• BeiDou-G2: Past and Present

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: In January 2022, the defunct satellite BeiDou-G2 was pulled out of geostationary orbit by Shijian-21 to a graveyard orbit. For safe docking and operation, it was necessary to determine the rotation state in advance. In this paper, we show the evolution of the rotation of the BeiDou-G2 satellite based on the photometry observation data for the past 10 years. The rotational speed of BeiDou-G2 was found to be annual oscillation, mainly due to the solar radiation. Based on the evolution of BeiDou-G2's rotation speed and its orbit, we confirmed that in the last 10 years, the satellite had six abnormal events. These abnormal events were mainly due to the increase in the rotation speed caused by suspected fuel leakages. Additionally, the abnormal events included one collision in 2012, which was inferred to be the trigger of the fuel leakages in the following years. No rotational speed abnormalities occurred again after 2017, probably due to the complete release of the residual fuel. The parameters and the propagating models after one incidence of solar panel damage in 2014 and one fragment in 2016, with the standard errors for propagating over 1 year of the rotational axis less than 3{\deg} and rotational speed being 0.11{\deg}/s, were believed to be able to satisfy the accuracy requirements of the rotation state well at the moment of docking.

  • A new species of Saurichthys from the Middle Triassic (Anisian) of southwestern China

    分类: 生物学 >> 动物学 提交时间: 2017-11-07 合作期刊: 《古脊椎动物学报》

    摘要: The saurichthyiform fishes were effective predators and hence the significant consumers in the aquatic ecosystems during the Early Mesozoic. They showed a notable diversification in the Anisian (Middle Triassic) Lagerst鋞ten of southwestern China. In this contribution, we report a new species of Saurichthys from the Anisian of Yunnan, China, that displays some peculiar modifications of the axial skeleton and the longate body of the group. This new species, Saurichthys spinosa is a small-sized saurichthyid fish characterized by a very narrow interorbital region of the skull roof, an anteriorly expansive and ventrally arched cleithrum, proportionally large abdominal vertebrae lacking neural spines and alternately bearing laterally-stretching paraneural plates, few fin rays in the median fins, and two paralleling rows of needle-like flank scales with strong thorns. This fish has slimmed down the body by reducing the depth of the head and the epaxial part of the trunk. The elongate paraneural plates inserted in the horizontal septum and the rigid interlocking of the flank scales render the fish a very stiff body, which is compatible with the functional consequence of the obvious decrease of the body (vertebral) segments. This discovery reveals the variability of the axial skeleton and the hydrodynamic properties of the saurichthyiform fishes. These factors, together with the innovations in the locomotion and feeding habit, might have intrinsically effected the evolutionary burst of the eastern Tethyan saurichthyiform fishes during the Anisian, a marked signature of the rapid radiation stage of the biotic recovery after the end-Permian extinction.

  • First record of Saurichthys (Actinopterygii: Saurichthyidae) from the Late Triassic of eastern Paleo-Tethys

    分类: 生物学 >> 动物学 提交时间: 2022-10-16 合作期刊: 《古脊椎动物学报》

    摘要: The saurichthyiform fishes, characterized by a pointed rostrum and a streamlined long and slender body plan, ranked among the top predators of the ichthyofauna in the Early Mesozoic oceanic ecosystem. In a cosmopolitan pattern, these fishes rapidly radiated after the end-Permian mass extinction (EPME) and diversified morphologically and ecologically during the Middle Triassic. Thereafter, they seemingly showed a notable shrinkage from a global distribution to an occurrence basically restricted to the western Paleo-Tethys realm since the Late Triassic. Specifically, there is no saurichthyiform fossil record so far from the marine Late Triassic of South China (eastern Paleo-Tethys), where contrastingly they were highly diversified in stratigraphically older Lagersttten (Middle Triassic Panxian-Luoping and Xingyi biotas). Here we report the discovery of Saurichthys taotie sp. nov. from the Guanling biota of Guizhou and Yunnan provinces, southwestern China. This new species is a medium-sized Saurichthys featured by subtriangular subopercles ornamented with densely arranged vertical striae, faint ornamentation on the posterior part of the skull roof, and strong longitudinal ridges decorating the anterodorsal surface of the rostrum. By marking its own groups first occurrence in the Late Triassic of eastern Paleo-Tethyan province, Saurichthys taotie suggests that the saurichthyiform fishes were actually much more widespread than previously thought during that geological stage when they showed a considerable decline in the diversity. By still possessing some features previously only seen in its Early Triassic congeners elsewhere, Saurichthys taotie sheds new light on the evolutionary and paleobiogeographical history of saurichthyiform fishes.

  • A power-free graphene mid-infrared photodetector array

    分类: 光学 >> 量子光学 提交时间: 2023-02-19

    摘要: Graphene plasmons can resonantly enhance the incident light absorption and offer a potential for tunable spectral selectivity for mid-infrared (MIR) detection. High-performance tunable graphene plasmonic devices are, however, typically based on electrode-patterned graphene, which requires high power input and are technologically challenging in compact assembly. Here we demonstrate a tunable MIR photodetector array operating at zero input bias voltage. Our devices consist of integrating monolayer graphene with periodically "type-printed" ferroelectric superdomain. The spatial graphene carrier density patterns can be readily modulated by artificially defined ferroelectric superdomain with periodic nanoscale-wide stripe domains, achieving nonuniform pattering of conductivity and subsequently enabling graphene plasmons excitation and confinement for a selective transmission resonance in MIR regime. A high photoresponsivity of ~30 mA W-1 at room temperature is achieved in our device. We also observe that our device array features a tunable detection performance with spectral selectivity from 7.2 to 8.5 {\mu}m by directly reconfiguring the periodicity of ferroelectric superdomain. Our strategy could lead to the development of smart fabrication of on-chip MIR photodetector array for application of tunable spectral systems with low-energy consumption.