朱诺号为什么要从「极地轨道」研究木星，这么做的好处是什么？ 第1页

以下内容全部来自NASA的Juno介绍手册

20160705更新，结尾加了彩蛋~

一句话概括：既要有足够近的观测，又要最大限度避开木星的高辐射带。

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极轨的设计当然和增加观测覆盖面等等有关，毕竟极区一般来说都是深空观测的盲区（或者数据质量很差），但最主要的还是和Juno的探测目标有关。

Juno的主要目标是揭示木星的形成和演化。具体探测任务就是要以前所未有的高精度探测木星的重力场、磁场、大气、成分、以及内部结构同外部磁场大气共同作用的耦合机制等等，重点是“高精度”。要达到这么高的目标精度，就必须离木星足够近，一直离得足够近吧又容易受到木星辐射带的损害，估计很快就坏掉了（多费钱啊……）于是只好通过极轨设计来勉强折衷一下咯。而且就算这样估计Juno的预期寿命也不会很长，Galileo飞了八年就是靠离得远啊。

↓ Juno的极轨设计，最近可达离木星4200km， 最远将达7900km（截自NASA's Eyes）

其他附加的好处自然也有一堆：

• 可以让Juno的三块巨大的太阳能电池板有持续光照（轨道全部避开了木星的阴影区；太阳到木星的光照强度只有到地球的1/25）
  
• 全球覆盖（观测可以重复覆盖所有纬度，加上木星的自转，还可以覆盖所有的经度）
  
• 可以近距离仔细观测木星的极区（高清影像，带电粒子，磁场，同步观测太阳系内最亮的极光等等）
  
• 获得极区的三围磁层结构（跟前面几条也差不多）
  
• ……

最后附Juno任务具体的科学目标，总之就是：绝大多数轨道设计都是在现有技术制约之下为实际探测目标服务的。

Juno Science Objectives
The primary science objectives of the mission are as follows:
Origin
Determine the abundance of water and place an upper limit on the mass of Jupiter’s possible
solid core to decide which theory of the planet’s origin is correct.
Interior
Understand Jupiter’s interior structure and how material moves deep within the planet by
mapping its gravitational and magnetic fields.
Atmosphere
Map variations in atmospheric composition, temperature, cloud opacity and dynamics to depths greater than 100 bars at all latitudes.
Magnetosphere
Characterize and explore the three-dimensional structure of Jupiter’s polar magnetosphere and auroras.
The overall goal of the Juno mission is to improve our understanding of the solar system by understanding the origin and evolution of Jupiter. It addresses science objectives central to three NASA Science divisions: Solar System (Planetary), Earth–Sun System (Heliophysics), and Universe (Astrophysics).
Juno’s primary science goal of understanding the formation, evolution, and structure of Jupiter is directly related to the conditions in the early solar system, which led to the formation of our planetary system. The mass of Jupiter’s possible solid core and the abundance of heavy elements in the atmosphere discriminate among models for giant planet formation. Juno constrains the core mass by mapping the gravitational field, and measures through microwave sounding the global abundances of oxygen (water) and nitrogen (ammonia).
Juno reveals the history of Jupiter by mapping the gravitational and magnetic fields with sufficient resolution to constrain Jupiter’s interior structure, the source region of the magnetic field, and the nature of deep convection. By sounding deep into Jupiter’s atmosphere, Juno determines to what depth the belts and zones penetrate. Juno provides the first survey and exploration of the three-dimensional structure of Jupiter’s polar magnetosphere.

加个彩蛋，发现NASA的NASA之眼更新了Juno模块，可以实时跟进Juno的进度哦~刚试了一下，还比较炫酷~下载地址可戳

↓ 打开是这样

↓ 进入Juno模块

↓ 从Juno的角度看木星

↓ 俯视木星系统，可以看到Juno的实时位置

↓ 同时显示木星的内部结构（我没显示，觉得有点丑）、辐射场、磁场、极光、木星环（我咋没看到）

↓ 换个视角