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号是两艘发射用以探索外太阳系行星及其星际环境的航天器之一。每一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、结构和成分；（2）描述行星卫星的形态、地质和物理状态；（3）提供行星、其卫星以及任何环的质量、大小和形状的改进值；（4）确定磁场结构，并表征其中捕获的带能粒子和等离子体的成分和分布。

航天器和子系统

每一艘旅行者号由一个十面体的舱体组成，舱体高度为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舱体顶部。科学仪器的主要部分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旋转的扫描平台，上面安装了成像和光谱遥感仪器。等离子体和带电粒子探测器也安装在了科学桁架的各个距离上。磁力计位于一个独立的桁架上，该桁架从科学桁架的对侧延伸出13米。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）也从航天器伸出，彼此垂直。航天器采用三轴自旋稳定，以实现长积分时间和扫描平台上仪器的选择性观测。

电源通过三个放射性同位素热电发电机为航天器系统和仪器提供。RTG串联安装在可展开的桁架上，该桁架通过附着在基本结构上的外伸支架铰接。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39千克。RTG使用放射性同位素（本例中为钚-238的氧化钚，或PuO2）作为热源，随着其衰变，释放出热量。使用双金属热电装置将热量转换为航天器的电能。随着放射性材料的消耗，RTG的总输出量随时间缓慢降低。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。随着功率的持续下降，航天器上的负载也必须相应减少。据1998年的估计，至少可以继续进行越来越有限的仪器操作，直到2020年。

通信通过高增益天线提供，并配备低增益天线作为备用。高增益天线支持X波段和S波段下行链路遥测。旅行者号是首次将X波段作为主要遥测链路频率的航天器。可以使用机载数字磁带记录器存储数据，以供稍后传送到地球。

由于旅行者号距离地球较远，导致指挥的时间延迟，因此其被设计为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预定时间发出指令。姿态和活动控制子系统（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

每艘旅行者号的一个侧面都安装了一个12英寸的金色镀铜盘。该磁盘记录了地球的声音和图像，旨在描绘地球上生命的多样性和文化。每个磁盘都装在一个保护性的铝制外壳中，以及一个卡式盒和针头。外壳上刻有解释航天器起源和如何播放磁盘的说明。在盖子的2厘米面积上电镀了一个超纯铀-238（放射性约为0.26纳居里，半衰期为45.1亿年），通过测量剩余的U238的子元素量，可以确定自发射以来经过的时间。磁盘上的115张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、自然和人造声音以及27首音乐片段）设计在1000转/分钟的速度播放。旅行者号并不是第一艘设计有此类信息传给未来的航天器。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板，尽管它们的雄心不如旅行者号。

任务概况

最初计划对外行星进行一次大巡游，包括1976-77年的木星、土星和冥王星的双发射，以及1979年的木星、天王星和海王星的双发射。由于预算限制，项目大幅缩减为两艘航天器，每艘航天器仅飞往木星和土星。新的任务被称为水手木星/土星任务，或MJS。在发射前六个月，该任务被更名为旅行者。缩减后的任务估计耗资2.5亿美元（至土星任务结束），仅为大巡游设计成本的1/3。

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。然而，最初看似吉祥的发射却证明了一系列问题的开始。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号晚发射16天，但旅行者1号的轨迹更快，于1977年12月15日，当两艘航天器都在小行星带时，超过了旅行者2号距离太阳的距离。

发射几个月后，1978年4月，旅行者2号的主要无线电接收器失效，自动启动备用接收器，但备用接收器也被证明是故障的。尝试恢复使用主要接收器的努力失败了，备用接收器被用于剩余的任务。尽管使用备用接收器使与航天器的通信更加困难，但工程师们找到了解决方案。

旅行者2号继续执行其主要任务，飞越了木星（1979年7月9日最近距离）和土星（1981年8月5日）。在这些飞越期间，旅行者2号获取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）。

尽管遭遇了困难，但科学家和工程师们已经能够使旅行者号获得巨大的成功。因此，批准了扩展任务，首先到天王星，然后到海王星，并继续在海王星之外进行观测。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）。由于这两个行星的距离更远，因此必须进行适应性调整，以适应较低的光照水平和减少的通信。旅行者2号成功获取了天王星及其卫星的大约8,000张图像。机载软件的改进和使用图像压缩技术使大约10,000张海王星及其卫星的图像得以拍摄。

旅行者2号上的所有实验都产生了有用的数据。

继续向外拓展

1989年，NASA在旅行者号与海王星相遇后将其重新命名为旅行者星际任务（VIM），旅行者2号继续运行，对星际磁场、等离子体和带电粒子环境进行测量，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的结束时，已耗资8.75亿美元用于建造、发射和运行两艘旅行者航天器。另外拨付了3000万美元用于VIM的前两年。

旅行者2号以约3.13 AU/年的速度从太阳飞离，向天空中的一个点飞去，该点的赤经为338度，赤纬为-62度（黄道纬度为-47.46度，黄经为310.89度）。