3,495 research outputs found
Extension of the sun-synchronous Orbit
No full textThrough careful consideration of the orbit perturbation force due to the oblate nature of the primary body a secular variation of the ascending node angle of a near-polar orbit can be induced without expulsion of propellant. Resultantly, the orbit perturbations can be used to maintain the orbit plane in, for example, a near-perpendicular (or at any other angle) alignment to the Sun-line throughout the full year of the primary body; such orbits are normally termed Sun-synchronous orbits [1, 2]. Sun-synchronous orbits about the Earth are typically near-circular Low-Earth Orbits (LEOs), with an altitude of less than 1500 km. It is normal to design a LEO such that the orbit period is synchronised with the rotation of the Earth‟s surface over a given period, such that a repeating ground-track is established. A repeating ground-track, together with the near-constant illumination conditions of the ground-track when observed from a Sun-synchronous orbit, enables repeat observations of a target over an extended period under similar illumination conditions [1, 2]. For this reason, Sun-synchronous orbits are extensively used by Earth Observation (EO) platforms, including currently the Environmental Satellite (ENVISAT), the second European Remote Sensing satellite (ERS-2) and many more. By definition, a given Sun-synchronous orbit is a finite resource similar to a geostationary orbit. A typical characterising parameter of a Sun-synchronous orbit is the Mean Local Solar Time (MLST) at descending node, with a value of 1030 hours typical. Note that ERS-1 and ERS-2 used a MLST at descending node of 1030 hours ± 5 minutes, while ENVISAT uses a 1000 hours ± 5 minutes MLST at descending node [3]. Following selection of the MLST at descending node and for a given desired repeat ground-track, the orbit period and hence the semi-major axis are fixed, thereafter assuming a circular orbit is desired it is found that only a single orbit inclination will enable a Sun-synchronous orbit [2]. As such, only a few spacecraft can populate a given repeat ground-track Sun-synchronous orbit without compromise, for example on the MLST at descending node. Indeed a notable feature of on-going studies by the ENVISAT Post launch Support Office is the desire to ensure sufficient propellant remains at end-of-mission for re-orbiting to a graveyard orbit to ensure the orbital slot is available for future missions [4]. An extension to the Sun-synchronous orbit is considered using an undefined, non-orientation constrained, low-thrust propulsion system. Initially the low-thrust propulsion system will be considered for the free selection of orbit inclination and altitude while maintaining the Sun-synchronous condition. Subsequently the maintenance of a given Sun-synchronous repeat-ground track will be considered, using the low-thrust propulsion system to enable the free selection of orbit altitude. An analytical expression will be developed to describe these extensions prior to then validating the analytical expressions within a numerical simulation of a spacecraft orbit. Finally, an analysis will be presented on transfer and injection trajectories to these orbits
Analytical sun synchronous low-thrust manoeuvres
No full textArticle describes analytical sun synchronous low-thrust manoeuvres
Solar sail mission applications and future advancement
No full textSolar sailing has long been envisaged as an enabling technology. The promise of open-ended missions allows consideration of radically new trajectories and the delivery of spacecraft to previously unreachable or unsustainable observation outposts. A mission catalogue is presented of an extensive range of potential solar sail applications, allowing identification of the key features of missions which are enabled, or significantly enhance, through solar sail propulsion. Through these considerations a solar sail application-pull technology development roadmap is established, using each mission as a technology stepping-stone to the next. Having identified and developed a solar sail application-pull technology development roadmap, this is incorporated into a new vision for solar sailing. The development of new technologies, especially for space applications, is high-risk. The advancement difficulty of low technology readiness level research is typically underestimated due to a lack of recognition of the advancement degree of difficulty scale. Recognising the currently low technology readiness level of traditional solar sailing concepts, along with their high advancement degree of difficulty and a lack of near-term applications a new vision for solar sailing is presented which increases the technology readiness level and reduces the advancement degree of difficulty of solar sailing. Just as the basic principles of solar sailing are not new, they have also been long proven and utilised in spacecraft as a low-risk, high-return limited-capability propulsion system. It is therefore proposed that this significant heritage be used to enable rapid, near-term solar sail future advancement through coupling currently mature solar sail, and other, technologies with current solar sail technology developments. As such the near-term technology readiness level of traditional solar sailing is increased, while simultaneously reducing the advancement degree of difficulty along the solar sail application-pull technology development roadmap
Solar sail trajectories at the Earth-Moon Lagrange points
No full textPaper presented during Session 3, Orbital Dynamics, Symposium C1, Astrodynamics, Paper Number 13. This paper investigates displaced periodic orbits at linear order in the circular restricted Earth-Moon system, where the third massless body is a solar sail. These highly non-Keplerian orbits are achieved using an extremely small sail acceleration. The solar sail Earth-Moon system differs greatly from the Earth-Sun system as the Sun line direction varies continuously in the rotating frame and the equations of motion of the sail are given by a set of nonlinear non-autonomous ordinary differential equations. By introducing a first-order approximation, periodic orbits are derived analytically at linear order. These approximate analytical solutions are utilized in a numerical search to determine displaced periodic orbits in the full nonlinear model. The importance of finding such displaced orbits is to obtain continuous communications between the equatorial regions of the Earth and the polar regions of the Moon. As will be shown, displaced periodic orbits exist at all Lagrange points at linear order
Novel mission concepts for polar coverage : An overview of recent developments and possible future applications
No full textThe paper provides a survey of novel mission concepts for continuous, hemispheric polar observation and direct-link polar telecommunications. It is well known that these services cannot be provided by traditional platforms: geostationary satellites do not cover high-latitude regions, while low- and medium-orbit Sun-synchronous spacecraft only cover a narrow swath of the Earth at each passage. Concepts that are proposed in the literature are described, including the pole-sitter concept (in which a spacecraft is stationary above the pole), spacecraft in artificial equilibrium points in the Sun-Earth system and non-Keplerian polar Molniya orbits. Additionally, novel displaced eight-shaped orbits at Lagrangian points are presented. For many of these concepts, a continuous acceleration is required and propulsion systems include solar electric propulsion, solar sail and a hybridisation of the two. Advantages and drawbacks of each mission concept are assessed, and a comparison in terms of high-latitude coverage and distance, spacecraft mass, payload and lifetime is presented. Finally, the paper will describe a number of potential applications enabled by these concepts, focusing on polar Earth observation and telecommunications
POLAR investigation of the Sun—POLARIS
No full textThe POLAR Investigation of the Sun (POLARIS) mission uses a combination of a gravity assist and solar sail propulsion to place a spacecraft in a 0.48 AU circular orbit around the Sun with an inclination of 75° with respect to solar equator. This challenging orbit is made possible by the challenging development of solar sail propulsion. This first extended view of the high-latitude regions of the Sun will enable crucial observations not possible from the ecliptic viewpoint or from Solar Orbiter. While Solar Orbiter would give the first glimpse of the high latitude magnetic field and flows to probe the solar dynamo, it does not have sufficient viewing of the polar regions to achieve POLARIS’s primary objective: determining the relation between the magnetism and dynamics of the Sun’s polar regions and the solar cycle
Solar sail capture trajectories at Mercury
No full textMercury is an ideal environment for future planetary exploration by solar sail since it has proved difficult to reach with conventional propulsion and hence remains largely unexplored. In addition, its proximity to the Sun provides a solar sail acceleration of order ten times the sail characteristic acceleration at 1 AU. Conventional capture techniques are shown to be unsuitable for solar sails and a new method is presented. It is shown that capture is bound by upper and lower limits on the orbital elements of the approach orbit and that failure to be within limits results in a catastrophic collision with the planet. These limits are presented for a range of capture inclinations and sail characteristic accelerations. It is found that sail hyperbolic excess velocity is a critical parameter during capture at Mercury, with only a narrow allowed band in order to avoid collision with the planet. The new capture methodis demonstrated for a Mercury sample return mission
Pattern transition in spacecraft formation flying via the artificial potential field method and bifurcation theory
No full textIn recent years many new and exciting space concepts have developed around spacecraft formation flying. This form of distributed system has the advantages of being extremely flexible and robust. This paper considers the development of new control methodologies based on the artificial potential function method and extends previous research in this area by considering bifurcation theory as a means of controlling the transition between different formations. For real, safety critical applications it is important to prove the stability of the system. This paper therefore aims to replace algorithm validation with mathematical proof through dynamical systems theory. Finally we consider the transition of formations at the Sun-Earth L2 point
Passive orbit control for space-based geo-engineering
No full textIn this Note we consider using solar sail propulsion to stabilize a spacecraft about an artificial libration point. It has been demonstrated that the constant acceleration from a solar sail can be used to generate artificial libration points in the Earth-Sun three-body problem. This is achieved by directing the thrust due to the sail such that it adds to the centripetal and gravitational forces. These libration points have the potential for future space physics and Earth observation missions. Of particular interest is the possibility of placing solar reflectors at the L1 artificial libration point to offset natural and human driven climate change. One engineering challenge that presents itself is that these artificial libration points are highly unstable and require active control for station-keeping. Previous work has shown that it is possible to stabilize a solar sail about artificial libration points using variations in both pitch and yaw angles. However, in a practical sense, solar sails are large structures and active control of the sail's attitude is a challenging engineering problem. Passive stabilization of such reflectors is to be investigated here to reduce the complexity of space-based geo-engineering schemes
La politica della legalità : il ruolo del giurista nell'età contemporanea
No full textAs the author of this book shows as he tests his theses, impartiality and justice require specific and scrupulous forms of civil engagement which render obsolete the traditional call for neutrality. At the same time, citizens must be made to understand how formal features of law that usually evoke their hostility actually guarantee their rights. The legal professional must learn to be a "relativist", obliged - through his choices - to establish priorities among distinct and often irreconcilable value domains
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