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"Orbits"
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Doomsday rocks from space
Discusses comets, asteroids, and meteors, including how and where they are formed in space, what scientists are doing to study and track them, and what danger these doomsday rocks are to Earth.
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Circular light orbits of a general, static, and spherical symmetrical wormhole with Formula omitted symmetry
Recently, the ring images or the shadow images of the centers of the galaxy M87 and the Milky way have been reported by Event Horizon Telescope Collaboration. It is believed that the ring images imply that the central objects form unstable light circular orbits. Some of wormholes with [Formula omitted] symmetry against a throat are wrongly excluded from the candidates at the centers of M87 and the Milky way due to the overlooking the unstable light circular orbits on the throat. A general asymptotically-flat, static, and spherical symmetrical wormhole without a thin shell has at least one unstable circular light orbit at the throat or elsewhere. If the wormhole has [Formula omitted] symmetry against the throat, it has the unstable circular light orbits on the throat or it has stable circular light orbits on the throat and unstable ones near the throat. We need to analyze the throat carefully to make sure we do not unfairly rule out the [Formula omitted]-symmetrical wormholes. In this study, we categorize the numbers of the circular light orbits of the [Formula omitted]-symmetrical wormhole and their stability from the derivatives of an effective potential at the throat and we investigate the circular light orbits around a Simpson-Visser black-bounce spacetime, a Damour-Solodukhin wormhole spacetime, a Reissner-Nordström black-hole-like wormhole spacetime or a charged Damour-Solodukhin wormhole spacetime as examples. We give complete treatments including degenerated circular light orbits made from more than one stable and unstable circular light orbits on and off the throat.
Journal Article
SUR CERTAINES CONTRIBUTIONS UNIPOTENTES DANS LA FORMULE DES TRACES D'ARTHUR
We establish a fine expansion for the geometric part of the Arthur-Selberg trace formula (as it was conjectured by Werner Hoffmann). For the general linear group, we deduce an expression for the contributions of regular by blocks unipotent orbits (orbits with one Jordan block with multiplicity). As a consequence, we find formulas for Arthur's global coefficients attached to such orbits.
Journal Article
Lyapunov exponent and charged Myers-Perry spacetimes
We compute the proper time Lyapunov exponent for the charged Myers-Perry black hole spacetime and investigate the instability of the equatorial circular geodesics (both time-like and null) via this exponent. We also show that for more than four spacetime dimensions (N [greater than or equal to] 3), there are no Innermost Stable Circular Orbits (ISCOs) in the charged Myers-Perry black hole spacetime. We further show that among all possible circular orbits, time-like circular orbits have longer orbital periods than null circular orbits (photon spheres) as measured by asymptotic observers. Thus, time-like circular orbits provide the slowest way to orbit around the charged Myers-Perry black hole.
Journal Article
The Effect of Observation Discontinuities on LEO Real-Time Orbital Prediction Accuracy and Integrity
Real-time, high-accuracy orbital products for low Earth orbit (LEO) satellites are essential for LEO-augmented real-time positioning, navigation and timing services. In particular, complete and continuous global navigation satellite system (GNSS) observations onboard tracked LEO satellites are necessary to guarantee precise orbit determination (POD) and generate short-term predicted orbits that can be fit with real-time ephemeris parameters. However, in practice, GNSS observations of LEO satellites often suffer from discontinuities due to tracking problems, data transmission problems, or downlinking strategies. Understanding the effect of these observation gaps on orbit accuracy is therefore essential for developing strategies to minimize accuracy degradation in real-time LEO satellite orbits. This study investigates trade-offs between two suites of strategies for addressing multi-hour observation data gaps followed by short segments of tail data during reduced-dynamic POD. The first strategy, EP, involves sacrificing the tail data and extending the prediction time. The second set of strategies retain the tail data but vary the POD strategies: the tested options include maintaining stochastic accelerations as estimable parameters (RP), not estimating stochastic accelerations (CP), or combining the RP-based orbits from the non-gap periods with the CP-based orbits during the gap (BP). Using real GNSS observations from the LEO satellite Sentinel-6A, we evaluated the accuracy and integrity of these strategies for 1-h orbital predictions with assumed gap lengths of 3, 5, 7, and 9 h and tail data lengths set to 15, 30, 45, and 60 min. Results show that the BP strategy achieves the highest prediction accuracy, with mean orbital user range errors (OUREs) of approximately 5.7 and 13.4 cm for a 3-h data gap followed by 60-min and 15-min tails, respectively. In contrast, the EP strategy demonstrates the highest integrity. For a 15-min tail, the 99.9% confidence level of the OURE for the EP strategy reaches approximately 3.1 and 8.7 dm for gap lengths of 3 h and 9 h, respectively. Overall, BP is the preferred strategy for maximizing prediction accuracy, while the EP strategy is preferable for short gaps and tails. The CP strategy provides a balanced approach, maintaining reasonably strong performance for both prediction accuracy and integrity.
Journal Article
High-order ionospheric delay correction of GNSS data for precise reduced-dynamic determination of LEO satellite orbits: cases of GOCE, GRACE, and SWARM
Ionospheric delay is one of the main error sources in the precise orbit determination (POD) of low earth orbit (LEO) satellites using spaceborne global navigation satellite system (GNSS) data. The ionospheric-free linear combination is usually used to eliminate the influence of the first-order main term, and the impact of higher-order ionospheric (HOI) delay is ignored. With the development of LEO satellite POD technology, calculating HOI delay at different orbital altitudes and exploring the variations in HOI delay have become key topics for further improving POD. The slant total electron content was calculated by using the smoothed satellite-borne GNSS data. The location of the ionospheric pierce point (IPP) and geomagnetic field intensity at the IPP were calculated by using the International Reference Ionosphere-2016(IRI-2016) and International Geomagnetic Reference Field: the 13th generation (IGRF-13) models. The second- and third-order ionospheric delays could be determined by using the above data. GOCE, GRACE-A and SWARM-A/B were selected as case studies. Comparing the HOI delays of these four satellites shows that the impact of HOI delay on LEO satellite GPS data is approximately on the order of millimeters to centimeters. The higher the orbit altitude is, the smaller the HOI delay. Reduced-dynamic orbit determination and analysis were performed using GPS observations with and without HOI delay. The results of overlapping orbit analysis, precision orbit comparison, and satellite laser ranging tests show that HOI delay correction can improve the inner and outer coincidence precision of LEO satellite POD and that the improvement decreases gradually with increasing LEO satellite orbit altitude. In summary, the impact of HOI delay on the POD precision of LEO satellites is at the submillimeter level. As the POD precision of LEO satellites moves toward the mm level with the development of spaceborne GNSS techniques, the impact of HOI delay on POD cannot be ignored.
Journal Article
Orbit Selection of Remote Sensing Satellite in Polar Regions
Orbit selection of the remote sensing polar satellite is critical for polar observation missions. This paper reviews the current status and characteristics of satellite orbits in polar region, and analyzes the pros and cons of four major types of remote sensing polar satellite orbits, namely sun - synchronous orbits, geosynchronous orbits, low - Earth polar orbits, and highly elliptical orbits. For future remote sensing polar satellites with high resolution, sun - synchronous orbits should be chosen for optical remote sensing missions, and low - Earth polar orbits are suitable for other remote sensing missions with no specific requirements for lighting conditions on the ground. Using constellation with multiple satellites and multi-orbit coordination can achieve full coverage and frequent revisits in polar regions. Finally, it is pointed out that there is an urgent need to build a dedicated remote sensing polar satellite system. Furthermore, there is a need to further explore new types of polar orbits and corresponding application scenario, as well as to develop commercial small satellites for polar region, in order to enhance the comprehensive capabilities of observation in polar region.
Journal Article
Stellar Spin-Orbit Misalignment in a Multiplanet System
Stars hosting hot Jupiters are often observed to have high obliquities, whereas stars with multiple coplanar planets have been seen to have low obliquities. This has been interpreted as evidence that hot-Jupiter formation is linked to dynamical disruption, as opposed to planet migration through a protoplanetary disk. We used asteroseismology to measure a large obliquity for Kepler-56, a red giant star hosting two transiting coplanar planets. These observations show that spin-orbit misalignments are not confined to hot-Jupiter systems. Misalignments in a broader class of systems had been predicted as a consequence of torques from wide-orbiting companions, and indeed radial velocity measurements revealed a third companion in a wide orbit in the Kepler-56 system.
Journal Article