Explore the vast range of titles available.

The positioning of highly elliptical orbit (HEO) spacecraft is very important in deep space exploration and other missions. The Beidou Navigation Satellite System (BDS-3), which was completed in 2020, has put forward new ideas for using GNSS technology to make up for the lack of earth-based TT&C systems. At the same time, the selection of the gravity field order and the perturbation forces must be considered when using dynamical methods for HEO spacecraft positioning. In this paper, simulation experiments are carried out based on the Two-Line Orbital Element (TLE) data of BDS. Two HEO satellites with different semimajor-axis (67509.6 km, 212857.3 km) are selected as the target spacecraft orbits. The single point positioning (SPP) method and Extended Kalman Filter (EKF) method are used for HEO spacecrafts positioning respectively. The experiments show that a 20×20 order gravity field model can be chosen for the orbital integral of HEO spacecraft orbits, and the integration error caused by solar gravity and lunar gravity should be taken into account with the change of orbital altitude. Adding Geostationary Orbit (GEO) and Inclined GeoSynchronous Orbit (IGSO) navigation satellites improved the visibility of these two orbits by 19.45% and 16.64%, respectively, and improved the GDOP value by 14.1% and 3.8%, respectively, compared to observing MEO satellites only. The use of the EKF method can effectively improve the positioning accuracy of the HEO spacecraft compared to the SPP method. For the spacecraft in HEO-1, the RMS value of the positioning errors of the EKF method was 67.82 m, and the accuracy of each direction was improved by 44.28%, 38.85%, and 38.62%, respectively. For the spacecraft in HEO-2, the positioning errors in the x direction of EKF method was within 12 km and within 2 km in the y and z directions. The accuracy of these three directions was improved by 84.91%, 79.24%, and 58.64%, respectively.

BRITE-Constellation (where BRITE stands for BRIght Target Explorer) is an international nanosatellite mission to monitor photometrically, in two colours, the brightness and temperature variations of stars generally brighter than mag(V) ≈ 4 with precision and time coverage not possible from the ground. The current mission design consists of six nanosats (hence Constellation): two from Austria, two from Canada, and two from Poland. Each 7 kg nanosat carries an optical telescope of aperture 3 cm feeding an uncooled CCD. One instrument in each pair is equipped with a blue filter; the other with a red filter. Each BRITE instrument has a wide field of view (≈24°), so up to about 15 bright stars can be observed simultaneously, sampled in 32 × 32 pixels sub-rasters. Photometry of additional fainter targets, with reduced precision but thorough time sampling, will be possible through onboard data processing. The BRITE sample is dominated by the most intrinsically luminous stars: massive stars seen at all evolutionary stages, and evolved medium-mass stars at the very end of their nuclear burning phases. The goals of BRITE-Constellation are to (1) measure p- and g-mode pulsations to probe the interiors and ages of stars through asteroseismology; (2) look for varying spots on the stars surfaces carried across the stellar disks by rotation, which are the sources of co-rotating interaction regions in the winds of the most luminous stars, probably arising from magnetic subsurface convection; and (3) search for planetary transits.

After decades of centralized control of economic activity in space, NASA and US policymakers have begun to cede the direction of human activities in space to commercial companies. NASA garnered more than 0.7 percent of GDP in the mid-1960s, but is only around 0.1 percent of GDP today. Meanwhile, space has become big business, with $300 billion in annual revenue. The shift from public to private priorities in space is especially significant because a widely shared goal among commercial space's leaders is the achievement of a large-scale, largely self-sufficient, developed space economy. Jeff Bezos, has stated that the mission of his firm Blue Origin is “millions of people living and working in space.” Elon Musk, founder of SpaceX, has laid out plans to build a city of a million people on Mars within the next century. Both Neil deGrasse Tyson and Peter Diamandis have been given credit for stating that Earth's first trillionaire will be an asteroid-miner. Such visions are clearly not going to become reality in the near future. But detailed roadmaps to them are being produced and recent progress in the required technologies has been dramatic. If such space-economy visions are even partially realized, the implications for society will be enormous. Though economists should treat the prospect of a developed space economy with healthy skepticism, it would be irresponsible to treat it as science fiction. In this article, I provide an analytical framework—based on classic economic analysis of the role of government in market economies—for understanding and managing the development of the space economy.

At the end of its mission, the MESSENGER spacecraft's orbit intersected Mercury's nightside magnetic equator at low altitudes below 420 km, enabling the first in situ observations of this region, where the magnetic field strength is typically sub‐dipolar. We present 5 events from these orbits where MESSENGER encountered Earth‐like dipolarization regions characterized by enhanced field strengths up to 20 nT above the intrinsic planetary field, and an average ≃23% $\\simeq 23\\%$ decrease and ≃44% $\\simeq 44\\%$ increase in plasma proton density and temperature, respectively, for 1–2 min periods, comparable to Hermean substorm timescales. The events span local times of 1.5 hr pre‐ and post‐midnight, and are present from the magnetic equator up to magnetic latitudes of at least 12° $12{}^{\\circ}$ north. Supported by estimates of decreased flux tube entropy during these events, we suggest these dipolarization regions are formed by the pileup of dipolarization fronts and formation of a substorm current wedge.

In view of the characteristics of the low-orbit spacecraft's orbit, the influence of the low-orbit spacecraft's orbit error on the time comparison method based on tri-frequency is theoretically analyzed. With the simulation and test, validing the effectiveness of the precise two-way time comparison method, and the accuracy of the method can reach the order of ps-level. Furthermore, with the simulated data, the influence of orbit error on the ultra-high precise space-ground time comparison is analyzed. The results show that the percentage of orbit noise in the total orbit error has a great influence on the accuracy of time comparison. In order to achieve a link accuracy better than ps, the percentage of orbit noise should be kept within 5%.

Due to its eccentric orbit, Mercury experiences a varying gravitational pull from the Sun along its orbit, leading to periodic surface tidal deformation. The previous measurement of Mercury's tidal h2 ${h}_{2}$ by Bertone et al. (2021, ) is based on minimizing height differences at cross‐overs of the Mercury Laser Altimeter (MLA) profiles. However, this method can suffer from significant interpolation errors. In this study, we apply an alternative approach, which is based on the co‐registration of reprocessed MLA profiles. For the reprocessing, we account for the pointing aberration and incorporate an updated spacecraft orbit model. Within the study region of 77°N to 84°N, we obtain a tidal h2 ${h}_{2}$ of 0.92± $\\pm $0.58 (3‐σ $\\sigma $). This value is compatible with current interior structure and rheology models, but significantly lower than the previous estimate of 1.55± $\\pm $0.65 (3‐σ $\\sigma $). When combined with recent tidal k2 ${k}_{2}$ estimates, our measurement favors a small to medium‐sized inner core.

Martian moons exploration, MMX, is the new sample return mission planned by the Japan Aerospace Exploration Agency (JAXA) targeting the two Martian moons with the scheduled launch in 2024 and return to the Earth in 2029. The major scientific objectives of this mission are to determine the origin of Phobos and Deimos, to elucidate the early Solar System evolution in terms of volatile delivery across the snow line to the terrestrial planets having habitable surface environments, and to explore the evolutionary processes of both moons and Mars surface environment. To achieve these objectives, during a stay in circum-Martian space over about 3 years MMX will collect samples from Phobos along with close-up observations of this inner moon and carry out multiple flybys of Deimos to make comparative observations of this outer moon. Simultaneously, successive observations of the Martian atmosphere will also be made by utilizing the advantage of quasi-equatorial spacecraft orbits along the moons’ orbits.

In this manuscript, we study the orbit pursuit-evasion game problem in the Cartesian model under J2 perturbations, give the dynamic equation of both spacecraft and derive the costate dynamics. This paper proposes a dimensionality reduction technology based on the transition problem and the saddle-point solution is obtained through the Costate Mapping Hybrid Optimal Method. Unlike the traditional direct method for solving costate, this method obtains the pursuer’s costate in the one-side interception problem by costate mapping and then considers the maneuver of the evader to solve the costate’s optimal value of the two-point boundary value problem. In numerical results, this method is consistent with other methods, and the computation time is about 2-4 seconds. The simulations show that this technology reduce the dimensionality of the problem and the sensitivity of the initial costate, the proposed method can obtain the saddle-point solution quickly and does not have the convergence problem. This work also compares the differences between the optimal solutions of the minimum-time interception problem and saddle-point solutions of the orbital game problem under different thrust configurations, then discusses the effects of the evader thrust and initial relative distance on the optimal control law of the pursuer.

Space junk represents a growing threat to commercialization and other activities in space. [PUBLICATION ABSTRACT]

Vesta's surface is characterized by abundant impact craters, some with preserved ejecta blankets, large troughs extending around the equatorial region, enigmatic dark material, and widespread mass wasting, but as yet an absence of volcanic features. Abundant steep slopes indicate that impact-generated surface regolith is underlain by bedrock. Dawn observations confirm the large impact basin (Rheasilvia) at Vesta's south pole and reveal evidence for an earlier, underlying large basin (Veneneia). Vesta's geology displays morphological features characteristic of the Moon and terrestrial planets as well as those of other asteroids, underscoring Vesta's unique role as a transitional solar system body.