Natural and sail-displaced doubly-symmetric Lagrange point orbits for polar coverage

This paper proposes the use of doubly-symmetric, eight-shaped orbits in the circular restricted three-body problem for continuous coverage of the high-latitude regions of the Earth. These orbits, for a range of amplitudes, spend a large fraction of their period above either pole of the Earth. It is shown that they complement Sun-synchronous polar and highly eccentric Molniya orbits, and present a possible alternative to low thrust pole-sitter orbits. Both natural and solar-sail displaced orbits are considered. Continuation methods are described and used to generate families of these orbits. Starting from ballistic orbits, other families are created either by increasing the sail lightness number, varying the period or changing the sail attitude. Some representative orbits are then chosen to demonstrate the visibility of high-latitude regions throughout the year. A stability analysis is also performed, revealing that the orbits are unstable: it is found that for particular orbits, a solar sail can reduce their instability. A preliminary design of a linear quadratic regulator is presented as a solution to stabilize the system by using the solar sail only. Finally, invariant manifolds are exploited to identify orbits that present the opportunity of a ballistic transfer directly from low Earth orbit

Extension of Earth-Moon libration point orbits with solar sail propulsion

This paper presents families of libration point orbits in the Earth-Moon system that originate from complementing the classical circular restricted three-body problem with a solar sail. Through the use of a differential correction scheme in combination with a continuation on the solar sail induced acceleration, families of Lyapunov, halo, vertical Lyapunov, Earth-centred, and distant retrograde orbits are created. As the solar sail circular restricted three-body problem is non-autonomous, a constraint defined within the differential correction scheme ensures that all orbits are periodic with the Sun’s motion around the Earth-Moon system. The continuation method then starts from a classical libration point orbit with a suitable period and increases the solar sail acceleration magnitude to obtain families of orbits that are parametrised by this acceleration. Furthermore, different solar sail steering laws are considered (both in-plane and out-of-plane, and either fixed in the synodic frame or fixed with respect to the direction of sunlight), adding to the wealth of families of solar sail enabled libration point orbits presented. Finally, the linear stability properties of the generated orbits are investigated to assess the need for active orbital control. It is shown that the solar sail induced acceleration can have a positive effect on the stability of some orbit families, especially those at the L2 point, but that it most often (further) destabilises the orbit. Active control will therefore be needed to ensure long-term survivability of these orbits

Cardiometabolic factors and risk of non-arteritic anterior ischemic optic neuropathy: a systematic review and meta-analysis

Purpose: The purpose of this systematic review and meta-analysis of the literature is to evaluate the association between cardiometabolic risk factors (hypertension, diabetes mellitus, hypercholesterolemia/dyslipidemia, HDL cholesterol, LDL cholesterol, lipoprotein(a), and triglycerides) and non-arteritic anterior ischemic optic neuropathy (NAION). Methods: Pertinent publications were identified through a systematic search in PubMed and EMBASE databases, without language restrictions. The pooled odds ratios (OR) and standardized mean differences (SMD), with their 95% confidence intervals (95% CI) were estimated using random effects (DerSimonian Laird) models, as appropriate. A set of subgroup analyses and meta-regression analysis models were performed. Results: Twenty-one studies (including 1560 patients with NAION and 2292 controls), examining the association between NAION and cardiometabolic risk factors, were eligible for the systematic review and meta-analysis. Hypertension (pooled OR = 1.50; 95% CI: 1.16–1.94), diabetes mellitus (pooled OR = 1.71; 95% CI: 1.33–2.21), and hypercholesterolemia/dyslipidemia (pooled OR = 2.00; 95% CI: 1.53–2.62) were associated with NAION. Among the components of dyslipidemia, higher serum triglycerides were associated with NAION, with a medium effect size (SMD = + 0.58, 95% CI: + 0.12 to + 1.04), whereas synthesis of four studies reporting on HDL and LDL cholesterol did not reveal any significant associations. A significant association between NAION and higher serum lipoprotein(a) levels (pooled OR = 2.88; 95%CI: 1.01–8.21) was also noted. Conclusions: This systematic review and meta-analysis found that NAION was associated with cardiometabolic factors, suggesting that vascular dysfunction may be implicated in the pathogenesis of the disease. Our findings may alert health care providers to try modifying these risk factors for NAION prevention. [Figure not available: see fulltext.]. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature