Interstellar medium parameters derived from low-frequency variability

Complete infraredJ andK light curves of the RS CVn system XY UMa have been obtained at the beginning of 1989. We point out some interesting morphological changes observed in these light curves.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Diffuse extragalactic gamma-radiation above ‘black-body cutoff’

The possible contribution of gamma-rays predicted within the universal cosmic-ray (CR) hypothesis to the energy range of CR spectrum above black-body cutoff is calculated. These gamma-rays arise from the relativistic electromagnetic cascade generated in the field of microwave background radiation (MBR). The ultra-high energy photons and electrons that initiate the cascade are produced at the decay of -mesons created in interactions of photons with the MBR. Simple analytic expression for cascade gamma-ray spectrum is obtained from the solution of kinetic equations for electrons and photons as well as for protons propagating in the MBR field. It is shown that at certain values of magnetic field and radio-wave density in the intergalactic space te flux of cascade gamma-rays may at least partly mask the black-body cutoff in the CR spectrum.Deceased, August 13, 1989.     

Strömgren photometry of a-stars: A test of physical parameter determination

By use of known published values forT _eff, logg, andM _v, a check on our procedure for determining the physical parameters of A v-type stars from Strömgren photometry has been performed. External errors for our calculated physical parameters have been obtained.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

A summary of major solar proton events

Solar proton events have been routinely detected by satellites since the 20th solar cycle; however, before that time only very major proton events were detected at the Earth. Even though the detection thresholds differed between the 19th and more recent cycles, more than 200 solar proton events with a flux of over 10 particles (cm² s ster)^–1 above 10 MeV have been recorded at the Earth in the last three solar cycles. At least 15% of these events had protons with energies greater than 450 MeV detected at the Earth. Other than an increase in solar proton event occurrence with increasing solar cycle, no recognizable pattern could be identified between the occurrence of solar proton events and the solar cycle. The knowledge we have gained from the data acquired over the past 40 years illustrates the difficulty in extrapolating back in time to infer the number and intensity of major solar proton events at the Earth.The U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.     

Ultraviolet spectroscopy of Asteroid (4) Vesta

We report a comprehensive review of the UV–visible spectrum and rotational lightcurve of Vesta combining new observations by Hubble Space Telescope and Swift Gamma-ray Burst Observatory with archival International Ultraviolet Explorer observations. The geometric albedos of Vesta from 220 nm to 953 nm are derived by carefully comparing these observations from various instruments at different times and observing geometries. Vesta has a rotationally averaged geometric albedo of 0.09 at 250 nm, 0.14 at 300 nm, 0.26 at 373 nm, 0.38 at 673 nm, and 0.30 at 950 nm. The linear spectral slope as measured between 240 and 320 nm in the ultraviolet displays a sharp minimum near a sub-Earth longitude of 20°, and maximum in the eastern hemisphere. This is consistent with the longitudinal distribution of the spectral slope in the visible wavelength. The photometric uncertainty in the ultraviolet is ∼20%, and in the visible wavelengths it is better than 10%. The amplitude of Vesta’s rotational lightcurves is ∼10% throughout the range of wavelengths we observed, but is smaller at 950 nm (∼6%) near the 1-μm band center. Contrary to earlier reports, we found no evidence for any difference between the phasing of the ultraviolet and visible/near-infrared lightcurves with respect to sub-Earth longitude. Vesta’s average spectrum between 220 and 950 nm can well be described by measured reflectance spectra of fine particle howardite-like materials of basaltic achondrite meteorites. Combining this with the in-phase behavior of the ultraviolet, visible, and near-infrared lightcurves, and the spectral slopes with respect to the rotational phase, we conclude that there is no global ultraviolet/visible reversal on Vesta. Consequently, this implies a lack of global space weathering on Vesta, as previously inferred from visible–near-infrared data.     

Measurement of the radius of Mercury by radio occultation during the MESSENGER flybys

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of Mercury in 2008–2009. During the first and third of those flybys, MESSENGER passed behind the planet from the perspective of Earth, occulting the radio-frequency (RF) transmissions. The occultation start and end times, recovered with 0.1 s accuracy or better by fitting edge-diffraction patterns to the RF power history, are used to estimate Mercury's radius at the tangent point of the RF path. To relate the measured radius to the planet shape, we evaluate local topography using images to identify the high-elevation feature that defines the RF path or using altimeter data to quantify surface roughness. Radius measurements are accurate to 150 m, and uncertainty in the average radius of the surrounding terrain, after adjustments are made from the local high at the tangent point of the RF path, is 350 m. The results are consistent with Mercury's equatorial shape as inferred from observations by the Mercury Laser Altimeter and ground-based radar. The three independent estimates of radius from occultation events collectively yield a mean radius for Mercury of 2439.2±0.5 km.     

On the galactic motion

A certain potential function is studied as a possible model for the galactic potential. Some solutions are obtained. Also the numerical study of some of the orbits and their stability is carried out.     

Parameters of Acoustic Signals Generated by the Atmospheric Meteoroid Explosion over Romania on January 7, 2015

System spectral analysis of temporal variations in the level of acoustic signals recorded at a number of European infrasound stations is carried out. The prevailing periods that varied within 3–5 s are found. Initial kinetic and acoustic energies of the Romanian meteoroid and infrasound parameters are calculated: celerity (280 m/s), acoustic efficiency, and stratospheric wind velocity (about 20 m/s). Parameters of cylindrical and explosive shock waves are found: duration, characteristic size, and amplitude. The dependence of the attenuation of infrasonic waves on distance is estimated. The results of the estimates are in good agreement with the results of the observations.     

Analytical study of the swing-by maneuver in an elliptical system

The objective of the present paper is to derive a set of analytical equations that describe a swing-by maneuver realized in a system of primaries that are in elliptical orbits. The goal is to calculate the variations of energy, velocity and angular momentum as a function of the usual basic parameters that describe the swing-by maneuver, as done before for the case of circular orbits. In elliptical orbits the velocity of the secondary body is no longer constant, as in the circular case, but it varies with the position of the secondary body in its orbit. As a consequence, the variations of energy, velocity and angular momentum become functions of the magnitude and the angle between the velocity vector of the secondary body and the line connecting the primaries. The “patched-conics” approach is used to obtain these equations. The configurations that result in maximum gains and losses of energy for the spacecraft are shown next, and a comparison between the results obtained using the analytical equations and numerical simulations are made to validate the method developed here.     

Determination of the optimal conditions for inclination maneuvers using a Swing-by

The search for methods to reduce the fuel consumption in orbital transfers is something relevant and always current in astrodynamics. Therefore, the maneuvers assisted by the gravity, also called Swing-by maneuvers, can be an advantageous option to save fuel. The proposal of the present research is to explore the influence of some parameters in a Swing-by of an artificial satellite orbiting a planet with one of the moons of this mother planet, with the goal of changing the inclination of the artificial satellite around the main body of the system. The fuel consumption of this maneuver is compared with the required consumption to perform the same change of inclination using the classical approach of impulsive maneuvers.