On the Stability of Artificial Equilibrium Points in the Circular Restricted Three Body Problem

The article analyses the stability properties of minimum-control artificial equilibrium points in the planar circular restricted three-body problem. It is seen that when the masses of the two primaries are of different orders of magnitude, minimum-control equilibrium is obtained when the spacecraft is almost coorbiting with the second primary as long as their mutual distance is not too small. In addition, stability is found when the distance from the second primary exceeds a minimum value which is a simple function of the mass ratio of the two primaries and their separation. Lyapunov stability under non-resonant conditions is demonstrated using Arnold’s theorem. Among the most promising applications of the concept we find solar-sail-stabilized observatories coorbiting with the Earth, Mars, and Venu

Power density of a bare electrodynamic tether generator

The maximum performance of bare electrodynamic tethers as power generating systems under OML-theory is analyzed. Results show that best performance in terms of power density is achieved by designing the tether in such a way to increase ohmic impedance with respect to plasma contact impedance, hence favoring longer and thinner tethers. In such condition the corresponding optimal value of the load impedance is seen to approach the ohmic impedance of the conducting tether. At the other extreme, when plasma contact impedance dominates (which is not optimal but can be relevant for some applications) optimum power generation is found by matching the load impedance with an effective tether-plasma contact impedance whose expression is derived

Non-averaged regularized formulations as an alternative to semi-analytical orbit propagation methods

This paper is concerned with the comparison of semi-analytical and non-averaged propagation methods for Earth satellite orbits. We analyse the total integration error for semi-analytical methods and propose a novel decomposition into dynamical, model truncation, short-periodic, and numerical error components. The first three are attributable to distinct approximations required by the method of averaging, which fundamentally limit the attainable accuracy. In contrast, numerical error, the only component present in non-averaged methods, can be significantly mitigated by employing adaptive numerical algorithms and regularized formulations of the equations of motion. We present a collection of non-averaged methods based on the integration of existing regularized formulations of the equations of motion through an adaptive solver. We implemented the collection in the orbit propagation code THALASSA, which we make publicly available, and we compared the non-averaged methods to the semi-analytical method implemented in the orbit propagation tool STELA through numerical tests involving long-term propagations (on the order of decades) of LEO, GTO, and high-altitude HEO orbits. For the test cases considered, regularized non-averaged methods were found to be up to two times slower than semi-analytical for the LEO orbit, to have comparable speed for the GTO, and to be ten times as fast for the HEO (for the same accuracy). We show for the first time that efficient implementations of non-averaged regularized formulations of the equations of motion, and especially of non-singular element methods, are attractive candidates for the long-term study of high-altitude and highly elliptical Earth satellite orbits.Comment: 33 pages, 10 figures, 7 tables. Part of the CMDA Topical Collection on "50 years of Celestial Mechanics and Dynamical Astronomy". Comments and feedback are encourage

Optimal Impulsive Collision Avoidance

The problem of optimal impulsive collision avoidance between two colliding objects in 3-dimensional elliptical Keplerian orbits is investigated with the purpose of establishing the optimal impulse direction and orbit location that give rise to the maximum miss distance following the maneuver. Closed-form analytical expressions are provided that predicts such distance and can be employed to perform a full optimization analysis. After verifying the accuracy of the expression for any orbital eccentricity and encounter geometry the optimum maneuver direction is derived as a function of the arc length separation between the maneuver point and the predicted collision point. The provided formulas can be used for high accuracy instantaneous estimation of the outcome of a generic impulsive collision avoidance maneuver and its optimizatio

Participation in the Analysis of the Far-Infrared/Submillimeter Interferometer

We have contributed to the development of the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) by analyzing various aspects related to the tethers that connect the spacecraft of this space interferometer. We have focused our analysis on key topics as follows: (a) helping in the configuration selection; (b) computing the system eigenfrequencies as a function of baseline length; (c) developing techniques and conceptual design of devices for damping the tether oscillations; (d) carrying out numerical simulations of tethered formation to assess the effects of environmental perturbations upon the baseline length variation; (e) devising control laws for fast retargeting of the interferometer at moderate baseline lengths; (f) estimating the survivability to micrometeoroid impacts of a tether at L2; and (g) developing a conceptual design of a high-strength and survivable tether

Earth Delivery of a Small NEO with an Ion Beam Shepherd

The possibility of capturing a small Near Earth Asteroid (NEA) and deliver it to the vicinity of the Earth has been recently explored by different authors. The key advantage would be to allow a cheap and quick access to the NEA for science, resource utilization and other purposes. Among the different challenges related to this operation stands the difficulty of robotically capturing the object, whose composition and dynamical state could be problematic. In order to simplify the capture operation we propose the use of a collimated ion beam ejected from a hovering spacecraft in order to maneuver the object without direct physical contact. The feasibility of a possible asteroid retrieval mission is evaluated

PENGARUH AUDIT INTERNAL DAN KUALITAS SISTEM INFORMASI AKUNTANSI PENJUALAN TERHADAP EFEKTIVITAS PENJUALAN (Studi Pada 3 Perusahaan BUMN Sektor Jasa di Kota Bandung)

Abstrak Penelitian ini bertujuan untuk mengetahui pengaruh audit internal, dan kualitas sistem informasi akuntansi penjualan terhadap efektivitas penjualan secara parsial maupun secara simultan. Penelitian menggunakan pendekatan kuantitatif. Teknik pengumpulan data yang digunakan adalah dengan menggunakan instrument kuesioner. Penelitian ini menggunakan data primer. Subjek penelitian ini adalah tiga (3) Persusahaan BUMN Sektor Jasa di Kota Bandung dan objek penelitiannya yaitu audit internal, kualitas sistem informasi akuntansi penjualan, dan efektivitas penjualan. Teknik pengambilan sampel yang digunakan adalah simple random sampling dengan jumlah responden sebanyak 57 orang. Analisis statistik yang digunakan dalam penelitian ini adalah uji asumsi klasik, analisis regresi, korelasi, analisis koefisien determinasi dan uji hipotesis yang diolah dengan menggunakan program SPSS 23,0 for windows. Hasil penelitian menunjukkan bahwa (1) secara parsial pengaruh audit internal sebesar 25,90% dan kualitas sistem informasi akuntansi sebesar 33% terhadap efektivitas. (2) secara simultan audit internal dan kualitas sistem informasi akuntansi penjualan terhadap efektivitas penjualan sebesar 58,9%, sedangkan sisanya sebesar 41,1% merupakan pengaruh faktor lain di luar variabel independen yang diteliti. Seperti Efektivitas Pengendalian Internal, Pengetahuan Akuntansi, Operasi Penjualan, Sistem Pengendalian Intern Penjualan, Efektivitas Pengendalian Piutang. Kata Kunci: Audit Internal, Kualitas Sistem Informasi Akuntansi Penjualan, Efektivita

Adaptive scheme for Accurate orbit propagation

Two extensions of the fast and accurate special perturbation method recently developed by Peláez et al. are presented for respectively elliptic and hyperbolic motion. A comparison with Peláez?s method and with the very efficient Stiefel- Scheifele?s method, for the problems of oblate Earth plus Moon and continuous radial thrust, shows that the new formulations can appreciably improve the accuracy of Peláez?s method and have a better performance of Stiefel-Scheifele?s method. Future work will be to include the two new formulations and the original one due to Peláez into an adaptive scheme for highly accurate orbit propagatio

Short-Term Space Occupancy and Conjunction Filter

Conjunction analysis for resident space objects (RSOs) requires estimating the range of altitudes that each object can cross, i.e. its space occupancy, throughout a screening time of, typically, a few days. The work proposes an analytical method to evaluate the space occupancy of an RSO for a time horizon of generic duration in a zonal problem model. The method, which allows one to compute the maximum and minimum occupied orbit radii by solving a quartic equation, is employed as a basis for the implementation of a new conjunction filter with a considerable performance improvement compared to the classical apogee-perigee filter. The effectiveness of the newly proposed filter is assessed in a low-Earth orbit scenario using a high-fidelity perturbation model and for a wide range of orbits and conjunction geometries. Finally, the space occupancy evaluation method is employed as a quick and effective tool to assess the degree of radial overlap of RSOs in low-earth orbit and its evolution in the last decade.Comment: preprint submitted to AIAA Journal of Guidance, Control and Dynamic