Voyager 2

Voyager 2 was one of a pair of spacecraft launched to explore the planets of the outer solar system and the interplanetary environment. Each Voyager had as its major objectives at each planet to: (1) investigate the circulation, dynamics, structure, and composition of the planet's atmosphere; (2) characterize the morphology, geology, and physical state of the satellites of the planet; (3) provide improved values for the mass, size, and shape of the planet, its satellites, and any rings; and, (4) determine the magnetic field structure and characterize the composition and distribution of energetic trapped particles and plasma therein.

Spacecraft and Subsystems

Each Voyager consisted of a decahedral bus, 47 cm in height and 1.78 m across from flat to flat. A 3.66 m diameter parabolic high-gain antenna was mounted on top of the bus. The major portion of the science instruments were mounted on a science boom extending out some 2.5 m from the spacecraft. At the end of the science boom was a steerable scan platform on which were mounted the imaging and spectroscopic remote sensing instruments. Also mounted at various distances along the science boom were the plasma and charged particle detectors. The magnetometers were located along a separate boom extending 13 m on the side opposite the science boom. A third boom, extending down and away from the science instruments, held the spacecraft's radioisotope thermoelectric generators (RTGs). Two 10 m whip antennas (used for the plasma wave and planetary radio astronomy investigations) also extended from the spacecraft, each perpendicular to the other. The spacecraft was three-axis spin stabilized to enable long integration times and selective viewing for the instruments mounted on the scan platform.

Power was provided to the spacecraft systems and instruments through the use of three radioisotope thermoelectric generators. The RTGs were assembled in tandem on a deployable boom hinged on an outrigger arrangement of struts attached to the basic structure. Each RTG unit, contained in a beryllium outer case, was 40.6 cm in diameter, 50.8 cm in length, and weighed 39 kg. The RTGs used a radioactive source (Plutonium-238 in the form of plutonium oxide, or PuO2, in this case) which, as it decayed, gave off heat. A bi-metallic thermoelectric device was used to convert the heat to electric power for the spacecraft. The total output of RTGs slowly decreases with time as the radioactive material is expended. Therefore, although the initial output of the RTGs on Voyager was approximately 470 W of 30 V DC power at launch, it had fallen off to approximately 335 W by the beginning of 1997 (about 19.5 years post-launch). As power continues to decrease, power loads on the spacecraft must also decrease. Current estimates (1998) are that increasingly limited instrument operations can be carried out at least until 2020.

Communications were provided through the high-gain antenna with a low-gain antenna for backup. The high-gain antenna supported both X-band and S-band downlink telemetry. Voyager was the first spacecraft to utilize X-band as the primary telemetry link frequency. Data could be stored for later transmission to Earth through the use of an on-board digital tape recorder.

Voyager, because of its distance from Earth and the resulting time-lag for commanding, was designed to operate in a highly-autonomous manner. In order to do this and carry out the complex sequences of spacecraft motions and instrument operations, three interconnected on-board computers were utilized. The Computer Command Subsystem (CCS) was responsible for storing commanding for the other two computers and issuing the commands at set times. The Attitude and Articulation Control Subsystem (AACS) was responsible for controlling spacecraft attitude and motions of the scan platform. The Flight Data Subsystem (FDS) controlled the instruments, including changes in configuration (state) or telemetry rates. All three computers had redundant components to ensure continued operations. The AACS included redundant star trackers and Sun sensors as well.

Message in a Bottle

Each Voyager has mounted to one of the sides of the bus a 12-inch gold-plated copper disk. The disk has recorded on it sounds and images of Earth designed to portray the diversity of life and culture on the planet. Each disk is encased in a protective aluminum jacket along with a cartridge and a needle. Instructions explaining from where the spacecraft originated and how to play the disk are engraved onto the jacket. Electroplated onto a 2 cm area on the cover is also an ultra-pure source of uranium-238 (with a radioactivity of about 0.26 nanocuries and a half-life of 4.51 billion years), allowing the determination of the elapsed time since launch by measuring the amount of daughter elements to remaining U238. The 115 images on the disk were encoded in analog form. The sound selections (including greetings in 55 languages, 35 sounds, natural and man-made, and portions of 27 musical pieces) are designed for playback at 1000 rpm. The Voyagers were not the first spacecraft designed with such messages to the future. Pioneers 10 and 11, LAGEOS, and the Apollo landers also included plaques with a similar intent, though not quite so ambitious.

Mission Profile

Originally planned as a Grand Tour of the outer planets, including dual launches to Jupiter, Saturn, and Pluto in 1976-77 and dual launches to Jupiter, Uranus, and Neptune in 1979, budgetary constraints caused a dramatic rescoping of the project to two spacecraft, each of which would go to only Jupiter and Saturn. The new mission was called Mariner Jupiter/Saturn, or MJS. It was subsequently renamed Voyager about six months prior to launch. The rescoped mission was estimated to cost $250 million (through the end of Saturn operations), only a third of what the Grand Tour design would have cost.

Voyager 2 was the first of the two spacecraft to be launched, with liftoff occurring 20 Aug. 1977. What was at first an auspicious launch, however, proved to be the beginning of a number of problems. The primary cause of the initial problems were attributed to commanding by the AACS, including difficulty in determining the full deployment of the science boom. These problems resulted in a delay of four days in the launch of Voyager 1 to ensure they wouldn't occur for it.

Although launched sixteen days after Voyager 2, Voyager 1's trajectory was the quicker one to Jupiter. On 15 Dec. 1977, while both spacecraft were in the asteroid belt, Voyager 1 surpassed Voyager 2's distance from the Sun.

Several months after launch, in April 1978, Voyager 2's primary radio receiver failed, automatically kicking in the backup receiver which proved to be faulty. Attempts to recover the use of the primary receiver failed and the backup receiver was used for the remainder of the mission. Although use of the backup receiver made communication with the spacecraft more difficult, engineers were able to find workarounds.

Voyager 2 proceeded with its primary mission and flew by Jupiter (closest approach on 09 July 1979) and Saturn (05 Aug. 1981). During these flybys, Voyager 2 obtained images roughly equal in number to Voyager 1 (18,000 at Jupiter, 16,000 at Saturn).

Voyager 2's launch date had preserved one part of the original Grand Tour design, i.e. the possibility of an extended mission to Uranus and Neptune. Despite the difficulties encountered, scientists and engineers had been able to make Voyager enormously successful. As a result, approval was granted to extend the mission, first to Uranus, then to Neptune and later to continue observations well past Neptune. Voyager 2 made successful flybys of Uranus (24 Jan. 1986) and Neptune (25 Aug. 1989). Because of the additional distance of these two planets, adaptations had to made to accomodate the lower light levels and decreased communications. Voyager 2 was successfully able to obtain about 8,000 images of Uranus and its satellites. Additional improvements in the on-board software and use of image compression techniques allowed about 10,000 images of Neptune and its satellites to be taken.

All of the experiments on Voyager 2 have produced useful data.

Onward and Outward

Rechristened the Voyager Interstellar Mission (VIM) by NASA in 1989 after its encounter with Neptune, Voyager 2 continues operations, taking measurements of the interplanetary magnetic field, plasma, and charged particle environment while searching for the heliopause (the distance at which the solar wind becomes subsumed by the more general interstellar wind). Through the end of the Neptune phase of the Voyager project, a total of $875 million had been expended for the construction, launch, and operations of both Voyager spacecraft. An additional $30 million was allocated for the first two years of VIM.

Voyager 2 is speeding away from the Sun at a velocity of about 3.13 AU/year toward a point in the sky of RA=338 degrees, Dec=-62 degrees (-47.46 degrees ecliptic latitude, 310.89 degrees ecliptic longitude).

旅行者2号是两艘被发射以探索外太阳系行星及其星际环境的航天器之一。每艘旅行者号在每颗行星的主要目标包括：（1）调查行星大气的循环、动力学、结构和组成；（2）描述行星卫星的形态、地质和物理状态；（3）提供行星、其卫星及任何环的质量、大小和形状的改进值；（4）确定磁场结构并表征其中捕获的能粒子与等离子体的组成和分布。

航天器和子系统

每艘旅行者号由一个十面体总线组成，总线高度为47厘米，从一面到另一面的宽度为1.78米。一个直径为3.66米的抛物面高增益天线安装在总线上方。大部分科学仪器安装在从航天器伸出约2.5米的科学桁架上。在科学桁架的末端是一个可调扫描平台，上面安装了成像和光谱遥感仪器。在科学桁架的各个距离处还安装了等离子体和带电粒子探测器。磁力仪位于一个独立的桁架上，沿桁架延伸13米，位于科学桁架相对的侧面。第三个桁架，从科学仪器向下延伸，携带了航天器的放射性同位素热电发电机（RTG）。两个10米的鞭状天线（用于等离子体波和行星射电天文学研究）也从航天器延伸出来，每个天线都垂直于另一个。航天器采用三轴自旋稳定，以便进行长积分时间和对扫描平台上仪器的选择性观测。

电源

通过使用三个放射性同位素热电发电机为航天器系统和仪器供电。RTG串联安装在可展开的桁架上，桁架通过附着在基本结构上的支架外伸装置铰接。每个RTG单元包含在一个铍制外壳中，直径为40.6厘米，长度为50.8厘米，重39公斤。RTG使用放射性源（本案中为钚-238的氧化钚形式，或PuO2），随着其衰变，会释放热量。一个双金属热电装置被用来将热量转换为航天器的电能。随着放射性材料的消耗，RTG的总输出量随着时间的推移而缓慢下降。因此，尽管旅行者号上RTG的初始输出功率约为发射时的470瓦30伏直流电，到1997年初（发射后约19.5年）已降至约335瓦。随着电力的继续下降，航天器上的电力负载也必须减少。1998年的估计表明，至少可以继续进行越来越有限的仪器操作，直至2020年。

通信

通过高增益天线进行通信，并配备低增益天线作为备份。高增益天线支持X频段和S频段下行遥测。旅行者号是第一个使用X频段作为主要遥测链路频率的航天器。数据可以通过机载数字磁带记录器存储，以供稍后传输到地球。

旅行者号由于其与地球的距离和由此产生的指挥延迟，被设计成以高度自主的方式运行。为了实现这一点并执行复杂的航天器运动和仪器操作序列，使用了三个相互连接的机载计算机。计算机指挥子系统（CCS）负责存储其他两个计算机的指令并在设定的时间发布指令。姿态和关节控制子系统（AACS）负责控制航天器姿态和扫描平台的运动。飞行数据子系统（FDS）控制仪器，包括配置（状态）或遥测速率的变化。所有三个计算机都具有冗余组件，以确保持续运行。AACS还包括冗余的星跟踪器和太阳传感器。

瓶中信

每艘旅行者号在总线的侧面安装了一个12英寸的金质铜盘。该磁盘记录了地球的声音和图像，旨在描绘地球上生命和文化多样性。每个磁盘都封装在一个保护性的铝制外壳中，以及一个盒式磁带和针。外壳上刻有解释航天器起源以及如何播放磁盘的说明。在封面上的2平方厘米区域内电镀了一个超纯铀-238（具有约0.26纳居里的放射性和4.51亿年的半衰期），允许通过测量剩余的U238和其子元素的数量来确定自发射以来的经过时间。磁盘上的115幅图像以模拟形式编码。声音选择（包括55种语言的问候语、35种声音、自然和人工声音以及27首音乐作品的片段）被设计在1000转/分钟的速度下播放。旅行者号并不是第一个设计有此类信息的航天器。先驱者10号和11号、LAGEOS和阿波罗着陆器也包含了具有类似意图的牌匾，尽管并不如此雄心勃勃。

任务概况

最初计划为外行星的巡回旅行，包括1976-77年对木星、土星和冥王星的两次发射，以及1979年对木星、天王星和海王星的两次发射，但由于预算限制，项目大幅缩减为两艘航天器，每艘航天器仅前往木星和土星。新的任务被称为旅行者木星/土星任务，或MJS。在发射前六个月，该任务被更名为旅行者。缩减的任务估计耗资2.5亿美元（至土星操作结束），仅为巡回旅行设计的成本的三分之一。

旅行者2号是两艘航天器中首先发射的，发射日期为1977年8月20日。最初，这是一次吉祥的发射，但后来证明是许多问题的开始。最初问题的主要原因是AACS的指挥，包括难以确定科学桁架的完全展开。这些问题导致旅行者1号的发射推迟了四天，以确保不会出现同样的问题。

尽管比旅行者2号晚16天发射，但旅行者1号的轨迹更快地到达了木星。1977年12月15日，当两艘航天器都在小行星带中时，旅行者1号超过了旅行者2号与太阳的距离。

发射几个月后，1978年4月，旅行者2号的主要无线电接收机失效，自动启动了备用接收机，该备用接收机也被证明是故障的。尝试恢复使用主要接收机的努力失败了，备用接收机被用于整个任务的剩余部分。尽管使用备用接收机使与航天器的通信更加困难，但工程师们能够找到解决方案。

旅行者2号继续执行其主要任务，在1979年7月9日和1981年8月5日分别飞越了木星和土星。在这些飞越期间，旅行者2号获得的图像数量与旅行者1号大致相等（木星18,000张，土星16,000张）。

尽管遇到了困难，但科学家和工程师们已经使旅行者号取得了巨大成功。因此，获得了批准，将任务扩展到天王星，然后到海王星，并最终继续在冥王星之外进行观测。旅行者2号成功飞越了天王星（1986年1月24日）和海王星（1989年8月25日）。由于这两颗行星距离更远，因此必须进行适应，以适应较低的光照水平和减少的通信。旅行者2号成功获得了大约8,000张天王星及其卫星的图像。机载软件的改进和使用图像压缩技术使得能够拍摄大约10,000张海王星及其卫星的图像。

旅行者2号上的所有实验都产生了有用的数据。

继续前行

在1989年，NASA在旅行者号与海王星相遇后，将其更名为旅行者星际任务（VIM）。旅行者2号继续运行，测量星际磁场、等离子体和带电粒子环境，同时寻找磁层顶（太阳风被更普遍的星际风吞没的距离）。到旅行者项目海王星阶段结束时，总共耗资8.75亿美元用于建造、发射和运行两艘旅行者航天器。另外分配了3000万美元用于VIM的前两年。

旅行者2号以每年约3.13 AU的速度远离太阳，朝着赤经338度，赤纬-62度的天空点前进。