了解宇宙是如何运行的（2010）

How the Universe Works S01E08

Part 3 April 18, 2021, 7:58 p.m.

它们从何而来?
Where did they come from?
为寻求答案
To try and find out,
太空总署派出卡西尼号♥探测器
NASA sent the Cassini probe
执行为期12年的任务
on a 12-year mission
研究土星、土星环和卫星
to study Saturn, its
rings, and its moons.
卡西尼号♥拍回了一幅最美丽的图片
We took with Cassini probably the most
beautiful picture that's ever beer taken,
不只我一个人这么说
and I'm not the only
one who has said this.
卡西尼被土星在太阳下的阴影盖着
Cassini was in the shadow
of Saturn cast by the sun.
所以见不到太阳
And so you don't see the sun,
见到背光的土星和美丽的光环
you see the backlit planet of
Saturn and its beautiful rings.
见到从土星边缘折射出来的阳光
You see the refracted image of the
sun poking out from the side of Saturn
在这美景当中, 有一颗小圆点
and nestled in all of that
splendour is this small little dot
那颗小圆点不是卫星
That tiny dot is not a moon.
那是接近十亿英里外的地球
That is the distant planet Earth
nearly a billion miles away.
我们对土星、土星环和卫星的认识
Most of what we know about
Saturn, of its rings and moons,
大部分来自卡西尼号♥
comes from Cassini.
之前我们以为只有8个环
Before Cassini, we thought
there were only eight rings.
今天见到30个以上
Today, we can see over 30.
土星哪里景物丰盛, 令人眼花缭乱
And what we have found at Saturn has been
just literally an embarrassment of riches.
这是以前见过的东西
We're seeing something
that we had seen before,
但现在见到的细节和清晰程度
but now we're seeing it with
a level of detail and clarity
令人瞠目结舌
that was just mind-blowing.
从前以为是土星约40亿年前形成后
Scientists used to think the rings
were made of the icy leftovers
剩下的冰粒子形成这些环
after Saturn was formed
about four billion years ago.
但历史这么悠久的物体
But anything that old should be covered
理应布满星尘和污垢
with cosmic dust and dirty.
因何土星环外表光洁如新?
So why does Saturn's rings appear
bright and clean, almost new?
指挥中心把卡西尼号♥移近土星环
To get the answer, mission control
manoeuvred Cassini close to the rings.
探测器发现
The probe saw that
环上冰块不停地撞击和破裂
all the ice pieces in the rings are
constantly colliding and breaking up.
每次撞击后都露出一些光洁的表面
And each collision exposes new
surfaces that are clean and polished.
以下是天文学家的推论
This is what astronomers think happened.
土星年轻时没有环, 只有大批卫星
When Saturn was young, it had
no rings, just lots of moons.
在某一刻, 有颗冰冷彗星不速而至
At some point, an icy comet
zoomed in from deep space
撞向其中一颗卫星
and smashed into one of those moons.
彗星分♥裂♥成数十亿碎片
The comet broke up
into billions of pieces.
撞击力亦把卫星推近土星
The impact also pushed
the moon closer to Saturn
土星的强大引力令卫星解体
where the planet's enormous
gravity broke it up.
卫星的碎砾和彗星的冰混合起来
Now debris from the moon
and ice from the comet mixed.
在引力拉动下, 逐渐形成环状
Gradually, Saturn's gravity pulled all
those fragments into rings around it
卫星全靠引力造就
The story of moons is
the story of gravity.
引力把它们固定在轨道上
Gravity holds them in orbit.
使卫星核心发热、造成不同的表面
It heats up their insides
and shapes their surfaces.
主宰卫星的一切, 甚至生死存亡
In the end, it controls everything about
moons, even their survival and destruction.
引力更可以掳劫小行星、彗星
Gravity can even create new moons
以至整颗行星, 把它们变成新卫星
by kidnapping asteroids,
comets, and even whole planets.
我们知道, 引力可以创造卫星
We know that gravity makes moons.
标准的办法是集结…
The standard way is to assemble them
行星诞生时留下的星尘去砌成
from debris leftover
when planets are formed.
但另一个办法是: 俘掳它们
But gravity makes moons a second
way, too: it captures them.
试想像一颗彗星或小行星
Imagine a wandering comet or asteroid,
somehow it gets knocked off course.
被撞离轨道, 靠近某颗行星
It wanders too close to a planet
引力像科幻小说中的牵引波束
Gravity acts like a science fiction
把它抓住
tractor beam and grabs it.
如果引力不够强, 它便可逃脱
Not quite enough
gravity, and it escapes.
如果引力太强, 它便撞向行星
Too much gravity, and it
collides with the planet
引力怡到好处, 它便环绕行星运转
Just enough, and the comet or asteroid
goes into orbit around the planet
成为新卫星
and becomes a new moon.
火星有两颗小卫星
Mars has two tiny moons
定名为火卫一和火卫二
named Phobos and Deimos,
都是被俘掳的小行星
both are captured asteroids.
一颗在环绕行星时不断向外推
One is pushing outward
as it circles the planet
终有一天会脱离, 继续遨游外层空间
and will eventually break free and
continue on its journey through space.
另一颗不断向内转向火星
The other is circling inwards, a
little closer to Mars all the time.
终有一天会相撞
Eventually, it will smash into it
只是颗3英里宽的小行星
it's an asteroid, really,
just three miles across.
但有时被称为地球的第二颗卫星
But it's sometimes described
as Earth's second moon.
1986年发现了小小的克鲁特尼之后
With the little object Cruithne,
which was discovered back in 1986,
我们开始研究
we start to get into this realm
卫星的定义
of, of what does it mean to be a moon.
不过数千年之前
Only a few thousand years ago,
克鲁特尼是颗普通的小行星
Cruithne was an ordinary asteroid
像数十亿同类, 环绕太阳运行
orbiting the sun like
billions of others,
但后来它脱离了小行星带的轨道
but eventually, it wobbled out
of its orbit in the asteroid belt
被地球的引力吸去
and got snagged by Earth's gravity.
但克鲁特尼的举动很不寻常
But then Cruithne did something unusual.
它不像正常的卫星那样环绕地球
Instead of orbiting around
the Earth like a normal moon,
而是跟在地球后面
Cruithne began to follow behind it
所以我们可把它当成地球的卫星
And so one might call this
sort of a moon of the Earth,
但又不是完全贴切
not exactly, though, because
that object is designed,
因为它是环绕太阳, 不是地球
you know, it's on its own independent
orbit around the sun, not the Earth.
有时小行星也俘掳卫星
Sometimes asteroids
capture their own moons.
1993年, 伽利略宇宙飞船
In 1993, the Galileo spacecraft
飞过小行星埃达
flew past the asteroid Ida
有出人意表的发现:
and found something nobody expected:
一颗细小、宽半英里的卫星
a tiny, half-mile wide moon.
这只是宇宙飞船经过的第二颗小行星
The fact that we saw a satellite
around only the second asteroid
便发现它有颗卫星
ever to be encountered with a spacecraft
显然, 小行星有卫星是寻常现象
immediately tells us that moons around
asteroids must be incredibly common.
被俘掳的卫星不一定细小
Not all captured moons are small.
最大的被俘掳卫星是海卫一
The mother of all
captured moons is Triton.
它环绕海王星运行, 十分巨大
It orbits the planet
Neptune and it is big,
直径大约1,700英里
about 1,700 miles in diameter.
但海卫一是颗不寻常的卫星
But Triton is a moon
with an unusual story.
海卫一令行星学家感到大惑不解
Triton was a very puzzling
problem for planetary scientists
因为传统观点是…
because our traditional views would
tend to make all the moons orbit
卫星跟着行星的方向转动
in the same direction of
the planet itself spins.
海卫一环绕海王星的方式却不同
In the case of Triton around
Neptune, it's the other way around.
海王星向这边转动
Neptune is spinning this way,
海卫一向相反方向运行
Triton is orbiting around
in the opposite direction.
这显示它不像大部分卫星…