Comments on Stabilization of a Satellite

An expression is derived for the cost resultingfrom the stabilization of a satellite using aspecific optimal control (SOC). An exampleis given in which the optimal and SOC yieldvirtually identical costs.     

7. Dynamics,winds, circulation and turbulence in the atmosphere of venus

With the possible exception of the lowest one or two scale heights, the dominant mode of circulation of Venus' atmosphere is a rapid, zonal, retrograde motion. Global albedo variations in the ultraviolet may reflect planetary scale waves propagating relative to the zonal winds. Other special phenomena such as cellular convection in the subsolar region and internal gravity waves generated in the interaction of the zonal circulation with the subsolar disturbance may also be revealed in ultraviolet imagery of the atmosphere. We discuss the contributions of experiments on the Orbiter and Entry Probes of Pioneer Venus toward unravelling the mystery of the planet's global circulation and the role played by waves, instabilities and convection therein.     

On the Design of Suboptimal Controllers for Minimal Time Feedback Regulators

A method for designing suboptimal minimal time controllers for linear, time-invariant regulator systems is presented. An economical closed-loop controller is obtained by using Lyapunov surfaces, corresponding to a quadratic Lyapunov function, as a continuous approximation for isochronal surfaces. Systems employing this controller are obtained by minimization, with respect to parameters of the quadratic Lyapunov function, of the worst case upper bound on the arrival time of an arbitrary initial state at the origin.     

Recognition of fossil prokaryotes in Cretaceous methane seep carbonates: relevance to astrobiology

Recovery of prokaryotic body fossils from methane seep carbonates such as those of the Cretaceous Tepee Buttes of Colorado serves as a model for sampling in future astrobiological missions. The fossils, found primarily at the interface between paragenetic fabrics, suggest a sharp physicochemical gradient. Evidence of these microbial fossils occurs at a variety of scales. In the field, microbialite is found as meter-scale thrombolitic zones and centimeterscale stromatolitic crusts lining voids inferred to be the sites of ancient methane seepage. Petrographic fabrics suggestive of microbialite include indistinct peloids (0.1-1 mm in diameter) and crusts of authigenic micrite. Primary evidence obtained from scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy analysis comprises pinnate bacteria (0.3 microm in diameter and 1-1.5 microm long), sheaths (2-4 microm in diameter), coccoids (0.5-1 microm in diameter, up to 40 per cluster), and the presence of framboidal pyrite (6-8 microm in diameter). These results are in agreement with studies of other ancient and modern seeps and suggest a morphological conservatism of microbial form that can be incorporated into studies of extraterrestrial environments where it is presumed that reduced gases drive the metabolic activity of prokaryote-like organisms. Target areas that could serve as conduits for reduced gas seeps include tectonic or impact-driven faulting, zones of cryosphere melting, or other disruptions in crustal coherence. Ancient seeps, preserved as localized anomalous evaporite deposits in the sedimentary cover, could be detected by remote sensing.     

Expanding shells of young pulsars as sources of high-energy neutrinos

The initial power outputs P_o of pulsars are not yet well known, but these seem to follow approximately a distribution law N(> P_o) ∝ P_o⁻ⁿ where 0.5 ≤ n ≤ 1.0. It seems likely that P_o ≥ 10³⁸ ergs/sec. With these assumptions, we estimate that the DUMAND detector can record ≥ 10³ high-energy (> 4 TeV) neutrino events in a four-month period per Galactic supernova; (in our Galaxy, these are estimated to occur at the rate of about 8 per century.) Neutrinos from supernova shells in the Virgo supercluster would be marginally detectable (one very bright supernova per decade at about 20 Mpc) if N(> P_o) ∝ P_o^−0.5, but undetectable if N(> P_o) ∝ P_o⁻¹. The diffuse flux summed over distant extragalactic supernovae is likely to be well below the detection threshold.     

General Relativity Accuracy Test (GReAT): New configuration for the differential accelerometer

We presents the results of an activity concerning the test of the Einstein Weak Equivalence Principle with an accuracy of about 5 × 10⁻¹⁵. The experiment will be performed in an “Einstein elevator” using a differential accelerometer with a final sensitivity of about 10⁻¹⁴ g_⊕/Hz^1/2. The differential accelerometer is spun about an horizontal axis at a frequency in the range 0.5–1 Hz in order to modulate, during the free fall, the signal from a possible violation of the Equivalence Principle. In the paper the perturbing effects with the same signature of the possible violation are analyzed and constrained. The experimental results obtained in the laboratory with a first prototype of the differential accelerometer are discussed, comparing this results with those obtained using a new prototype.     

Eigenstructure Assignment for Linear Systems

The use of feedback (full state, output, and constrained output) is considered as a means of shaping the transient response of linear time invariant systems. The underlying importance of the eigenstructure (eigenvalues/eigenvectors) is highlighted. Also, the important results and techniques are presented along with a brief literature review. An extensive flight control example is presented which should give direction to the application of eigenstructure assignment in diverse areas.     

An Improved Two-Element Almost Omnidirectional Receiver

The sum of received power has been suggested as a method for providing improved signal detectability from a plurality of sensors while retaining omnidirectionality. This paper describes an alternative procedure for the two-element case that, while not quite omnidirectional, offers a decided advantage in detection performance over the power sum scheme. It involves taking the logical union of the thresholded power of the sum of the sensor outputs and the thresholded power of the difference of the sensor outputs. A comparison of the two systems is presented and it is shown that the latter scheme is almost always better than the former.     

10. The surface and interior of venus

Present ideas about the surface and interior of Venus are based on data obtained from (1) Earth-based radio and radar: temperature, rotation, shape, and topography; (2) fly-by and orbiting spacecraft: gravity and magnetic fields; and (3) landers: winds, local structure, gamma radiation. Surface features, including large basins, crater-like depressions, and a linear valley, have been recognized from recent ground-based radar images. Pictures of the surface acquired by the USSR's Venera 9 and 10 show abundant boulders and apparent wind erosion.On the Pioneer Venus 1978 Orbiter mission, the radar mapper experiment will determine surface heights, dielectric constant values and small-scale slope values along the sub-orbital track between 50°S and 75°N. This experiment will also estimate the global shape and provide coarse radar images (40–80 km identification resolution) of part of the surface. Gravity data will be obtained by radio tracking. Maps combining radar altimetry with spacecraft and ground-based images will be made. A fluxgate magnetometer will measure the magnetic fields around Venus.The radar and gravity data will provide clues to the level of crustal differentiation and tectonic activity. The magnetometer will determine the field variations accurately. Data from the combined experiments may constrain the dynamo mechanism; if so, a deeper understanding of both Venus and Earth will be gained.     

Dust hazard near Halley Comet in case of the VEGA project

The motion of dust particles near Halley Comet is studied and the probability of dust impacts with the spacecraft in case of the VEGA (Venus-Halley)- project is determined. The formation of a crater due to a particle impact with the dust shield is considered and the necessity for using a dual-sheet bumper shield is substantiated. The thickness of a front sheet that plays a role of the particle evaporator is estimated theoretically. The numerical experiment is carried out that simulates the dynamics of collision and evaporation of a particle. Three factors causing perforations of the rear sheet are discussed, i.e. dust penetrated through holes in the front sheet, gas jets and spall fragments of the front-sheet. The consideration of these factors makes it possible to estimate basic parameters of the dual-sheet bumper shield. Flexural vibrations of the front sheet under action of the reverse gaseous jet from the rear sheet are discussed that can affect essentially the shield strength. The perturbing effect of the dust and gas fluxes on the spacecraft is studied.