Orbital dynamics of high area-to-mass ratio spacecraft under the influence of J2 and solar radiation pressure

This paper investigates the effect of planetary oblateness and solar radiation pressure on the orbit of high area-to-mass spacecraft. A planar Hamiltonian model shows the existence of equilibrium orbits with the orbit apogee pointing towards or away from the Sun. These solutions are numerically continued to non-zero inclinations and considering the obliquity of the ecliptic plane relative to the equator. Quasi-frozen orbits are identified in eccentricity, inclination and angle between the Sun-line and the orbit perigee. The long-term evolution of these orbits is then verified through numerical integration. A set of ‘heliotropic’ orbits with apogee pointing in direction of the Sun is proposed for enhancing imaging and telecommunication on the day side of the Earth. The effects of J2 and solar radiation pressure are exploited to obtain a passive rotation of the apsides line following the Sun; moreover the effect of solar radiation pressure enables such orbits at higher eccentricities with respect to the J2 only case

A passive high altitude deorbiting strategy

A deorbiting strategy for small satellites, in particular CubeSats, is proposed which exploits the effect of solar radiation pressure to increase the spacecraft orbit eccentricity so that the perigee falls below an altitude where atmospheric drag will cause the spacecraft orbit to naturally decay. This is achieved by fitting the spacecraft with an inflatable reflective balloon. Once this is fully deployed, the overall area-to-mass ratio of the spacecraft is increased; hence solar radiation pressure and aerodynamic drag have a greatly increased effect on the spacecraft orbit. An analytical model of the orbit evolution due to solar radiation pressure and the J2 effect as a Hamiltonian system shows the evolution of an initially circular orbit. The maximum reachable orbit eccentricity as a function of semi-major axis and area-to-mass ratio can be found and used to determine the size of balloon required for deorbiting from circular orbits of different altitudes. A system design of the device is performed and the feasibility of the proposed deorbiting strategy is assessed and compared to the use of conventional thrusters. The use of solar radiation pressure to increase the orbit eccentricity enables passive deorbiting from significantly higher altitudes than conventional drag augmentation devices

A passive satellite deorbiting strategy for MEO using solar radiation pressure and the J2 effect

The growing population of space debris poses a serious risk to the future of space flight. To effectively manage the increase of debris in orbit, end-of life disposal has become a key requirement for future missions. This poses a challenge for Medium Earth Orbit (MEO) spacecraft which require a large Δv to re-enter the atmosphere or reach the geostationary graveyard orbit. This paper further explores a passive strategy based on the joint effects of solar radiation pressure and the Earth’s oblateness acting on a high area-to-mass ratio object. The concept was previously presented as an analytical planar model. This paper uses a full 3D model to validate the analytical results numerically for equatorial circular orbits first, then investigating higher inclinations. It is shown that for higher inclinations the initial position of the Sun and right ascension of the ascending node become increasingly important. A region of very low required area-to-mass ratio is identified in the parameter space of a and inclination which occurs for altitudes below 10,000 km

Solar radiation pressure augmented deorbiting from high altitude sun-synchronous orbits

This paper discusses the use of solar radiation pressure (SRP) augmented deorbiting to passively remove small satellites from high altitude Sun-synchronous orbits. SRP-augmented deorbiting works by deploying a light-weight reflective inflatable device to increase the area-to-mass-ratio of the spacecraft. The interactions of the orbital perturbations due to solar radiation pressure and the Earth’s oblateness cause the eccentricity of the orbit to librate at a quasi-constant semi-major axis. A large enough area-to-mass-ratio will ensure that a maximum eccentricity is reached where the spacecraft will then experience enough aerodynamic drag at the orbit pericentre to deorbit. An analytical model of the orbital evolution based on a Hamiltonian approach is used to obtain a first guess for the required area-to-mass-ratio to deorbit. This first guess is then used in a numerical propagation of the orbital elements using the Gauss’ equations to find the actual requirements as a function of altitude. The results are discussed and altitude regions for Sun-synchronous orbits are identified in which the proposed method is most effective. Finally, the implementation of the device is discussed. It is shown that passive solar radiation pressure deorbiting is a useful alternative to propulsive end-of-life manoeuvres for future high altitude Sun-synchronous missions

Orbit evolution, maintenance and disposal of SpaceChip swarms

The combined effect of solar radiation pressure and atmospheric drag is investigated for future mission conceptsfor swarms of satellites-on-a-chip (SpaceChips). The natural evolution of the swarm is exploited to perform spatially distributed measurements of the upper layers of the atmosphere. The energy gain from asymmetric solar radiation pressure can be used to balance the energy dissipation from atmospheric drag. An algorithm for long-term orbit control is then designed, based on changing the reflectivity coefficient of the SpaceChips. The subsequent modulation of the solar radiation pressure allows stabilisation of the swarm in the orbital element phase space. It is shown that the normally short orbit lifetime for such devices can be extended through the interaction of solar radiation pressure and atmospheric drag and indeed selected and the end-of-life re-entry of the swarm can be ensured, by exploiting atmospheric drag

Orbit control of high area-to-mass ratio spacecraft using electrochromic coating

This paper presents a novel method for the orbit control of high area-to-mass ratio spacecraft, such as spacecraft-on-a-chip, future „smart dust‟ devices and inflatable spacecraft. By changing the reflectivity coefficient of an electrochromic coating of the spacecraft, the perturbing effect of solar radiation pressure (SRP) is exploited to enable long-lived orbits and to control formations, without the need for propellant consumption or active pointing. The spacecraft is coated with a thin film of an electrochromic material that changes its reflectivity coefficient when a small current is applied. The change of reflectance alters the fraction of the radiation pressure force that is transmitted to the satellite, and hence has a direct effect on the spacecraft orbit evolution. The orbital element space is analysed to identify orbits which can be stabilised with electrochromic orbit control. A closed-loop feedback control method using an artificial potential field approach is introduced to stabilise these otherwise unsteady orbits. The stability of this solution is analysed and verified through numerical simulation. Finally, a test case is simulated in which the control method is used to perform orbital manoeuvres for a spacecraft formation

Geo-engineering using dust grains in heliotropic elliptical orbits

This paper examines the concept of a Saturn-like Earth ring comprised of dust grains to offset global warming. A new family of non-Keplerian periodic orbits, under the effects of solar radiation pressure and the Earth’s oblateness J2 perturbation, is selected to increase the lifetime of the passive cloud of particles and, thus, increase the efficiency of this geo-engineering strategy. An analytical model is used to predict the evolution of the dust due to solar-radiation pressure and the J2 effect. The attenuation of the solar radiation can then be calculated from the ring model. In comparison to circular orbits, eccentric orbits yield a more stable environment for small grain sizes and therefore achieve higher efficiencies when the orbital decay of the material is considered. Moreover, the special orbital dynamics experienced by high area-to-mass ratio objects, influenced by solar radiation pressure and the J2 effect, ensure the ring will maintain a permanent heliotropic shape, with dust spending the largest portion of time on the Sun facing side. It is envisaged that small dust grains can be released with an initial Δv to enter an eccentric orbit with Sun-facing apogee. Finally, an estimate of 5.94x1011 kg is computed as the total mass required to offset the effects of global warming

StrathSat-R : Deploying inflatable CubeSat structures in micro gravity

This paper presents the concepts, objectives and design of a student-led sounding rocket experiment which shall test novel inflatable devices in space conditions. This experiment is envisaged as the first step towards developing a CubeSat programme at the University of Strathclyde, which can exploit the novel concepts developed and the technical skills gained. The experiment itself aims to test novel, student developed, inflatable space structures in micro gravity and reduced pressure conditions. It consists of three distinct sections, the ejection housing on the rocket and the two ejectable modules that are based on CubeSat architecture. Shortly before reaching apogee, the two modules are ejected from the rocket and will deploy their own inflating structure during free flight. After landing, the ejectable modules recovery will rely upon a GPS position relayed to the team from the module by Globalstar transmission and a RF beacon for tracking with the recovery helicopter. The two modules carry two different structures resulting in distinct mission objectives: The aim of FRODO is to deploy an experimental passive de-orbiting system for high altitude spacecraft which will in the future utilise solar radiation pressure for orbit removal. The aim of SAM is to serve as a technology demonstrator for the residual air deployment method of a smart bio-inspired space structure. This paper contains details about the science objectives of the mission and how they will be achieved, its experimental design and the management of the student-led project

Effect of remote ischaemic conditioning on clinical outcomes in patients with acute myocardial infarction (CONDI-2/ERIC-PPCI): a single-blind randomised controlled trial.

BACKGROUND: Remote ischaemic conditioning with transient ischaemia and reperfusion applied to the arm has been shown to reduce myocardial infarct size in patients with ST-elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PPCI). We investigated whether remote ischaemic conditioning could reduce the incidence of cardiac death and hospitalisation for heart failure at 12 months. METHODS: We did an international investigator-initiated, prospective, single-blind, randomised controlled trial (CONDI-2/ERIC-PPCI) at 33 centres across the UK, Denmark, Spain, and Serbia. Patients (age >18 years) with suspected STEMI and who were eligible for PPCI were randomly allocated (1:1, stratified by centre with a permuted block method) to receive standard treatment (including a sham simulated remote ischaemic conditioning intervention at UK sites only) or remote ischaemic conditioning treatment (intermittent ischaemia and reperfusion applied to the arm through four cycles of 5-min inflation and 5-min deflation of an automated cuff device) before PPCI. Investigators responsible for data collection and outcome assessment were masked to treatment allocation. The primary combined endpoint was cardiac death or hospitalisation for heart failure at 12 months in the intention-to-treat population. This trial is registered with ClinicalTrials.gov (NCT02342522) and is completed. FINDINGS: Between Nov 6, 2013, and March 31, 2018, 5401 patients were randomly allocated to either the control group (n=2701) or the remote ischaemic conditioning group (n=2700). After exclusion of patients upon hospital arrival or loss to follow-up, 2569 patients in the control group and 2546 in the intervention group were included in the intention-to-treat analysis. At 12 months post-PPCI, the Kaplan-Meier-estimated frequencies of cardiac death or hospitalisation for heart failure (the primary endpoint) were 220 (8·6%) patients in the control group and 239 (9·4%) in the remote ischaemic conditioning group (hazard ratio 1·10 [95% CI 0·91-1·32], p=0·32 for intervention versus control). No important unexpected adverse events or side effects of remote ischaemic conditioning were observed. INTERPRETATION: Remote ischaemic conditioning does not improve clinical outcomes (cardiac death or hospitalisation for heart failure) at 12 months in patients with STEMI undergoing PPCI. FUNDING: British Heart Foundation, University College London Hospitals/University College London Biomedical Research Centre, Danish Innovation Foundation, Novo Nordisk Foundation, TrygFonden