Simulation of hypervelocity impacts using a contact charge

Two sets of hypervelocity impact experiments have been performed in the open using a contact charge technique and recorded using fast-framing cameras. It has been possible to record the uninterrupted ballistic trajectories of fragments from the catastrophically disrupted targets, together with their velocity and rotational properties directly after the impact, as well as their size. By performing these experiments in the open and on fairly soft ground, secondary fragmentation normally caused by impact onto the walls or floor of a test chamber has been minimised. A total of 10 experiments have been performed using targets of artificial rock which were either homogeneous, cored or carefully pre-fractured. We report here on the analysis of some of these data using a computer and special software written and developed by our group, with an indication of the results obtained.     

On the accuracy of line-of-sight velocity measurements using telluric lines as reference lines

It is demonstrated that telluric lines H₂O and O₂ are displaced as much as 0.2 mÅ as the solar disk is being scanned. The temporal variations of such displacements have a quasi-periodic character.     

Enhancement of on-board forecasting accuracy of GEO SC COG motion using the compensative transversal acceleration

A method is suggested for enhancing the on-board forecasting accuracy of the COG motion of a GEO SC with a long time of independent operation. The suggested method consists of introducing so-called compensative transversal acceleration (CTA), along with zonal harmonics into the right sides of the differential equations of SC motion among other disturbances due to the Earth’s gravitational field eccentricity. The CTA compensates the integral effect of the sectoral and tesseral harmonics; its value is constant for a specified point of GEO SC location (standing point) and is calculated on the Earth from numerical integration of differential equations of motion taking into account the complete set of gravitational field harmonics. The CTA value is transmitted on-board of an SC as program command data. The method is implemented in algorithms of on-board forecasting of Electro-L SC motion and can be used to enhance the on-board forecasting accuracy of the COG motion of GEO SCs with a long time of independent operation.     

On the accuracy of lunar ephemerides using the data provided by the future Russian lunar laser ranging system

The potential effect of the future Russian lunar laser ranging system (LLRS) on the accuracy of lunar ephemerides is discussed. In addition to the LLRS in Altai, several other observatories suitable for the LLRS installation are considered. The variation of accuracy of lunar ephemerides in the process of commissioning of new LLRS stations is estimated by mathematical modeling. It is demonstrated that the error in the determination of certain lunar ephemeris parameters may be reduced by up to 16% after seven years of operation of the Altai LLRS with a nearly optimal observational program.     

Simulation of Titan haze formation using a photochemical flow reactor: The optical constants of the polymer

Solar UV is the principal energy source impinging the atmosphere of Titan while the energy from the electrons in Saturn's magnetosphere is less than 0.5% of the UV light. Titan haze analogs were prepared by the photolysis of a mixture of gases that simulate the composition of its atmosphere (nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene). The real (n) and imaginary (k) parts of the complex refractive index of haze analogs formed from four different gas mixtures were calculated from the spectral properties of the solid polymer in UV-visible, near infrared and infrared wavelength spectral regions. The value of n was constant at 1.6±0.1 throughout the 0.2-2.5 μm region. The variation of k with wavelength for the values derived for Titan has a lower error than the absolute values of k so the more significant comparisons are with the slopes of the k(λ) plots in the UV-VIS region. Three of the photochemical Titan haze analogs had slopes comparable to those derived for Titan from the Voyager data (Rages and Pollack, 1980, Icarus 41, 119-130; McKay and Toon, 1992, in: Proceedings of the Symposium on Titan, in: ESA SP, Vol. 338, pp. 185-190). The slopes of the k(λ) plots for haze analogs prepared by spark discharge (Khare et al., 1984, Icarus 60, 127-137) and plasma discharge (Ramirez et al., 2002, Icarus 156, 515-529) were also comparable to Titan's. These finding show that the k(λ) plots do not differentiate between different laboratory simulations of atmospheric chemistry on Titan in the UV-VIS near IR region (0.2-2.5 microns). There is a large difference between the k(λ) in the infrared between the haze analogs prepared photochemically and analogs prepared using a plasma discharges (Khare et al., 1984, Icarus 60, 127-137; Coll et al., 1999, Planet. Space Sci. 47, 1331-1340; Khare et al., 2002, Icarus 160, 172-182). The C/N ratio in the haze analog prepared by discharges is in the 2-11 range while that of the photochemical analogs is in the 18-24 range. The use of discharges and UV light for initiating the chemistry in Titan's atmosphere is discussed.     

Effect of the alidade thermal behavior on the pointing accuracy of a large radio telescope

The alidade's non-uniform temperature field of a large radio telescope is very obvious under solar radiation. Estimating a radio telescope's pointing errors, caused by the alidade deformation under solar radiation, is significant to improve the telescope's pointing accuracy. To study the effect of the alidade thermal behavior on the pointing accuracy of a large radio telescope, a temperature experiment is first carried out in a 70-m radio telescope on a sunny day. According to the measured results, the temperature distribution rule of the alidade is summarized initially. In addition, the alidade's temperature field is calculated by finite element thermal analysis. The simulated results are proved to be in good agreement with the experimental results. Finally, the alidade deformation under solar radiation is computed by finite element thermalstructure coupling analysis. The telescope's pointing errors caused by alidade deformation are estimated via the alidade's node displacements. The final results show that the effect of alidade thermal behavior on the telescope's elevation pointing errors ?ε_2 + ?ε_r is much more than the effect on the telescope's crosselevation pointing errors ?ε_1. The maximum of ?ε_2 + ?ε_r is more than 45, while the maximum of ?ε_1 is less than 6. This study can provide valuable references for improving the pointing accuracy of large radio telescopes.     

Simulation of extensive air showers at ultra-high energy using the CORSIKA Monte Carlo code

Extensive simulations have been carried out with CORSIKA version 5.62 to investigate the general properties of giant cosmic air shower in the energy range 1–100 EeV. The comparison between protons, heavy nuclei and γ initiated showers exhibits unexpected and interesting features. The apparent muon electron ratio at great distances (1.5 km from axis) tends to be comparable at ultra-high energy in both photon-induced cascades and hadronic cascades (compensation between the enhancement versus energy of photo-production cross-section and of the decrease of both pair production cross-section and the bremsstrahlung cross-section, with Landau–Pomeranchuk–Migdal effect); for proton and nuclei primaries, a correlation with lateral electron profile suggests a new energy estimator, in complement to electrons size or density at 600 m, suitable for the determination of the total primary energy spectrum. Another tendency is the local contrast in the abundance of positive and negative muons (with a possible ellipticity in the lateral muon distribution) induced by the geomagnetic field, especially visible for some azimuthal and zenith angles. These distortions are more intense for heavy primaries; they can be exploited on the most favorable horizontal axis or areas, for the discrimination between nuclei and protons.     

Effects of a micrometeorite stream on the accuracy of astrometric measurements by scanning space-borne instruments

The effect of micrometeorite impacts upon the surface of a spacecraft on the accuracy of astrometric measurements made by scanning with instruments onboard the spacecraft is considered. This effect is shown to be marginal for HIPPARCOS measurements. However, disregarding this kind of effect for all the currently projected spacecraft may result in the declared measurement accuracy being unachievable. Spacecraft maintaining constant spatial orientation during measurements are essentially not subject to the errors caused by collisions with micrometeorites.     

Limits to the accuracy of the 10.7 cm flux

The 10.7 cm flux data, which are widely used as an index of solar activity, are actually spot measurements of the solar flux density at 10.7 cm wavelength, made three times each day, usually at 17:00, 20:00, and 23:00 UT. These values, or the 20:00 UT determination alone, are frequently used as the average flux for that day. Since each spot measurement takes about one hour to make, and the Sun's emissions at that wavelength can vary over time scales shorter than the intervals between the measurements, the data are unavoidably undersampled. Radio emissions from transient events, such as flares, are defined as contaminants of the flux, and largely-empirical procedures have evolved which are used to filter them from the data. The utility of theF _10.7 index over more than 40 years suggests that the consequences of the under-sampling and the use of largely-empirical data filters are not serious. However, as new applications of the flux data appear, and existing ones become more quantitative, we need to better understand the accuracy of data as estimates of the 10.7 cm flux index, and to know how much precision we can reasonably expect to attain. In this paper we describe part of a study aimed at estimating how good the spot measurements are as estimators of the ‘daily-average’ flux. By a combination of measurement and modelling, the contributions to the flux monitor output truly due to the Sun are separated from the non-solar signals. We then derive the daily average 10.7 cm flux values and compare them with the spot measurements. We find that in general, the spot measurements are usually within a percent or so of the daily-average fluxes.     

The celestial bodies motion theories: on the required accuracy of the motion theories of the perturbing bodies

The equation for calculation of the required accuracy of the perturbing bodies motion theories is obtained. The equation relates the accuracy required to take into account perturbing acceleration, acting on the perturbed body, with the accuracy of the motion theory of the perturbing body. The solutions for estimation of the required accuracy both for the inner and the external cases in the spherical coordinates are coincided. The solution for the calculation of the required accuracy for the general case (combining the inner and the external cases) in Cartesian coordinates is obtained. The special cases for the solution in Cartesian coordinates are studied. As an example, the estimations of the required accuracy of the motion theories of the solar system planets for some perturbed bodies (the near-Earth asteroid 4179 Toutatis, the main belt asteroid 208 Larcimosa, the trojan asteroid 588 Achilles, the centaur asteroid 5145 Pholus, the Kuiper belt asteroid 1995 QZ9, the comet Halley) are obtained. The conditions of the use of the obtained results are discussed.     

On the accuracy in the numerical solution of theN-body problem

There exist several widely used methods that give a qualitative estimation of the accuracy of the results in the numerical solution of theN-body problem. The reverse and closure tests are examined here critically. The author has developed a method for the estimation of global errors propagated in the numerical solution of ordinary and partial differential equations that has proven to be rather efficient in numerous cases (see P. E. Zadunaisky [17]). Applications of the method to several cases of theN-body problem are presently made and the advantages and limitations of the method are shown in a set of examples.     

Simulation of the full two rigid body problem using polyhedral mutual potential and potential derivatives approach

Herein we investigate the coupled orbital and rotational dynamics of two rigid bodies modelled as polyhedra, under the influence of their mutual gravitational potential. The bodies may possess any arbitrary shape and mass distribution. A method of calculating the mutual potential’s derivatives with respect to relative position and attitude is derived. Relative equations of motion for the two body system are presented and an implementation of the equations of motion with the potential gradients approach is described. Results obtained with this dynamic simulation software package are presented for multiple cases to validate the approach and illustrate its utility. This simulation capability is useful both for addressing questions in dynamical astronomy and for enabling spacecraft missions to binary asteroid systems.     

The accuracy of proper orbital elements and the properties of asteroid families: Comparison with the linear theory

The accuracy and reliability of the proper orbital elements used to define asteroid families are investigated by simulating numerically the dynamical evolution of families assumed to arise from the “explosion” of a parent object. The orbits of the simulated family asteroids have then been integrated in the frame of the elliptic restricted three-body problem Sun-Jupiter-asteroid, for times of the order of the circulation periods of perihelia and nodes. By filtering out short-periodic perturbations, we have monitored the behavior of the proper eccentricities and inclinations, computed according to the linear secular perturbation theory. Significant long-period variations have been found especially for families having nonnegligible eccentricities and/or inclinations (like the Eos family), and strong disturbances due to the proximity of mean motion commensurabilities with Jupiter have been evidenced (for instance, in the case of the Themis family). These phenomena can cause a significant “noise” on the proper eccentricities and inclinations, probably affecting in some cases the derived family memberships. They can also give rise to a spurious anisotropy in the fragment ejection velocity fields computed from the dispersion in proper elements observed in each family, and this could explain the puzzling anisotropies of this kind actually found in real families by D. Brouwer (1951, Astron. J. 56, 9–32) and by V. Zappalà, P. Farinella, Z. Knežević, and P. Paolicchi (1984), Icarus 59, 261–285).