行星（2019）

美国国家航♥空♥航♥天♥局“信使”号♥水星探测器发射升空
Lift off of Messenger on Nasa's mission to Mercury.
水 星
金 星
火 星
木 星
土 星
天 王 星
海 王 星
行星 第5集
在十分遥远的地方
Far, far away...
越过火星
..beyond Mars...
穿过木星的风暴和土星环
..past the storms of Jupiter, and the rings of Saturn...
太空中的环境变得截然不同
..conditions become very different.
这里的温度骤降 不同行星之间的距离
Temperatures plummet, and distances between worlds
不再是数千万公里 而是以数十亿公里来计算
are measured not in millions, but in billions of kilometres.
太阳系所有行星中最神秘的一颗就在这里
Here lie the most mysterious planets of them all.
天王星
Uranus.
一颗有着大理石纹理的淡蓝色星球
A pale blue marble hanging in the dark,
悬在黑暗冰冷的太空深处
frozen depths of space.
在更加遥远的地方
And, further out...
则是太阳系最后一颗真正的行星
..the solar system's final true planet...
海王星
..Neptune.
再往远处去 人们一度以为 那里只有体积很小 死气沉沉
Beyond, we thought we'd only ever find tiny, lifeless worlds...
内核完全冰冻的天体
..frozen to the core.
但我们完全错了
How wrong we were.
黑暗之地
太阳系外缘
在我们生活的这个时代 人类文明触及到了前所未有的空间范围
We live at a time when our civilisation has unprecedented
我们对火星保持了几乎长久性的探测
reach, and we have a near-permanent presence at Mars,
包括火星轨道和火星表面 我们发射的探测器
both in orbit and on the surface, and we've had spacecraft orbiting
已能环绕水星 金星 木星和土星完成长时间运行
Mercury, Venus, Jupiter and Saturn for extended periods.
但若要超出土星的距离 空间探测就变得困难许多
But, once we get beyond Saturn, things become much more difficult.
到达那里的距离极其遥远
The distances are vast.
飞行时间不是几个月
Travel times are measured not in one months,
而是几年甚至几十年 我们一次只能探访
but in years, decades, even. And that means that we've only
一颗太阳系外层的行星
visited the outer planets once.
每一次造访都是进入未知世界的远行
Each visit is a voyage into the unknown, as we strive
因为我们要前往人类所探索的最遥远的行星
to reach the most distant planetary systems ever visited.
当我们到达那里时 探测器的速度太快了
And, when we do get there, our spacecraft are travelling so fast
以至于它们只能径直飞过
that they fly straight through,
仅在行星范围内停留数小时
only spending a few hours in the system.
不过这几个小时里收集的数据
But those few hours of precious data have allowed us to begin
让我们得以开始了解那些隐藏在太阳系
to understand the mysteries that lie there on the outer edges
边缘空间的秘密
of the solar system.
“旅行者”2号♥探测器 1977年
点火完毕 发射完毕
We have ignition and we have liftoff.
用以探测太阳系外层空间
返回的报告表明两台固体发动机
Reports coming back indicate those twin, large, solid motors
运转完全正常
are functioning perfectly.
人类第一次飞向黑暗之地的任务能够完成 全都要归功于
Our first mission into the dark was only possible thanks to some
这些行星自身提供的帮助
help from the planets themselves.
每个行星的公转速度都是不同的
Each planet orbits at a different speed.
大部分时间里它们会分散在太阳的周围
And, for most of the time, they're scattered around the Sun.
但每隔175年
But once every 175 years...
就会出现一个相当奇妙的现象
..something rather wonderful happens.
太阳系外层的这几颗行星会排成一条线
The outer planets align...
这就打开了一条
..creating a path...
从地球出发 途经木星和土星的通道
..that leads from Earth past Jupiter and Saturn...
并且能一直通向天王星和海王星
..all the way to Uranus and Neptune.
“旅行者”2号♥正好赶在这个恰当的时机发射
Voyager 2 had been launched at precisely the right moment
以利用这一罕见的行星连珠现象
to take advantage of this rare planetary alignment.
它用了不到两年的时间到达木星
It arrived at Jupiter in a little under two years.
木星的质量是其它七大行星
Two and a half times more massive
质量总和的2.5倍还多
than all the other planets combined...
“旅行者”2号♥利用木星的引力把自己弹了出去
..Jupiter gave Voyager a gravitational kick.
又过了两年 “旅行者”2号♥见证了
Another two years on, and Voyager witnessed
太阳系中最美丽的景象之一
one of the most beautiful sights in the solar system.
另一个气态巨行星的冰环
The icy rings of another gas giant...
这就是土星
..Saturn.
然后探测器继续前进
And, then, on and on...
深入黑暗世界
..further into the darkness.
在离开地球将近9年后 “旅行者”2号♥逐渐靠近了
Almost nine years after leaving Earth, Voyager approached
一个全新类型的行星
an entirely new class of planet.
天王星 距离太阳29亿公里
与木星和土星一样 天王星表面主要是
Just like Jupiter and Saturn, the planet's upper atmosphere
氢气和氦气形成的旋涡流
is composed mostly of swirling hydrogen and helium gas.
在表面之下隐藏的是罕见的冰状混合物 含有甲烷
And hidden beneath lies an exotic, icy mix of methane,
氨和水
ammonia and water.
但与气态巨行星不同的是 天王星几乎没什么明显的动态特征
But, unlike the other gas giants, Uranus is almost featureless.
“旅行者”2号♥在整个观测过程中
For all the time that Voyager stared at the planet,
只看到了10个云层
it saw just ten cloud formations.
人们很快发现了其中的原因
And we soon discovered why.
天王星只有零下224摄氏度
Uranus, at -224 degrees Celsius,
是太阳系最冷的行星
is the coldest planet in the solar system.
也是第一颗被发现的冰巨星
The first of the ice giants,
它处于永久性深度冻结状态
in a permanent state of deep freeze.
“旅行者”2号♥在天王星附近只逗留了6个小时
Voyager spent just six hours with Uranus...
随着视野扩大 它看清了整个天王星系统
..and, as its gaze widened, it took in the entire system.
和土星一样
Just like Saturn...
天王星也有行星环
..Uranus has rings.
它的行星环十分微弱 暗淡 很难从地球上
The rings are so dark, so faint, that they're very difficult
观察到
to see from Earth.
事实上 人们直到1977年才发现它们
In fact, we didn't discover them until 1977.
这些行星环一定是由某种不易反射太阳光的物质组成的
They must be made of some kind of material that doesn't reflect
但人们不知道那是什么
a lot of sunlight back, and we don't know what that is.
一定不是像土星环那样的水冰
It can't be water ice like the rings of Saturn,
不然它们的亮度会更亮
because they would be much brighter.
这些光环极脆 极薄
They're also extremely delicate and thin,
而且十分令人不解的是 我们本以为环内的微粒
and, again, that's a great mystery, cos you'd expect the particles
会发生碰撞 行星环会随着时间推移而延展
to collide and the rings to spread out over time.
但这种情况并未发生
That doesn't happen.
所以这些光环一定是被什么东西固定住了
So, something must be holding those rings in place,
“旅行者”2号♥拍到的这张照片
and the answer can be seen in this photograph
可以给出答案
taken by Voyager 2.
可以看到一个很细的光环 它叫ε环
You can see a bright, thin ring, which is known as the epsilon ring,
在它上面和下面可以看到两颗卫星
and, above it and below it, you can see two moons.
里面那颗是天卫六 外面那颗是天卫七
The inner one is called Cordelia, and the outer one is called Ophelia.
现在我们需要了解一下运行的轨道
Now, the thing you need to know about orbits, a moon or a ring
运行轨道离行星近的卫星或行星环粒子
particle that's orbiting closer to a planet is moving more slowly
要比轨道远离行星的卫星或行星环粒子移♥动♥速度慢
than a moon or ring particle that's orbiting further away.
所以如果ε环内的粒子
And, so, if there's a particle in the epsilon ring that maybe
发生碰撞损失能量后 开始往下落
has a collision, loses a bit of energy and starts to drop down,
天卫六就会帮助粒子加速 重新回到环内
then Cordelia tends to accelerate that particle back up into the ring
在引力作用下 使它加速
again to speed it up by its gravitational influence.
反过来如果ε环外部边缘的粒子
Whereas, if it's a particle on the outer edge of the epsilon ring,
获得了更多能量 并开始上升
it gets a bit more energy and raises up in altitude,
那么天卫七就会让它减速
then Ophelia tends to slow it down,
并重新回到光环里
and, so, it falls back into the ring again.
通过这种机制 这些卫星保证了行星环
And, by that mechanism, these moons keep the ring nice
完整有序地环绕在天王星周围 也正是这个原因
and neat around Uranus, and, for that reason,
它们又被称为牧羊卫星
these moons have become known as shepherd moons.
目前我们还未发现更多的牧羊卫星
Now, we haven't discovered any more shepherd moons.
要知道“旅行者”2号♥只在天王星系统待了几个小时
Voyager was only in the system for a few hours.
不过我们认为还存在更多 它们环绕天王星运转
But we assume that they're there, orbiting around Uranus,
确保其它的行星环各归其位
keeping the other rings precisely in place.
消失的卫星并非天王星唯一让人疑惑不解的谜团
The missing moons are not the only mysterious thing about the planet.
这颗行星以独一无二的方式环绕太阳运转
Uranus orbits the Sun in a unique way,
这是很久以前发生的一系列事件导致的结果
a consequence of events that happened long ago.
46亿年前
天王星诞生于年轻太阳周围
Uranus was born from the vast cloud of material
巨大的团状物质之中
that surrounded the young Sun.
久而久之 这团物质
Over time, this material grew together
在引力作用下聚集到一起
under the influence of gravity...
每颗行星都是这样形成的
..forming each of the planets...
所有这些原始的气体尘埃云
..all orbiting in the same anticlockwise direction
都以逆时针方向旋转
as the primordial cloud of gas and dust.
这样的自转一直持续至今
This rotation remains to this day.