Cassini

The Cassini Orbiter mission consists of delivering the Huygens probe, which was provided by the European Space Agency, ESA, to Titan, and then remaining in orbit around Saturn for detailed studies of the planet and its rings and satellites. The principal objectives are to:

* 1) Determine the three-dimensional structure and dynamical behavior of the rings
* 2) Determine the composition of the satellite surfaces and the geological history of each object
* 3) Determine the nature and origin of the dark material on the Iapetus leading hemisphere
* 4) Measure the three-dimensional structure and dynamical behavior of the magnetosphere
* 5) Study the dynamical behavior of Cronian atmosphere at cloud level
* 6) Study the time variability of Titan's clouds and hazes
* 7) Characterize Titan's surface on a regional scale

* Spacecraft and Subsystems
* =========================

The spacecraft was originally planned to be the second three-axis stabilized, radioisotope thermoelectric generator, RTG, powered Mariner Mark II, a class of spacecraft developed for missions beyond the orbit of Mars. However, various budget cuts and rescopings of the project forced a more specialized design, postponing indefinitely any implementation of the Mariner Mark II series.

Cassini is the largest interplanetary spacecraft ever constructed by NASA. It measured 6.8 m in length with a 4 m high gain antenna. At launch, the spacecraft had a mass of 5,655 kg of which 3,132 kg were propellant.

The orientation of Cassini was maintained through the use of either three reaction wheel assemblies mounted along orthogonal axes on the spacecraft, for fine pointing control or via 16 0.5 N thrusters for coarse pointing control. The thrusters were arranged in four groups of four and used hydrazine. The orientation was determined through the use of either three inertial reference units that used solid-state gyroscopes or a star tracker, which detects stars in its field of view and compares them with an on-board catalog of 5,000 stars. The thrusters were also used in the alteration of the spacecraft trajectory of less than 5 m/s. For trajectory corrections of greater than 5 m/s, one of two identical main engines, of which one served as a backup, was used. The engines were gimbaled so that thrust could can be maintained through the spacecraft center of mass and burn the bipropellants nitrogen tetroxide and monomethyl hydrazine.

Power was provided to the spacecraft through the use of three RTGs. Each RTG used the heat generated by the decay of 10.9 kg of plutonium dioxide, PuO2, to generate electrical power to be used throughout the spacecraft. At the beginning of the mission, each RTG was capable of producing 300 W of electrical power. By the end of the nominal 11 yr mission, the output was expected to degrade to around 210 W per RTG. The resultant electricity, a regulated 30 V DC source, was used not only to provide power to the various science instruments and spacecraft subsystems, but also to one-time pyrotechnic devices that were used in the course of the mission, such as to separate the the spacecraft from the Centaur launch vehicle or to separate the Huygens probe from Cassini.

Although some data will be transmitted in real time, much science data and spacecraft health and status information were recorded on a solid-state data recorder. Although such systems had been used previously on other missions, Cassini was the first to use one in deep space. The spacecraft was equipped with two recorders, each of which had a capacity of 2 Gb in the form of dynamic random access memory, DRAM. Because such memory is vulnerable to radiation effects, the recorders were encased in half-inch thick aluminum. Nonetheless, degradation of the recorders due to solar and cosmic ray activity was expected to reduce their capacity by about 10% by the end of the mission. Recorded data were then periodically transmitted to Earth via the HGA and erased.

卡西尼探测器任务旨在将欧洲航天局（ESA）提供的惠更斯探测器送达土星卫星泰坦，并在土星轨道上停留以进行对该行星及其环和卫星的详细研究。其主要目标包括：

* 1) 确定环系的三维结构和动力学行为
* 2) 确定卫星表面的组成以及每个天体的地质历史
* 3) 确定伊阿珀图斯半球暗物质的本性和起源
* 4) 测量磁层的三维结构和动力学行为
* 5) 研究克洛尼亚大气在云层水平的动力学行为
* 6) 研究泰坦云层和雾霾的时间变化性
* 7) 在区域尺度上表征泰坦的表面

* 航天器和子系统
* =========================

航天器最初计划为第二个三轴稳定、同位素热电发电机（RTG）供电的航海者马克II型，这是一类专为火星轨道以外的任务开发的航天器。然而，由于项目预算削减和重新规划，迫使采用更专业的设计，无限期推迟了航海者马克II系列的任何实施。

卡西尼是美国国家航空航天局（NASA）建造的最大星际航天器。其长度为6.8米，配备4米高的全向天线。发射时，航天器的重量为5,655千克，其中3,132千克为推进剂。

卡西尼的定向通过在航天器上沿正交轴安装的三组反应轮组件，用于精确指向控制，或通过16个0.5N推力器，用于粗略指向控制来维持。推力器被分成四组，每组四个，并使用肼作为推进剂。定向是通过使用三个使用固态陀螺仪的惯性参考单元或星跟踪器来确定的，星跟踪器检测其视场内的恒星，并将其与航天器上的5,000颗恒星目录进行比较。推力器还用于改变航天器小于5米/秒的轨迹。对于大于5米/秒的轨迹校正，使用两个相同的备用主发动机之一，其中一个作为备份。发动机可以偏转，以保持推力通过航天器质心，并燃烧四氧化二氮和单甲基肼的双推进剂。

航天器的电力通过三个RTG提供。每个RTG利用10.9千克钚二氧化物（PuO2）衰变产生的热量来产生用于整个航天器的电能。在任务开始时，每个RTG能够产生300瓦的电能。到11年标准任务结束时，预计输出将降至每个RTG约210瓦。由此产生的电能，一个调节的30伏直流电源，不仅用于为各种科学仪器和航天器子系统供电，而且还用于任务过程中的一次性烟火装置，例如将航天器与天顶号运载火箭或将惠更斯探测器与卡西尼分离。

尽管一些数据将实时传输，但大量科学数据和航天器健康及状态信息被记录在固态数据记录器上。虽然此类系统已在其他任务中使用过，但卡西尼是第一个在深空中使用此类记录器的。航天器配备了两个记录器，每个记录器以动态随机存取存储器（DRAM）的形式具有2千兆字节的容量。由于此类内存易受辐射效应的影响，记录器被封装在半英寸厚的铝中。尽管如此，由于太阳和宇宙射线活动，预计到任务结束时，记录器的容量将减少约10%。记录的数据随后定期通过高增益天线（HGA）传输到地球，并删除。