船瀬 龍

Water is one of the most suitable propellants for small spacecraft because it can be stored at low pressure, is safe and abundant on earth. Although propulsion systems using water as a propellant were already operated in space, but the desired thrust was not obtained because the gas-liquid separation was not successful, The University of Tokyo has developed a propulsion system with a vaporization chamber for gas-liquid separation, AQUARIUS. In this research, it was demonstrated whether the required performance could be obtained by changing the method of supplying the propellant to the propulsion system provided with the vaporization chamber.

DELPHINUS is a camera system mounted on EQUULEUS, which is planned to be launched using NASA's Space Launch System EM-I in 2021. DELPHINUS aims to investigate size distribution, influx ratio, and daily variation of meteoroids in the cislunar space through observations of lunar impact flashes (LIFs) from the far side of the moon. DELPHINUS will observe the moon's surface with the 60-fps camera modules to capture the flashes that are short duration phenomena. All image data cannot be downlinked due to constraints in memory size and communication capability. Therefore, an on-board image processing algorithm was developed to reduce downlinked data size by extracting only necessary pixel data including LIFs. Three experiments using three simulators were demonstrated to verify the real-time processing performance and detection capability. This paper reports the details of the proposed algorithm and the verification results.

For the realization of the complicating missions by small spacecrafts, it is necessary to establish the micro-propulsion technology. AQUARIUS (AQUA ResIstojet propUlsion System) is a micro-propulsion system which is going to be installed on a 6U CubeSat EQUULEUS (EQUilibriUm Lunar-Earth point 6U Spacecraft) and launched in 2020. AQUARIUS uses water as a propellant. The advantages of using water mainly come from two points. First, it exits as a liquid in standard state. Second, it is non-toxic substance. These advantages lead to the easier handling, which results in the cost reduction and the period shortening of the development. However, one of the major disadvantages is that its latent heat 2400 J/g is large compared to the other propellants used in resistojet thrusters. In AQUARIUS, the waste heat of the high temperature devices was reutilized to compensate the latent heat. AQUARIUS is composed of mainly three components: a tank for containing water, a vaporization chamber for vaporization of water, and a thruster head for thrust generation. Up to now, AQUARIUS Flight Model was assembled, and the several kinds of experiments were conducted. In this study, the latest development status of it was shown, mainly focusing on the two types of thrust measurement because the thermal environment was different between AQUARIUS alone and that installed on the spacecraft. From the results, it was estimated that the operational parameters were appropriately chosen, and that the reutilization of the waste heat was successfully conducted in the spacecraft.

<p>超小型衛星向けの推進系の開発が世界中で行われつつある中，世界初の50 kg級の超小型深宇宙探査ミッションPROCYONのように，実際に科学・探査ミッションを行うために，実証目的ではなくミッション要求を満たせるような推進系を搭載する例も出てきている．本稿では，東京大学におけるこれまでのミッション経験や検討中の将来ミッションをベースに，今後の宇宙科学・探査ミッションの例として，小天体探査・惑星探査等の深宇宙探査ミッションや，フォーメーションフライトミッションを取り上げ，今後必要となるであろう超小型推進系の性能例やその考え方を提示した．超小型衛星ミッションでは，厳しいリソース制約の中で設計を最適化する必要があるため，推進系には，スラスタ単独の性能向上以外の観点での要求が多数存在する．今後，探査機システム側との密な連携により，真に使い勝手のよい高性能な推進系の研究開発が進むことを期待したい．</p>

Navigation and mutual communication are indispensable functions to manage a spacecraft formation flight. To achieve these functions, this paper presents a navigation and communication network architecture based on ultra-wide band (UWB) communication devices, which have been widely used for many applications like indoor-localization and navigation. By measuring distances between pairs of spacecraft, it is possible to estimate the relative positions and velocities of spacecraft in the formation. When it comes to a large-scale formation, a limited number of distances between two spacecraft can be measured by UWB devices at any one time due to physical limitations, such as available communications bandwidth. In this sense, it is necessary to select measured pairs of spacecraft to efficiently achieve high estimation accuracy. We propose an algorithm for the optimal selection of measure pairs of spacecraft to increase the estimation accuracy of the positions of spacecraft in the formation. The performance of the proposed method is evaluated in the simulation.

In order to deeply understand orbital disturbances, the flight data of the PROCYON, which is the 50kg-class interplanetary micro-spacecraft was analyzed. In the telemetry data, we found two unexpected behaviors of angular momentum in Z-axis as compared with the accurate solar radiation pressure model. In order to clarify the causes of the angular momentum anomalies, several small disturbances like thermal radiation pressure, deformation of the structure, and interplanetary magnetic field effect, which are usually ignored are discussed in this study. The thermal radiation and deformation of the structure can explain the over-large Z-axis anomaly. The interplanetary magnetic field effect is correlated with the sudden change of Z-axis torque anomaly in several cases, but the cause of the anomaly is not completely revealed yet.

Autonomous orbit determination method using active maneuvers and intersatellite ranging between multiple spacecraft is applicable to general dynamics situations, but large uncertainty of information about maneuvers results in inaccurate orbit estimation. This paper proposes an accurate and robust estimation method using sequential filter, RTS smoother, and EM algorithm. Proposed method estimates not only states but also maneuver results. Results from simulations of Mars-Phobos system show that the proposed method improve orbits determination accuracy.

Recent low-thrust space missions have highlighted the importance of designing trajectories that are robust against uncertainties. In its complete form, this process is formulated as a nonlinear constrained stochastic optimal control problem. This problem is among the most complex in control theory, and no practically applicable method to low-thrust trajectory optimization problems has been proposed to date. This paper presents a new algorithm to solve stochastic optimal control problems with nonlinear systems and constraints. The proposed algorithm uses the unscented transform to convert a stochastic optimal control problem into a deterministic problem, which is then solved by trajectory optimization methods such as differential dynamic programming. Two numerical examples, one of which applies the proposed method to low-thrust trajectory design, illustrate that it automatically introduces margins that improve robustness. Finally, Monte Carlo simulations are used to evaluate the robustness and optimality of the solution.

Temporarily-captured orbiters (TCOs) are a new population of asteroids that are temporarily gravitationally bound around the Earth-Moon system. Because of its small geocentric distance and energy, short-term exploration with small Ay is expected possible. This study aims to construct low-energy transfers to 2006 RH120, one of the TCOs, from low-Earth orbit using an analogy with the Earth-Moon low-energy transfers. The initial guess was sought by back-propagating perturbed 2006 RH120's trajectory, then it was optimized through the direct multiple shooting method. The result shows that various transfers are possible with rendezvous Ay below 100 m/s, and they form diverse family structures.