小行星探测电推进系统方案研究

小行星探测以及资源开发与利用对国家抢占深空探测主动权和制高点有着不可估量的战略意义。电推进具有高比冲、长寿命和高度自主巡航等特点,小行星探测器采用电推进执行巡航阶段轨道机动任务,将大幅减少推进剂重量和提高载重比。调研了国外小行星探测的电推进系统方案,针对我国小行星探测对电推进系统的任务需求及现有电推力器的技术基础,提出了5种电推进系统方案,并进行多维度对比,对最优方案进行了设计和关键技术梳理。     

近地小行星防御策略分析

近年来,各国针对近地小行星的威胁及相应防御策略的研究越来越多。调研了国内外近地天体观测发展现状和多种近地小行星撞击地球的防御策略,通过比较分析防御策略的实现原理、关键技术和存在的问题,对各种策略的适用性、可行性和有效性进行了评估。通过对各种策略的分析和总结,结合我国近地小行星防御策略的研究现状,尝试性提出了采用多种策略组合的方式对有潜在威胁的小行星采取相关有效方案,为航天器的研制和防御策略的实施争取更多的时间。     

基于星联网的深空自主导航方案设计

为了降低地面测控系统的负担、提高深空探测器的导航效率,提出了基于星联网的航天器自主导航概念,对星联网的应用体系进行了设计。借助脉冲星、星间链路等手段实现星联网系统中基准航天器完全自主的高精度导航,用户航天器通过与基准航天器或其他用户航天器的交互通信与测量就可以实现自身状态估计。以地月转移任务为例,设计了星联网系统在地月空间的具体应用方案,分析了地月空间基准航天器的配置与自主导航方法,阐述了用户航天器的单层与多层导航策略。对基于脉冲星与星间链路观测的基准航天器自主导航进行了仿真,验证了观测基准航天器或者其他用户航天器时,地月转移段航天器自主导航的可行性。结果表明:基准航天器可以达到20 m的定位精度,用户航天器可以达到优于30 m的定位精度。基于星联网的航天器自主导航是可行的,发展星联网可以为我国构建天基自主基准导航系统提供有力支持。     

光学遥感器整体优势突显中巴地球资源卫星的特色

中巴地球资源卫星高层技术决策立足两个发展中国家的具体国情,着眼于自身整体优势和国际应用,根据国际地球资源卫星发展趋势和客观规律,准确地把握了中巴地球资源卫星技术发展方向。科学合理地进行了多谱段及其不同分辨率的遥感器配置,使中巴地球资源卫星多种有效载荷技术性能和指标互补,综合技术指标先进,光学遥感器技术指标整体优势已经突显出中巴地球资源卫星的特色,瞻望发展前景遥感器技术水平将明显提高,卫星应用范围将日趋扩大。     

Landing of the asteroid probe with three-legged cushioning: Planar asymmetric dynamics and safety margin

The probes landing on the surfaces of the asteroids can increase the scientific return of the exploration missions and also promote the development of deep space resources. Because of its excellent applicability to the uneven terrain and a lighter configuration than the four-legged mechanisms, the three-legged cushioning mechanisms are suitable for dissipating the impact energy and then quickly stabilizing the probe attitude when the probe lands on the micro-gravitational surfaces of the asteroids. Research on the landing dynamics of the probe facilitates to the design of the landing-cushioning mechanism and the optimization of its configuration, as well as the assessment of the landing safety. Comparing with the previous extensive related literature focusing on landing dynamics of the probes assisted by the four-legged cushioning mechanisms, this paper studies creatively the planar dynamics considering the asymmetric characteristic and the leg-leg coupling to understand the landing process of the asteroid probe with the three-legged cushioning mechanism and thereby to optimize the configuration of cushioning mechanism and assess safety margin of the landing. According to the touchdown status, the asymmetric landing modes are classified and the coupling issue in the construction of the landing models is explained. Consequently, two types of dynamics models describing the two-stages touchdown cushioning process of the probe are established. Then, five significant configuration factors of the cushioning mechanism are extracted, and their values combinations are designed according to the Taguchi orthogonal method. On this basis, the maximum safe landing attitude angles of the probe are solved by using these values combinations as the input conditions under the dangerous situations in different landing modes. The range analysis and nonlinear fitting methods are employed to discuss the influence of the configuration factors on the landing safety margin, and the favorable parameter values of the configuration factors are determined. Next, the influence of the ground obstacle on the landing safety margin and several methods to improve the margin are researched. Finally, the complete attitude changes of the probe in two representative landing cases are analyzed. The results studied in this paper can contribute to configuration optimization of the three-legged cushioning mechanisms and safety assessment of the legged probes landing on the asteroids, as well as to provide a reference for discussing the leg-leg coupling issue received less attention in landing dynamics of the probes with the four-legged cushioning mechanisms.     

Trajectory prediction in pipeline form for intercepting hypersonic gliding vehicles based on LSTM

The interception problem of Hypersonic Gliding Vehicles (HGVs) has been an important aspect of missile defense systems. In order to provide interceptors with accurate information of target trajectory, a model based on an improved Long Short-Time Memory (LSTM) network for trajectory prediction pipeline is proposed for the interception of a skip gliding hypersonic target. Firstly, for trajectory prediction required by intercepting guidance laws, the altitude, velocity and velocity direction of the target are formulated in the form of analytic functions, consisting of linear decay terms and amplitude decay sinusoidal terms. Then, the dynamic characteristics of the model parameters are analyzed, and the target trajectory prediction pipeline is proposed with the prediction error considered. Finally, an improved LSTM network is designed to estimate parameters in a dynamically-updated manner, and estimation results are used for the calculation of the final trajectory prediction pipeline. The proposed prediction algorithm provides information on the velocity vector for midcourse guidance with the effect of prediction errors on interception taken into account. Simulation is conducted and the results show the high accuracy of the algorithm in HGVs’ trajectory prediction which is conducive to increasing the interception success rate.     

Very low thrust station-keeping for low Earth orbiting satellites

Traditional station-keeping for Earth observation satellites with chemical thrusters generally involves maneuvers every couple months that are able to change significantly the semi-major axis and the inclination. These strategies do not scale down to very low thrust level (a few hundreds of μN) electrical thrusters. This paper presents both in-plane and out-of-plane strategies that spread corrections over very long arcs and discretize them to tiny maneuvers every couple orbits, taking into account mission-constraints on maneuvers locations. These strategies scale up to medium thrust strategies, filling the gap between propulsion technologies. The out-of-plane strategy although features a new no-deadband property and controls the full orbital momentum. All strategies allow control very close to the reference (a few hundreds meters in osculating parameters) and very low cost.     

Improving the low orbit satellite tracking ability using nonlinear model predictive controller and Genetic Algorithm

The satellite motion on the reference orbit (RO) with less energy consumption has always persuaded researchers to design optimal control systems. The nonlinear nature and time-varying equations of motion make this quest more challenging. The present study proposes a novel control system for satellite motion on the RO by considering a comprehensive model of its dynamics in orbit and a Nonlinear Model Predictive Controller (NMPC). The NMPC calculates the sub-optimal control inputs of satellite motion reference on the elliptic orbit by minimizing a convex cost function at each stage. Moreover, all weighting parameters of the cost function are optimized by the Genetic Algorithm (GA) to produce less perturbation and guarantee the best NMPC performance. Finally, the implemented NMPC has been compared to a Linear MPC (LMPC). The results show that not only can the NMPC resist against larger errors and perturbations, but it can also compensate for those errors by returning the satellite to its main orbit and maintaining it.     

Closed-form solution of attitude command generation for spin-to-spin maneuver

In recent earth observing missions, agile satellites enable various imaging modes beyond the traditional along-track strip imaging. However, it requires maneuvering with boundary conditions of considerable angular velocity, i.e., spin-to-spin maneuvering. This paper proposes an attitude command generation method for spin-to-spin maneuvering that can provide feedforward commands for the attitude control loop. A general solution for arbitrary flight time is provided which steers a satellite to the given final attitude and angular velocity at the prescribed time. In addition, an alternative method is proposed that further improves the maneuvering speed, which is applicable to small-angle maneuvering cases. The proposed solutions are both closed-form which are more intuitive and easier to comprehend than numerical solutions. It also has a great advantage in computational efficiency, which could enable its use on-board in real time. Numerical examples demonstrate the performance of the proposed methods in a single maneuvering case as well as in a consecutive maneuvering case integrated with a realistic earth observing scenario.     

Asteroid interception and disruption for terminal planetary defense

Modern techniques for planetary defense from comets and asteroids involve the deflection of the bolide via kinetic, gravitational, ablative, or radiative means. While potentially effective, none of these methods are capable of operating in a terminal interdiction mode wherethe threat is discovered with little time prior to impact. We present a practical and effective method for planetary defense which enables extremely short interdiction time scales, but can also operate within longer time scales and can be effective for extremely large threats. Called PI (“Pulverize It”), the method makes use of an array of hypervelocity penetrators which uses the kinetic energy of the asteroid or comet to disrupt it. In the terminal interdiction mode, the fragments of maximum $～10$ m diameter disperse laterally as they continue towards the Earth, and then enter the Earth’s atmosphere where they burn up as a series of airburst events which spatially and temporally de-correlate the energy of the original parent bolide for any arbitrary observer on the ground in the form of acoustical shockwaves and optical pulses. We show that terminal interdiction modes ranging from 2 minutes prior to impact for 20-meter class bolides (such as the Chelyabinsk asteroid), 1 day prior to impact for 100 m-class asteroids, 10 days prior to impact for Apophis-class asteroids ($～370$ m), and even 60 days prior to impact for 1 km-class threats are all possible, though longer warning times are always preferred. Using only technologies readily available today, the PI method allows for a cost-effective and practical roadmap towards robust planetary defense capability.