) Radiation reaction decreases the angular velocity of the shell and causes
differential rotation (which is distinct from but similar in magnitude to the
differential drift reported by Rezzolla et al.) prior to saturation of the
r-mode growth. This is contrary to the phenomenological treatments to date,
which assume that the loss of stellar angular momentum is accounted for by the
r-mode growth. We demonstrate, for the first time, that r-mode radiation
reaction leads to differential rotation.
(e) We show that for l=2 r-mode electromagnetic radiation reaction is
equivalent to gravitational radiation reaction in the lowest post-Newtonian
order.Comment: 8 pages, no figures, uses MNRAS style, abstract abridged to fit into
24 line
Introduction to the physics of weightlessness
Equations of motion for describing weightlessness phenomena during space fligh
Critical phenomena at the threshold of black hole formation for collisionless matter in spherical symmetry
We perform a numerical study of the critical regime at the threshold of black
hole formation in the spherically symmetric, general relativistic collapse of
collisionless matter. The coupled Einstein-Vlasov equations are solved using a
particle-mesh method in which the evolution of the phase-space distribution
function is approximated by a set of particles (or, more precisely,
infinitesimally thin shells) moving along geodesics of the spacetime.
Individual particles may have non-zero angular momenta, but spherical symmetry
dictates that the total angular momentum of the matter distribution vanish. In
accord with previous work by Rein et al, our results indicate that the critical
behavior in this model is Type I; that is, the smallest black hole in each
parametrized family has a finite mass. We present evidence that the critical
solutions are characterized by unstable, static spacetimes, with non-trivial
distributions of radial momenta for the particles. As expected for Type I
solutions, we also find power-law scaling relations for the lifetimes of
near-critical configurations as a function of parameter-space distance from
criticality.Comment: 32 pages, 10 figure
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