New applications of the H-reversal trajectory using solar sails

Advanced solar sailing has been an increasingly attractive propulsion system for highly non-Keplerian orbits.Three new applications of the orbital angular momentum reversal(H-reversal) trajectories using solar sails are presented:space observation,heliocentric orbit transfer and collision orbits with asteroids.A theoretical proof for the existence of double H-reversal trajectories(referred to as‘H2RTs’) is given,and the characteristics of the H2RTs are introduced before a discussion of the mission applicati...     

Planar heliocentric roto-translatory motion of a spacecraft with a solar sail of complex shape

A complete treatment of the general motion of rotation and translation of a solar-sail spacecraft is proposed for the non-flat sail of complex shape. The planar heliocentric roto-translatory motion is considered, orbit-rotational coupling in the problem of altitude and orbital sail motion is investigated for the two-folding sail formed by two unequal reflective rectangular plates oriented at a right angle. The problem of orbit-rotational coupling is essentially a planar one: both sail plates are orthogonal to the orbital plane. The possibility of the non-controlled interplanetary transfer with such two-folding sail at its passive radiational orientation is established analytically from point of view of orbit-rotational coupling. Optimal geometric proportions of this sail are found at minimum-time interplanetary transfers.     

Analytical results for solar sail optimal missions with modulated radial thrust

The aim of this paper is to analyze the optimal trajectories of a spacecraft subjected to a modulated radial thrust, whose magnitude is inversely proportional to the square of the distance from the primary body. This case is representative of a Sun-facing solar sail with a passive attitude control system. In this study the sailcraft is assumed to perform a finite number of reorientation maneuvers to set the propelling acceleration to zero and generate suitable coasting arcs along the trajectory. Accordingly, the resulting generalized orbit is a sequence of either propelled or ballistic conic arcs, whose main characteristics (in terms of semimajor axis, eccentricity, and perihelion radius) can be calculated in closed form. As a result, the sailcraft optimal performance can be studied using an analytical approach. In particular, some compact relationships are drawn and discussed that allow one to find the optimal sailcraft characteristics required to reach a prescribed final orbit.     

Dynamics and control of a solar collector system for near Earth object deflection

A solar collector system is a possible method using solar energy to deflect Earth-threatening near-Earth objects.We investigate the dynamics and control of a solar collector system including a main collector (MC) and secondary collector (SC).The MC is used to collect the sunlight to its focal point,where the SC is placed and directs the collected light to an asteroid.Both the relative position and attitude of the two collectors should be accurately controlled to achieve the desired optical path.First,the dy...     

The angular momenta of solar system bodies: Implications for asteroid strengths

The angular momentum H is plotted versus mass M for the planets and for all asteroids with known rotation rates and shapes, primarily taken from D. C. McAdoo and J. A. Burns [Icarus18, 285–293 (1973)]. An asteroid's angular momentum is derived from its rotation rate as determined by the period of its lightcurve, its shape as indicated by the lightcurve amplitude, and where possible its size as given by polarimetry or radiometry. The asteroid is assumed to be rotating about its axis of maximum moment of inertia. As previously found by F. F. Fish [Icarus7, 251–256 (1967]) and W. K. Hartmann and S. M. Larson [Icarus7, 257–260 (1967)], H is approximately proportional to $\text{M}{}^{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}$, which shows that the asteroids and most planets spin with nearly the same rate. The very smallest asteroids on the plot deviate from the above reaction, usually containing excess angular momentum. This suggests that collisions have transferred substantial angular momentum to the smallest asteroids, perhaps causing their internal stress states to be substantially modified by centrifugal effects.The forces produced by gravitation are then compared to centrifugal effects for a rotating, triaxial ellipsoid of density 3 g cm^?3. For all asteroids with known properties the gravitational attraction is shown to be larger than the centrifugal acceleration of a particle on the surface: thus the observed asteroid regoliths are gravitationally bound. Poisson's equation for the gravitational potential is investigated and it is shown by mathematical and physical arguments that any arbitrarily shaped ellipsoid with the attractive surface force boundary condition found above will have only attractive internal forces. Thus the internal stress states in asteroids are always compressive so that asteroids could be internally fractured without losing their integrity.     

Towards a model for the solar dynamo

We study a mean field model of the solar dynamo, in which the non-linearity is the action of the azimuthal component of the Lorentz force of the dynamo-generated magnetic field on the angular velocity. The underlying zero-order angular velocity is consistent with recent determinations of the solar rotation law, and the form of the alpha effect is chosen so as to give a plausible butterfly diagram. For small Prandtl numbers we find regular, intermittent and apparently chaotic behaviour, depending on the size of the alpha coefficient. For certain parameters, the intermittency displays some of the characteristics believed to be associated with the Maunder minimum. We thus believe that we are capturing some features of the solar dynamo.     

Attitude stability of a spacecraft with two flexible solar arrays on a stationary orbit around an asteroid subjected to gravity gradient torque

In the gravity field of an asteroid with the second order and degree harmonics C ₂₀ and C ₂₂, the attitude stability of a spacecraft with two flexible solar arrays on a stationary orbit subjected to the fourth-order gravity gradient torque is investigated in this paper. The sufficient conditions of attitude stability of the spacecraft are obtained, the effect of the direction of the flexible solar arrays and some special cases are discussed. Taking the asteroids 4769 Castalia, 25143 Itokawa and the imaginary asteroids as examples, the attitude stability domains, determined by the sufficient conditions, of the spacecrafts moving on stationary orbits around them are presented. It is found that the attitude stability domains of the spacecraft with two flexible solar arrays are evidently different when the solar arrays are installed in different directions; the effect of the harmonics C ₂₀ and C ₂₂ of the asteroids has the significant influence on the attitude stability domains of the spacecrafts with flexible appendages moving on stationary orbits; in the certain case, the effect of the harmonics C ₂₀ and C ₂₂ of the asteroids has no influence on the attitude stability domains of the rigid spacecrafts moving on stationary orbits, but in the other cases, the effect of the harmonics C ₂₀ and C ₂₂ of the asteroids has also the significant influence on the attitude stability domains of the rigid spacecrafts moving on stationary orbits; whether the harmonics C ₂₀ and C ₂₂ of the asteroids are considered or not, the effect of flexible appendages decreases the attitude stability domains.     

Variations of the IMF Bz component over a solar activity cycle for different regimes of solar wind formation

The behavioural features of the IMF B_z component for different solar wind velocity regimes have been studied. The study revealed a significant difference in variations of the B_z component between high-speed and low-speed regimes. Formation mechanisms for the IMF meridional component as well as the relationship of B_z with dynamical properties of the large-scale magnetic fields on the Sun are discussed.     

Composite lightcurve of asteroid Moskva (787)

89 CCD frames of the asteroid Moskva (787) were obtained over six nights. Lightcurves have been acquired for each night. We suggest a possible composite lightcurve with a corresponding rotational period of 0.4 days. This is to the knowledge of the authors the first lightcurve of Moskva(787) to be published.     

On the possibility of a bimodal solar dynamo

A simple way to couple an interface dynamo model to a fast tachocline model is presented, under the assumption that the dynamo saturation is due to a quadratic process and that the effect of finite shear layer thickness on the dynamo wave frequency is analogous to the effect of finite water depth on surface gravity waves. The model contains one free parameter which is fixed by the requirement that a solution should reproduce the helioseismically determined thickness of the tachocline. In this case it is found that, in addition to this solution, another steady solution exists, characterized by a four times thicker tachocline and 4–5 times weaker magnetic fields. It is tempting to relate the existence of this second solution to the occurrence of grand minima in solar activity. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evidence for a long-term variation of the dynamo action responsible for the solar magnetic cycle

By using the sunspot time series as a proxy, we have made a detailed analysis of the mean solar magnetic field over the last two and half centuries, by means of a reconstruction of its phase space. We find evidence of a long-term trend variation of some of the solar physical processes (over a few decades) that might be responsible for the apparent erratic behaviour of the solar magnetic cycle. The analysis is done by means of a careful study of the axisymmetric dynamo model equations, where we show that the temporal counterpart of the magnetic field can be described by a self-regulated two-dimensional dynamic system, usually known as a Van der Pol–Duffing oscillator. Our results suggest that during the last two and half centuries, the velocity of the meridional flow, v _p, and the efficiency of the α mechanism responsible for the conversion of toroidal magnetic field into poloidal magnetic field might have suffered variations that can explain the observed variability in the solar cycle.     

Heating of the solar and stellar coronae: a review

Despite great advances in observations and modelling, the problem of solar and stellar heating still remains one of the most challenging problems of space physics. To find a definite answer to what sort of mechanisms act to heat the plasma to a few million degrees requires a collaborative effort of small scales observations, large capacity numerical modelling and complicated theoretical approaches. A unique theory should incorporate aspects such as the generation of energy, its transport and dissipation. Up to now, the first two problems are rather clarified. However, the modality of transfer of magnetic or kinetic energy into heat is a question still awaiting for an answer. In the present paper we review the various popular heating mechanisms put forward in the existing extensive literature. The heating processes are, somewhat arbitrarily, classified as hydrodynamic, magnetohydrodynamic or kinetic based on the characteristics of the model medium. These mechanisms are further divided based on the time scales of the ultimate dissipation involved (i.e. AC and DC heating, turbulent heating). In particular, special attention is paid to discuss shock dissipation, mode coupling, resonant absorption, phase mixing, and, reconnection. Finally, we briefly review the various heating mechanisms proposed to heat other stars. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of solar radiation pressure on asteroid surface hopping transfers for high area/mass ratio rovers

The high area/mass ratio hopping rovers have potential applications in future asteroid surface exploration. This paper systematically investigates the effects of solar radiation pressure(SRP) on ballistic surface hopping transfers for the asteroid 101955 Bennu. Effects of SRP on the traveled distance and the trajectory design of hopping transfers are analyzed and summarized. The simulation results indicate that it is necessary to take SRP into account to ensure the success of hopping transfers and the proper use of SRP can help design the trajectories of hopping transfers with low initial impulses and short transfer times. It also reveals the potential possibility in using SRP to control the post-hopping transfers with specific control policies in the future surface exploration of asteroids.