106,266 research outputs found
Mission analysis and systems design of a near-term and far-term pole-sitter mission
No full textThis paper provides a detailed mission analysis and systems design of a near-term and far-term polesitter mission. The pole-sitter concept was previously introduced as a solution to the poor temporal resolution of polar observations from highly inclined, low Earth orbits and the poor high-latitude coverage from geostationary orbit. It considers a spacecraft that is continuously above either the north or south pole and, as such, can provide real-time, continuous and hemispherical coverage of the polar regions. Being on a non-Keplerian orbit, a continuous thrust is required to maintain the pole-sitter position. For this, two different propulsion strategies are proposed, which result in a near-term pole-sitter mission using solar electric propulsion (SEP) and a far-term pole-sitter mission where the SEP thruster is hybridized with a solar sail. For both propulsion strategies, minimum propellant pole-sitter orbits are designed. In order to maximize the spacecraft mass at the start of the operations phase of the mission, the transfer from Earth to the pole-sitter orbit is designed and optimized assuming either a Soyuz or an Ariane 5 launch. The maximized mass upon injection into the pole-sitter orbit is subsequently used in a detailed mass budget analysis that will allow for a trade-off between mission lifetime and payload mass capacity. Also, candidate payloads for a range of applications are investigated. Finally, transfers between north and south pole-sitter orbits are considered to overcome the limitations in observations due to the tilt of the Earth’s rotational axis that causes the poles to be alternately situated in darkness. It will be shown that in some cases these transfers allow for propellant savings, enabling a further extension of the pole-sitter mission
Novel pole-sitter mission concepts for continuous polar remote sensing
No full textThe pole-sitter concept is a solution to the poor temporal resolution of polar observations from highly inclined, low Earth orbits and the poor high latitude coverage from geostationary orbit. It considers a spacecraft that is continuously above either the North or South Pole and, as such, can provide real-time, continuous and hemispheric coverage of the polar regions. Despite the significant distance from the Earth, the utility of this platform for Earth observation and telecommunications is clear, and applications include polar weather forecasting and atmospheric science, glaciology and ice pack monitoring, ultraviolet imaging for aurora studies, continuous telecommunication links with polar regions, arctic ship routing and support for future high latitude oil and gas exploration. The paper presents a full mission design, including launch (Ariane 5 and Soyuz vehicles), for two propulsion options (a near-term solar electric propulsion (SEP) system and a more advanced combination of a solar sail with an SEP system). An optional transfer from the North Pole to South Pole and vice-versa allows viewing of both poles in summer. The paper furthermore focuses on payloads that could be used in such a mission concept. In particular, by using instruments designed for past deep space missions (DSCOVR), it is estimated that resolutions up to about 20 km/pixel in the visible wavelengths can be obtained. The mass of these instruments is well within the capabilities of the pole-sitter design, allowing an SEP-only mission lifetime of about 4 years, while the SEP/sail propulsion technology enables missions of up to 7 years
Mission analysis and systems design of a near-term and far-term pole-sitter mission
No full textThis paper provides a detailed mission analysis and systems design of a near-term and far-term pole-sitter mission. The pole-sitter concept was previously introduced as a solution to the poor temporal resolution of polar observations from highly inclined, low Earth orbits and the poor high latitude coverage from geostationary orbit. It considers a spacecraft that is continuously above either the North or South Pole and, as such, can provide real-time, continuous and hemispherical coverage of the polar regions. Being on a non-Keplerian orbit, a continuous thrust is required to maintain the pole-sitter position. For this, two different propulsion strategies are proposed, which result in a near-term pole-sitter mission using solar electric propulsion (SEP) and a far-term pole-sitter mission where the SEP thruster is hybridized with a solar sail. For both propulsion strategies, minimum propellant pole-sitter orbits are designed. In order to maximize the spacecraft mass at the start of the operations phase of the mission, the transfer from Earth to the pole-sitter is designed and optimized assuming either a Soyuz or an Ariane 5 launch. The maximized mass upon injection into the polesitter orbit is subsequently used in a detailed mass budget analysis that will allow for a trade-off between mission lifetime and payload mass capacity. Also, candidate payloads for a range of applications are investigated. Finally, transfers between north and south pole-sitter orbits are considered to overcome the limitations in observations due to the tilt of the polar axis that causes the Poles to be alternately situated in darkness. It will be shown that in some cases these transfers allow for propellant savings, enabling a further extension of the pole-sitter mission
Einstein-Yang-Mills black holes in anti de-Sitter space
No full textIn this thesis we consider Einstein-Yang-Mills black holes in asymptotically anti-de Sitter space, in the presence of an su(N) gauge �eld. For a purely magnetic gauge �eld we de�ne a set of charges, namely the mass and N - 1 gauge invariant magnetic charges, and show that they characterize stable black holes.
We then go on to consider dyonic black holes which carry both electric and magnetic charge. We investigate spherically symmetric black holes and solitons, and �nd equations of motion for solutions with su(N) gauge �elds. These equations are solved numerically to �nd black hole and soliton solutions with su(2) and su(3) gauge groups.
We then turn to dyonic black holes with planar event horizons and investigate their suitability as gravitational analogues to high temperature superconductors under the AdS/CFT correspondence. We generalise a previously known ansatz for su(2) gauge groups to su(N), and show that there is a critical temperature above which non-abelian solutions do not exist. Below this critical temperature, we show that they are thermodynamically favoured over equivalent Reissner-Nordstr�om solutions, and have in�nite D.C. conductivity
Extending AdS/CFT: dual states for new geometries
No full textIn this thesis we present research that extends our knowledge of the AdS/CFT correspondence; in particular we look at various non-supersymmetric Spacetimes and their conjectured dual field theory states. We consider known U(l) xU(l) invariant spaces and investigate the requirements for smoothness, which results in the construction of new smooth non-supersymmetric soliton solutions with Dl, D5 and momentum charges. We are able to identify dual states for these geometries in the field theory describing D1-D5 systems. Also discussed are interesting aspects of these Spacetimes and new orbifold solutions which are valid string backgrounds. In addition to this, we study time-dependent Spacetimes which are asymptotically locally anti-de Sitter. There are two different Spacetimes with the same asymptotics: the 'bubble of nothing' solutions and higher dimensional BTZ black holes, which are both asymptotically locally anti de Sitter and whose conformal boundaries are both conformal to de Sitter space times a circle. We use the AdS/CFT correspondence to give a description of the spacetimes in the dual field theory. We are also able to relate horizons and their thermodynamic quantities in the bulk and boundary spacetimes and are able to assign entropy to non-compact horizons
Quantum scalar field theory on anti-de Sitter space
No full textThis thesis considers a real massive free quantum scalar field propagating with arbitrary coupling to -dimensional anti-de Sitter space.
Analytical expressions are found for the field modes and Feynman Green function. The condition for the equivalence of rotational vacuum states is also established. The rotational and thermal anti-commutator functions are then derived.
A method is developed for computing the Hadamard renormalised vacuum and thermal expectation values of the quadratic field fluctuations and the stress-energy tensor. Results are obtained for to , satisfying Wald's axioms and exhibiting the trace anomaly
A near term pole-sitter using hybrid solar sail propulsion
No full textIn this paper we investigate optimal pole-sitter orbits using hybrid solar sail and solar electric propulsion(SEP). A pole-sitter is a spacecraft that is constantly above one of the Earth's poles. Optimal orbits, that minimize propellant mass consumption, are designed using a shape-based first guess followed by an optimal control problem solved with a direct method. SEP and hybrid spacecraft are compared in terms of payload mass fraction and mission lifetime, investigating the conditions under which the hybrid sail allows saving on the spacecraft initial mass. It is proposed that a hybrid solar sail and SEP system may be a means of enabling challenging long-duration, high energy missions by using a modest solar sail to enhance the performance of existing SEP technology
Design of optimal Earth pole-sitter transfers using low thrust propulsion
No full textRecent studies have shown the feasibility of an Earth pole-sitter mission using low-thrust propulsion. This mission concept involves a spacecraft following the Earth's polar axis to have a continuous, hemispherical view of one of the Earth's poles. Such a view will enhance future Earth observation and telecommunications for high latitude and polar regions. To assess the accessibility of the pole-sitter orbit, this paper investigates optimum Earth pole-sitter transfers employing low-thrust propulsion. A launch from low Earth orbit (LEO) by a Soyuz Fregat upper stage is assumed after which a solar-electric-propulsion thruster transfers the spacecraft to the pole-sitter orbit. The objective is to minimise the mass in LEO for a given spacecraft mass to be inserted into the pole-sitter orbit. The results are compared with a ballistic transfer that exploits the manifolds winding off the pole-sitter orbit. It is shown that, with respect to the ballistic case, low-thrust propulsion can achieve significant mass savings in excess of 200 kg for a pole-sitter spacecraft of 1000 kg upon insertion. To finally obtain a full low-thrust transfer from LEO up to the pole-sitter orbit, the Fregat launch is replaced by a low-thrust, minimum time spiral through an orbital averaging technique, which provides further mass savings, but at the cost of an increased time of flight
Gravity in spacetimes with cosmological constants
No full textThis thesis is composed of two parts: gravity in the spacetime with a negative/positive cosmological constant. The first part, which is the negative case, devotes to constructing the IIB supergravity dual solution in AdS/CFT correspondence for N = (1, 0) and N = (1/2, 0) non-anticommutative deformed super Yang-Mills theory. The non-anticommutativity is realised on N D3-branes in certain constant self-dual RR 5-form background fields. These background fields can be sourced by a set of additional D3-branes intersecting the N D3's. By taking the near horizon limit to the brane configurations, the supergravity solutions are obtained. The mapping between the bulk scalar fields and the boundary operators for N = (1, 0) case is investigated, and it is found that the spectrum of a particular class of the BPS operators is not deformed by the non-anticommutativity. The second part is for the positive cosmological constant case. In this part, a black fusiform solution with appositive cosmological const in d =5, N = 4 de Sitter supergravity is constructed. The solution is obtained via the braneworld Kaluza-Klein reduction ansatz, and preserves half of the de Sitter supersymmetry. It is static, with the gravitational contraction being balanced by the cosmological repulsion. The black fusiform has an event horizon and a cosmological horizon, and is asymptotically non-de Sitter. The horizons are of an in x S(^2) topology, and contain singularities at the opposite ends due to the nature of the reduction ansatz. Despite the singularities, the solution exhibits some physically properties compatible with that of the regular asymptotically de Sitter spacetimes. The entropy and mass observe the N-bound proposal and the maximal mass conjecture respectively. It also carries a charge which contributes to the 1st law of black hole mechanics
EFT beyond the horizon:stochastic inflation and how primordial quantum fluctuations go classical
No full textWe identify the effective theory describing inflationary super-Hubble scales and show it to be a special case of effective field theories appropriate to open systems. Open systems allow information to be exchanged between the degrees of freedom of interest and those that are integrated out, such as for particles moving through a fluid. Strictly speaking they cannot in general be described by an effective lagrangian; rather the appropriate `low-energy' limit is instead a Lindblad equation describing the evolution of the density matrix of the slow degrees of freedom. We derive the equation relevant to super-Hubble modes of quantum fields in near-de Sitter spacetimes and derive two implications. We show the evolution of the diagonal density-matrix elements quickly approaches the Fokker-Planck equation of Starobinsky's stochastic inflationary picture. This provides an alternative first-principles derivation of this picture's stochastic noise and drift, as well as its leading corrections. (An application computes the noise for systems with a sub-luminal sound speed.) We argue that the presence of interactions drives the off-diagonal density-matrix elements to zero in the field basis. This shows why the field basis is the `pointer basis' for the decoherence of primordial quantum fluctuations while they are outside the horizon, thus allowing them to re-enter as classical fluctuations, as assumed when analyzing CMB data. The decoherence process is efficient, occurring after several Hubble times even for interactions as weak as gravitational-strength. Crucially, the details of the interactions largely control only the decoherence time and not the nature of the final late-time stochastic state, much as interactions can control the equilibration time for thermal systems but are largely irrelevant to the properties of the resulting equilibrium state.We identify the effective field theory describing the physics of super-Hubble scales and show it to be a special case of a class of effective field theories appropriate to open systems. Open systems are those that allow information to be exchanged between the degrees of freedom of interest and those that are integrated out, such as would be appropriate for particles moving through a fluid. Strictly speaking they cannot in general be described by an effective lagrangian, rather the appropriate ‘low-energy’ limit is instead a Lindblad equation describing the time-evolution of the density matrix of the slow degrees of freedom. We derive the equation relevant to super-Hubble modes of quantum fields in de Sitter (and near-de Sitter) spacetimes and derive two of its implications. We show that the evolution of the diagonal density-matrix elements quickly approach the Fokker-Planck equation of Starobinsky’s stochastic inflationary picture. This allows us both to identify the leading corrections and provide an alternative first-principles derivation of this picture’s stochastic noise and drift. (As applications we show that the noise for massless fields is independent of the details of the window function used, and also compute how the noise changes for systems with a sub-luminal speed of sound, c < 1.) We then argue that the presence of interactions drive the off-diagonal density-matrix elements to zero in the fieldWe identify the effective theory describing inflationary super-Hubble scales and show it to be a special case of effective field theories appropriate to open systems. Open systems allow information to be exchanged between the degrees of freedom of interest and those that are integrated out, such as for particles moving through a fluid. Strictly speaking they cannot in general be described by an effective lagrangian; rather the appropriate `low-energy' limit is instead a Lindblad equation describing the evolution of the density matrix of the slow degrees of freedom. We derive the equation relevant to super-Hubble modes of quantum fields in near-de Sitter spacetimes and derive two implications. We show the evolution of the diagonal density-matrix elements quickly approaches the Fokker-Planck equation of Starobinsky's stochastic inflationary picture. This provides an alternative first-principles derivation of this picture's stochastic noise and drift, as well as its leading corrections. (An application computes the noise for systems with a sub-luminal sound speed.) We argue that the presence of interactions drives the off-diagonal density-matrix elements to zero in the field basis. This shows why the field basis is the `pointer basis' for the decoherence of primordial quantum fluctuations while they are outside the horizon, thus allowing them to re-enter as classical fluctuations, as assumed when analyzing CMB data. The decoherence process is efficient, occurring after several Hubble times even for interactions as weak as gravitational-strength. Crucially, the details of the interactions largely control only the decoherence time and not the nature of the final late-time stochastic state, much as interactions can control the equilibration time for thermal systems but are largely irrelevant to the properties of the resulting equilibrium state
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